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REFRIGERATION, HEATPUMP CYCLES

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REFRIGERATORS

AND HEAT PUMPS

The objective of a refrigerator is to remove heat

(QL) from the cold medium; the objective of a heat

pump is to supply heat (QH) to a warm medium.

The transfer of heat from a low-temperatureregion to a high-temperature one requires

special devices called refrigerators.

Refrigerators and heat pumps are essentially

the same devices; they differ in their

objectives only.

for fixed values ofQL and QH

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THE REVERSED

CARNOT CYCLE

Schematic of a

Carnot refrigerator

and T-s diagram

of the reversed

Carnot cycle.

Both COPs increase

as the difference

between the two

temperatures

decreases, that is, as

TL rises orTH falls.

The reversed Carnot cycle is the most efficient

refrigeration cycle operating between TL and TH.

However, it is not a suitable model for refrigeration

cycles since processes 2-3 and 4-1 are not practical

because

Process 2-3 involves the compression of a liquidvapor

mixture, which requires a compressor that will handle

two phases, and process 4-1 involves the expansion of

high-moisture-content refrigerant in a turbine.

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HISTORY

- In Egypt (2 century) cooling effect - vaporization water- 1755 - William Cullen produced ice using vacuum pumps and

phase transformation

- 1777 Walther Hermann Nerst added to water H2SO4

- 1834.a. Jacob Perkins the first prototype as today we use

- 1844.a. Jon Corienair conditions

- 1864.a. absorber effect, Littman

-

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THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLEThe vapor-compressionrefrigerationcycle is the ideal model for refrigeration

systems. Unlike the reversed Carnot cycle, the refrigerant is vaporized completely

before it is compressed and the turbine is replaced with a throttling device.

Schematic and T-s diagram for the ideal

vapor-compression refrigeration cycle.

This is themost widelyused cycle forrefrigerators,A-C systems,and heatpumps.

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ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE

Schematic and T-s diagram for the actual

vapor-compression refrigeration cycle.

An actual vapor-compression refrigeration cycle differs from the ideal one in

several ways, owing mostly to the irreversibilities that occur in various

components, mainly due to fluid friction (causes pressure drops) and heat transferto or from the surroundings. The COP decreases as a result of irreversibilities.

DIFFERENCES

Non-isentropiccompression

Superheated vaporat evaporator exit

Subcooled liquid atcondenser exit

Pressure drops incondenser and

evaporator

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The Compressor

Thecompressoristhe heartofthe

system. Thecompressordoes just

whatitsnameis. Itcompresses

thelow pressurerefrigerant vapor

fromtheevaporatorand

compressesitintoa high pressure

vapor.

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The Condenser

The Discharge Lineleavesthecompressorand runstotheinletofthecondenser.

Becausetherefrigerant wascompressed,itisa hot high pressure vapor(aspressuregoes up temperaturegoesup).

The hot vaporentersthecondenserandstartstoflow through thetubes.

Coolairisblownacrosstheoutsideofthefinned tubesofthecondenser(usuallybyafanorwaterwith apump).

Sincetheairiscoolerthantherefrigerant, heat jumpsfromthetubingtothecoolerair(energygoesfrom hottocold latent heat).

Asthe heatisremoved fromthe

refrigerant, itreachesits saturatedtemperatureand startsto flash(changestates), intoa high pressureliquid.

The high pressureliquid leavesthecondenserthrough the liquid lineandtravelstothe metering device.Sometimesrunningthrough afilterdryer

first, toremoveany dirtorforeignparticles.

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Metering Devices

Metering devicesregulate how much

liquid refrigerantenterstheevaporator .

Common used metering devicesare,smallthincoppertubesreferred toascaptubes, thermallycontrollerdiaphragm valvescalled TXVs(thermalexpansion valves)and singleopening orifices.

Themetering devicetriestomaintainapresettemperature differenceorsuperheat, betweentheinletand outletopeningsoftheevaporator.

Asthemetering devicesregulatestheamountofrefrigerantgoingintotheevaporator, the deviceletssmallamountsofrefrigerantoutintotheline

and loosesthe high pressureit hasbehind it.

Now we havealow pressure, coolerliquid refrigerantenteringtheevaporativecoil (pressure went down sotemperaturegoes down).

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Thermal expansion Valves

A verycommontypeofmetering deviceis

called a TX Valve (Thermostatic ExpansionValve). This valve hasthecapabilityofcontrollingtherefrigerantflow. Iftheload ontheevaporatorchanges, the valvecanrespond tothechangeand increaseordecreasetheflow accordingly.

The TXV hasasensingbulbattached totheoutletoftheevaporator. Thisbulbsensesthesuctionlinetemperatureand sendsasignal

tothe TXV allowingittoadjusttheflow rate.Thisisimportantbecause, ifnotall, therefrigerantintheevaporatorchangesstateintoagas, therecould beliquid refrigerantcontentreturningtothecompressor. Thiscanbefataltothecompressor. Liquid cannotbecompressed and whenacompressortriestocompressaliquid, mechanicalfailingcan happen. Thecompressorcansuffermechanical damageinthe valvesandbearings. Thisiscalledliquid slugging.

Normally TXV'saresettomaintain 10degreesofsuperheat. Thatmeansthatthegasreturningtothecompressorisatleast 10degreesawayfromtheriskofhavinganyliquid.

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The Evaporator

Theevaporatoris wherethe heatisremoved fromyourhouse , businessor

refrigerationbox.

Low pressureliquid leavesthemetering

deviceand enterstheevaporator.

Usually, afan willmove warmairfrom

theconditioned spaceacrossthe

evaporatorfinned coils. Thecoolerrefrigerantintheevaporator

tubes, absorbthe warmroomair. The

changeoftemperaturecausesthe

refrigerantto flashorboil, and

changesfromalow pressureliquid toa

low pressurecold vapor.

Thelow pressure vaporispulled intothe

compressorand thecyclestartsover.

Theamountofheatadded totheliquid

tomakeitsaturated and changestatesis

called SuperHeat.

One waytochargeasystem with

refrigerantisbysuperheat.

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Refrigerant

A liquid that has a low boiling point.

There are several refrigerant manufacturers.

Heat pumps still use R22 refrigerants. R22 performs well over

the range of temperatures that heat pumps operate at.

R22 is known as a hydrochlorofluorocarbon (HCFC) refrigerant

and has an ozone depletion (ODP) factor of 0.05.

Many heat pumps today use R-407C or R-410A, which are hydrofluorocarbons (HFC).Both R-407C and R-410A have zero ozone depletion potential (ODP), and slightlylower global warming potential (GWP) in the case of R-407C, than R-22. R410Ahas a slightly higher GWP than R22.

Performance (heating capacity and efficiency) is about the same with R-407C andabout 4% better with R410A compared to R-22.

R-22 will be phased out for new equipment by January 1, 2010.

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SELECTING THE RIGHT REFRIGERANT Several refrigerants may be used in refrigeration systems such as

chlorofluorocarbons (CFCs), ammonia, hydrocarbons (propane, ethane, ethylene,

etc.), carbon dioxide, air (in the air-conditioning of aircraft), and even water (inapplications above the freezing point).

R-11, R-12, R-22, R-134a, and R-502 account for over90 percent of the market.

The industrial and heavy-commercial sectors use ammonia (it is toxic).

R-11 is used in large-capacity water chillers serving A-C systems in buildings.

R-134a (replaced R-12, which damages ozone layer) is used in domestic

refrigerators and freezers, as well as automotive air conditioners. R-22 is used in window air conditioners, heat pumps, air conditioners of commercial

buildings, and large industrial refrigeration systems, and offers strong competitionto ammonia.

R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercialrefrigeration systems such as those in supermarkets.

CFCs allow more ultraviolet radiation into the earths atmosphere by destroying theprotective ozone layer and thus contributing to the greenhouse effect that causesglobal warming. Fully halogenated CFCs (such as R-11, R-12, and R-115) do themost damage to the ozone layer. Refrigerants that are friendly to the ozone layerhave been developed.

Two important parameters that need to be considered in the selection of arefrigerant are the temperatures of the two media (the refrigerated space and the

environment) with which the refrigerant exchanges heat.

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HEAT PUMP SYSTEMS

A heat pump can be

used to heat a house

in winter and to cool

it in summer.

The most common energy source forheat pumps is atmospheric air (air-to-air systems).

Water-source systems usually usewell water and ground-source(geothermal) heat pumps use earth asthe energy source. They typically havehigher COPs but are more complexand more expensive to install.

Both the capacity and the efficiency of

a heat pump fall significantly at lowtemperatures. Therefore, most air-source heat pumps require asupplementary heating system suchas electric resistance heaters or a gasfurnace.

Heat pumps are most competitive inareas that have a large cooling loadduring the cooling season and arelatively small heating load during theheating season. In these areas, theheat pump can meet the entire coolingand heating needs of residential or

commercial buildings.

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INNOVATIVE VAPOR-COMPRESSION

REFRIGERATION SYSTEMS

The simple vapor-compression refrigeration cycle is the most widely

used refrigeration cycle, and it is adequate for most refrigeration

applications.

The ordinary vapor-compression refrigeration systems are simple,

inexpensive, reliable, and practically maintenance-free.

However, for large industrial applications efficiency, not simplicity, is

the major concern.

Also, for some applications the simple vapor-compression

refrigeration cycle is inadequate and needs to be modified.

For moderately and very low temperature applications some

innovative refrigeration systems are used. The following cycles are

generally employed:

Cascade refrigeration systems

Multistage compression refrigeration systems

Multipurpose refrigeration systems with a single compressor

Liquefaction of gases

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GasforHeat Pumps

Heat pumps fired by natural gas have been commerciallydeveloped.

One type uses the absorption cycle, where the energy for refrigerantcompression is provided by a gas burner.

Another variation is the engine-driven heat pump cycle. Here anatural gas engine is used to drive the compressor. Duringoperation, heat is recovered from the engine jacket cooling waterand engine exhaust.

Gas heat pumps are less common than electric heat pumps.

Performance compared to electric heat pumps is lower, with lowerCOPs for both absorption and engine-driven units than for

conventional electric heat pumps.

They promise to reduce global warming through more efficientconversion of natural gas and reduced emissions from electricpower plants as they do not use electricity to drive the heat pump.

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Gasengine driver

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District heatingand heatpump

Oslo