4. Hvac Practical Journal.pdf

TABLE OF CONTENTS S. No.

Object

01

Demonstration of vapor compression refrigeration cycle with visual observation of the important processes. Determination of the effect of evaporating and condensing temperatures on the refrigeration rate and condenser heat output. Investigation of the effect of compressor pressure ratio on system performance. Generation of a refrigeration cycle diagram on a pressure-enthalpy chart. To study the various parts of Compressor used in the domestic Refrigerators and Air-conditioners. Dismantling and Reassembling parts and record the reading on pressure gauge (Low pressure and High pressure). To assess the performance of a typical water chiller by applying varying loads to the system. To study the construction and Working Mechanism of DEW POINT. To study the various parts, Construction and Working Mechanism of “Aspiration Psychrometer” & To calculate the various properties of moisture air (with the help of dry and wet bulb temperature) To study the various parts, Construction and Working Mechanism of “Hair Hygrograph” & To record the “Relative Air Humidity” in the function of time and to maintain the Weekly Graph. To study the main parts, construction, and working mechanism of the domestic refrigeration trainer. To study main components construction and working mechanism of air conditioner. (To record the suction & exhaust pressure of the domestic air conditioner). To study the Construction and Working Mechanism of Vacuum Pump. To charge the Refrigerant (R-12) gas in a Domestic Refrigerant System.

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1

Practical No. 1 Object: Demonstration of vapor compression refrigeration cycle with visual observation of the important processes. Equipment/Parts required: Refrigeration Cycle Demonstration Unit (R633)

Fig.1.1 Theory: The refrigeration or heat pump cycle: A refrigerator is defined as a machine whose prime function is to remove heat from a low temperature region. Since energy cannot be destroyed, the heat taken in at a low temperature plus any other energy input must be dissipated to the surroundings. If the temperature at which the heat is dissipated is high enough to be useful, e.g. for space heating, the machine is then called a heat pump. By selective design of the components the cycle may be optimized either for heat pump applications or for refrigeration applications. Indeed, under certain applications both useful functions may be performed by one machine where circumstances permit. For example, in a dairy where refrigeration is required for milk cooling and storage and hot water is required for bottle or tank washing. The Clausias statement of the Second law of thermodynamics states that heat will not pass from a cold to a hot region without the aid of an “external agency”. Thus, a refrigerator will require an “external agency”, i.e. an input of high grade energy for it to operate. This energy input may be in the form of work, or a heat transfer at a high temperature. The most common type of refrigerator or heat pump uses a WORK INPUT and operates on the VAPOR COMPRESSION CYCLE. 2

Vapor compression cycle: The work input to the vapor compression cycle drives a compressor which maintains a low pressure in an EVAPORATOR and a high pressure in CONDENSER. The temperature at which a liquid will evaporate (or a vapor will condense) is dependent on the pressure, thus if a suitable fluid is introduced it will evaporate at a low temperature in the low pressure evaporator (taking in heat) and will condense at a higher temperature in the high pressure condenser (rejecting heat). The high pressure liquid formed in the condenser must then be returned to the evaporator at a controlled rate. Thus, the simple vapor compression refrigeration cycle has four main components. An evaporator where heat is taken in at a low temperature as the liquid evaporates at a low pressure. A compressor which uses a work input to reduce the pressure in the evaporator and increase pressure of the vapor being transferred to the condenser. A condenser where the high pressure vapor condenses, rejecting heat to its surroundings. A flow control device which controls the flow of liquid back to the evaporator and brings about the pressure reduction. The refrigeration cycle is most interesting from the thermodynamic view point. It is one of the few practical plants which operates on a true thermodynamic cycle and involves nucleate boiling and film wise condensation. Steady flow processes, i.e. throttling, compression and heat exchange. Flow control The thermodynamic properties, i.e. pressure, specific volume, temperature, specific enthalpy and specific entropy of a pure substance at all conditions between sub-cooled and super-heated vapor. Description: (Refer schematic diagram of refrigeration cycle demonstration unit R633 Fig. 1.2) All components are mounted on an attractive durable glass reinforced plastic panel. The evaporator is a vertical glass cylinder with plated end plates. A helical coil of copper tube conveys water through a pool of refrigerant in the cylinder. The compressor draws vapor from the evaporator

Fig.1.2 3

Thereby reducing pressure in the evaporator and this causes the refrigerant to boil at a low temperature. In order to boil, or change phase from liquid to vapor, heat is required and this is extracted from the water passing through the copper coil and to a lesser extent from the surroundings. As heat has been extracted from the water, its temperature is reduced. Vapor from the evaporator is drawn into the compressor casing and then into the compressor itself where its pressure is raised before being discharged into the condenser. Having had work done on the gas, its temperature and pressure is increased. The condenser is also a vertical glass cylinder fitted with plated metal end plates, the upper one supporting a helical coil of tube through which cooling water flows. The hot high pressure vapor from the compressor cools and condenses as it transfers heat to the cooling water inside the nickel plated copper coil. As heat is transferred to the cooling water, its temperature is increased. The cooled high pressure liquid collects in the bottom of the condenser and its level controls a float operated expansion valve. This valve reaches an equilibrium position and discharges refrigerant liquid back to the evaporator at the same rate as it is formed. As the warm high pressure liquid passes through the valve seating its pressure decreases to that in the evaporator and its temperature must fall to the saturation temperature at the lower pressure. The fall of temperature is accompanied by the formation of vapor bubbles and these may be seen through the sight glass fitted in the pipe returning the liquid/vapor mixture to the evaporator. On entering the evaporator, the low pressure liquid and vapor separate, the liquid passing into the “pool” for re-evaporation, while the vapor mixes with the vapor produced by the boiling action of the water coil. The vapor mixture then returns to the compressor to repeat the cycle.

Fig.1.3 4

Demonstration of vapor compression refrigeration cycle: The experiment should begin with the unit at rest, having been left in the shutdown condition for some time in order for all the components to be at a similar ambient temperature. Open the ball valves on the cylinders as for normal operation but do not turn on the unit and do not turn on the water supply to the evaporator and condenser coils. Turn on the unit and water supplies for normal operation. 1. Note that as the compressor draws vapors from the evaporator the pressure in the evaporator falls. Similarly, as the vapor is compressed by the compressor and passed to the condenser, the pressure in the condenser rises. 2. As the pressure in the evaporator falls, the liquid will begin to boil due to the reduced pressure. During the boiling action the refrigerant changes from the liquid to the vapor phase. (Reference should be made to the p-h diagram). In order to change phase at constant pressure, energy is required to increase the enthalpy of the vapor. This energy is taken from the water passing through the evaporator coil, depending upon the water inlet temperature and the local ambient temperature. If the water supply temperature is high (approx.: 16⁰C or more) then the boiling action should be readily visible from several points on the submerged evaporator coil. In order to promote evaporation from the coil surface, the coil has been specially treated to provide many bubble nucleation sites. If the water temperature is low then the evaporator pressure will need to reach a lower value and boiling may occur from single sites on the coil, from the surface of the liquid adjacent to the coil / surface interface or from the base plate of the evaporator. To induce further evaporation from other sites, open the ball valve at the base of the evaporator. DO NOT OPEN THE CHARGING VALVE as this will allow air to enter the system. Opening the ball valve at the base of the evaporator will cause the oil return capillary to become part of the evaporator and the resulting large increase in heat transfer surface area relative to the small volume of liquid in the capillary will result in vapor appearing from the base of the chamber. If the capillary tube is touched under these operating conditions, then the surface will feel cold. When the vapor bubbles are being produced from the evaporator coil within the evaporator chamber then heat is being extracted from the cooling water flowing through the coil. If the evaporator inlet water temperature t1 is examined after several minutes‟ operation and compared with the water discharge temperature t2, the discharge temperature should be found to be slightly lower than the inlet temperature. In order to increase the apparent temperature difference, the evaporator water flow rate may be reduced. If the evaporator cooling water flow rate is stopped completely then it is likely that boiling from the water coil will stop and the evaporator pressure will reduce further until another source of heat is found. This is most likely to be the base plate as heat is conducted from the outside air. In addition, depending upon the local ambient conditions, water vapor will also condense on the outside surface of the glass cylinder and base plate. The change in phase of the water vapor to a liquid will in itself provide heat to cause evaporation of the refrigerant in the chamber. In addition, if the evaporator cooling water flow is stopped then the rate of condensation forming on the 5

condenser coils will also reduce due to the reduced vapor generation rate in the evaporator and the reduced volumetric efficiency of the compressor and increase in specific volume of the vapor generated in the evaporator chamber at low pressure. 3. With the evaporator and condenser water again flowing in the normal operation conditions, the condenser pressure will be seen to be higher than that of the evaporator. This is obviously due to the compressor. The ratio of condenser pressure to the evaporator pressure, Pc / Pe is known as compressor pressure ratio. This will vary under different operating conditions and may be investigated. After the unit has been running for several minutes under normal conditions, the condenser cooling water inlet temperature and the discharge temperature should be compared. It will be found that the discharge temperature t3 is greater than the inlet temperature t4. This is due to the heat given up by the hot high pressure gas entering the condenser from the compressor. Depending upon operating conditions and the length of time the unit has been operating, the gas entering the condenser may be in a superheated condition. (Refer the p-h diagram). If in the superheated condition, initially the gas will de-superheat and its temperature will reach the saturation temperature corresponding to the chamber pressure. At this point the vapor will condense onto the water cooled coil and this will drip down to the base of the chamber. It is the cooling and condensing phase change that supplies the heat to raise the cooling water temperature. If the chamber is at a temperature above that of the surrounding atmosphere, then an amount of unidentified quantity of heat will be given up to the atmosphere. However, this should be small relative to the heat given to the cooling water. 4. If the condenser cooling water flow rate is reduced, then the condenser pressure will rise rapidly relative to the time taken for the evaporator pressure to reduce when the evaporator water supply was turned off. It will also be noted that the mean temperature of the vapor in the condenser t6 will also rise. If the temperature pocket protruding into the chamber is condensing vapor, then the temperature recorded at this point should correspond to the saturation temperature of the refrigerant at the indicated pressure. Note that the indicated pressure is gauge pressure and the pressures referred to on the p-h diagram are absolute pressures. In order to allow the thermometer pocket and thermometer to reach a representative temperature it will be necessary to hold the pressure constant for a brief period after each rise in condenser pressure. 5. The high pressure liquid leaves the condenser through the expansion valve which is controlled by a simple float at the base of the condenser. As soon as the liquid passes through the expansion valve its pressure drops to approximately the pressure inside the evaporator. This causes the liquid to immediately start to change phase from liquid to vapor. As in the evaporator energy is required to bring about the phase change and some of this is taken from the base plate of the condenser as the expansion valve is attached to the base plate.

6

Extracting heat from the base plate reduces its temperature and this in turn reduces the temperature of the condenser liquid at the base of the condenser. This results in the liquid being “sub-cooled” below its saturation temperature. If the optional temperature indicator is fitted, then an additional thermocouple t8 is supplied to be fitted in the base of the condenser chamber. Hence, the sub-cooled liquid temperature may be measured and this together with the other measured temperatures and pressures allows a complete refrigeration cycle diagram to be plotted on the pressureenthalpy diagram supplied. In the condenser therefore, the refrigerant changes from superheated vapor on entry, through to saturated vapor then to saturated liquid and ultimately to sub-cooled liquid before it leaves the condenser chamber. As the refrigerant passes along the pipe leading from the expansion valve to the evaporator heat will be extracted from the surroundings and the liquid will be further converted to a vapor. The sight glass just before entry to the evaporator allows the liquid/vapor mixture to be observed.

7

Practical No. 2 Object: Determination of the effect of evaporating and condensing temperatures on the refrigeration rate and condenser heat output. Equipment/parts required: Refrigeration Cycle Demonstration Unit (R633)

Fig. 2.1 Theory: The effect of evaporating temperature on the refrigeration rate can be investigated, but due to the limited effect on evaporating temperature of all but very large changes in cooling water flow, it is more graphic to investigate condensing temperature first. If time permits, the corresponding effects of evaporating temperature may then be investigated. The effect of increasing the condensing temperature on many refrigeration systems and heat pumps is a reduction in the heat discharged from the condenser and in many cases a smaller reduction in the refrigerating effect at the evaporator. Similar reductions will be observed if the evaporating temperature is lowered. The effects are due primarily to the reduction in volumetric efficiency of the compressor at high pressure ratios (Pc / Pe) and the reduction in specific volume of the refrigerant gas as the evaporating temperature reduces. A simple explanation of this is that for each suction stroke of the compressor a lower mass of gas (for the same volume) is drawn into the cylinder to be compressed. The effect of increasing condenser pressure may be investigated in the following manner. 8

Procedure: 1. Start the unit for normal operation and ensure that the unit is air free by venting air from the condenser. Once air free, increase the condenser cooling water flow to the flow meter maximum (50 g/s). The pressure at which the condenser stabilizes depends upon the water inlet temperature. 2. Set the evaporator water flow to approximately 20 – 30 g/s and allow the unit to run for approximately 15 – 20 minutes. The time taken to stabilize depends upon the local ambient conditions and the cooling water inlet temperature. 3. Record all the system parameters as illustrated in the observation table. 4. Reduce the condenser cooling water flow rate until the condenser pressure increases by approximately 5 – 10 kN/m2. Allow the unit to stabilize and again record the parameters. 5. Repeat for increasing condenser pressures to the minimum readable value on the condenser water flow meter is reached, or the condenser pressure reaches 200 kN/m2 gauge pressure. Observations: Description of Parameters: m1 = mass flow rate of water inside the tube of evaporator m2 = mass flow rate of water inside the tube of Condenser P1 = Condenser Pressure P2 = Evaporator Pressure T1 = Temperature of water at the inlet of evaporator tube T2 = Temperature of water at the outlet of evaporator tube T3 = Temperature of water at the outlet of condenser tube T4 = Temperature of water at the inlet of condenser tube T5 = Temperature of refrigerant inside evaporator tube T6 = Temperature of refrigerant inside condenser tube T7 = Temperature of refrigerant at the outlet of compressor tube T8 = Temperature of refrigerant at the outlet of condenser tube Q1 = Heat absorb by Condenser water= heat released by Refrigerant Q2 = Heat released by evaporator water= heat absorbed by Refrigerant Cw= 4.18 kJ/kg °C Cp, air = 1.005 kJ/kg °C Patm= 101.35kPa Pabs = Patm + Pgauge Observation Table S. No.

M1 g/sec

M2 g/sec

P1 kPa

P2 kPa

T1

T2

T3

T4

T5

T6

T7

T8

o

o

o

o

o

o

o

o

C

C

C

C

C

C

C

1 2

9

C

Calculations: Evaporator

Condenser

Results: Evaporator Temperature (T5) Condenser Temperature (T6) Heat Transfer in Evaporator (Qe) Heat Transfer in Condenser (Qc) Graph between Heat Transfer rates in condenser and evaporator v/s Condensing Temperature

Conclusion: ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________

10

Practical No. 3 Object: Investigation of the effect of compressor pressure ratio on system performance. Equipment/Parts required Refrigeration Cycle Demonstration Unit (R633)

Fig.3.1 Theory: The effect of increasing the condensing temperature for a constant given evaporating temperature is to increase the compression ratio pc / pe that the compressor is required to deliver. Due to the effects of valves and the necessary piston to cylinder head clearances the volumetric efficiency of reciprocating compressors tends to fall with increasing pressure ratio. Volumetric efficiency = Actual volume delivered Compressor swept volume In terms of a refrigeration system, Volumetric efficiency = Mass flow of refrigerant x specific volume of refrigerant at inlet Compressor swept volume Hence, if the volumetric efficiency falls with increasing pressure ratio then the effect will be a reduction in the effective mass flow of refrigerant. The mass flow of refrigerant through the compressor relates directly to the amount condensing on the condenser coil and this in turn relates to the rate of heat transfer to the cooling water. 11

In addition, the above equation indicates the effect of reducing the specific volume of the refrigerant entering the compressor by lowering the evaporating temperature. The effect of pressure ratio on system performance may be investigated by the following method. Procedure: 1. Start the unit for normal operation and ensure that the unit is air free by venting air from the condenser. Once air free, increase the condenser cooling water flow to the flow meter maximum (50 g/s). The pressure at which the condenser stabilizes depends upon the water inlet temperature. 2. Set the evaporator water flow to approximately 20 – 30 g/s and allow the unit to run for approximately 15 – 20 minutes. The time taken to stabilize depends upon the local ambient conditions and the cooling water inlet temperature. 3. Record all the system parameters as illustrated in the observation table. 4. Reduce the condenser cooling water flow rate until the condenser pressure increases by approximately 5 – 10 kN/m2. Allow the unit to stabilize and again record the parameters. 5. Repeat for increasing condenser pressures to the minimum readable value on the condenser water flow meter is reached, or the condenser pressure reaches 200 kN/m2 gauge pressure. Observations: Description of Parameters: m1 = Evaporator Water Flow Rate (gm/s) m2 = Condenser Water Flow Rate (gm/s) P1 = Condenser/Suction Pressure P2 = Evaporator/Delivery Pressure T1= Evaporator Water Inlet Temp. T2= Evaporator Water Outlet Temp. T3= Condenser Water Outlet Temp. T4= Condenser Water Inlet Temp. T5= Evaporator Temperature T6= Condenser Temperature T7= Compressor Temperature T8= Expansion Valve Temperature Observation Table S. No.

M1 g/sec

M2 g/sec

P1 kPa

P2 kPa

T1

T2

T3

T4

T5

T6

T7

T8

o

o

o

o

o

o

o

o

C

C

C

C

C

C

C

1 2

12

C

Calculations: Evaporator

Condenser

Compressor

Note that the pressure ratio should be derived using absolute pressure not gauge pressure.

Results: Compressor Pressure Ratio

Pc / Pe

Heat Transfer in Evaporator

Qe / W

Heat Transfer in Condenser

Qc / W

In order to investigate volumetric efficiency, it would be necessary to measure the rotational speed of the compressor and to know the mass flow of the refrigerant. With a hermetic compressor measurement of the compressor rotational speed is not possible. 13

Graph between system performance and compressor pressure ratio:

Conclusion: ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________

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Practical No. 4 Object: Generation of a refrigeration cycle diagram on a pressure-enthalpy chart. Equipment/ Parts required: Refrigeration Cycle Demonstration Unit (R633)

Fig.4.1 Theory: The vapor compression refrigeration cycle is of paramount importance in terms of food and drug preservation, air-conditioning and heat pumps. In order to analyze the system performance in terms of the thermodynamic cycle it is common for engineers to record system pressures and temperatures and then to plot the various state points on a pressureenthalpy chart of the working fluid. The working fluid in the Hilton Refrigeration Cycle Demonstration Unit Series R633 is R141b. This has the chemical name 1, 1, - Dichloro-1-fluoroethane. In order to plot a cycle diagram for the unit the following procedure should be adopted. Procedure: 1. Start the unit for normal operation and ensure that the unit is air free by venting air from the condenser. Once air free, increase the condenser cooling water flow to a mid-range value. The pressure at which the condenser stabilizes depends upon the water inlet temperature.

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2. Set the evaporator water flow to a mid-range value and allow the unit to run for approximately 15 – 20 minutes. The time taken to stabilize depends upon the local ambient conditions and the cooling water inlet temperature. 3. Record all the system parameters as illustrated in the observation table. 4. In order to demonstrate that the cycle varies for different operating conditions it is recommended that the condenser pressure is varied by adjustment of the condenser cooling water flow rate. The unit should be allowed to stabilize and the system parameters recorded. 5. The procedure may also be repeated at different evaporating temperatures and the results plotted on a pressure-enthalpy chart as described below. 6. The results from the observation table will be plotted on the p-h chart. The state points a, b and c on the diagram are located in the following manner: 1. Point “a” is at the intersection of the evaporator chamber pressure Pe = 32 kN/m2 absolute and the evaporating temperature t5 = 4⁰C. 2. Point b is at the intersection of the compressor chamber pressure Pc = 70 kN/m2 absolute and the compressor discharge temperature t7 = 41.7⁰C. 3. Point c is at the intersection of the compressor chamber pressure Pc = 70 kN/m2 absolute and the condensed liquid temperature t8 = 19.5⁰C. The expansion is assumed to be adiabatic and therefore a line of constant enthalpy may be drawn vertically down from point c to intersect with the evaporator pressure line Pe. Considering the processes that are happening at each state point in turn: 1. At point (a) the vapor from the evaporator is drawn into the compressor and the pressure is raised from pe to pc. It is evident from the line of constant entropy intersecting with point a (1.9 kJ/kg/ k) that the compression is not isentropic as the pressure rise is completed at an entropy of approximately 1.92 kJ/kg/k. If required, the isentropic efficiency of compression may be evaluated. 2. The vapor leaving the compressor at point b is superheated as it is to the right of the saturated vapor line. The vapor cools slightly in the pipe to the condenser and then in the condenser de-superheating and condensing take place at essentially constant pressure. 3. At point c the liquid at the base of the condenser is slightly sub-cooled due to the effect of the cooling water temperature being below the saturation temperature of the condenser chamber pressure (water temperature 11.6 to 16.7⁰C. Saturation temperature at pc = 70 kN/m2 = 21.7⁰C). In addition, some sub-cooling in this unit arrangement is added due to the expansion valve being physically attached to the condenser base plate. The temperature drops caused by the expansion conducts heat through the base plate from the condensed liquid reducing the liquid temperature further. 4. The pressure drop caused by the expansion brings the refrigerant into the vapor/liquid mixture region between the saturated liquid and saturated vapor lines. The mixture of vapor and liquid may be seen in the sight glass adjacent to the evaporator.

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The liquid/vapor mixture may be seen returning to the evaporator through the fitting in the top plate. Note: Though the experiment may only be carried out in the above detail utilizing the optional temperature indicator, state points a and b may be determined using the standard thermometers. The temperature of the condensed liquid may, however, be estimated from the saturation pressure in the condenser if it is assumed that there is no sub-cooling. In the above example, t8, the condensed liquid temperature, is measured as 19.5⁰C, but the saturation temperature at 70 kN/m2 pressure is 21.7⁰C. Clearly, some sub-cooling has occurred.

PRESSURE –ENTHALPY CHART OF FORANE 141b:

17

Practical No. 5 Object: To study the various parts of Compressor used in the domestic Refrigerators and Air-conditioners. Dismantling and Reassembling parts and record the reading on pressure gauge (Low pressure and High pressure). Equipment/parts required: Reciprocating Compressor Main parts: Connecting Rod

Crank Shaft

Crank Case

Rotor

Stator

Suction & Discharge tube

Piston

Cylinder Head

Gudgeon pin

Suction valve

Various Gas kit, bolts, pins & plug.

Theory: Compressors are machine used to increase the pressure of working fluid. There are various compressors designs: Rotary vane; Centrifugal & Axial flow (typically used on gas turbines); Lobe (Roots blowers), and Reciprocating. The main advantage of reciprocating compressor is that it can achieve high pressure ratios (but at comparatively low mass flow rates) and is relatively cheap. It is a piston and cylinder device with (automatic) spring controlled inlet and exhaust valves. Delivery is usually to a receiver. The receiver is effectively a store of energy used to derive (e.g.) compressed air tools. A compressor is used in refrigeration and cooling system to compress vaporized refrigerant. Refrigeration components, which increases the density, temperature and pressure of entering refrigerant through compression and discharger through a hot dense gas. A mechanical device that pressurizes a gas in order to turn it into a liquid, thereby allowing heat to be removed or added. A compressor is the main component of conventional heat pumps and air conditioners. In an air conditioning system, the compressor normally sits outside and has a large fan (to remove heat). Reciprocating compressors usually compress air but are also used in refrigeration where they compress a superheated vapor (to which the gas laws strictly do not apply). In order to be practical there is a clearance between the piston crown and the top of the cylinder. Air „trapped‟ in this clearance volume is never delivered, it expands as the piston moves back and limits the volume of fresh air which can be induced to a value less than the swept volume.

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Fig.5.1 The induced volume flow is an important purchasing parameter. It is called the “Free Air Delivery” (FAD), and it measures the capacity of a compressor in terms of the air flow it can handle. It is normally measured at standard level.

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Procedure: 1. 2. 3. 4. 5. 6. 7. 8.

Cutting of welded body of compressor. Separating the cover housing. Total dismantling of all parts. Cleaning the parts with any suitable oil (kerosene oil). Reassembling of all the parts in casing according to their original sequence. Filling the required compressor oil. Connecting the electrical circuits. Observing the working mechanism of various components.

Safety precautions: 1. Always cut the cover housing at the correct lining. 2. While dismantling keep all the parts such as brushes, bolts and pins in a tray in the proper place that they should not be misplaced. 3. After cleaning use of the correct grade of grease for lubrication. 4. Use the correct form of compressor oil before final building.

Results:

Low Pressure: __________

High Pressure: __________

20

Practical No. 6 Object: To assess the performance of a typical water chiller by applying varying loads to the system. Equipment/Parts required: Water chiller trainer (812)

Fig.6.1 Theory: The vapor compression refrigeration cycle is utilized in many industrial, commercial and domestic situations. When used for water chilling there are additional design parameters that must be considered relating to the fact that water will freeze in contact with any surface that is at or below 0⁰C. The Hilton Water Chiller Trainer 812 is designed for introducing vocational level students to the application of vapor compression refrigeration to water chilling applications. The unit uses recognizable industrial components and is designed to allow students to assess the performance of a typical water chiller by applying varying loads to the system. The unit is also equipped with standard access couplings that students will subsequently find on real plant. A hermetic compressor (1) raises the pressure of refrigerant (HFC134a) gas and discharges this into a water cooled condenser (2). This reduces the gas temperature slightly and condenses the gas to its liquid state. The high pressure liquid flows to the liquid receiver (4) and then through a filter / drier (5) and sight glass (6). The pressure of the condenser is indicated by a gauge (23). The pressure is also sensed by an automatic valve (12) which adjusts the flow of cooling water through the condenser in order to maintain the pressure / condensing temperature of the refrigerant at a constant value. If the pressure in the condenser rises above the set value, the water flow is increased and the 21

reverse occurs if it falls below the set value. The water flow may be monitored on the flow meter display (28). After the filter / drier the high pressure liquid refrigerant flows through a flow meter (21) and then to a thermostatic expansion valve (7). Here the liquid gas is allowed to expand from the condenser pressure to a lower pressure in the evaporator (8) and in doing so it is able to change phase from liquid to gas by absorbing heat at low temperature. Heat is absorbed from the water circulating through the evaporator and this cools the water. The minimum pressure of the refrigerant in the evaporator can be controlled by adjusting an evaporator pressure regulator (20). This can be used to prevent the refrigerant reaching a very low pressure / temperature that may cause the circulating water to freeze. The refrigerant gas leaving the pressure regulator passes to a suction accumulator (9) which prevents any remaining refrigerant liquid reaching the compressor suction port. Any liquid in the accumulator absorbs heat from the surroundings and evaporates. This prevents damage to the compressor. Once returned to the compressor the refrigerant cycle repeats. In order to load the evaporator a heating system (17) is located on the right hand side of the panel. A circulating pump (18) takes water from the tank and circulates this through the evaporator. In order to thermally load the evaporator (1 kW, 0.5 kW) heaters are located in the tank (17) and these may be switched on using the switches (25) on the control panel. Water flow is controlled by a valve (19). A separate high pressure and low pressure cut out (11) limits upper and lower safe pressures in the condenser and evaporator. A high temperature cut out (16) limits the upper temperature in the heating system. It is essential for operator safety and to prevent damage to the machine that the limit switch is set to not more than 50⁰C. The control panel (22) contains the instrumentation and electrical controls for the unit; the condenser cooling water flow indicator (28), the evaporator water flow indicator (29), the digital temperature indicator (27) and the temperature selector switch (26). The temperature control thermostat (15) controls the compressor and hence the temperature of the chilled water. Operation: Starting: 1. Turn on the main switch (23) and the circulating pump (18) will start. 2. Open the flow control valve (19) and water should be seen flowing through the evaporator. This will be displayed on the evaporator flow meter display (29). 3. Note that the compressor may not start unless the thermostat (15) is set to a low value. Note also that the condenser cooling water flow rate may be small until the compressor has been operating as the water flow control is by thermostatic valve (12). 4. When the compressor starts, the condenser pressure gauge (23) will indicate an increasing pressure and the evaporator pressure gauge (24) a reducing pressure. The condenser pressure may be adjusted by turning the screw in the center of the thermostatic valve (12). 5. To increase the condenser pressure, turn the screw clockwise and to reduce turn anticlockwise. Note that once the pressure is close to the required value only very small adjustments will be required. 22

6. If it is required to load the evaporator, turn on the heater switches (25) as required. Note that there will be a delay before the effect of any load is seen on the system evaporator. 7. Note that if no heat load is applied for a long period, the water temperature can reach a low value and even with a 30 % glycol mixture it is possible for the evaporator to freeze. If this occurs, it will be necessary to turn off the unit and allow the evaporator to defrost. 8. The condenser high pressure cut out is set for 18 bar (1800 kN/m2). If this point is reached the compressor will shut down until the condenser pressure has fallen to a lower value. The unit is now ready for full operation. Shutting Down: 1. When shutting down the unit, first turn off the evaporator heater switches (25) and then reduces the condenser pressure by adjusting the thermostatic water valve (12). 2. Finally, press the stop button and turn off the main switch then turn off the water supply at the isolator. Procedure: 1. Start the unit as explained above under operation (starting) heading. 2. Decide upon the heat load to be put onto the evaporating system and turn on the necessary heater switches (25). The options are 1 kW, 1 kW and 0.5 kW in any permutation. 3. Decide upon the condensing pressure required and adjust the thermostatic water valve (12) to achieve the required pressure. Note that there may initially be some delay before a higher pressure is achieved due to the thermal inertia of the system. 4. For manual operation once the temperatures (T1 to T10) pressures (evaporator and condenser) and refrigerant flow rate are constant then a complete set of readings may be taken using the blank data record sheet. 5. Once a set of stable data has been recorded students may either carry out the example calculations or alternatively undertake a series of tests in order to see how changing conditions affect the system performance. There after the same calculations may be undertaken on the performance related data. Observation: M1 = mass flow of water inside the evaporator M2 = mass flow of water inside the condenser Mref = mass flow of refrigerant in system P1 = Low pressure of the system P2 = High pressure of the system T1 = temperature of refrigerant at the inlet of compressor T2 = temperature of refrigerant at the exit of compressor T3 = temperature of refrigerant at the inlet of expansion valve T4 = temperature of refrigerant at the exit of expansion valve T5 = temperature of refrigerant at the inlet of suction accumulator T6 = temperature of refrigerant at the outlet of suction accumulator T7 = temperature of water at the inlet of condenser T8 = temperature of water at the outlet of condenser T9 = temperature of water at the inlet of evaporator T10 = temperature of water at the outlet of evaporator 23

Load kW

M1 M2 Mref g/sec g/sec g/sec

P1 bar

P2 bar

Observation Table T1 T2 T3 o o o C C C

T4 o C

T5 o C

T6 o C

T7 o C

T8 o C

T9 o C

T10 o C

0 1.5

Calculations: For 0kW

For 1.5 kW

Graph: C.O.P v LOAD

24

Pressure –enthalpy charts of R134a (For 0kW and 1.5kW)

25

Practical No. 7 Object: To study the construction and Working Mechanism of DEW POINT. Equipment/Parts required: Dew Point Apparatus Main parts: Retaining Rings Glass Windows Intake Nozzle Stand Thermometer

Joining Washer Spacing Ring End Cap Dipper Synthetic Rubber Washer

Theory: To record the Dew Point Temperature of room air with the help of dew point apparatus, using ether as refrigerant. Using this data, calculate following properties of air (with the help of Psychometric Chart). 1. W.B.T(Wet Bulb Temperature) 2. D.B.T(Dry Bulb Temperature) 3. R.H(Relative Humidity) 4. S.V(Specific Volume) 5. E.A.S(Enthalpy at Saturation) 6. Humidity Ratio Dew point: The temperature at which water vapor in any sample of air begins to condense. Dew point Temperature: The temperature indicated by a thermometer around the bulb of which is placed a wet cloth sleeve; a strong current of air, causing evaporation at the wet bulb lowers the thermometer reading by a definite amount, called the wet-bulb depression, which depends upon the amount of moisture present in the air. Wet bulb thermometer: A thermometer whose bulb is covered with a piece of water-soaked cloth. The lowering of temperature results from the evaporation of water around the bulb indicates the air‟s relative humidity. For example, water at 100 degrees at the boiling point, at standard sea-level atmospheric pressure. Dry Bulb Temperature: The temperature of the sensible heat of the air, as measured by an ordinary or dry-bulb thermometer. Humidity: Water in the physical state of vapor mixed in the air. In Refrigeration air conditioning equipment usually reduces the humidity of the air processed by the system. The relatively cold (below the dew point) evaporator coil condenses water vapor from the processed air, (much like an ice cold drink will condense water on the outside of the glass), sending the water to a drain and removing water vapor from the cooled space and lowering the relative humidity. Since humans perspire to provide natural cooling by the evaporation of perspiration from the skin, drier air (up to a point) improves the comfort provided. The 26

comfort air conditioner is designed to create a 40 to 60 percent relative humidity in the occupied space. Absolute Humidity: The weight of water vapor usually stated in grains per cubic foot of air. Relative Humidity: The ratio of the weight of water vapor in a sample of air to the weight of water vapor that same sample of air contains when saturated; usually stated as a percentage. Specific Humidity: The weight of water vapor, usually stated in grains per pound of dry air. Refrigerant: “Freon” is trade name for family of fluorocarbon refrigerants manufactured by DuPont and other companies. Those refrigerants were commonly used due to their superior stability and safety properties. Unfortunately, evidence has accumulated that these chlorine baring refrigerants reach the upper atmosphere when they escape. The chemistry is poorly understood but general consensus seems to be that CFCs break up in the stratosphere due to UV radiation, releasing their chlorine atoms. These chlorine atoms act as catalysts in the breakdown of ozone, which does severe damage to the ozone layer that shields the earth‟s surface from strong UV radiation. The chlorine will remain active as a catalyst until and unless it binds with another particle forming a stable molecule. CFC refrigerants in common but receding usage include R-11 and R-12. Newer and more environmentally safe refrigerants include HCFCs (R-22, used in most in most homes today) and HFCs(R-134a, used mostly in cars) have replaced most CFC use. Saturation temperature: The temperature at which a liquid substance boils under a given pressure. Procedure: 1. Check the thermometer to be at room temperature. 2. Remove the small pipe from the funnel behind thermometer. 3. Fill the tank with ether to use as refers. 4. Attach two polyphone tubes to the two horizontal metal tubes of front chamber of the apparatus. 5. Place two clips on the end of tubes. 6. After filling the ether tightly close. 7. Attach an aspirator on it. 8. Now operate the aspirator to cool the air through the ether. 9. As soon as the dew point formation is saturated on the mirror, record the temperature from thermometer. 10. Convert the temperature into 0C and calculate other also. Safety precautions: 1. Calibrate the thermometer of apparatus before starting. 2. Always keep the mirror clean. 3. Check the thermometer bulb is just touching the tank and fully immerse in ether. 4. Reading should be recorded as early as dew formation. 27

Observation: Dew Point Temperature

=

________

Results: 1.

Wet Bulb Temperature

=

__________

2.

Dry Bulb Temperature

=

_________

3.

R.H(Relative Humidity)

=

__________

4.

S.V(Specific Volume)

=

__________

5.

E.A.S(Enthalpy at Saturation) =

__________

28

Practical No. 8 Object: To study the various parts, Construction and Working Mechanism of “Aspiration Psychrometer”. To calculate the various properties of moisture air (with the help of dry and wet bulb temperature) and with the help of above data, calculate the following properties of air using psychrometric chart: 1. W.B.T(Wet Bulb Temperature) 2. D.B.T(Dry Bulb Temperature) 3. R.H(Relative Humidity) 4. S.V(Specific Volume) 5. E.A.S(Enthalpy at Saturation) 6. Humidity Ratio Equipment/Parts required: Aspiration Psychrometer

Fig.8.1 Specifications: Measuring range: Minimum Reading: Air flow rate:

-30 to 50 oC 0.2 oC 1.5m/sec to 2.0 m/sec.

Main parts: Driving gear Winding key Handle

Lid Mechanism Suspension bracket

Housing. Fan.

Theory: Aspiration psychrometer: Itis an instrument having two thermometers and one driving mechanism used for the determination of various properties of air and their mixture (moist air). Definition: Psychometry (sy-krometry) means literally, the measurement of cold from the Greek psychros, cold. It is the special name that has been given to the modern science that deals with air and water vapor mixtures. The amount of water vapor in the air has a great influence on human comfort. Such atmospheric moisture is called humidity, and the common expression, it isn‟t the heat, it‟s the humidity, is an indication of the popular recognition of the discomfort producing effects of moisture laden air in hot weather. 29

Psychometry is the science and practice of air mixtures and their control. The science deals mainly with dry air, water vapor mixture, with the specific heat of dry air and its volume. It also deals with the heat of water heat of vaporization or condensation and the specific heat of steam in reference to moisture mixed with dry air. Psychrometry is a specialized area of thermodynamics. Psychrometric chart: The psychrometric chart is a graph of the properties (temperature, relative humidity etc.) of air. It is used to determine how these properties vary as the amount of moisture (water vapor) in the air changes. Dew point temperature: It is the temperature below which moisture will condense out of air. Relative humidity: It is a measure of how much moisture is present compared to how much moisture the air could hold at that temperature. Humidity: It is the concentration of water vapor in the air. The concentration can be expressed as absolute humidity, specific humidity or relative humidity. The amount of humidity in an air controlled environment is an important issue in the air-conditioning industry. Humidity can make the warm temperature of the surrounding air feel like it is warmer than the actual temperature, because the cooling effect of evaporation from the skin is reduced. Humidity ratio: The ratio of the mass of the water vapor to the mass of dry air contained in the sample. Dry bulb temperature: The dry bulb temperature (DBT) of the air is the temperature as measured by an ordinary dry bulb thermometer. When measuring the dry bulb temperature of the air, the bulb of thermometer should shade to reduce the effect of direct radiation. Wet bulb temperature: The wet bulb temperature (WBT) of the air is the temperature as measured by wet bulb thermometer. Or Wet bulb temperature reflects the cooling effect of evaporating water. Specific volume: The specific volume is the no: of cubic feet of mixture per lb. of dry air. Enthalpy: Heat content or total heat, including both sensible and latent heat. The amount of heat contained in a refrigerant at any given temperature with reference to -40 oF. Specific enthalpy: Specific enthalpy of a working fluid, h is the property of the fluid which is defined as: H= U+PV Where, U= Specific internal energy P= Pressure V= specific Volume Specific Enthalpy has the same dimension as [energy/mass]. The SI unit of specific enthalpy is J/Kg. Other units are: 30

1 KJ/kg= 1000 J/Kg

1 erg/g= 1 E-4 J/Kg

1 Btu/lb.m= 2326 J/Kg

1 Cal/g = 4184 J/Kg

Enthalpy at saturation: enthalpy at saturation is also known as the enthalpy of water vapors and is based on a zero value of saturated liquid at 32 oC. Percentage saturation: It is the ratio of the specific humidity of the air to the specific humidity at the saturated air at the same time. Procedure: 1. After winding up the key to mechanism the instrument is held with the help of hanging bracket. 2. Check the initial reading of both thermometers. 3. Moist the wet bulb temperature by filling the water. 4. Hold the apparatus with its handle in straight position. 5. Examine the rotation of fan and wait till stop completion. 6. Record the reading from both thermometer and compile. Safety precautions: 1. 2. 3. 4.

Check the accuracy of both of the thermometers. Psychrometers should be holding exactly invert position. The glass should be attached with bulb. Record the reading of thermometer when fan shop.

Observations: After considering the above procedure, we have noted following readings: Wet bulb temperature

=

__________

Dry bulb temperature

=

__________

Results: With the help of above two readings and psychometric chart, we calculated: 1. 2. 3. 4.

Dew Point Temperature R.H(relative humidity) S.V(specific volume) E.A.S(enthalpy at saturation)

= = = =

_____________ _____________ ____________ ____________

31

Practical No 9 Object: To study the various parts, Construction and Working Mechanism of “Hair Hygrograph” & to record the “Relative Air Humidity” in the function of time and to maintain the Weekly Graph. Equipment/Parts required: Hair Hygrograph

Fig. 9.1 Specification: Measuring Range: Basic time mark: Time for one turn of drum: Basic humidity mark: Main parts: Base Handle Spring Spanner Key Setting Screw Stay back rod

0-100% 2 hours 176 hours 1%

Side Wall Safe Guard Locking Lever Recording Drum Pen arm

Lid Bracket Fixing Rule Hair Stand Recording Pen

Theory: An instrument used to measure Relative Humidity of air, or an instrument designed to measure the air‟s water content. The sensing part of the instrument can be hair (hair hygrometer or a plate coated with carbon (electrical hygrometer), or an infrared sensor (infrared hygrometer). There are three types of hygrometers: The hair hygrometer uses a human hair as the sensing instrument. The hair lengthens when the air is moist and contracts when the air is dry, but remains unaffected by air temperature. However, the hair hygrometer cannot respond to rapid fluctuations in humidity. 32

An electric hygrometer uses a plate coated with carbon. Electrical resistance of the carbon coating changes as the moisture content of the air changes – changes that translate into relative humidity. This type of hygrometer is used frequently in the radiosonde. An infrared hygrometer uses a beam of light containing two separate wave lengths to gauge atmospheric humidity. One of the wavelengths is absorbed by water vapor; the other is unaffected, providing an extremely accurate index of water vapor for paths of a few thousand feet. Human hair, a hygroscopic substance, changes in length proportionally to the relative humidity of the atmosphere. In the instrument, a bundle of hairs is linked to a pen-arm by which a continuous record of the relative humidity is left on a chart mounted on clock-driven drum. This instrument is used in Weather services, Cold storage, Refrigeration and Air conditioning, drying plant, Spinning mills, Telephone exchanges, Cigarette factory. Humidity: It is the amount of water vapor in the air. The higher the temperature, the greater the number of water molecules the air can hold. For example, at 60 oF (15 oC), a cube of air one yard on each side can hold up to 4.48 ounces of water, at 104 oF(40 oC), the same cube of air can hold up to 17.9 ounces of water. Relative humidity: It describes the amount of water in the air compared with how much the air can hold at the current temperature. Example 50% relative humidity means the air holds a half of the water vapor that it‟s capable of holding; 100% relative humidity means the air holds all the water it can. At 100% humidity, no more evaporation can occur until the temperature rises, or until the water vapor leaves the air through condensation. Absolute humidity: It is the ratio of the mass of water vapor present in a system of moist air to the volume occupied by the mixture that is the density of water vapor. It is expressed in percent in the ratio of actual partial pressure exerted by the water vapors in any volume of the air to the partial pressure that would be exerted by the water vapors, if the water vapors in the air were saturated at the temperature of air. R H = (actual partial pressure / partial pressure at saturation) * 100 Procedure: 1. First of all, clean the pen and after filling it with proper ink, attraction opens arm. 2. With the help of stay back rod separate the recording pen from recording drum. 3. After fixing the paper on recording drum, wind the drum with the help of winding key spanner 4. After winding drum release the pen arm from the stay back rod. 5. Release the lid and lock with locking lever. 6. Fix the guard at proper position. 7. Now the apparatus is ready for recording. 8. Place it at a suitable place & observe its rotation periodically.

33

Safety precautions: 1. Calibrate the clock mechanism for accurate reading with the help of motor clock at least after a month. 2. Protect the apparatus against any stock 3. Do not lubricate any part of the apparatus except clock. 4. Always use proper instrumentation ink. Observations: After one week remove the chart from drum and prepare the final chart showing the time & relative humidity of each day.

Results: Saturday

=

__________

Sunday

=

_________

Monday

=

_________

Tuesday

=

_________

Wednesday

=

_________

Thursday

=

_________

Friday

=

_________

The average humidity of the week = _________

34

Practical No.10 Object: To study the main parts constructions and working mechanism of domestic refrigeration trainer. And to record the high and low pressures and temperatures of the unit. Equipment/Parts required: Domestic refrigeration trainer. Main parts: Compressor Capillary Tube Main Switch Panel Ampere Meter

Condenser Evaporator Filter (Dryer) Flow meter Temperature Probe A, B Wattmeter Electric Temperature Indicator (Thermocouple).

Theory: Refrigeration is the process of removing the heat from matter. The matter can any be a solid, liquid, or a gas. Removing heat from the matter and cools it or lower its temperature. There are a number of ways of lowering temperature some of which are of historical interest only. In the circuit of mechanism through which the refrigerant Freon 12flows there are five main elements. Starting from the point where we wish to remove heat they are 1) Evaporator, 2) Compressor, 3) condenser, 4) liquid receiver and 5) expansion valve. In addition, various control and safety, a connected into the circuit.

Fig.10.1 Compressor: It is reciprocating type compressor having 1/6 horsepower motor are enclosed in housing. It is designed for Vapor and gas cooling R-12 refrigerant is used. This vapor does not remain n in the evaporator. The compressor is operating and the suction which it exerts (on the evaporator side of its circuit) pulls the heat- laden vapor out of the evaporator, through the piping and into the compressor. The compressor therefore, is the mechanism that keeps the Freon 12 in circulation through the system. In the compressor cylinders, the Freon 12 is compressed from a low pressure vapor to high pressure vapor, line between the low and high pressure side is the discharge valve of the compressor. 35

Condenser: The Freon vapor, now at high pressure passes next into the condenser. The excess heat thus flowing out of the vapors both superheat and latent heat of vaporization and therefore the vapor condense back to the liquid state. The liquid Freon 12 is now at high pressure and high temperature. Evaporator: Tin plate having a bank, or a coil of copper tubing. It is filled with Freon 12 at low pressure and temperature. Heat flowing from the air spaces or articles to be cooled into the coil causes the liquid Freon 12 to boil. Boiling can take place only as a result of entrance into liquid of its latent heat of vaporization, and this latent heat can come only from the surrounding substances are lowered. The latter portion of the evaporator coil is therefore filled with Freon 12 vapor at low- pressure. Capillary Tube: I.D=0.032 inches, length=10ft. The size of the capillary tube is determined by a capacity of evaporation. It should allow the proper amount of liquid refrigerant enter the expansion valve at high pressure and high temperature. This valve regulates the flow of the refrigerant into the evaporator. The liquid outlet from this expansion valve is a small opening called orifice. In passing through the orifice, the liquid is subjected to a throttling action, and there is dispersed into a finely divided form. The Freon 12 is now again a liquid at low pressure and low temperature, and is again entering the evaporator, its cycle completed, and ready to be repeated. Every part of the cycle is, of course taking place simultaneously and continuously throughout the circuit as long as refrigeration is wanted. The entire operation is automatic. Refrigeration Ton: A unit used in measuring the elimination of heat one refrigeration ton is the removal of heat that would be required to melt one ton of ice at 32 degrees F in 24 hours. Procedure: 1. 2. 3. 4. 5. 6. 7.

Put on the electric switch and start the compressor unit. After few minutes, the cooling will start at evaporator. Try to hear the spring noise of refrigerant into the evaporator from the capillary tube. Observe the whole operation of the unit particularly the ice defrosts on evaporator. After 15 minutes approx. when the ice deposited is completed in the evaporator. Record the high pressure and low pressure reading from both the manometers. Stop the unit by putting off electric connection.

Result: High pressure

=

__________

Low pressure

=

__________

High temperature

=

__________

Low temperature

=

__________

36

Practical No. 11 Object: To study main components construction and working mechanism of air conditioner. (To record the suction & exhaust pressure of the domestic air conditioner). Equipment/Parts required: Domestic air Conditioner Trainer. Main parts: Compressor Condenser Pan Motor Evaporator Capillary Tube Flow Meter Filter (Dryer) Humidity Indicator Main Switch Temperature Probe A, B Electrical Temperature Indicator (Thermocouple) Theory: Air conditioning: It is a field of engineering dealing with design, construction and operating of equipment used in establishing and maintaining desirable indoor air conditions. These conditions vary according to special requirements of the installations, which may be in theater, factory, store submarine or any other enclosure occupied by human being. The process of treating air to control simultaneously its temperature, humidity, cleanliness and distribution. In the modern engineering practice of air conditioning two phases are involved: 1. Actual conditioning of air that is, there alteration under control of its temperature, humidity purity and oxygen content. 2. Ventilation or replacement of stale air in an enclosure by conditioned air, or HVAC (pronounced either H-V-A-C) stand for “heating, ventilation and air conditioning”. Refrigeration cycle: In the refrigeration cycle, a heat pump pumps heat from a lower temperature source into a higher temperature heat sink. Heat would naturally flow in the opposite direction. This is the most common type of air conditioning. A refrigerator works in much the same way, as it pumps the heat out of the interior into the room in which it stands. The most common refrigeration cycle uses an electric motor to drive a compressor. In an automobile the compressor is driven by a pulley on the engine‟s crankshaft with both using electric motors for air circulation, since evaporation absorbs heat and condensation releases it air conditioning are designed to use a compressor to cause pressure changes between two compartments and actively pump a refrigerant around. A refrigerant is pumped into the cooled compartment (the evaporator coil) where the low pressure and low temperature cause refrigerant to evaporate into a vapor, taking heat with it in the other compartment the condenser) the refrigerant vapor is compressed and forced through another heat exchange coil condensing into a liquid rejecting the heat previously absorbed from the cooled space). Thermostats: They control the operation of HVAC system turning on the heating or cooling systems to bring the building to set temperature. Typically, the heating and cooling systems have separate control systems (even though they may share a thermostat) so that temperature is controlled one way. That is in winter a building that is too hot will not be cooled by the 37

thermostat. Thermostats may also be incorporated into facility energy management systems in which the power utility customer may control the overall energy expenditure. In addition, a growing number of power utilities have made available a device which when professionally installed, will control or limit the power to an HVAC system during peak use timers in order to avoid necessitating the use of rolling blackouts. Humidifier: A device to increase the humidity within a room or a building by means of the discharge of water vapors. Humidifiers may also consist of individual room size units or larger units attached to a forced hot air furnace to condition the entire building seem more comfortable during the dry winter months. In fact, a room usually feels warmer if the humidity level is higher. However too much humidification will cause moisture to build up in the walls and ceilings and result in possible rot. Dehumidifier: It is an air conditioning like device that controls the humidity of a room or building. They are deployed in basements. Which is because of their lower temperature have a higher relative humidity. (Conversely a humidifier increases the humidity of the building.) Humidity: Refrigeration air conditioning equipment usually reduces the humidity of the air processed by the system. The relatively cold (below the dew point) evaporator coil condenses water vapor from the processed air (much like an ice cold drink will condense water on the outside of a glass) sending the water to drain and removing water vapor from the cooled space and lowering the relative humidity since humans perspire to provide natural cooling space and lowering the relative humidity, since humans perspire to provide natural cooling by the evaporation of perspiration from the skin drier air ( up to a point) improves the comfort provided. The comfort air conditioner is designed to create a 40% to 60% relative humidity in the occupied space. Refrigerants: Freon is a trade name for a family of fluorocarbon refrigerants manufactured by DUPONT and other companies. These refrigerants were commonly used due to their superior stability and safety properties. Unfortunately, evidence has accumulated that these chlorine bearing refrigerants reach the upper atmosphere when they escape the chemistry is poorly understood but in general consensus seems to be that CFCs break up in the stratosphere act as a catalyst in the breakdown of ozone, which does severe damage to the ozone layer that shields the earth‟s surface from the strong UV radiation. The chlorine will remain active as a catalyst until and unless it binds with another particle forming a stable molecule. CFC refrigerants in common but receding usage include R-11 and R-12. Newer and more environmentally safe refrigerants include HCFCs (R-22 used in most homes today) and HFCs (R-134a used in most cars) have replaced most CFX use. Compressor: It is a reciprocating type compressor having ½ horsepower motor is enclosed in housing. It is designed for vapor and gas cooling R-22 refrigerant is used. Condenser: Condenser pipe diameter is 3/8 inch.

38

Evaporator: The plates having copper tube. It is a part of whole system which provides a means of what a refrigerant to vaporize into the vapors. Hence evaporator cools down the surrounding air. Evaporator diameter is 3/8 inch. Capillary tube: I.D = 1/8 inch, length =5ft The size of capillary tube is determined by a capacity of evaporation. It should allow the proper amount of liquid refrigerant enter in the evaporator to ensure proper performance. Procedure: 1. 2. 3. 4. 5. 6.

Put on the electric switch and start the compressor of the unit. After few minutes, the cooling will start at evaporator. Try to hear the spring noise of refrigerant into the evaporator from the capillary tube. Observe the whole operation of the unit particularly the ice defrosts on the evaporator. Record the high pressure and low pressure reading from the both manometers. Stop the unit by putting g off electric connections.

Safety precautions: 1. 2. 3. 4.

Always check whole system as already earthed before starting Always stand 2 ft. away from the system. Before starting check the system for any possible leakage. Record the reading of high pressure and low pressure just after defrosting of ice evaporator. 5. Before staring the unit, check the ampere gauge and voltage supply.

Results: Suction pressure

=

__________

Exhaust pressure

=

___________

Condensing temperature T1 =

____ _____

Condensing temperature T3 =

__________

Conclusion: ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________

39

Practical No.12 Object: To study the Construction and Working Mechanism of Vacuum Pump. Equipment/parts required: Vacuum Pump Main parts: Base Rubber Washer Belt Safe Guard Nozzle

Starting Switch V-Belt Suction Nozzle

Electric Motor Pump Exhaust

Fig.12.1 Theory: Vacuum Pump: To create a vacuum in a system it is necessary to move all the molecules of gas out of the system. The molecules will move only if there is a pressure difference between the two regions of the space (see Figure 4). The low pressure region is the space with the smaller number of molecules, while the high pressure region is the space with the larger number of molecules. Any device which can introduce a pressure difference between the two regions in the space is called a pump. The Pump which creates the vacuum in the certain system is called a vacuum pump. Example: You need only a cup of beverage and a straw. Try to suck a drink by the straw and feel how the mouth muscle moves. When one sucks on a straw the mouth muscles create the region of low pressure, while atmospheric pressure on the surface of your beverage pushes it up the straw. A pump operates on the same principles. 40

Operation of vacuum pump: The transfer pump is also called kinetic pumps since they impart the momentum to the gas which is being pushed in such a way that the gas is transferred continuously from the inlet of the pump to the outlet. This is usually done by mechanical moving parts of the pump, as shown in figure. The moving (usually rotating) parts of the pump accelerate the molecule of the gas and make the region of low pressure. Therefore, the molecules from the tank will start moving towards the region of low static pressure, with the procedure continuously repeating until all (or most) of the molecules are taken from the container where we would like to have a vacuum. When we got the wanted level of vacuum we isolate the tank by a high vacuum valve. This valve stops any exchange of gas between the container and the pump. Procedure: 1. Check the specific vacuum of a pump and fill it if necessary. 2. Collect the suction nozzle of vacuum pump by means of vacuum pipe to the time of the system through manifold. 3. Record high pressure gauge and low pressure reading. 4. Start the motor of vacuum pump & observe the top rotation of the pump & moment of the gauge. 5. Check for any leakage in the system 6. Stop the motor of the vacuum pump, when the gauge shows the required vacuum of 28 psi & the exhaust nozzles stop pumping the air. Safety precautions: 1. 2. 3. 4. 5.

Be sure to perform only operations you know how to do safely. Never wear clothes or other articles that dangle and could catch on the equipment. Wipe up any oil that is on the floor around the machine. Keeps hands away from the motor belt during operation. Clean and wipe the machine when the job is finished.

Result: Vacuum Pressure

=

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Practical No. 13 Object: To charge the Refrigerant (R-12) gas in a Domestic Refrigerant System. Equipment/Parts required: Model Refrigerator Main parts: Domestic Refrigerator Manifold Gauges Gas Cylinder / Container

Gas charging lines Ampere meter Screw range, Screw driver & Spanners etc.

Theory: The chemical name of Freon 12 is dichlorodifluoromethane (CCl2F2). At atmospheric pressure, Freon 12 boils at -21.66 degrees F and freezes at -311 degrees F. Its latent heat of vaporization at atmospheric pressure is about 72Btu per pound. “Freon is a trade name for a family of fluorocarbon refrigerants. The refrigerants were commonly used due to their superior stability and safety properties. Unfortunately, evidence has accumulated that these chlorine bearing refrigerants reach the upper atmosphere when they escape. The chemistry is poorly understood but general consensus seems to be that CFCs break up in the atmosphere due to UV-radiation, releasing their chlorine atoms. These chlorine atoms act as catalysts in the breakdown of ozone, which does severe damage to the ozone layer that shields the Earth‟s surface from the strong UV radiation. The chlorine will remain active as catalyst until and unless it binds with another particle forming a stable molecule. CFC refrigerants in common but receding usage R-11 and R-12. Newer and more environmentally safe refrigerants include HCFCs (R-22, used in most homes today) and HFCs (R-134a, used in most cars) have replaced most CFC use. Advantages: 1. 2. 3. 4. 5. 6. 7.

It is safe refrigerant & tasteless It is nonflammable It is non explosive It is non corrosive Its vapor is nontoxic in quantities up to 20% by volume It will not harm foods, fabrics, furs and so forth It is odorless in concentrations of 20% or less by volume. In high concentrations if has a slight odor of carbon tetrachloride, of which it is a derivative.

Additional advantages for operation: 1. It has a low boiling point, -21.66 degrees F atmospheric pressure 2. It acts rapidly in freezing other substances; its latent heat at 5 degrees F is 69.47 Btu, or much lower than that of other refrigerants. 3. At the low pressure points of refrigeration cycle, it operates at pressures only slightly above atmospheric pressure, thus reducing the possibility of air entering the system in the event of leakage. 42

Disadvantages of Freon R12: 1. While Freon 12 is nonflammable. It decomposes in contact with an open flame at high temperature (1000 degrees F), highly toxic gas, and other decomposition products. 2. It is an excellent loosened of scale and dirt that may be left in or may get into the system. Such material is carried around the system and is finally deposited in the strainers. However, some damage may also result during its journey. In installation or care, great care should be taken to prevent the entry of any foreign matter into the system. 3. It does not mix with the water. One important reason why air must be kept out of the system is that air almost always contains some water vapor. This water vapor tends to condense and freeze, thus interfering with second operation, and damaging various walls and other parts of system. 4. It is absorbed by lubricating oil. Procedure: 1. From the previous practical the vacuum is already produced in the system. 2. Now check any suitable electronic device that is halogen detector, torch or firm simply soap bubble test. For any possible leakage in the system. After confirming the leak proof condition, now start the process of gas charging 3. Connect the discharging nozzle of gas cylinder to suction line of the system through the gas charging of the pipe by opening of the gas cylinder 4. Start entering the R-12 gas from cylinder slowly by opening of the gas cylinder 5. Start the compression of the system and open the full valve of system 6. Continue the process till the pressure gauge shows the required pressure of refrigerant. Safety precautions: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Check regularly the ampere range of the voltage of the motor compressor Sufficient amount of the gas & refrigerant should always be available in the cylinder Observe the spring noise (shooting) of refrigerant in the evaporator Keep Freon 12 out of eyes Keep lubricant oil used in plant away from eyes Do not drop Freon 12 cylinders Do not fill Freon 12 cylinders beyond 80% capacity Keep open flame away from Freon 12 cylinders Never attempt to taste Freon 12

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