Refrigeration Compressors A compressor is the most important and often the costliest component (typically 30 to 40 percent p ercent of total cost) of any vapour compression compression refrigeration re frigeration system system (VCRS). The function of a compressor co mpressor in a VCRS is to continuously draw t he refrigerant vapour from the evaporator, so that a low pressure and low temperature can be maintained in the evaporator at which the refrigerant can boil extracting extract ing 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 type ii. Rotor-dynamic type In positive displacement type compressors, compression is achieved by trapp ing a refrigerant vapour into an enclosed space and then reducing its volume.Since a fixed amount of refrigerant is 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 nd a fresh charge o f low-pressure refrigerant refrigerant is drawn in and the cycle continues. Since S ince the flow of refrigerant to the compressor is not steady, t he positive displacement type compressor is a pulsating flow device. However, since the operating speeds are normally very high the flow appears to be almost steady on macroscopic time scale. Since S ince the flow is pulsating on a microscopic time scale, po sitive displacement type compressors are prone to high wear, vibration and noise level. Depending upo n the construction, positive displacement type compressors used in refrigeration and air conditioning can be c lassified into: i. Reciprocating type ii. Rotary type with sliding vanes iii. Rotary screw type iv. Orbital compressors v. Acoustic compressors In rotor-dynamic compressors, the pressure rise of refrigerant is achieved by imparting kinet ic energy to a steadily flowing flowing stream strea m of refrigerant by a rotating mechanical element and then converting into pressure as the refrigerant flows through a diverging passage.
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 capac ity 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 dr iven or gear driven. Open type t ype compressors are normally used in medium to large capacity refrigeration system for a ll refrigerants refrigerants and for ammonia (due to its incompatibility with hermetic motor materials). materials). Open type compressors are characterized by high efficiency, flexibility, better compressor cooling and serviceability. However, since the s haft has to extend through t hrough the seal, refrigerant leakage from the system cannot be eliminated completely. Hence refrigeration systems using open type compressors require a re frigerant frigerant reservoir to take care of the refrigerant leakage for some so me time, and then regular maintenance for charging the system with refrigerant, changing of seals, gaskets etc. In hermetic compressors, the motor and the co mpressor are enclosed in the same housing to prevent refrigerant leakage. The housing hou sing has weld connections for refrigerant inlet and outlet and for power input socket.As a result of this, t his, there is virtually no possibility of refrigerant leakage from the compressor. All motors reject a part of the po wer supplied to it due to eddy currents and friction, 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 the surrounding air for efficient operation. In hermetic co mpressors mpressors heat cannot be rejected to the surrounding air since both bot h are enclosed in a shell. Hence, the cold
suction gas is made to flow flow over o ver the motor and the compressor before entering the compressor. This keeps the motor cool. The motor winding is in direct contact with the refrigerant hence only those refrigerants, r efrigerants, which have high dielectric strength, can be used in hermetic her metic compressors. The cooling rate depends d epends 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 low enough the insulation on the winding of the motor can burn out and short-circuiting may occur. Hence, hermetically sealed compressors give satisfactory and safe per formance over a very narrow range of design temperature te mperature and should not be used for off-design o ff-design conditions. The COP of the hermetic co mpressor based systems is lower than that of the open compressor based systems since a part o f the refrigeration effect is lost in cooling the motor and t he 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 o f hermetic compressors is ideal in systems, which use capillary tubes as expansion devices and are critically charged systems. Hermetic compressors are normally not serviceable. 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 c ylinder head is usually removable so that the valves and the piston can be serviced. This Th is 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 and "No More Than" Than"
values. Products returned as matche matche Power required to run the compressor 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
Your choices are... Scroll
Refrigerant gas is moved as intake air a ir is compressed between the surfaces of mating involute invo lute spirals, one of which is moving to progressively push the evacuated 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.
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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 Refrigeration compressors and air a ir conditioning compressors also remove vapor from the evapo rator. Most refrigeration compressors (refrigerant compressors) are large, mechanical units that form the heart of industrial cooling systems or heating, ventilation, and air cond itioning (HVAC) systems. Air conditioning compressors are also large-scale mechanical devices; however, t hese compressors are designed specifically for air conditi cond itioning oning systems and do not provide heating or ventilation functions. There are three main catego ries of refrigeration compressors and air conditioning compressors: screw,, scroll and piston screw and piston.. Screw compressors pass refrigerant vapor through screw sp indles which compress the gas. Although most screw compress co mpressors ors have two spindles, products with three or more spindles are also available. Scroll co mpressors mpressors are the most common type of refrigerant compressor. They have a relatively low rate of leakage and provide high efficiency. Piston compressors or reciprocating compressors are specifically designed for industrial and commercial applications. They compress gases to high pressures and are designed for continuous operation. Other types of specialized refrigeration compressors and air cond itioning compressors are also available. Selecting refrigeration compressors and air conditioning compressors requires an analysis of refrigerant options and nominal cooling specifications. Refrigerant cho ices include R12, R13, R22,, R134a R22 R134a,, R404a R404a,, R407c R407c,, R502, and R507 R507.. There are four nominal cooling specifications to
consider: capacity, input power, condensing temperature, and evaporative temperature. Capacity measures 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 revo lutions 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 and 24 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-source specifications are available for European and Asian markets.
Parts of a Refrigerator
uiz Corner Q uiz How much you know about motors found in your refrige rator and what they do? Test your knowledge with our Electric Motor Quiz!
As we learned in the introduction, the basic idea behind a refrigerator re frigerator 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 alcohol feels even cooler because it evaporates at a lower temperature. The liquid, or refrigerant, used in a refrigerator evaporates at an extremely low t emperature, so it can create freezing freezing temperatures inside the refrigerator. If you place yo ur refrigerator's refrigerant 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-condi (or air-conditioning tioning system syste m): y y y y y
Compressor Heat-exchanging pipes - serpentine or coiled set of pipes outside the unit Expansion valve Heat-exchanging pipes - serpentine or coiled set of pipes inside the unit Refrigerant - liquid that evaporates inside the refrigerator to create the cold temperatures
Many industrial installations use pure ammonia as t he refrigerant. Pure ammonia evaporates at -27 degrees d egrees Fahrenheit (-32 degrees Celsius).
