Vapour-compression refrigeration or vapor-compression refrigeration system (VCRS), in which the refrigerant undergoes phase changes, is one of the many refrigeration cycles and is the most widely used method for air conditioning of buildings and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for chilled or frozen 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 are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems. Cascade refrigeration systems may also be implemented using two pressure levels, where the heat flow from cold to warm is split over two separate refrigeration systems.
Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it to a higher temperature level. A device that performs this function may also be called an air conditioner, refrigerator, air source heat pump, geothermal heat pump, or chiller (heat pump).
Description
Vapor-compression uses a circulating fluid 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, a thermal expansion valve (also called a throttle valve), and an evaporator. Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vaporSaturated 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. 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 a temperature and pressure at which it can be condensed with either cooling water or cooling air flowing across the coil or tubes or to the higher temperature stage of a cascade system, where the low temperature condensor is the high temperature evaporator.
The superheated vapor then passes through the condenser. This is where heat is transferred from the circulating refrigerant to an external medium, allowing the gaseous refrigerant to cool and condense into a liquid. The rejected heat is carried away by either the water or the air, depending on the type of condenser.
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 refrigerant liquid and vapor mixture is then routed through the coil or tubes in the evaporator. Air in the enclosed space circulates across the coil or tubes due to either thermal convection or a fan. Since the air is warmer than the cold liquid refrigerant, heat is transferred from the air to the refrigerant, which cools the air and warms the refrigerant, causing evaporation, returning it to a gaseous state. While liquid remains in the refrigerant flow, its temperature will not rise above the boiling point of the refrigerant, which depends on the pressure in the evaporator. Most systems are designed to evaporate all of the refrigerant to ensure that no liquid is returned to the compressor. Of course also ather media than air can be cooled in the evaporator as water or any circulating fluid in the heatexchanger.
To complete the refrigeration cycle, the refrigerant vapor from the evaporator is again a saturated vapor and is routed back into the compressor. Over time, the evaporator may collect ice or water from ambient humidity. The ice is melted through defrosting. The water from the melted ice or the evaporator then drips into a drip pan, and the water is carried away by gravity or a condensate pump.
Refrigerants
The selection of working fluid has a significant impact on the performance of the refrigeration cycles and as such it plays a key role when it comes to designing or simply choosing an ideal machine for a certain task. One of the most widespread refrigerants is "Freon". 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 at room temperature and atmospheric pressure, nor obviously toxic as were the fluids they replaced, such as sulfur dioxide. Haloalkanes are also an order(s) of magnitude more expensive than petroleum-derived flammable alkanes of similar or better cooling performance.
Unfortunately, chlorine- and fluorine-bearing refrigerants reach the upper atmosphere when they escape. In the stratosphere, substances like CFCs and HCFCs break up due to UV radiation, releasing their chlorine free-radicals. These chlorine free-radicals act as catalysts in the breakdown of ozone through chain reactions. One CFC molecule can cause thousands of ozone molecules to break down. This causes severe damage to the ozone layer that shields the Earth's surface from the Sun's strong UV radiation and has been shown to lead to increased rates of skin cancer. 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 refrigerants that have reduced ozone depletion effects compared to CFCs have replaced most CFC use. Examples include HCFCs (such as R-22, used in most homes) and HFCs (such as R-134a, used in most cars). HCFCs in turn are being phased out under the Montreal Protocol and replaced by hydrofluorocarbons (HFCs), which do not contain chlorine atoms. However, CFCs, HCFCs, and HFCs all have very large global warming potential (GWP).
More benign refrigerants are currently the subject of research, such as supercritical carbon dioxide, known as R-744.r744.com – Everything R744 , The Natural Refrigerant R744 (CO)2, 2006–2012 These have similar efficiencies compared to existing CFC- and HFC-based compounds, and have many orders of magnitude lower global warming potential. General industry and governing body push are toward more GWP-friendly refrigerants. In industrial settings ammonia, as well as gasses like ethylene, propane, iso-butane and other hydrocarbons are commonly used (and have their own R-x customary numbers), depending on required temperatures and pressures. Many of these gases are flammable, explosive, or toxic; making their use restricted (i.e. well-controlled environment by qualified personnel, or a very small amount of refrigerant used). HFOs which can be considered to be HFCs with some carbon-carbon bonds being double bounds, do show promise of lowering GWP so little to be of no further concern. In the meantime, various blends of existing refrigerants are used to achieve the required properties and efficiency, at a reasonable cost and lower GWP.
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 low-temperature, low-pressure saturated vapor. From point 1 to point 2, the vapor is isentropically compressed (compressed at constant entropy) and exits the compressor as a high-pressure, high-temperature vapor.
From point 2 to point 3, the vapor travels through part of the condenser which removes the heat by cooling the vapor. Between point 3 and point 4, the vapor travels through the remainder of the condenser and is condensed into a high-temperature, high-pressure subcooled liquid. Subcool is the amount of sensible heat removed from the liquid below its maximum saturation. The condensation process occurs at essentially constant pressure.
Between points 4 and 5, the subcooled 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 (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 evaporation process occurs at essentially constant temperature. After evaporation is completed, the vapor will start to increase in temperature. The amount of sensible heat added to the vapor above its saturation point, i.e. its boiling point, is called superheat.
The resulting superheated vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle.
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 irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).
Types of gas compressors
The most common compressors used in refrigeration are reciprocating and scroll compressors, but large chillers or industrial cycles may use rotary screw or centrifugal compressors. Each application prefers one or another due to size, noise, efficiency, and pressure issues. Compressors are often described as being either open, hermetic, or semi-hermetic, to describe how the compressor and/or motor is situated in relation to the refrigerant being compressed. Variations of motor/compressor types can lead to the following configurations:
Hermetic motor, hermetic compressor Hermetic motor, semi-hermetic compressor Open motor (belt driven or close coupled), hermetic compressor Open motor (belt driven or close coupled), semi-hermetic compressor
Typically in hermetic, and most semi-hermetic compressors (sometimes known as accessible hermetic compressors), the compressor and motor driving the compressor are integrated, and operate within the refrigerant system. The motor is hermetic and is designed to operate, and be cooled by, the refrigerant being compressed. The obvious disadvantage of hermetic motor compressors is that the motor drive cannot be maintained in situ, and the entire compressor must be removed if a motor fails. A further disadvantage is that burnt out windings can contaminate whole refrigeration systems requiring the system to be entirely pumped down, and the refrigerant replaced.
An open compressor has a motor drive which is outside of the refrigeration system, and provides drive to the compressor by means of an input shaft with suitable gland seals. Open compressor motors are typically air-cooled and can be fairly easily exchanged or repaired without degassing of the refrigeration system. The disadvantage of this type of compressor is a failure of the shaft seals, leading to loss of refrigerant.
Open motor compressors are generally easier to cool (using ambient air) and therefore tend to be simpler in design and more reliable, especially in high pressure applications where compressed gas temperatures can be very high. However the use of liquid injection for additional cooling can generally overcome this issue in most hermetic motor compressors.
Reciprocating compressors