Vapor-compression refrigeration

Vapor-compression refrigeration1 is one of the abounding algidity cycles accessible for use. It has been and is the a lot of broadly acclimated adjustment for air-conditioning of ample accessible buildings, offices, clandestine residences, hotels, hospitals, theaters, restaurants and automobiles. It is aswell acclimated in calm and bartering refrigerators, all-embracing warehouses for algid or arctic accumulator of foods and meats, air-conditioned trucks and railroad cars, and a host of added bartering and automated services. Oil refineries, petrochemical and actinic processing plants, and accustomed gas processing plants are a part of the abounding types of automated plants that generally advance ample vapor-compression algidity systems.

Refrigeration may be authentic as blurred the temperature of an amid amplitude by removing calefaction from that amplitude and appointment it elsewhere. A accessory that performs this action may aswell be alleged a calefaction pump.

Description of the vapor-compression refrigeration system

The vapor-compression uses a circulating aqueous refrigerant as the average which absorbs and removes calefaction from the amplitude to be cooled and after rejects that calefaction elsewhere. Figure 1 depicts a typical, single-stage vapor-compression system. All such systems accept four components: a compressor, a condenser, a Thermal amplification valve (also alleged a burke valve or Tx Valve), and an evaporator. Circulating refrigerant enters the compressor in the thermodynamic accompaniment accepted as a saturated vapor2 and is aeroembolism to a college pressure, consistent in a college temperature as well. The hot, aeroembolism breath is afresh in the thermodynamic accompaniment accepted as a superheated breath and it is at a temperature and burden at which it can be abridged with about accessible cooling baptize or cooling air. That hot breath is baffled through a condenser area it is cooled and abridged into a aqueous by abounding through a braid or tubes with air-conditioned baptize or air-conditioned air abounding beyond the braid or tubes. This is area the circulating refrigerant rejects calefaction from the arrangement and the alone calefaction is agitated abroad by either the baptize or the air (whichever may be the case).

The abridged aqueous refrigerant, in the thermodynamic accompaniment accepted as a saturated liquid, is next baffled through an amplification valve area it undergoes an brusque abridgement in pressure. That burden abridgement after-effects in the adiabatic beam dehydration of a allotment of the aqueous refrigerant. The auto-refrigeration aftereffect of the adiabatic beam dehydration lowers the temperature of the aqueous and breath refrigerant admixture to area it is colder than the temperature of the amid amplitude to be refrigerated.

The algid admixture is afresh baffled through the braid or tubes in the evaporator. A fan circulates the balmy air in the amid amplitude beyond the braid or tubes accustomed the algid refrigerant aqueous and breath mixture. That balmy air evaporates the aqueous allotment of the algid refrigerant mixture. At the aforementioned time, the circulating air is cooled and appropriately lowers the temperature of the amid amplitude to the adapted temperature. The evaporator is area the circulating refrigerant absorbs and removes calefaction which is after alone in the condenser and transferred abroad by the baptize or air acclimated in the condenser.

To complete the algidity cycle, the refrigerant breath from the evaporator is afresh a saturated breath and is baffled aback into the compressor.

edit Refrigerants

"Freon" is a barter name for a ancestors of haloalkane refrigerants bogus by DuPont and added companies. These refrigerants were frequently acclimated due to their above adherence and assurance properties: they were not combustible at allowance temperature and atmospheric pressure, nor acutely baneful as were the fluids they replaced, such as sulfur dioxide. Haloalkanes are aswell an order(s) of consequence added big-ticket than petroleum acquired combustible alkanes of agnate or bigger cooling performance. Unfortunately, chlorine- and fluorine-bearing refrigerants ability the high atmosphere if they escape. In the stratosphere, CFCs breach up due to UV-radiation, absolution their chlorine atoms. These chlorine atoms act as catalysts in the breakdown of ozone, appropriately causing astringent accident to the blast band that bouncer the Earth's apparent from the Sun's able UV radiation. The chlorine will abide alive as a agitator until and unless it binds with addition particle, basic a abiding molecule. CFC refrigerants in accepted but abbreviating acceptance cover R-11 and R-12. Newer refrigerants with bargain blast burning aftereffect such as HCFCs (R-22, acclimated in a lot of homes today) and HFCs (R-134a, acclimated in a lot of cars) accept replaced a lot of CFC use. HCFCs in about-face are getting phased out beneath the Montreal Protocol and replaced by hydrofluorocarbons (HFCs), such as R-410A, which abridgement chlorine. However, CFCs, HCFCs, and HFCs all accept ample all-around abating potential.

Newer refrigerants are currently the accountable of research, such as supercritical carbon dioxide, accepted as R-744.3 These accept agnate efficienciescitation needed compared to absolute CFC and HFC based compounds, and accept abounding orders of consequence lowerglobal abating potential.

Thermodynamic analysis of the system

The thermodynamics of the breath compression aeon can be analyzed on a temperature against anarchy 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 breath is isentropically aeroembolism (i.e., aeroembolism at connected entropy) and exits the compressor as a superheated vapor.

From point 2 to point 3, the superheated breath campaign through allotment of the condenser which removes the superheat by cooling the vapor. Between point 3 and point 4, the breath campaign through the butt of the condenser and is abridged into a saturated liquid. The abstract action occurs at about connected pressure.

Between credibility 4 and 5, the saturated aqueous refrigerant passes through the amplification valve and undergoes an brusque abatement of pressure. That action after-effects in the adiabatic beam dehydration and auto-refrigeration of a allocation of the aqueous (typically, beneath than bisected of the aqueous flashes). The adiabatic beam dehydration action is isenthalpic (i.e., occurs at connected enthalpy).

Between credibility 5 and 1, the algid and partially vaporized refrigerant campaign through the braid or tubes in the evaporator area it is absolutely vaporized by the balmy air (from the amplitude getting refrigerated) that a fan circulates beyond the braid or tubes in the evaporator. The evaporator operates at about connected pressure. The consistent saturated refrigerant breath allotment to the compressor basin at point 1 to complete the thermodynamic cycle.

It should be acclaimed that the aloft altercation is based on the ideal vapor-compression algidity aeon which does not yield into annual absolute apple items like frictional burden bead in the system, slight centralized irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).

Types of gas compressors

The a lot of accepted compressors acclimated in chillers are reciprocating, rotary screw, centrifugal, and annal compressors. Each appliance prefers one or accession due to size, noise, ability and burden issues. Compressors are about declared as getting either open, hermetic, or semi-hermetic, to call how the compressor and/or motor is anchored in affiliation to the refrigerant getting compressed. Variations of motor/compressor types can advance to the afterward configuations:

Abstruse motor, abstruse compressor

Abstruse motor, semi-hermetic compressor

Attainable motor (belt apprenticed or abutting coupled), abstruse compressor

Attainable motor (belt apprenticed or abutting coupled), semi-hermetic compressor

Typically in hermetic, and a lot of semi-hermetic compressors (sometimes accepted as attainable abstruse compressors), the compressor and motor active the compressor are integrated, and accomplish aural the refrigerant system. The motor is abstruse and is advised to operate, and be cooled by, the refrigerant getting compressed. The accessible disadvantage of abstruse motor compressors is that the motor drive cannot be maintained in situ, and the absolute compressor have to be removed if a motor fails. A added disadvantage is that burnt out windings can alloy accomplished algidity systems acute the arrangement to be absolutely pumped down and the refrigerant replaced.

An attainable compressor has a motor drive which is alfresco of the algidity system, and provides drive to the compressor by agency of an ascribe shaft with acceptable gland seals. Attainable compressor motors are about air-cooled and can be adequately calmly exchanged or repaired afterwards degassing of the algidity system. The disadvantage of this blazon of compressor is a abortion of the shaft seals, arch to accident of refrigerant.

Open motor compressors are about easier to air-conditioned (using ambient air) and accordingly tend to be simpler in architecture and added reliable, abnormally in top burden applications area aeroembolism gas temperatures can be actual high. However the use of aqueous bang for added cooling can about affected this affair in a lot of abstruse motor compressors.

edit Reciprocating compressors

Main article: Reciprocating compressor

Reciprocating compressors are piston-style, absolute displacement compressors.

edit Rotary circling compressors

Figure 3: Circling Compressors

Main article: Rotary circling compressor

Rotary circling compressors are aswell absolute displacement compressors. Two coaction screw-rotors circle in adverse directions, accoutrement refrigerant vapor, and abbreviation the aggregate of the refrigerant forth the rotors to the acquittal point.

edit Centrifugal compressors

Main article: Centrifugal compressor

Centrifugal compressors are activating compressors. These compressors accession the burden of the refrigerant by imparting acceleration or activating energy, application a alternating impeller, and converting it to burden energy.

edit Annal compressors

Figure 4: Operating assumption of a Annal Compressor

Main article: Annal compressor

Scroll compressors are aswell absolute displacement compressors. The refrigerant is aeroembolism if one circling orbits about a additional anchored spiral, creating abate and abate pockets and college pressures. By the time the refrigerant is discharged, it is absolutely pressurized.

edit Others

Main article: Diaphragm compressor

Main article: Axial-flow compressor

Main article: Diagonal or mixed-flow compressor

Main article: Aqueous ring compressor

Main article: Roots blower

edit Compressor Lubrication

In adjustment to anoint the affective locations of the compressor, an oil is added to the refrigerant during accession or commissioning. The blazon of oil may be mineral or constructed to clothing the compressor type, and aswell called so as not to acknowledge with the refrigerant blazon and added apparatus in the system. In baby algidity systems the oil is accustomed to broadcast throughout the accomplished circuit, but affliction have to be taken to architecture the pipework and apparatus such that oil can cesspool aback beneath force to the compressor. In beyond added broadcast systems, abnormally in retail refrigeration, the oil is commonly captured at an oil separator anon afterwards the compressor, and is in about-face re-delivered, by an oil akin administration system, aback to the compressor(s). Oil separators are not 100% able so arrangement pipework have to still be advised so that oil can cesspool aback by force to the oil separator or compressor.

Some newer compressor technologies use alluring bearings and crave no lubrication, for archetype the Danfoss Turbocor ambit of centrifugal compressors. Avoiding the charge for oil lubrication and the architecture requirements and ancillaries associated with it, simplifies the architecture of the refrigerant arrangement and reduces aliment requirements.

edit Control

In simple bartering algidity systems the compressor is commonly controlled by a simple burden switch, with the amplification performed by a capillary tube or simple thermostatic amplification valve. In added circuitous systems, including assorted compressor installations, the use of cyberbanking controls is typical, with adjustable set credibility to ascendancy the burden at which compressors cut in and cut out, and temperature ascendancy by the use of cyberbanking amplification valves.

In accession to the operational controls, abstracted top burden and low burden switches are commonly utilised to accommodate accessory aegis to the compressors and added apparatus of the arrangement from operating alfresco of safe parameters.

In added avant-garde cyberbanking ascendancy systems the use of amphibian arch pressure, and proactive assimilation pressure, ascendancy routines acquiesce the compressor operation to be adapted to accurately accommodated differing cooling demands whilst abbreviation activity consumption.

Other features and facts of interest

The schematic diagram of a single-stage algidity arrangement apparent in Figure 1 does not cover added accessories items that would be provided in a ample bartering or automated breath compression algidity system, such as:

A accumbent or vertical burden vessel, able internally with a demister, amid the evaporator and the compressor basin to abduction and abolish any residual, entrained aqueous in the refrigerant breath because aqueous may accident the compressor. Such vapor-liquid separators are a lot of generally referred to as "suction band accumulators". (In added automated processes, they are alleged "compressor assimilation drums" or "knockout drums".)

Ample bartering or automated algidity systems may accept assorted amplification valves and assorted evaporators in adjustment to air-condition assorted amid spaces or rooms. In such systems, the abridged aqueous refrigerant may be baffled into a burden vessel, alleged a receiver, from which aqueous refrigerant is aloof and baffled through assorted pipelines to the assorted amplification valves and evaporators.

Filter Dryers, installed afore the compressors to bolt any damp or contaminents in the arrangement and appropriately assure the compressors from centralized damage

Some algidity units may accept assorted stages which requires the use of assorted compressors in assorted arrangements.4

The cooling accommodation of algidity systems is generally authentic in units alleged "tons of refrigeration". The a lot of accepted analogue of that assemblage is: 1 ton of algidity is the amount of calefaction abatement appropriate to benumb a abbreviate ton (i.e., 2000 pounds) of baptize at 32 °F in 24 hours. Based on the calefaction of admixture for baptize getting 144 Btu per pound, 1 ton of algidity = 12,000 Btu/h = 12,660 kJ/h = 3.517 kW. A lot of residential air conditioning units ambit in accommodation from about 1 to 5 bags of refrigeration.

A abundant beneath accepted analogue is: 1 tonne of algidity is the amount of calefaction abatement appropriate to benumb a metric ton (i.e., 1000 kg) of baptize at 0 °C in 24 hours. Based on the calefaction of admixture getting 334.9 kJ/kg, 1 tonne of algidity = 13,954 kJ/h = 3.876 kW. As can be seen, 1 tonne of algidity is 10 percent beyond than 1 ton of refrigeration.

Economic analysis

Advantages

Very complete technology.

Relatively inexpensive.

Can be apprenticed anon application automated activity (water, car/truck motor) or with electrical energy.

Able up to 60% of Carnot's abstract absolute (as evaluated in ASHRAE testing conditions: dehydration temperature of -23.3 °C, abbreviating 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, abounding algidity systems use compressors accepting lower efficiencies of amid 40-55%, aback the 60% able ones amount about alert as abundant as the lower ability ones.

edit Disadvantages

Many systems still use HCFC refrigerants, which accord to burning of the Earth's blast layer. In countries adhering to the Montreal Protocol, HCFCs are due to be phased out and are abundantly getting replaced by ozone-friendly HFCs. However, systems application HFC refrigerants tend to be hardly beneath able than systems application HCFCs. HFCs aswell accept an acutely ample all-around abating abeyant (GWP) because they abide in the atmosphere for abounding years and allurement calefaction added finer than carbon dioxide.

With disruption of the cachet quo already a certainty, another non-haloalkane refrigerants are accepting popularity. In particular, once-abandoned refrigerants such as hydrocarbons (HCs, such as butane) and CO2 are advancing aback into broader use. For example, Coca-Cola's automat machines at the 2006 FIFA World Cup in Germany acclimated algidity utilizing CO26 Ammonia NH3 is one of the oldest refrigerants, has an accomplished performance, about no abuse botheration but with two disadvantages: is locally toxic; adverse with chestnut tubing 7 Is broadly acclimated in bartering plant.

History

The American artist Oliver Evans, acclaimed as the "father of refrigeration," invented the vapor-compression algidity apparatus in 1805. Heat would be removed from the ambiance by recycling vaporized refrigerant, area it would move through a compressor and condenser and would eventually backslide aback to a aqueous anatomy in adjustment to echo the algidity action over again. However, no such algidity assemblage was congenital by Evans.8 In 1834, Jacob Perkins adapted Evans' aboriginal design, architecture the world's aboriginal refrigerator and filing the aboriginal acknowledged apparent for algidity application vapor-compression.9 John Gorrie, an American doctor from Florida, invented the aboriginal automated algidity assemblage in 1841, based on Evans' aboriginal apparatus to accomplish ice in adjustment to air-conditioned the air for chicken agitation patients. Gorrie's automated algidity assemblage was issued a apparent in 1851.10 In 1853, Alexander C. Twining of Cleveland, Ohio patented an aboriginal vapor-compression refrigerator that was absolutely able of bearing a ton of ice per day.11

In 1856, James Harrison, an immigrant from Scotland active in Australia, developed an ice authoritative apparatus application ammonia and an ether compressor. It was acclimated in the brewing and meat packing industries of Geelong, Victoria.12 Ferdinand Carré of France developed a somewhat added circuitous arrangement in 1859. Unlike beforehand compression-compression machines, which acclimated air as a coolant, Carré's accessories independent rapidly accretion ammonia.