Vapor-compression refrigeration

Vapor-compression refrigeration^[1][2] is one of the many refrigeration cycles available for use. It has been and is the most widely used method for air-conditioning of large public buildings, private residences, hotels, hospitals, theaters, restaurants and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. Oil refineries, petrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems.

Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere. A device that performs this function may also be called a heat pump.

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Vapor-compression refrigeration cycle

In the vapor-compression refrigeration cycle, heat is transferred from a lower temperature source to a higher temperature heat sink. Heat naturally flows in the opposite direction, and due to the second law of thermodynamics work is required to move heat from cold to hot. A food refrigerator or freezer works in much the same way; it moves heat out of the interior into the room in which it stands.

This most common refrigeration cycle uses an electric motor to drive a compressor. In an automobile the compressor is usually driven by a belt connected to a pulley on the engine's crankshaft, with both using electric motors for air circulation. Since evaporation occurs when heat is absorbed, and condensation occurs when heat is released, air conditioners are designed to use a compressor to cause pressure changes between two compartments, and actively pump a refrigerant around. A refrigerant is pumped into the low pressure compartment (the evaporator coil), where, despite the low temperature, the low pressure causes the refrigerant to evaporate into a vapor, taking heat with it. In the other compartment (the condenser), the refrigerant vapour is compressed and forced through another heat exchange coil, condensing into a liquid, rejecting the heat previously absorbed from the cooled space. The heat exchanger in the condenser section (the heat sink mentioned above) is often cooled by a fan blowing outside air through it, or in some cases, such as marine applications, by other means such as water.

Description of the vapor-compression refrigeration system

The vapor-compression refrigeration system uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. Figure 1 depicts a typical, single-stage vapor-compression system. All such systems have four components: a compressor, a condenser, an expansion valve (also called a throttle valve), and an evaporator. Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapor and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at temperature and pressure at which it can be condensed with typically available cooling water or cooling air. That hot vapor is routed through a condenser where it is cooled and condensed into a liquid by flowing through a coil or tubes with cool water or cool air flowing across the coil or tubes. This is where the circulating refrigerant rejects heat from the system and the rejected heat is carried away by either the water or the air (whichever may be the case).

The condensed liquid refrigerant, in the thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporation of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated.

The cold mixture is then routed through the coil or tubes in the evaporator. A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapor mixture. That warm air evaporates the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser.

To complete the refrigeration cycle, the refrigerant vapor from the evaporator is again a saturated vapor and is routed back into the compressor.

Note: Saturated vapors and saturated liquids are vapors and liquids at their saturation temperature and saturation pressure. A superheated vapor is at a temperature higher than the saturation temperature corresponding to its pressure.

Refrigerants

"Freon" is a trade name for a family of haloalkane refrigerants manufactured by DuPont and other companies. These refrigerants were commonly used due to their superior stability and safety properties: they were not flammable nor obviously toxic as were the fluids they replaced. Unfortunately, these chlorine-bearing refrigerants reach the upper atmosphere when they escape. In the stratosphere, CFCs break up due to UV-radiation, releasing their chlorine atoms. These chlorine atoms act as catalysts in the breakdown of ozone, which does severe damage to the ozone layer that shields the Earth's surface from the Sun's strong UV radiation. The chlorine will remain active as a catalyst until and unless it binds with another particle, forming a stable molecule. CFC refrigerants in common but receding usage include R-11 and R-12. Newer and more environmentally-safe refrigerants include HCFCs (R-22, used in most homes today) and HFCs (R-134a, used in most cars) have replaced most CFC use. HCFCs in turn are being phased out under the Montreal Protocol and replaced by hydrofluorocarbons (HFCs), such as R-410A, which lack chlorine.

Newer refrigerants are currently the subject of research, such as supercritical carbon dioxide, known as R-744.^[3] These have similar efficiencies compared to existing CFC and HFC based compounds.

Thermodynamic analysis of the system

The thermodynamics of the vapor compression cycle can be analyzed on a temperature versus entropy diagram as depicted in Figure 2. At point 1 in the diagram, the circulating refrigerant enters the compressor as a saturated vapor. From point 1 to point 2, the vapor is isentropically compressed (i.e., compressed at constant entropy) and exits the compressor as a superheated vapor.

From point 2 to point 3, the superheated vapor travels through part of the condenser which removes the superheat by cooling the vapor. Between point 3 and point 4, the vapor travels through the remainder of the condenser and is condensed into a saturated liquid. The condensation process occurs at essentially constant pressure.

Between points 4 and 5, the saturated liquid refrigerant passes through the expansion valve and undergoes an abrupt decrease of pressure. That process results in the adiabatic flash evaporation and auto-refrigeration of a portion of the liquid (typically, less than half of the liquid flashes). The adiabatic flash evaporation process is isenthalpic (i.e., occurs at constant enthalpy).

Between points 5 and 1, the cold and partially vaporized refrigerant travels through the coil or tubes in the evaporator where it is totally vaporized by the warm air (from the space being refrigerated) that a fan circulates across the coil or tubes in the evaporator. The evaporator operates at essentially constant pressure. The resulting saturated refrigerant vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle.

It should be noted that the above discussion is based on the ideal vapor-compression refrigeration cycle which does not take into account real world items like frictional pressure drop in the system, slight internal irreversiblity during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).

Types of gas compressors

Main article: Gas compressor

The most common compressors used in chillers are reciprocating, rotary screw, centrifugal, and scroll compressors. Each application prefers one or another due to size, noise, efficiency and pressure issues.

Reciprocating compressors

Main article: Reciprocating compressor

Reciprocating compressors are piston-style, positive displacement compressors.

Rotary screw compressors

Main article: Rotary screw compressor

Rotary screw compressors are also positive displacement compressors. Two meshing screw-rotors rotate in opposite directions, trapping refrigerant vapor, and reducing the volume of the refrigerant along the rotors to the discharge point.

Centrifugal compressors

Main article: Centrifugal compressor

Centrifugal compressors are dynamic compressors. These compressors raise the pressure of the refrigerant by imparting velocity or dynamic energy, using a rotating impeller, and converting it to pressure energy.

Scroll compressors

Main article: Scroll compressor

Scroll compressors are also positive displacement compressors. The refrigerant is compressed when one spiral orbits around a second stationary spiral, creating smaller and smaller pockets and higher pressures. By the time the refrigerant is discharged, it is fully pressurized.

Others

Main article: Diaphragm compressor

Main article: Axial-flow compressor

Main article: Diagonal or mixed-flow compressor

Main article: Liquid ring compressor

Main article: Roots blower

Other features and facts of interest

The schematic diagram of a single-stage refrigeration system shown in Figure 1 does not include other equipment items that would be provided in a large commercial or industrial vapor compression refrigeration system, such as:

• A horizontal or vertical pressure vessel, equipped internally with a demister, between the evaporator and the compressor inlet to capture and remove any residual, entrained liquid in the refrigerant vapor because liquid may damage the compressor. Such pressure vessels are most often referred to as "suction line accumulators". (In other industrial processes they are called "compressor suction drums" or "knockout drums".)

• Large commercial or industrial refrigeration systems may have multiple expansion valves and multiple evaporators in order to refrigerate multiple enclosed spaces or rooms. In such systems, the condensed liquid refrigerant may be routed into a pressure vessel, called a receiver, from which liquid refrigerant is withdrawn and routed through multiple pipelines to the multiple expansion valves and evaporators.

• Some refrigeration units may have multiple stages which requires the use of multiple compressors in various arrangements.^[4]

More details about the design and performance of vapor-compression refrigeration system are available in the classic "Perry's Chemical Engineers' Handbook".^[5]

The cooling capacity of refrigeration systems is often defined in units called "tons of refrigeration". The most common definition of that unit is: 1 ton of refrigeration is the rate of heat removal required to freeze a short ton (i.e., 2000 pounds) of water at 32 °F in 24 hours. Based on the heat of fusion for water being 144 Btu per pound, 1 ton of refrigeration = 12,000 Btu/h = 12,660 kJ/h = 3.517 kW. Most residential air conditioning units range in capacity from about 1 to 5 tons of refrigeration.

A much less common definition is: 1 tonne of refrigeration is the rate of heat removal required to freeze a metric ton (i.e., 1000 kg) of water at 0 °C in 24 hours. Based on the heat of fusion being 334.9 kJ/kg, 1 tonne of refrigeration = 13,954 kJ/h = 3.876 kW. As can be seen, 1 tonne of refrigeration is 10 percent larger than 1 ton of refrigeration.

An interesting history of the evolution of refrigeration technology is available on the Internet.^[6]

Applications

Economic analysis

Advantages

• Very mature technology.
• Relatively inexpensive.
• Can be driven directly using mechanical energy (water, car/truck motor) or with electrical energy.
• Efficient up to 60% of Carnot's theoretical limit (as evaluated in ASHRAE testing conditions: evaporation temperature of -23.3 °C, condensing temperature of 54.4°C, and ambient temperature of 32°C) based on some of the best compressors produced by Danfoss, Matsushita, Copeland, Embraco, Bristol and Tecumseh compressor manufacturers. However, many refrigeration systems use compressors having lower efficiencies of between 40-55%, since the 60% efficient ones cost almost twice as much as the lower efficiency ones.

Disadvantages

Many systems still use HCFC refrigerants, which contribute to depletion of the Earth's ozone layer. In countries adhering to the Montreal Protocol, HCFCs are due to be phased out and are largely being replaced by ozone-friendly HFCs. However, systems using HFC refrigerants tend to be slightly less efficient than systems using HCFCs. HFCs also have an extremely large global warming potential (GWP) because they remain in the atmosphere for many years and trap heat more effectively than carbon dioxide.

With disruption of the status quo already a certainty, alternative non-haloalkane refrigerants are gaining popularity. In particular, once-abandoned refrigerants such as hydrocarbons (HCs, such as butane) and CO₂ are coming back into broader use. For example, Coca-Cola's vending machines at the World Cup 2006 in Germany used refrigeration utilizing CO₂^[8]

See also

• Absorption refrigeration or Einstein refrigerator or Gas absorption refrigerator
• Air conditioning
• Flash evaporation
• Heat pump
• HVAC
• Magnetic refrigeration
• Refrigeration
• Refrigeration cycle

References

• "The ideal vapor compression refrigeration cycle", University of Nevada (US)

• "The Refrigeration Cycle", from HowStuffWorks

• Scientific Papers about CO₂ Heat Pumps / Refrigeration