Refrigeration Compressors and Air Conditioning Compressors Information
Refrigeration compressors and air conditioning compressors provide air conditioning, heat pumping, and refrigeration for large-scale facilities and equipment. They use compression to raise the temperature of a low-pressure gas, and also remove vapor from the evaporator. Most refrigeration compressors (refrigerant compressors) are large, mechanical units that form the heart of industrial cooling, heating, ventilation, and air conditioning (HVAC) systems. Many 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 ventilation functions.
Refrigerant compressors work by taking in low pressure gas on the inlet and compressing it mechanically. Different types of compression mechanisms are what differentiate compressors (discussed below). This compression creates a high temperature, high pressure gas - an essential step in the overarching refrigeration cycle.
The refrigeration cycle or heat pump cycle is the model describing the transfer of heat from regions of lower temperature to regions of higher temperature. It defines the operating principles behind refrigerators, air conditioners, heaters, and other "heat pumping" devices.
This diagram presents a visual overview of the refrigeration cycle:
The letters A-D indicate the different system components. The numbers 1-5 indicate the different physical states of the refrigerant fluid as it moves through the system.
State 1 is the state after the refrigerant passes through an evaporator (D), where warm air heats the fluid and converts it completely to vapor.
State 2 is the state after the fluid passes through a compressor (A) which increases the fluid's pressure and temperature up to superheated levels.
States 3 and 4 are when the fluid passes through an evaporator (B), which transfers heat to the ambient and condenses the fluid to liquid.
State 5 is the state after the fluid passes through an expansion valve or metering device (C), which lowers the pressure of the fluid. This cools the fluid and subsequently turns the liquid into a liquid/vapor mixture.
This video provides further explanation of the refrigeration cycle:
Video Credit: Learn Enginering / CC BY-SA 4.0
Temperature-entropy and pressure-enthalpy diagrams are often used to construct and describe these systems. They define the properties of the fluid at different stages in the system.
The diagram below depicts temperature-entropy a typical refrigeration cycle:
The following diagram shows pressure-entropy of a typical refrigeration cycle:
Types of Compressors
There are a number of different types of compressors used for refrigeration and air conditioning. Like pumps, all "heat pumps" can first be categorized as either positive displacement or non-positive displacement (centrifugal). Positive displacement compressors have chambers which decrease in volume during compression, while non-positive displacement compressors have fixed-volume chambers. Beyond this distinction, each type differs based on its specific mechanism for fluid compression. The five main types of compressors are piston, rotary, screw, scroll, and centrifugal.
Piston compressors, also called reciprocating compressors, use a piston and cylinder arrangement to provide compressive force - like combustion engines or piston pumps. The reciprocating motion of the piston due to external power compresses the refrigerant inside the cylinder. Piston compressors have a low initial cost and a simple, easy to install design. They have a large power output range and can reach extremely high pressures. However, they have high maintenance costs, potential vibrational issues, and are not typically designed to run continuously at full capacity.
Rotary compressors have two rotating elements, like gears, between which the refrigerant is compressed. These compressors are very efficient because the actions of taking in refrigerant and compressing refrigerant occur simultaneously. They have very few moving parts, low rotational speeds, low initial and maintenance costs, and are forgiving in dirty environments. However, they are limited to smaller volumes of the gas and produce less pressure than other types of compressors.
The following diagram shows rotary vane compressor operation.
Screw compressors use a pair of helical rotors or screws which mesh together to compress the refrigerant between them. They can produce high pressure for a small quantity of gas and consume less power than reciprocating compressors. They have low to medium initial and maintenance costs and few moving parts. However, they have difficulty in dirty environments, high rotational speeds, and shorter life expectancies than other designs.
Scroll compressors use two offset spiral disks nested together to compress the refrigerant. The upper disk is stationary while the lower disk moves in orbital fashion. Scroll compressors are quiet, smooth-operating units with few moving parts and the highest efficiency ratio of all compressor types. They also are more flexible for handling refrigerants in the liquid. However, as fully hermetic designs, scroll compressors cannot be easily repaired. They also typically cannot rotate in both directions. Scroll compressors are commonly used in automobile air conditioning systems and commercial chillers.
Centrifugal compressors use the rotating action of an impeller wheel to exert centrifugal force on refrigerant inside a round chamber (volute). Unlike other designs, centrifugal compressors do not operate on the positive displacement principle, but have fixed volume chambers. They are well suited to compressing large volumes of refrigerant to relatively low pressures. The compressive force generated by an impeller wheel is small, so systems that use centrifugal compressors usually employ two or more stages (impellers wheels) in series to generate high compressive forces. Centrifugal compressors are desirable for their simple design, few moving parts, and energy efficiency when operating multiple stages.
Usually compressors are designed to work with a particular type of refrigerant. Selecting a proper refrigeration compressor or air conditioning compressor requires finding a compressor rated for the desired refrigerant for the application. Refrigerants are given names such as R-12 or R-134a from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). Ideal refrigerants have favorable thermodynamic properties and are chemically inert (noncorrosive), environmentally friendly (degradable), and safe (nontoxic, nonflammable). The desired fluid should have a boiling point somewhat below the target temperature, a high heat of vaporization, moderate liquid density, high gas density, and a high critical temperature.
There are a number of specifications to consider when selecting compressors. These include capacity, condensing temperature, evaporating temperature, flow rate, and power.
Charts like this one are provided by the compressor's manufacturer, allowing engineers to make these adjustments properly once in the system:
Table Credit: Carlyle Compressor Company
Capacity (Btu/hr) measures the ability of a refrigerant compressor to remove heat from the refrigerant gas. Nominal capacity ratings are based on a standard set of conditions which include condensing temperature (CT), evaporative temperature (ET), refrigerant, and motor revolutions per minute (rpm). Typically, refrigeration compressors and air conditioning compressors can run at many different values for these parameters, with corresponding changes in their cooling capacity. Once in use, compressors can be tweaked and adjusted to the desired capacity and operating conditions.
Condensing temperature is the range of condensing temperatures over which the compressor is rated to operate.
Evaporating temperature is the range of evaporative temperatures over which the compressor is rated to operate.
Flow rate is the rate (by mass) at which the fluid is passed through the compressor, measured in pounds per hour (lb/hr) or kilograms per hour (kg/hr).
Power (W) is the input power required to run the compressor motor at a specific operating point.
Refrigeration compressors and air conditioning compressors also carry power-source specifications defined by voltage / frequency / 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 can incorporate a number of features which may be important for certain applications.
Thermal shut off - compressor features controls which turn the compressor off at high temperatures to prevent it from overheating. They also can provide restart once the compressor has cooled down below a certain temperature.
Sealing - describes how the compressor and motor drive are situated in relation to the gas or vapor being compressed. Sealed compressors do not allow the gas a route to leak out of the system. Compressors may be one of three types: open, hermetic, and semi-hermetic.
Open types have a separate housing for the compressor and the motor. They rely on lubricant in the system to splash on pump components and seals. If not operated frequently, the system can leak its operating gases. Open compressors can be driven by non-electric power sources such as combustion engines.
Hermetic types seal the compressor and motor together in the same housing. These compressors are leak-free and can sit for long periods unused, but cannot be maintained or repaired.
Semi-hermetic types also contain the motor and compressor in one housing, but instead of a one-piece housing they incorporate gasketed/bolted covers. These can be removed for maintenance and repair of the compressor or motor.
Low noise - compressor operation generates less noise for applications where a quiet environment is desired.
Light weight - compressor is compactly built or constructed with low-density materials for cooling systems which require low weight components.
Variable speed - compressor has speed adjustment for running at various operating flow rates and conditions.
Standards related to refrigeration and air conditioning compressors include:
BS EN 13771-1 -- Compressor And Condensing Units for Refrigeration - Performance Testing and Test Methods - Part 1: Refrigerant Compressors
DIN 51503-2 -- Testing Used Lubricants For Refrigeration Compressors
GOST 22502 -- Condensing Units with Hermetic Refrigeration Compressors for Commercial Refrigeration Equipment
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