R & Ac Ass(2003)

Refrigeration “Refrigeration is the process of removing heat from an enclosed space, or from a substance, and moving it to a place where it is unobjectionable”. The primary purpose of refrigeration is lowering the temperature of the enclosed space or substance and then maintaining that lower temperature. The term cooling refers generally to any natural or artificial process by which heat is dissipated. The process of artificially producing extreme cold temperatures is referred to as cryogenics.

Cold is the absence of heat, hence in order to decrease a temperature, one "removes heat", rather than "adding cold." In order to satisfy the Second Law of Thermodynamics, some form of work must be performed to accomplish this. This work is traditionally done by mechanical work but can also be done by magnetism, laser or other means 06-ME-20 1

Lab Assignment # 01

Refrigeration system components There are five basic components of a refrigeration system, these are: - Evaporator - Compressor - Condenser - Expansion Valve - Refrigerant; to conduct the heat from the product In order for the refrigeration cycle to operate successfully each component must be present within the refrigeration system.

The Evaporator The purpose of the evaporator is to remove unwanted heat from the product, via the liquid refrigerant. The liquid refrigerant contained within the evaporator is boiling at a low-pressure. The level of this pressure is determined by two factors: - The rate at which the heat is absorbed from the product to the liquid refrigerant in the evaporator - The rate at which the low-pressure vapour is removed from the evaporator by the compressor To enable the transfer of heat, the temperature of the liquid refrigerant must be lower than the temperature of the product being cooled. Once transferred, the liquid refrigerant is drawn from the evaporator by the compressor via the suction line. When leaving the evaporator coil the liquid refrigerant is in vapour form.

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A/C evaporator

Types of evaporators Evaporators used for industrial refrigeration and air-conditioning purposes are very large and also called chillers. They are usually made in the form of shell and tube types with two possible arrangements: namely, dry expansion evaporators and flooded evaporators. For smaller and home purposes there are three types of evaporators: baretube type, plate-surface type and finned evaporators:

•

bare-tube evaporators the refrigerant flows through the bare-tube and the fluid to be chilled flows directly over it.

•

Plate-surface evaporators are used in household refrigerators. These evaporators are formed by welding together two plates that have grooves on their surface. When they are welded, the closed grooves form a sort of the tubing through which the refrigerant flows.

•

Finned evaporators are commonly used in window, split and packaged air-conditioners. They are in the form of a copper coil over which several fins are welded to increase the cooling area of the evaporator. Hot air passes over this evaporator and gets chilled as it enters the room.

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Some other types are: •

Natural/forced circulation evaporator

•

Falling film evaporator

•

Rising film (Long Tube Vertical) evaporator

•

Plate evaporator

The Compressor The purpose of the compressor is to draw the low-temperature, low-pressure vapour from the evaporator via the suction line. Once drawn, the vapour is compressed. When vapour is compressed it rises in temperature. Therefore, the compressor transforms the vapour from a low-temperature vapour to a high-temperature vapour, in turn increasing the pressure. The vapour is then released from the compressor in to the discharge line.

compressers

Types of compressor The three basic types of air compressors are •

reciprocating

•

rotary screw

•

rotary centrifugal

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Reciprocating Air Compressors Reciprocating air compressors are positive displacement machines, meaning that they increase the pressure of the air by reducing its volume. This means they are taking in successive volumes of air which is confined within a closed space and elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by a piston within a cylinder as the compressing and displacing element. Reciprocating compressors are of two types: i) Open type: The speed of the open compressor can be adjusted as per the capacity requirements. If it is a multi-cylinder compressor, a certain number of cylinders can be bypassed to adjust the capacity and reduce

power

consumption.

ii) Hermetics – totally sealed (welded) and semi-hermetic:

Hermetic Compressor In the hermetically sealed compressor, the electric motor and compressor are both in the same airtight (hermetic) housing and share the same shaft. Figure shows a hermetically sealed unit. Note that after assembly, the two halves of the case are welded together to form an airtight cover. The compressor, in this case, is a double-piston reciprocating type. Other compressors may be of the centrifugal or rotary types.

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Cooling and lubrication are provided by the circulating oil and the movement of the refrigerant vapor throughout the case. The advantages of the hermetically sealed unit (elimination of pulleys, belts and other coupling methods, elimination of a source of refrigerant leaks) are offset somewhat by the inaccessibility for repair and generally lower capacity.

Rotary Screw Compressors Rotary air compressors are positive displacement compressors. The most common rotary air compressor is the single stage helical or spiral lobe oil flooded screw air compressor. These compressors consist of two rotors within a casing where the rotors compress the air internally. There are no valves. These units are basically oil cooled (with air cooled or water cooled oil coolers) where the oil seals the internal clearances.

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Centrifugal Compressors The centrifugal air compressor is a dynamic compressor which depends on transfer of energy from a rotating impeller to the air. Centrifugal compressors produce high-pressure discharge by converting angular

momentum

imparted

by

the

rotating

impeller

(dynamic

displacement). In order to do this efficiently, centrifugal compressors rotate at higher speeds than the other types of compressors. These types of compressors are also designed for higher capacity because flow through the compressor is continuous.

The Condenser The purpose of the condenser is to extract heat from the refrigerant to the outside air. The condenser is usually installed on the reinforced roof of the building, which enables the transfer of heat. Fans mounted above the condenser unit are used to draw air through the condenser coils. The temperature of the high-pressure vapour determines the temperature at which the condensation begins. As heat has to flow from the condenser to the air, the condensation temperature must be higher than that of the air; usually between -12°C and -1°C. The high-pressure vapour within the condenser is then cooled to the point where it becomes a liquid refrigerant once more, whilst retaining some heat. The liquid refrigerant then flows from the condenser in to the liquid line.

Types of condensers There are two basic types of condensers: •

air cooled,

•

water cooled and evaporative.

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Air cooled condensers: Air cooled condensers are used in small units like household refrigerators, deep freezers, water coolers, window air-conditioners, split air-conditioners, small packaged air-conditioners etc. These are used in plants where the cooling load is small and the total quantity of the refrigerant in the refrigeration cycle is small. Air cooled condensers are also called coil condensers as they are usually made of copper or aluminum coil. Air cooled condensers occupy a comparatively larger space than water cooled condensers. Air cooled condensers are of two types: • Natural convection • Forced convection In the natural convection type, the air flows over it in natural a way depending upon the temperature of the condenser coil. In the forced air type, a fan operated by a motor blows air over the condenser coil.

Air cooled Condensers

Water cooled condensers: Water cooled condensers are used for large refrigerating plants, big packaged air-conditioners, central air-conditioning plants, etc. These are used in plants where cooling loads are excessively high and a large quantity 06-ME-20 8

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of

refrigerant

flows

through

the

condenser.

There are three types of water cooled condensers: • Tube-in-tube or double pipe type • Shell and coil type • Shell and tube type. In all these condensers the refrigerant flows through one side of the piping while the water flows through the other piping, cooling the refrigerant and condensing it.

Water cooled condenser

The Expansion devices Within the refrigeration system, the expansion valve is located at the end of the liquid line, before the evaporator. The high-pressure liquid reaches the expansion valve, having come from the condenser. The valve then reduces the pressure of the refrigerant as it passes through the orifice, which is located inside the valve. On reducing the pressure, the temperature of the refrigerant also decreases to a level below the surrounding air. This lowpressure, low-temperature liquid is then pumped in to the evaporator.

Types of expansion devices 06-ME-20 9

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There are different types of throttling devices, but in refrigerating and airconditioning systems, the two most commonly used types are:

Capillary tube: Instead of the orifice, the capillary is small diameter tubing that offers the restricted flow of the refrigerant. Its internal diameter ranges from 0.020 to 0.090 inches depending upon the capacity of the refrigerating or airconditioning system. The pressure drop attained through the capillary depends upon its diameter and length. Capillary tubing made of copper is most

commonly

used.

Capillary tubing is used for small refrigerating and air-conditioning systems like household refrigerators, water coolers, deep freezers, window airconditioners, split air-conditioners, small packaged air-conditioners etc. For systems in which capillary tubing is fitted, technicians have to be very careful of refrigerant charging as the overcharging can lead excessive high discharge pressures from the compressor, which leads to over loading of the compressor and the chances of refrigerant leakages from the system are also increased.

Capillary tubes

Thermostatic expansion valves: 06-ME-20 10

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The thermostatic expansion valve is not controlled by the temperature. It works automatically maintaining proper flow of the refrigerant depending upon the heat load in the evaporator. Apart from reducing the pressure of the refrigerant, the thermostatic expansion valve also keeps the evaporator active. These days thermostatic expansion valves used with solenoid valves are

more

common.

Thermostatic expansion valves are used extensively in medium and large sized refrigerating and air-conditioning systems. They can be used for large water chilling plants, brine chilling plants, large packaged air-conditioners, central air-conditioning plants etc.

Types of expansion device The following types of expansion device are generally available: •

Capillary tube

•

Restrictor expansion device

•

Thermostatic expansion valve

•

Externally equalised expansion valve

•

Electronic expansion valve

•

Hand operated expansion valve

•

Automatic expansion valve

•

Flooded evaporator float control

•

Low pressure float valve control

The Refrigerant 06-ME-20 11

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The type of refrigerant used will depend on the pressure capabilities of the system and the temperatures that have to be achieved during refrigeration.

The refrigeration cycle The refrigeration cycle (shown in Diagram 1 below) begins with the refrigerant in the evaporator. At this stage the refrigerant in the evaporator is in liquid form and is used to absorb heat from the product. When leaving the evaporator, the refrigerant has absorbed a quantity of heat from the product and is a low-pressure, low-temperature vapour. This low-pressure, low-temperature vapour is then drawn from the evaporator by the compressor. When vapour is compressed it rises in temperature. Therefore, the compressor transforms the vapour from a low-temperature vapour to a high-temperature vapour, in turn increasing the pressure. This hightemperature, high-pressure vapour is pumped from the compressor to the condenser; where it is cooled by the surrounding air, or in some cases by fan assistance. The vapour within the condenser is cooled only to the point where it becomes a liquid once more. The heat, which has been absorbed, is then conducted to the outside air. At this stage the liquid refrigerant is passed through the expansion valve. The expansion valve reduces the pressure of the liquid refrigerant and therefore reduces the temperature. The cycle is complete when the refrigerant flows into the evaporator, from the expansion valve, as a lowpressure, low-temperature liquid.

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Diagram:

Instrumentation of refrigeration system Pressure switch A Pressure switch is a form of switch that makes electrical contact when a certain set pressure has been reached on its input. This is used to provide on/off switching from a pneumatic or hydraulic source. The switch may be designed to make contact either on pressure rise or on pressure fall.

Pneumatic Uses of pneumatic pressure switches include: •

switching off an electrically driven gas compressor when a set pressure is achieved in the reservoir

•

in-cell charge control in a Nickel-metal hydride battery

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Hydraulic Hydraulic pressure switches have various uses in automobiles, for example: •

to switch on a warning light if engine oil pressure falls below a safe level

•

to switch on brake lights automatically by detecting a rise in pressure in hydraulic brake pipes

•

In dust control systems (bag filter), a pressure switch is mounted on the header which will raise an alarm when air pressure in the header is less than necessary to gain or decline energy beyond the set value

Thermometer A thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to measure") is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor (e.g. the bulb on a mercury thermometer) in which some physical change occurs with temperature, plus some means of converting this physical change into a value (e.g. the scale on a mercury thermometer). Thermometers increasingly use electronic means to provide a digital display or input to a computer.

Pressure guage Pressure gauges and switches are among the most often used instruments in a plant. But because of their great numbers, attention to maintenance-and reliability--can be compromised. As a consequence, it is not uncommon in older plants to see many gauges and switches out of service. This is unfortunate because, if a plant is operated with a failed pressure switch, the safety of the plant may be compromised. Conversely, if a plant can operate safely while a gauge is defective, it shows that the gauge was not needed in 06-ME-20 14

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the first place. Therefore, one goal of good process instrumentation design is to install fewer but more useful and more reliable pressure gauges and switches.

Mechanical side with Bourdon tube

Indicator side with card

and dial

Pressure stat Pressure stat is a device used for regulating the pressure of a system so that the pressure of a system is maintained to a desired pressure. This pressure can be set manually.

Thermostat A thermostat is a device for regulating the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The name is derived from the Greek words thermos "hot" and statos "a standing". The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer fluid as needed, to maintain the correct temperature.

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Touch-screen thermostat

Thermostat

Types of thermostat Some types of thermostat are given below: •

Mechanicaly o

•

o

Wax pellet

o

Gas expansion

Electricaly o

Bimetallic thermostatic components

o

Simple two wire thermostats

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Bimetal



Millivolt thermostats



volt thermostats



Line voltage thermostats

Lab Assignment # 01