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Refrigeration Compressors

A compressor is the most important and often the costliest component (typically 30 to 40 percent

of total cost) of any vapour compression refrigeration system (VCRS).

The function of a compressor in a VCRS is to continuously draw the refrigerant vapour from theevaporator, so that a low pressure and low temperature can be maintained in the evaporator atwhich the refrigerant can boil extracting heat from the refrigerated space.

The compressor then has to raise the pressure of the refrigerant to a level at which it can

condense by rejecting heat to the cooling medium in the condenser.

Classification of compressors: 

Compressors used in refrigeration systems can be classified in several ways:

a) Based on the working principle: 

i. Positive displacement typeii. Rotor-dynamic type

In positive displacement type compressors, compression is achieved by trapping a refrigerant

vapour into an enclosed space and then reducing its volume.Since a fixed amount of refrigerantis trapped each time, its pressure

rises as its volume is reduced.

When the pressure rises to a level that is slightly higher than the condensing pressure, then it is

expelled from the enclosed space and a fresh charge of low-pressure refrigerant is drawn in andthe cycle continues. Since the flow of refrigerant to the compressor is not steady, the positivedisplacement type compressor is a pulsating flow device.

However, since the operating speeds are normally very high the flow appears to be almost steadyon macroscopic time scale. Since the flow is pulsating on a microscopic time scale, positive

displacement type compressors are prone to high wear, vibrationand noise level. Depending upon the construction, positive displacement type

compressors used in refrigeration and air conditioning can be classified into:

i. Reciprocating type

ii. Rotary type with sliding vanesiii. Rotary screw typeiv. Orbital compressors

v. Acoustic compressors

In rotor-dynamic compressors, the pressure rise of refrigerant is achieved by imparting kineticenergy to a steadily flowing stream of refrigerant by a rotating mechanical element and then

converting into pressure as the refrigerant flows through a diverging passage.

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Unlike positive displacement type, the roto-dynamic type compressors are steady flow devices,hence are subjected to less wear and vibration. Depending upon the construction, rotor-dynamic

type compressors can be classified into:

i. Radial flow type, or 

ii. Axial flow type

Centrifugal compressors (also known as turbo-compressors) are radial flow type, roto-dynamic

compressors. These compressors are widely used in large capacity refrigeration and air conditioning systems. Axial flow compressors are

normally used in gas liquefaction applications.

b) Based on arrangement of compressor motor or external drive: 

i. Open type

ii. Hermetic (or sealed) type

iii. Semi-hermetic (or semi-sealed) type

In open type compressors the rotating shaft of the compressor extends through a seal in the

crankcase for an external drive. The external drive may be an electrical motor or an engine (e.g.diesel engine).

The compressor may be belt driven or gear driven. Open type compressors are normally used inmedium to large capacity refrigeration system for all refrigerants and for ammonia (due to its

incompatibility with hermetic motor materials).

Open type compressors are characterized by high efficiency, flexibility, better compressor 

cooling and serviceability. However, since the shaft has to extend through the seal, refrigerantleakage from the system cannot be eliminated completely.

Hence refrigeration systems using open type compressors require a refrigerant reservoir to takecare of the refrigerant leakage for some time, and then regular maintenance for charging the

system with refrigerant, changing of seals, gaskets etc.

In hermetic compressors, the motor and the compressor are enclosed in the same housing to

 prevent refrigerant leakage. The housing has weld connections for refrigerant inlet and outlet andfor power input socket.As a result of this, there is virtually no possibility of refrigerant leakage

from the compressor.

All motors reject a part of the power supplied to it due to eddy currents andfriction, that is, inefficiencies. Similarly the compressor also gets heated-up due to

friction and also due to temperature rise of the vapor during compression.

In Open type, both the compressor and the motor normally reject heat to thesurrounding air for efficient operation. In hermetic compressors heat cannot be

rejected to the surrounding air since both are enclosed in a shell. Hence, the cold

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suction gas is made to flow over the motor and the compressor before enteringthe compressor.

This keeps the motor cool. The motor winding is in direct contact

with the refrigerant hence only those refrigerants, which have high dielectric

strength, can be used in hermetic compressors.

The cooling rate depends upon the flow rate of the refrigerant, its temperature and the thermal

 properties of the refrigerant. If flow rate is not sufficient and/or if the temperature is not lowenough the insulation on the winding of the motor can burn out and short-circuiting may occur.

Hence, hermetically sealed compressors give satisfactory and safe performance over a verynarrow range of design temperature and should not be used for off-design conditions.

The COP of the hermetic compressor based systems is lower than that of theopen compressor based systems since a part of the refrigeration effect is lost in

cooling the motor and the compressor. However, hermetic compressors are

almost universally used in small systems such as domestic refrigerators, water coolers, air conditioners etc, where efficiency is not as important as customer convenience (due to absence of continuous maintenance).

In addition to this, the use of hermetic compressors is ideal in systems, which use capillary tubes

as expansion devices and are critically charged systems. Hermetic compressors are normally notserviceable. They are not very flexible as it is difficult to vary their speed to control the cooling

capacity.

In some (usually larger) hermetic units, the cylinder head is usually removableso that the valves and the piston can be serviced. This type of unit is called a

semi-hermetic (or semi-sealed) compressor.

Capacity of the compressor to remove heat from the refrigerant gas. This is most frequently a nominal

rating, that is, based on a standard set of conditions including Condensing Temperature (CT),

Evaporative Temperature (ET), refrigerant, motor RPM, etc. Typically a compressor can be run at many

different values for these parameters, with corresponding changes in the cooling capacity. Typical units

are kilowatts, tons of refrigeration, or BTU/hour.

Search Logic: User may specify either, both, or neither of the "At Least" and "No More Than"

values. Products returned as matche Power required to run the compressor motor at the rating point.

s will meet all specified criteria.

Condensing Temperature: 

Range of condensing temperature over which the compressor is rated.

Evaporative Temperature: 

Range of evaporative temperature over which the compressor is rated.

Compressor Construction

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 Your choices are...

Scroll  Refrigerant gas is moved as intake air is compressed

 between the surfaces of mating involute spirals, one

of which is moving to progressively push theevacuated air out to the exhaust.

Screw  One screw spindle compresses intake gas or multiple spindles with intermeshing screws rotate

oppositely, creating axially progressing "chambers,"moving the gas from suction to discharge.

Piston  Motor-driven piston(s) mechanically reduces gas

volume inside cylinder; often are rated for high pressure.

=

 About Refrigeration Compressors and Air Conditioning Compressors

Refrigeration compressors and air conditioning compressors provide air conditioning, heat

 pumping, and refrigeration for large-scale facilities and equipment. They compress low-pressure,low-volume gas into high-pressure and high-temperature gas. Refrigeration compressors and air 

conditioning compressors also remove vapor from the evaporator. Most refrigeration

compressors (refrigerant compressors) are large, mechanical units that form the heart of industrial cooling systems or heating, ventilation, and air conditioning (HVAC) systems. Air conditioning compressors are also large-scale mechanical devices; however, these compressors

are designed specifically for air conditioning systems and do not provide heating or ventilationfunctions.

There are three main categories of refrigeration compressors and air conditioning compressors:screw, scroll and piston. Screw compressors pass refrigerant vapor through screw spindles which

compress the gas. Although most screw compressors have two spindles, products with three or more spindles are also available. Scroll compressors are the most common type of refrigerant

compressor. They have a relatively low rate of leakage and provide high efficiency. Pistoncompressors or reciprocating compressors are specifically designed for industrial and

commercial applications. They compress gases to high pressures and are designed for continuousoperation. Other types of specialized refrigeration compressors and air conditioning compressors

are also available.

Selecting refrigeration compressors and air conditioning compressors requires an analysis of refrigerant options and nominal cooling specifications. Refrigerant choices include R12, R13,

R22, R134a, R404a, R407c, R502, and R507. There are four nominal cooling specifications to

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consider: capacity, input power, condensing temperature, and evaporative temperature. Capacitymeasures the ability of a refrigerant compressor to remove heat from the refrigerant gas.

 Nominal ratings are based on a standard set of conditions which include condensing temperature(CT), evaporative temperature (ET), refrigerant, and motor revolutions per minutes (rpm).

Typically, refrigeration compressors and air conditioning compressors can run at many different

values for these parameters, with corresponding changes in their cooling capacity.

Refrigeration compressors and air conditioning compressors also carry power-source

specifications that include voltage, frequency and phase. Common choices include 12 VDC and24 VDC, as well as 115/60/1, 230/50/1, 208-230/60/1, 208-230/60/3, 380/50/3, 460/60/3 and

575/60/3. Refrigeration compressors and air conditioning compressors with other power-sourcespecifications are available for European and Asian markets.

Parts of a Refrigerator

Q uiz Corner

How much you know about motors found in your refrigerator and what they do? Test your knowledge

with our Electric Motor Quiz! 

As we learned in the introduction, the basic idea behind a refrigerator is to use the evaporation of a liquid to absorb heat. You probably know that when you put water on your skin it makes you

feel cool. As the water evaporates, it absorbs heat, creating that cool feeling. Rubbing alcoholfeels even cooler because it evaporates at a lower temperature. The liquid, or refrigerant, used

in a refrigerator evaporates at an extremely low temperature, so it can create freezingtemperatures inside the refrigerator. If you place your refrigerator's refrigerant on your skin

(definitely NOT a good idea), it will freeze your skin as it evaporates.

There are five basic parts to any refrigerator (or air-conditioning system):

y  Compressor 

y  Heat-exchanging pipes - serpentine or coiled set of pipes outside the unit 

y  Expansion valve 

y  Heat-exchanging pipes - serpentine or coiled set of pipes inside the unit 

y  Refrigerant - liquid that evaporates inside the refrigerator to create the cold temperatures

Many industrial installations use pure ammonia as the refrigerant. Pure ammonia

evaporates at -27 degrees Fahrenheit (-32 degrees Celsius).