Air Conditioning

INTRODUCTION An operation and maintenance manual usually includes such items like starting and shut down sequences, equipment maintenance, etc. However, what is often missed is the communication of the design intent of the Engineer/Consultant, to the operating personnel. There had been instances of improper plant operation, because the operating personnel had not been made aware of how the systems were designed to operate. This Manual has been prepared to eliminate such deficiencies. Further, Air Conditioning and refrigeration Engineering is a specialised linefrom the design stage to the ultimate service operations. It calls for detailed application design, system and component selection, special care in installation, commissioning and ultimately the day to day operation of the Plant, its maintenance and trouble shooting. However well the plant has been designed and installed, the full benefit of the Plant can only be obtained by diligent operation and systematic maintenance. In Central Air Conditioning systems-each Plant is tailor made to a great extent to suit the job details and meet the maximum load conditions that can be envisaged during the course of a year and also another special features that may be required for a particular application. However, since the principle of working, of the systems are practically same for the various applications, this Operation and Maintenance Manual will be useful for all types of applications.

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DESIGN DETAIL Areas Air Conditioned

(i) (ii) (iii)

Sq. mtr. Sq. mtr. Sq. mtr.

Design conditions Summer DB WB RH

Winter DB WB RH

Monsoon DB WB RH

Outdoor Indoor

Internal Load

:

No. of people

:

Lighting load

:

Equipment load

:

Condenser water (inlet) Temp

:

Condenser water temp range

:

Compressor Sat. Suction Temp.

:

Total plant capacity required (Tons)

:

Total installed capacity (tons)

:

No. of plants to be operated at maximum load conditions

:

No. of standby plants:

:

Refrigeration plants

:

Condenser water pump

:

Chilled water pump

:

*

:

Cooling coil

: Direct Expansion type.

Chilled water type

Condensing unit

: Water cooled.

Air cooled.

Chiller

: Direct Expansion type

Type of Plant

* Strike out those not applicable.

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PLANT DETAIL EQUIPMENT

MAKE

TYPE/MODEL/ ENCLOSURE

HP/NOM. CAPACITY

QTY.

Compressor Compressor motor Condenser Chiller Cooling tower Cond. water pump Chilled water pump Cond. water pump motor Chilled water pump motor Cooling tower fan motor Air Handling Units : Size Quantity Motor HP Filters - Qty 510 x 510 x 50 635 x 405 x 50 635 x 510 x 50 E lectrical Panel Compressor Condenser Switch/Fuse starter ra ting O/L, U/V SPP V/A meter S.S.

Evaporator

Fresh Air

Humidifica- Remark tion

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GENERAL DESCRIPTION OF AIR CONDITIONING SYSTEMS The term ‘Air Conditioning’ encompasses both cooling and heating. But, heating will be needed only to a very limited geographical areas in our Country and hence, this Manual deals with the cooling aspect of the Air Conditioning Plant. The Air Conditioning Plant is basically a heat exchange equipment. It takes away the heat from the area/substance to the cooled and the heat thus taken is rejected into the atmosphere. That is to say, there is heat exchange taking place in removing the heat from the area to be cooled and again in rejecting the heat to the atmosphere. The component of the plant, which removes heat from the area to be cooled and dehumidified, is the EVAPORATOR along with the air handling apparatus and air distribution network. The component through which the heat is rejected to the atmosphere is the CONDENSER. In the air cooled system, the heat is rejected from the Condenser Coil to the atmosphere by maintaining a continuous flow of atmospheric air over the Condenser Coil for which, therefore, condenser fans are necessary. In the Water Cooled System, the heat is rejected to the atmosphere through the condenser and the COOLING TOWER. Water pump/pumps is/.are used for circulating the cooling water through the condenser and cooling tower. There must be a medium to absorb the heat at the evaporator (i.e. At the low temperature level) and to reject this heat to the atmosphere. This medium is called the Refrigerant, which is a volatile liquid having a low boiling point. The refrigerant is fed into the evaporator in the liquid form at low pressure and temperature, where in absorbing heat, it changes to its vapour state. To move the heat laden refrigerant gas from the evaporator to the condenser (where the heat is rejected to the atmosphere) and also to raise its pressure so that the gaseous refrigerant can be converted to its liquid state (for reuse in the evaporator), the COMPRESSOR is used. The refrigerant is at high pressure in the Condenser, while in the evaporator it has to be at low pressure to enable to absorb the heat at the low temperature level. For reducing the pressure of the refrigerant from the high side to that of the low side and also to automatically monitor the rate of flow of the refrigerant to meet the load requirements, we have the THERMOSTATIC EXPANSION VALVE. In addition to the above main components, there are the safety and operating controls and other ancillaries to complete the system. It will be evident that the Air-conditioning system consists of a number of components selected and connected together. For satisfactory performance of the plant, naturally, the performance characteristics of each of the components should match well with that of the whole system capacity., Therefore, the selection (application engineering) of the various components for each application, has to be done in an exhaustive manner. It is here that the high degree of experience and expertise of D.O.T. Electrical Wing plays a major role.

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DIRECT EXPANSION SYSTEM : The air from the space to be cooled is circulated over the evaporator coil, inside which the refrigerant is boiling and absorbing the heat from air. The Evaporator Coil is fitted in the air handling unit which is located in the air handling unit room and air is distributed to the area to be cooled by a network of ducts. The Compressor-Condenser set (called the Condensing unit) is located in a separate plant room. The two components of the refrigeration plant i.e. the Condensing Unit and evaporator coil are interconnected by means of the refrigerant piping.

MAKE UP & QUICK FILL CONNECTION DRAIN

RETURN AIR DUCT OR PASSAGE

SUPPLY AIR DUCT

S.A. DIFFUSER

A.H.U. ROOM

PLANT ROOM FRESH AIR DUCT WITH DAMPER

FAN SECTION

EVAPORATOR COIL AIR FILTER

R.A. DIFFUSER

AIR-CONDITIONED ROOM COND. THERMOSTATIC EXPANSION VALVE

LIQUID LINE STRAINER

COND. WATER PUMP

DX SYSTEM

CHILLED WATER SYSTEM : The Air-conditioning plant engineered and installed for you has the Chilled Water System. Water is chilled in a Central Composite Refrigeration Plant and the chilled water is circulated through the cooling coils in air handling units located in various places or zones of the building. This system is adopted for multi-story/large buildings, as it is not advisable and economical to run long lengths of refrigerant piping from the condensing unit (Compressor-condenser assembly) to the various evaporator coils located in different parts or zones of the building. Further, this system will be more suited to take care of diversities in load, inherent in large buildings. Schematic representation of the system is given on page 7 . CONTROLS : There are two categories of controls :-

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(a)

6

Safety Controls to protect the system as under :HP/LP CUTOUT : This stops the compressor either when the discharge pressure exceeds a set safety point or when the suction pressure falls below a certain set point. OVER LOAD RELAYS : To stop the motor when the current exceeds the rated full load current. OIL SAFETY SWITCH : To stop the compressor whenever the oil pressure does not develop as required. ANTI-FREEZE THERMOSTAT : Provided in the outlet of the chiller to protect the chiller against freezeup. It stops the compressor in chilled water system, when the leaving water temperature falls below 380 F.

(b)

Operating Controls : These vary according to the requirement of each job. In large size plants (say at capacities of 40 tones and above) the compressors are provided with capacity control to unload cylinder when the load drops down. These are automatic devices which load and unload the cylinders according to load variations. Provision of the capacity controls also helps in unloaded starting of the plant, thereby reducing the heavy draw of current of the compressor motor at the time of starting. CRANK CASE ELECTRIC HEATERS : These heaters are provided in the Crank Case of the compressor to keep the oil warm when the compressor is idle. The special refrigeration oil used in the Compressors have a tendency to absorb refrigerant gas when oil temperature falls down. Absorption of refrigerant gas dilutes the oil and the lubricating property of the oil will be substantially reduced; this will affect the bearing life of the compressor. The crank case heater warm up the oil and reduces the absorption of refrigerant gas during the idle period of the compressor. The heater is connected in such a way that it comes ‘ON’ automatically as soon as the compressor is stopped/stops.

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SUPPLY AIR DUCT

7

RETURN AIR DUCT OR PASSAGE A.H.U. ROOM FAN SECTION

EVAPORATO R COIL AIR FILTER

AIR-CONDITIONED ROOM

SUPPLY AIR DUCT

RETURN AIR DUCT OR PASSAGE A.H.U. ROOM FAN SECTION

EVAPORATOR COIL AIR FILTER

AIR-CONDITIONED ROOM

SUPPLY AIR DUCT

RETURN AIR DUCT OR PASSAGE A.H.U. ROOM FAN SECTION

EVAPORATO R COIL AIR FILTER

AIR-CONDITIONED ROOM

SUPPLY AIR DUCT

RETURN AIR DUCT OR PASSAGE A.H.U. ROOM FAN SECTION

EVAPORATO R COIL AIR FILTER

AIR-CONDITIONED ROOM

DRAIN

SUPPLY AIR DUCT

RETURN AIR DUCT OR PASSAGE A.H.U. ROOM FAN SECTION

PLANT ROOM

EVAPORATO R COIL AIR FILTER

AIR-CONDITIONED ROOM

COND.

COND. WATER PUMP

CHILLED WATER SYSTEM

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COMPRESSOR OIL : The refrigeration oil used in compressors is a speciality item, to withstand the changes in temperature conditions in the system. The Air Conditioning Plant being a heat exchange equipment, its efficiency and the capacity it can deliver naturally will depend upon the efficiency of the heat exchanging components, i.e., the evaporator and condenser and also the efficiency of the compressor which maintains the required flow rate and pressure for condensation of the refrigerant. In the case of water cooled plants, since this heat is ultimately rejected to the atmosphere through the cooling tower (again a heat exchanger) the performance of the complete plant will be linked with the efficiency of the cooling tower and the condenser water circulation system. Therefore, to get the maximum output of the plant, the various components mentioned above have to be kept in good condition and thus maintenance plays a very important role. Maintenance is detailed in the subsequent chapters.

OPERATION OF THE PLANT

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This chapter deals with the day to day operation of the plant. Operation of the plant is not just starting and stopping the plant, but also includes some routine maintenance work- on a day to day weekly and short period basis. The result of faulty operation (including the routine maintenance) may escape the attention, because it is not as dramatic as a breakdown, but in the long run can work out to be quite costly. Therefore, proper operation is an important aspect. There may be symptoms of some minor malfunction, which may appear to be small or insignificant; yet such small things can grow into major troubles in course of time; resulting in costly and time-consuming repairs, loss of time, dislocation and lots of extra work, usually at a most inconvenient time. So the operating personnel should be sharp in detecting any minor defects and should take timely action to prevent such minor defects grow into serious problems. In an air conditioning plant, pressure, temperature, current etc. can never be constant. These readings vary according to such factors as outside conditions, room load, etc. The readings can vary during the course of a day itself. Quite often it is assumed that as long as the temperature in the conditioned area is within design limits, there is nothing wrong with the plant. But this is a wrong assumption, because even a slightly inefficient plant can give satisfactory inside-conditions if the load is less or outside-conditions are comparatively low. It is the reading of the various temperatures and pressures of the plant which can lead us to a positive conclusion about the conditions of the plant. Therefore, one of the important aspects of operation is the logging of the various readings (log sheet formats are given at the end of this chapter). Needless to point out that readings should be taken correctly and the operating personnel should have the ability to analyse the readings to establish whether the plant is working satisfactorily or not. OPERATING INSTRUCTIONS : Before starting the Plant : 1.

Ensure all dampers in air ducts are open.,

2.

Ensure all valves in the (a)

Refrigeration Systems,

(b)

Condenser Water Lines, and

(c)

Chilled Water Linesare open, except those of the standby items of equipment.

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3.

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Check up water level in the cooling tower and ensure that the make up water system is working satisfactorily-such as float ball is OK and float valve is free in movement and there is water flow when the ball is pressed down, etc., and there is regular make up water supply to cooling tower. During operation, some water is lost by way of evaporation and drift losses. Unless these losses are made up continuously, the water level in the cooling tower will fall down and the condenser will not get full and continuous water flow thus affecting the plant; the discharge pressure will shoot up and the plant will stop on High Pressure cut out switch.

4.

In chilled water plants, check the water level in the expansion tank and ensure that the make up water system is working and there is regular water supply. Water level in the chilled water system can fall down due to pump gland drips. If the level in the expansion tank is not maintained, air can enter the chilled water system affecting the system performance substantially and can even lead to freeze up of the chiller.

5.

Check all air filters and water strainers and clean whenever found dirty.

6.

Ensure that all doors and windows of the air conditioned area are closed.

7.

Ensure that the crank Case of the Compressor is warm to the physical touch. If it is not warm, check for defects in the Crank Case heater/circuit. Do not start the compressor until the defect is rectified and the Crank Case warms up. Otherwise, poor lubrication will result, reducing the life of the bearings of the Compressor substantially.

8.

Check supply voltage. It should be within 400 to 440 volts. If the voltage is less than 390 volts, do no start the Plant. The windings of the motors can get affected if run on low voltage conditions.

STARTING SEQUENCE 1.

Switch ‘ON’ the mains

2. 2.1 2.2

Start Air handling unit motors Condenser water pump/s. Check that sufficient water pressure is built up.

IMPORTANCE/SIGNIFICANCE OF OPERATIONAL STEPS Observe the voltage. If it is less than 380 volts, DO NOT START the plant.

Correct pressure of the condenser water system: the water pressure built by the condenser water pumps (i.e. The difference between the discharge and suction pressures of the pump) will depend upon the total length of the water

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pipes in the system, number of water passes provided in the condenser etc. The pressure observed at the time of commissioning and handling over the plant should be the best guide. As a general guidance, the pressure built up by the pump will be around 1.75 to 2.46 kg/cm2 (25 to 35 PSI). 2.3

Cooling tower fan/s

Inspect the cooling tower fan guard/s periodically and ensure that they are in good condition.

2.4

Chilled water pump/s - where chilled water systems are installed. Confirm that sufficient pressure is built up by the pump/s.

Correct pressure of the chilled water, the pressure built by the chilled water pump/s will depend upon the length of piping, fittings, chiller, etc. Here again the pressure observed at the time of handing over the plant is the best guide.

2.5

Switch ‘ON’ the compressor control switch.

The compressor control circuit is protected by a fuse and a switch is provided for service purpose.

2.6

Switch ‘ON’ the refrigerant solenoid valve or the pilot solenoid valve (whichever is used).

Refrigerant solenoid valves are provided in the main liquid line usually where the plant size is small. Pilot solenoid valves are generally provided for bigger plants.

2.7

Start the compressor motor. In the case of slip ring motors, stator-rotor starters are used. After switching on the stator switch of the starter, turn the Rotor Wheel or handle gradually and move smoothly to the full running position. Caution : The movement of Rotor Wheel/Handle should be neither too fast nor too slow. Never leave the wheel/handle in an intermediate position. As the compressor is started the compressor oil pressure should build up. Check that correct net oil pressure is build up. Once the oil pressure is built up, the cylinders of the compressor will

The rotor circuit in a slip ring motor starter is provided with number of resistances. At the point of starting, maximum amount of resistance is in series with the rotor and as the rotor wheel is turned towards the full speed (run) position, the resistances are progressively cut off and rotor windings get connected in Star. If the rotor wheel is kept in an intermediate position, the resistance wires in the rotor starter can get burnt.

3.1

Since the oil in the Crank Case of the Compressor is subjected to the suction pressure, the NET OIL PRESSURE developed by the (compressor) oil pump is the difference between the reading shown on the oil pressure gauge and the

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load up.

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suction pressure at the time of taking the reading. For example : If the oil pressure gauge reading is 110 PSIG (7.73 kg/cm2) and the suction pressure at that time is 70 PSIG (4.92kg/cm2 ) the NET OIL PRESSURE is : 7.73-4,.92 = 2.81 kg/cm2 (110-70 = 40 PSI). It is the NET OIL PRESSURE which supplies sufficient lubrication to the bearings and loading mechanism. Suction pressure of the system can be varying according to the load conditions and consequently the reading shown by the oil pressure gauge also will be varying. Hence to ensure that the compressor lubrication system is working satisfactorily one has to arrive at the NET OIL PRESSURE every time. The net oil pressure should be around 2.8 kg/cm2 (40 PSI).

3.2

Check the oil level in the oil sight glass of the compressor. The oil level should be 3/8 or 3/8 of 1/2 the sight glass.

The oil level in the compressor oil sight glass should be within 3/8 to 1/2 of the glass. In operation, certain amount of oil gets entrained in the refrigerant vapour in the compressor and is carried away to the system alongwith the refrigerant. It is obvious that the oil carried into the system should return to the compressor, continuously and automatically, to maintain sufficient oil level in the compressor. While designing the system and the refrigerant piping, it is ensured that the oil return will be proper. However, due to certain malfunctions of the system that can develop due to improper maintenance or shortage of refrigerant in the system due to leakage, etc., The oil return can get affected. Hence, it is important to observe the oil level in the compressor periodically. If the oil level tends to go down, it is a sure

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indication of a system malfunction. Immediate action should be taken to find the reason for the improper oil and rectify the defect. Otherwise a major compressor breakdown can occur, resulting in costly and time consuming repairs. 4.

After the plant operation has stabilized, check all the pressures and temperatures and ensure that the plant is working satisfactorily.

Correct temperatures and pressures. It may take about half hour for the plant to stabilize on starters. Readings are to be taken (and received) after the plant has stabilized. It should be noted that the temperatures and pressures will vary according to various factors. Therefore, there is nothing like a constant reading for all parameters. Judgment is needed to analyse the readings and to conclude the performance of the plant. Some broad guidelines are given below to analyse the readings. It should be noted that these are to be taken only as a general guideline. (a) Discharge Pressure : This depends mainly on the temperature of the condensing medium (i.e. Water or air) and to some extent on suction pressure. Once the system has stabilized and the load is within limits, the discharge pressure is primarily governed by the temperature of the condensing medium. In water cooled condensers generally the condensing temperature of the refrigerant in the condenser (i.e. the saturation temperature corresponding to the discharge pressure) will be 100 C to 11.10 C (180F to 200F) above the entering water temperature to the condenser (or) about 5.50C (100F) above the condenser water outlet temperature. For example: Inlet water temperature to condenser: 850F. Then the condensing temperature should be bout 39.40C (1030F) which is about

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14.5 kg/cm2 (207 PSI). (b) Again the inlet water temperature is governed by the outside WET BULB Temperature. In atmospheric type cooling towers, the water temperature can be brought to about 4.40C to 5.50C (80F to 100F) of wet bulb i.e. Outside WB + 8 to 100F and in induced draft cooling towers the water temperature can be outside WB + 2.70C to 3.30C (50F to 60F). (c) Cooling tower performance also can be judged by the relations between outside W.B. Temperature and cooling tower sump temperature (this difference is called the ‘wet bulb approach’ of the cooling tower. (d) The air cooled condenser- the condensing temperature will be about 16.70C to 19.40C (300F to 350F) above the outside DRY BULB temperature. Note that in water cooled unit the condensing temperature is governed by the outside WET BULB temperature, while in air cooled units it is the outside DRY BULB temperature that has to be taken as guide line. (e) Suction pressure : It is governed by the load on the plant or in short the temperature in the condition area (+) outside conditions. Once the room temperature has fairly stabilized, the correctness of the suction pressure can be verified by the canvas connection temperature i.e. the temperature of air coming out of the air handling unit. The saturation temperature of refrigerant (corresponding to suction pressure) should be about 8.30C to 100C (150F to 180F) below the canvas correcting temperature.

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Refrigerant R-22 saturation temperature - pressure chart is given at the end of this chapter. As the plant is started, the room temperature will gradually fall down and so also the suction pressure. (f) Oil pressure : It is the NET OIL PRESSURE developed by the oil pump that is important. The net oil pressure should be 2.8 kg/cm2 (40 PSI) more than the suction pressure. (g) The pressure developed by the condenser and chilled water pumps, as already explained earlier, will depend on the length of the water circuits etc., and since these pressures cannot change for a particular system, the pressures observed at the time of handing over should be the best guide. 5.

Record hourly readings of temperature, pressures, current and other data in the log sheet. Sample log sheet format is given at the end of this chapter.

6.

Check for any unusual noise/vibration in the plant. If any is noticed, trace out the reason for this and rectify. If you are not able to identify the cause of any vibration/noise, contact maintenance. Agency / manufacturer immediately.

7.

During the operation of the Plant, if it stops suddenly, try to trace out the reasons for this to

It is very important to take the various readings correctly and record them in the log sheet. These log readings help in analysing variation of readings on a day to day basis and to establish the condition of the plant. When a problem arises with the plant, the log readings of earlier days or corresponding days in the previous years, will help to lead us to the problem area. Therefore, the readings should be taken faithfully and the log book kept in good condition.

Whenever a plant suddenly stops, it is the general practice of operating personnel to first reset the oil safety switch and try

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happen, before starting the plant again. If on restarting, the plant trips again, please get in touch with maintenanca agency / manufacturer immediately.

STOPPING SEQUENCE 1.

Switch off the refrigerant solenoid valve. Compressor will go off on the low pressure section as the system gets pumped down.

2.1

Switch off the compressor mains.

2.2

Check that the Crank Case electric heater comes on as soon as compressor stops and heater is working.

3. 4.

Stop chilled water pump/s. Stop air handling unit/s.

5.

Stop condenser water pump/s and cooling tower fan/s.

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starting the plant. This is a wrong procedure. Resetting of oil safety switch should be the last item in the step by step procedure in finding the reasons for the sudden stoppage. The plant could have stopped on the pressure switches, water flow switches, antifreeze thermostat, motor overload, etc. First check these. If ultimately it is found that all these are OK then try resetting the oil switch. If it is established that the plant has tripped on the oil safety switch, then a very close watch on the oil pressure (net oil pressure) is absolutely essential. If the net oil pressure tends to go down it is sure indication of a problem in compressor. Immediately trace out the problem and rectify. Otherwise, a major compressor breakdown can occur. If in difficulty, contact maintenance agency. IMPORTANCE/SIGNIFICANCE OF OPERATIONAL STEPS

The importance of the Crank Case electric heater has been explained in detail, in chapter 3. Ensure that the Crank Case heater comes ‘ON’ when the compressor is stopped.

SOME IMPORTANT POINTS TO BE TAKEN CARE OF : 1.

Where standby plant/s, pump/s etc. are provided systematically change over the work of the plants periodically say once in a week or even every day. This will ensure uniform wear and tear of the plants. Further this also helps in

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ensuring that all the plants are in good repair and the standby is in good condition should a necessity occur. 2.

Never ‘SWITCH OFF’ the main switch on the main electrical board or switch off any component of the system, when the plant is in operation, for ease in shutting down the plant. There had been instances of operators doing this way. Such an action will put a heavy strain on the isolating switch/es and they can even get completely damaged.

3.

All the water valves in the system can be kept open all the time and need not be closed daily. But it is essential to close and open each valve periodically to ensure that the valves work and do not get stuck by scale formation or dirt accumulation.

4.

Keep the plant room clean. Do not use the plant room, particularly the air handling unit rooms, as lumber rooms.

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SPECIMEN LOG SHEET FOR TEMPERATURE READINGS

DATE : TIME CANVAS DB WB

AHU-1 RETURN DB WB RH

CANVAS DB WB

AHU-2 RETURN DB WB RH

Outside Temp DB

WB

RH

Inside Temp DB

WB

RH

Inside Temp DB

WB

RH

Remarks

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SPECIMEN LOG SHEET FOR PLAT ROOM READINGS (CHILLED WATER PLANT) CH. W. CH. W. COND COND C.T. PUMP 1 PUMP 2 PUMP 1 PUMP 2 FAN PRESS.

Out Amps. In

Out In

Out In

Out In

Out In

Out In

Out In

Out In

In Out In

PRESS. TEMP. PRESS. TEMP. PRESS. TEMP. PRESS. TEMP. PRESS.

PRESS.

PRESS.

Amps.

COND-2

Out Amps.

COND-1

Out Amps. In

COMP-2 CHILLER-1 CHILLER-2 S.P. D.P. O.P. Amps.

COMP-1 S.P. D.P. O.P. Amps.

TIME

Out Amps. In

DATE:

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MAINTENANCE DAY TO DAY MAINTENANCE : It is a well known fact that all mechanical equipments need systematic maintenance. In the case of Air-conditioning plant this aspect assumes greater importance. Unless the work of maintenance is carried out in a planned manner needless to point out that the full capacity of the plant not be obtained. Further by proper maintenance work major-break down of the plant can be substantially reduced. It has been explained in the Chapter-3 that an Air-conditioning plant is a heat exchange equipment. So to get the bast out of the plant, the heat exchangers should be kept in good condition in addition to the maintenance work required for any Electro-Mechanical equipment. Maintenance pertaining to the Air-conditioning plant falls into two-categories. 1.

Running/operating maintenance, and

2.

Planned and preventive maintenance on a periodical basis.

While the operating maintenance, being simple to be carried out by the operating personnel of the plant, the second category of work calls for expertise handling, thus calling for the services of a experienced Mechanic. For bigger plants, service engineers will be needed. This chapter deals with the step by step running/operating maintenance DAILY : 1.

WATER LEVEL IN COOLING TOWER : Check water level before starting the plant. Also ensure that the makeup water system is working properly i.e. the float valve is working satisfactorily and there is sufficient water in the make-up water tank for the full day operation of the plant. As explained in Chapter 4, if this step is not taken care of, the plant performance will be affected and the compressor can stop on the High Pressure Switch.

2.

IN CHILLED WATER SYSTEM : Check up water level in the expansion tank and ensure the make-up water system is working satisfactorily. If this is not ensured, there are chances of the freeze up of the chiller.

3.

CHECK CRANK CASE HEATER :

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It is very important that the crank case heater comes on automatically when the compressor is stopped. This has to be checked on a day to day basis without fail. Do not start the compressor, unless the crank case is warm to the physical touch. The necessity of the crank case heater coming into operation during the idle period of the compressor has been explained in Chapter 3. 4.

OIL LEVEL IN COMPRESSOR : This should be within 3/8 to 1/2 of the oil sight glass. If this level is not maintained, it is a positive indication of a malfunction in the plant. If the operating personnel cannot locate and rectify the problem, service personnel should be informed immediately. CAUTION : If the oil level in compressor goes down, NEVER FILL IN OIL to make up the level. The only correct step is to find out the reason/s for the poor oil return and rectify it.

5.

Be alert to look for/observe unusual noise/vibration than normal. Even if a small vibration sets in than normal, look for the reason and rectify, rather than leaving it as negligible.

6.

LOG BOOK : It may appear as a bit odd to talk about logging readings as a maintenance step. But, remember that, it is the readings that tell the real condition of the plant. If the readings show any abnormality, there is some malfunctioning somewhere and this has to be investigated and rectified without delay to avoid a major breakdown. This log book forms an important maintenance tool for day to day operation of the plant, as well as for future reference.

7.

Check for over heating of any part in the plant.

WEEKLY : 1.

CLEAN AIR FILTERS : A dirty air filter can reduce plant capacity. More the dirt accumulation on the air filter, greater will be the reduction in capacity of the plant. Air filters, if not cleaned regularly will get saturated with dirt and thereafter dirt will pass on to the cooling coil fins, which will affect plant capacity much more and in extreme cases can even lead to liquid flood back to the compressor, which is highly injurious to the compressor. Cooling coils being wet, this dirt/dust will form a crust in

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the coil. Hence, air filters are to be cleaned regularly. Weekly cleaning has been suggested here. But where plants are installed in dusty environments, it may be necessary to clean the air filters more often. For example after a dust storm in Northern India, it is a must to clean the air filters. Air filters should be changed, when it is found that cleaning does not remove all the dust. 2.

LEAK TESTING FOR REFRIGERANT LEAK : A halide torch should be used, as this is a sensitive service tool to locate even minor leak. While leak testing, the approach should be to find a leak rather than taking it for granted that there won’ t be any leak and doing the work of leak test as a virtual. A leakage-even a minor one- can lead to lot of problems, such as poor oil return, heating of compressor apart from poor cooling, etc. Further, Refrigerant is a costly item and so less leak becomes a big cost saving centre in the operation of the plant.

3.

WATER PUMP GLANDS : Check for excessive water leak through the gland (certain amount of water drip through the gland is necessary to keep the gland cool). However if the drip develops into a regular flow, it is an indication that the gland is not holding well. The gland nuts can be tightened to reduce the leak. When tightening the nuts does not improve the situation, the gland packing has to be renewed. If this is not done in time, water consumption will go up and in chilled water pump, a water leak means, loss of refrigeration. The gland nuts should be tightened evenly to avoid the gland flange touching the shaft. Always check for heating of the gland after tightening. If the gland gets heated, it is an indication of overtightening of the nuts.

4.

Clean water strainers wherever provided. A clogged strainer reduces water flow rate and thus affect the plant performance. Further, if the strainers are not cleaned regularly, the dirty muck will form a crust and it becomes too difficult to clean. This can even puncture the strainer mesh, necessitating replacement of the strainer element. Though the strainer appear to be small item in a complete plant, remember that they play a very important role in keeping the heat exchange surfaces clean and efficient.

5.

BELT TENSION OF BELT DRIVES :

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Check the tension of belts and tighten whenever found loose. A loose belt will reduce transmission efficiency and its own life will be reduced; further the drive pulleys can get heated up. Replace belt when it is not possible to tighten them any further. In mullet-belt drives, change the complete set of belts. Select the belts for a matched set. If this is not done, the load will be taken only by the smaller length belts, thereby affecting transmission efficiency and belt life. 6.

It is the pressure gauges and thermometers which give us a correct indication of the plant performance and condition. Therefore, ensure that these are in good order.

7.

Check the spray of the cooling tower nozzles.

8.

Drain, clean and refill the cooling tower sump. Cooling towers, being in the open, collect lot of dust and muck, hence it is necessary to clean once in a week. Analyse the pressure and temperature readings of the plant from the log book and establish that the plant is working satisfactorily. Corrective action should be taken promptly when readings show even a minor malfunction

PREVENTIVE MAINTENANCE : As the name implies, Preventive Maintenance is a planned service programme. This helps in minimizing major breakdown, to operate the plant at design efficiency and also to save energy. As explained in Chapter 5, this maintenance work calls for the services of a well experienced Serviceman/Service Engineer. A step by step procedure in this regard in given below as a guideline :MAINTENANCE

FREQUENCY

1.

Periodical Inspection and service

1.1

Check the various pressures, temperature and Quarterly the current drawn by the motors. Analyse the months) readings to ascertain the condition of the plant.

1.2

On analysing the readings, if some malfunctioning is indicated, investigate and rectify the faults.

1.3

Ensure that oil return is proper.

1.4

Check crank case heater for proper operation.

(once

in

3

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1.5

Carry out a through test for refrigerant leak, by halide torch.

1.6

Inspect the condition of the air filters. Replace them if they are found to be beyond cleaning/sagging-out of shape .

1.7

Check operation of all safety and operating controls.

1.8

Check belt tension and adjust. Replace worn out belts (in sets only).

1.9

Check alignment of all belt and direct drives and rectify where found necessary. Dial gauge should be used to check the alignment of direct drive in Compressors.

1.10

Inspect the cooling tower for proper operation and ensure that the ‘wet bulb approach’ of the tower is within limits.

1.11

Check all bearing surfaces for abnormal heating. Where such symptoms are observed, investigate and take corrective action.

1.12

Check drains for free flow and clean wherever necessary.

1.13

Check for abnormal vibration and noise. Check for tightness of all fastening bolts.

1.14

Check cooling coil fins for dirt accumulation. Clean if found necessary.

1.15

Check that the electrical connections are tight.

1.16

Using an Electrical blower, blow dust accumulated within the motor. Dust accumulation on windings will retard the cooling of the motor windings and thus affect the life of the windings.

2.

Keep the heat exchange surfaces clean.

2.1

CONDENSER :

2.1.1 Water cooled condenser. Check the “leaving temperature difference” (Condensing temp -

24

Half yearly or yearly-Please see note at the end.

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Leaving Condenser Water temp.). If it is more than 6.60 to 70C (12 to 12.50F) Condenser Water tubes will have to be cleaned. Note : The frequency of cleaning the condenser tubes will depend upon the quality of water used. When water used is fairly soft, yearly cleaning will be found satisfactory. The frequency of cleaning will have to be established by observing the operation over a period of time. 2.1.2 AIR COOLED CONDENSER : The temperature difference between the Half yearly of oftener. Condensing temperature and outside dry bulb temperature is the criterion to determine whether the condenser fins need cleaning. The temperature difference established at the time of commissioning and handing over of the plant is the best guide line. Water under pressure sprayed over the coil is the positive method of cleaning a condenser coil. The (Pressure) water it should be applied in the opposite direction of the operating direction of the air flow of the coil. The frequency of cleaning will depend upon he installation. 2.2

COOLING COIL : The temperature difference between the canvas Half yearly or oftener. connection temperature and evaporator temperature is the criterion to determine whether the coil needs cleaning. The temperature difference found at the time of commissioning and handing over the plant is the best guide. Cooling coils also are to be cleaned with water under pressure.

3.

Lubricate all moving parts. The compressor oil however need be changed only if the condition of oil is bad.

4.

Check and clean contacts in starters/contactors.

Half yearly.

Note : Do not dress the Contacts with emery or sand paper. The contacts have hard wearing coatings. If emery or abrasive paper is used, this coatings may get removed. The shape of the contact point surfaces are made slightly sloping so that the moving contact surface slides on the surface of the fixed contact and maintains a positive contact. Thus the initial points of contact where arcing occurs on making and breaking (which causes the pitting of the contact

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surfaces) are not the areas through which current passes in operation. These specially shaped surfaces can get out of proper shape if emery paper is used. Therefore cleaning should be done only with cloth. Replace badly pitted contacts.

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TROUBLE SHOOTING Even with a well maintained and operated plant troubles can develop. When trouble occurs, a systematic study of all symptoms (by taking temperature and pressure readings) should be made to come to a conclusion as to the cause of the trouble. Many of the breakdowns are caused originally by simple faults. Hence, while trouble shooting, look for simple points first. Never jump to conclusion from some symptoms only. Analyse all symptoms and come to a logical conclusion. A properly maintained log book will help to a great extent to pin point the probable area of trouble. Breakdown or major malfunctioning generally will not occur all of a sudden. A small defect will develop into a major one, if not detected and rectified in time. For example, a very minute gas leak over a period of time can lead to lot of problems, such as, insufficient cooling, poor oil return, excessive heating of compressor head etc. If only regular leak test was taken and the minute leak detected and rectified, all the major troubles cited could have been avoided., A trouble shooting chart is given on the next page as a guide line.

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TROUBLE SHOOTING CHART SYMPTOM OR DIFFICULTY 1.

POSSIBLE CAUSES

Compressor (a) Main switch open refuses to start

REMEDIAL MEASURES Close switch.

(b) Fuse blown

Correct the replace fuse.

fault

and

(c) Thermal cutout relay open

Check and attend electric supply

(d) Defective contactor

Repair or replace.

(e) High pressure cutout open

Correct high pressure as in symptom 4 and reset high pressure cutout.

(f) Low pressure cutout open

Correct low pressure as in symptom 7 and reset low pressure cutout.

(g) Oil pressure failure switch Correct low oil pressure as open in symptom 11 and reset the low oil pressure cutout. (h) Thermostat set too high

Reset thermostat.

(i) Liquid line solenoid not Repair or replace. open

2.

Plant Vibrating

(j) Motor Electrical trouble

Repair or replace motor.

(k) Loose wiring

Check all wire junctions

(a) Foundation bolts not tight Tighten the bolts. enough, hence vibration (b) V belt drives or couplings Align the V belts drives or gone out of alignment couplings as the case may be. (c) Pulley/fly wheel loose on the Tighten locking nut. compressor shaft (d) Motor pulley loose

Change or adjust motor pulley.

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3.

Compressor noisy

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(a) Too high an oil level causing Correct the oil level. liquid hammering (b) Liquid flood back to the Check for reasons for compressor liquid flood back and rectify. (c) Excessive wear on bearing

4.

High Discharge Pressure

Overhaul the compressor.

(a) Discharge shut off valves Open valve. not fully opened. (b) Air or non-condensables in Purge the non-condensable the system from the system. (c) Condenser entering water (I) Inspect cooling tower temperature high spray or water distribution in cooling tower. Rectify defects in the cooling tower. (ii) Outside wet bulb temperature higher than normal. This being a natural phenomena there is no possible remedy. However, this occurs very rarely and if at all it happens, it would be only for a short duration. (d) Condenser Water tubes Descale the condenser. fouled or sealed up. In such a case the difference between inlet and outlet water temperature will be lower than normal. Also the LTD will be high. (L.T.D.: Leaving Temperature Difference i.e. Condensing temperature (minus) condenser water leaving temperature) (e) Insufficient water flow through the condenser. This

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will be indicated by a higher than normal temperature difference between inlet and outlet water temperatures of the condenser. This could be due to : (i) Partially opened water Open fully. valves . (ii) Low water level in Rectify. Ensure cooling tower . make-up water. (iii) Blocked water strainers

enough

Clean strainers.

(iv) Passages with water Open pump, inspect and pump impeller blocked. clean. (v) Pump inefficient. (f) System overcharged 5.

Low Discharge Pressure

Overhaul pump. Remove excess refrigerant from the system.

(a) Too much cooling water to Regulate the water supply. the condenser. (b) Cooling water exceptionally Reduce the water supply. cold. (c) Compressor unloaded.

running Refer item 9.

(d) Suction shut off valve not Open the valve. fully open (e) Insufficient system

6.

refrigerant

in Charge the refrigerant after thorough leak test.

(f) Leaky discharge valve

Remove cylinder cover. Inspect the valve plates and rings and renew if necessary.

(g) Low suction pressure

See symptom 7 below.

High suction (a) Excessive load on the plant This may be of pressure than designed temporary duration.

a

(b) Too much liquid being fed Adjust the expansion valve through expansion valve.

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(c) Compressor unloaded

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operating Refer item 9.

(d) Leaky suction valve.

7.

Remove the cylinder covers, inspect valve plates and rings and renew if necessary.

Low suction (a) Liquid line filter dirty. pressure

Clean or replace.

(b) Compressor suction strainer Clean or replace. dirty. (c) Expansion valve blocked.

Clean expansion valve.

(d) Shortage of refrigerant in the Charge refrigerant, after system. thorough leak test. (e) Incorrect adjustment expansion valve.

8.

of Open expansion valve or readjust to feed more refrigerant.

(f) Evaporator dirty.

Clean.

(g) Compressor not unloading.

Check mechanism.

(h) Discharge pressure low .

See symptom 5 above.

(i) Too much oil in the system.

Drain the oil gradually from compressor keeping an eye on oil return.

unloading

Compressor (a) Capacity control gone out of Adjust. not unloading adjustment. (b) Cut-outs for capacity Reset the cut-outs automatic control not set capacity control. correctly.

9.

Compressor not loading

(c) Rocker arms displaced

Check and adjust.

(a) Inadequate oil pressure

See symptom 11 below.

(b) Unloading mechanism stuck

Rectify or replace.

(c) Oil lines or oil strainer of Clean lines and strainers. capacity control dirty . (d) Solenoid valve coil defective Check and replace.

of

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(e) Plunger of solenoid valve Clean . stuck. 10.

11.

Compressor (a) Expansion valve in correctly loading and adjusted . unloading intervals too (b) Oil pressure erratic. short Low pressure

Recheck and set expansion valve. Check compressor oil level stop oil foaming and clean oil lines.

oil (a) Compressor running I wrong Rectify. direction. (b) Oil strainer dirty .

Clean oil strainer.

(c) Oil lines choked.

Clean oil lines.

(d) Oil pump not satisfactorily.

operating Check and repair.

(e) Oil regulating valve gone Adjust or repair out of adjustment. regulating valve rotating clockwise increase pressure. (f) Bearing clearance too high. 12.

Compressor (a) Oil trapped in the system. oil level going own. (b) Piston rings leaking.

13.

Compressor working continuously

loose

oil by to

Overhaul the Compressor. Investigate reasons for poor oil return and rectify.

and Renew piston rings and if necessary the piston.

(c) Worn-out cylinder liners.

Replace cylinder liners.

(a) Lack of refrigerant.

Leak test refrigerant.

(b) Thermostat adjusted low.

Readjust.

and

charge

(c) Compressor delivery valves Inspect the valves renew or leaking. replace. 14.

Compressor

(a) Cycling on high or low Refer items 6 or 7. pressure switches. (b) Thermostat differential low.

Readjust differential.

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Note : 1.

Before investigating the causes of the symptom or difficulty experienced, make sure that all pressure gauges and all cutouts are functioning satisfactorily.

2.

The above is only an illustrative list.

3.

Wherever necessary refer to Compressor Manual

33

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SOME SPECIAL SERVICE OPERATIONS PUMPING DOWN When the plant is to be shut-down for prolonged period or if the refrigeration system is to be opened out for repairs and overhaul, it is necessary to pump down all the refrigerant from the Evaporator and piping and store it in the condenser/receiver. This is called pumping down. Following procedure should be followed when it is necessary to pump down the refrigerant. 1.

Close the liquid outlet valve on the condenser/receiver and start the compressor. The compressor should be run until the suction pressure gauge indicates a pressure of 2 psig. The compressor should then be stopped and discharge/shut-off valve should be quickly closed.

2.

To pump down a system it will generally be necessary to operate the compressor below the normal cutout setting on the low pressure switch. It will, therefore, be necessary to remove the cover from the pressure switch and manually block the contacts in close position. Wherever the refrigeration system is equipped with a magnetic stop valve, it will be necessary to see that this stop valve is kept open during the period of pump-down. This may be done by keeping the solenoid coil energised during the pump down cycle. Certain types of solenoid valves are equipped with manually operated stems which may be used to keep the valve open during pumping down.

PUMPING OUT THE REFRIGERANT : It may sometimes be necessary to remove the entire charge of refrigerant from the system and store it in another receiver. In that event, first, pump-down the gas as outlined above. Close compressor suction valve and stop machine. Back seat the compressor discharge service valve and connect the line from the empty cylinder to the gauge connection. Place the receiver in an ice water bath, purge the air from the line and turn slowly the compressor discharge service valve away from the back seat position. Open the valve on the refrigerant receiver. Keep the condenser warm by keeping the condenser water pump running to assist the removal of refrigerant. Wherever a separate compressor is available, this may be used to pump out the refrigerant from the system into the empty receiver. REPLACEMENT OF DEFECTIVE PARTS : It may be sometimes necessary to open up a closed refrigeration system for replacement of defective parts. In the event, the system should first be pumped down as detailed above. Never leave the refrigerant lines open as dirt and

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moisture may settle on the system and causes heavy damage when the plant is started up. When reassembling the system, it is advisable that the liquid side should be closed up. Before completely sealing the system, crack the liquid outlet valve to purge air from the system. Close the valve immediately and make the final joint. However, where the system has been kept open for a long time, it is necessary to evaluate the system before charging gas. Test the system for leaks with Halide torch. If everything is satisfactory the compressor may be started in the following way :1.

Open the compressor suction and discharge valves.

2.

Open hot gas inlet valve on the condenser/receiver, if any.

3.

Open liquid outlet valve. This will build-up a pressure in the system and cause the low pressure switch to function and start the compressor. (Note : If the low pressure switch contacts were kept mechanically ‘closed’ during pump down cycle, this must be removed. Also, if the liquid solenoid valve was manually opened, it should be reset for automatic operation).

4.

Check oil level in the crank case and the refrigerant charge. If necessary, add oil and gas. In case of refrigerant shortage, bubbles may appear on the liquid indicator in the refrigerant liquids line.

REFRIGERANT CHARGING : If the system has lost refrigerant gas due to leaks, broken connections, etc., this gas must be replaced in order to ensure proper operation. It is important that no gas be added until all leaks have been repaired. Shortage of refrigerant may be indicated by any of the following symptoms :1.

Distinct hissing sound from the expansion valve.

2.

Low suction pressure and low condensing pressure. This condition may cause ‘short cycling’ of the system.

3.

Numerous bubbles at the liquid level indicator on the liquid line.

4.

Presence of the oil at leaking joint, connection, bolt and head etc.

To add refrigerant to the system, follow the procedure outlined below :1.

Back seat the compressor suction shut-off valve. Remove the plug from the gauge port.

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2.

Connect the charging line from the refrigerant drum to the gauge port but before tightening the connections, ensure that all air is purged from the line by cracking the outlet valve on the refrigerant drum.

3.

Keep the refrigerant drum in a vertical position with the outlet connection on top so that, only gas can enter the system.

4.

Start the compressor. Crack the suction valve off the back seat and open the refrigerant drum valve. Gas will start flowing into the system. Charge the system slowly. Stop the charging every two or three minutes. Allow the system to stabilize and check suction, pressure and liquid indicator, etc. Repeat this process of charging until system is fully charged.

5.

Watch the liquid level indicator and the suction pressure gauge. When the bubbles disappear and the suction pressure is normal, close the drum valve, back seat the compressor suction shut-off valve, remove the charging line and plug the gauge port.

6.

Check the oil level. Note : Whenever a system is provided with a charging valve in liquid line, it is recommended that this should be utilised in preference to the gauge pot on the suction valve.

PURGING OF NON-CONDENSABLE GASES : A refrigerant system must be kept free from gases other than the refrigerant. Listed below are several sources of non-condensable in a system : 1.

Leakage of air onto a system operating at a vacuum through leaky joints or on the packing.

2.

Residual air in hose connections when adding oil or when charging refrigerant.

3.

System not properly evaluated after the system has been opened out for repairs.

4.

Possible non-condensable in the gas cylinder itself. (Note : To avoid this, always get refrigerant from well established and reputed suppliers).

Non-condensable gases will cause high condensing pressures This will reduce the refrigeration capacity, increase the operating cost and cause erratic operation of the system. It is necessary therefore that non-condensable gases must be purged from the system as soon as their presence is evidenced. In

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order to determine whether air is present in the condenser, following procedure should be followed: Pump down the unit. Run the condenser water pump for approximately half an hour to allow the condenser to cool down. Install a pressure gauge on the discharge service valve. After the water has circulated in the condenser for sometime, its temperature and the pressure in the condenser will both be about steady. Determine the saturation pressure of the refrigerant corresponding to the condenser water temperature, leaving and entering water temperature will of course be the same. If the actual pressure in the condenser is much above this saturation pressure, it is clear that non-condensable gasses are present and the system should be purged through the purge valve provided on top of the condenser. On self contained systems where the condenser is an integral part of the refrigeration unit and is slung under the compressor baseplate, the discharge service valve being the highest point over the condenser, presents a convenient point to purge. 1.

Back seat the service valve.

2.

Discount the gauge from its outlet connection on this valve.

3.

Slowly turn the valve system off the back seat from one to two turns. Air will blow off if this is done slowly. On central remote installations, a separate purge valve is generally provided which is located at a highest point on the discharge line. On such system this valve should be used for purging non-condensable gases.

ADDING OIL TO THE COMPRESSOR : If the crank case oil level is low, add oil to the system to bring oil to the correct level. Use capella ‘D’ or equivalent lubricant oil. Oil must be taken out only from sealed airtight tins. Do not use oil from open drums or containers.