8/3/2019 Refrigeration 2ab

1/20

REFRIGERATION, HEAT

PUMP CYCLES

Dr. Varaprasada Rao

8/3/2019 Refrigeration 2ab

2/20

REFRIGERATORSAND 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 requiresspecial devices called refrigerators.

Refrigerators and heat pumps are essentiallythe same devices; they differ in theirobjectives only.

for fixed values of QL and QH

8/3/2019 Refrigeration 2ab

3/20

THE REVERSEDCARNOT CYCLE

Schematic of aCarnot refrigeratorand T-sdiagramof the reversed

Carnot cycle.

Both COPs increase

as the differencebetween the twotemperaturesdecreases, that is, asTLrises or THfalls.

The reversed Carnot cycle is the most efficientrefrigeration cycle operating between TL and TH.

However, it is not a suitable model for refrigerationcycles since processes 2-3 and 4-1 are not practical

becauseProcess 2-3 involves the compression of a liquidvapormixture, which requires a compressor that will handletwo phases, and process 4-1 involves the expansion ofhigh-moisture-content refrigerant in a turbine.

8/3/2019 Refrigeration 2ab

4/20

4

HISTORY

- In Egypt (2 century) cooling effect - vaporization water

- 1755 - William Cullen produced ice using vacuum pumps andphase 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

-

8/3/2019 Refrigeration 2ab

5/20

THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLEThe vapor-compression refrigeration cycle is the ideal model for refrigerationsystems. Unlike the reversed Carnot cycle, the refrigerant is vaporized completelybefore it is compressed and the turbine is replaced with a throttling device.

Schematic and T-sdiagram for the ideal

vapor-compression refrigeration cycle.

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

8/3/2019 Refrigeration 2ab

6/20

An ordinaryhouseholdrefrigerator.

The P-hdiagram of an ideal vapor-

compression refrigeration cycle.

The ideal vapor-compression refrigeration cycle involves an irreversible (throttling)process to make it a more realistic model for the actual systems.

Replacing the expansion valve by a turbine is not practical since the addedbenefits cannot justify the added cost and complexity.

Steady-flowenergy balance

8/3/2019 Refrigeration 2ab

7/20

ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE

Schematic and T-sdiagram for the actual

vapor-compression refrigeration cycle.

An actual vapor-compression refrigeration cycle differs from the ideal one inseveral ways, owing mostly to the irreversibilities that occur in variouscomponents, mainly due to fluid friction (causes pressure drops) and heat transfer

to 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

8/3/2019 Refrigeration 2ab

8/20

8

The Compressor

The compressor is the heart of thesystem. The compressor does justwhat its name is. It compresses

the low pressure refrigerant vaporfrom the evaporator andcompresses it into a high pressurevapor.

8/3/2019 Refrigeration 2ab

9/20

9

The Condenser

The Discharge Line leaves thecompressor and runs to the inlet of thecondenser.

Because the refrigerant was compressed,it is a hot high pressure vapor (aspressure goes up temperature goesup).

The hot vapor enters the condenser andstarts to flow through the tubes.

Cool air is blown across the out side ofthe finned tubes of the condenser(usually by a fan or water with a pump).

Since the air is cooler than therefrigerant, heat jumps from the tubing tothe cooler air (energy goes from hot tocoldlatent heat).

As the heat is removed from the

refrigerant, it reaches its saturatedtemperature and starts to flash(change states), into a high pressureliquid.

The high pressure liquid leaves thecondenser through the liquid line andtravels to the metering device.Sometimes running through a filter dryer

first, to remove any dirt or foreignparticles.

8/3/2019 Refrigeration 2ab

10/20

10

Metering Devices

Metering devices regulate how muchliquid refrigerant enters the evaporator .

Common used metering devices are,small thin copper tubes referred to ascap tubes, thermally controllerdiaphragm valves called TXVs(thermal expansion valves) and singleopening orifices.

The metering device tries to maintain apreset temperature difference or superheat, between the inlet and outletopenings of the evaporator.

As the metering devices regulates theamount of refrigerant going into theevaporator, the device lets smallamounts of refrigerant out into the line

and looses the high pressure it hasbehind it.

Now we have a low pressure, coolerliquid refrigerant entering the evaporativecoil (pressure went down sotemperature goes down).

8/3/2019 Refrigeration 2ab

11/20

11

Thermal expansion Valves

A very common type of metering device iscalled a TX Valve (Thermostatic ExpansionValve). This valve has the capability ofcontrolling the refrigerant flow. If the load onthe evaporator changes, the valve canrespond to the change and increase ordecrease the flow accordingly.

The TXV has a sensing bulb attached to theoutlet of the evaporator. This bulb senses thesuction line temperature and sends a signal

to the TXV allowing it to adjust the flow rate.This is important because, if not all, therefrigerant in the evaporator changes stateinto a gas, there could be liquid refrigerantcontent returning to the compressor. Thiscan be fatal to the compressor. Liquid cannot be compressed and when a compressortries to compress a liquid, mechanical failingcan happen. The compressor can suffermechanical damage in the valves and

bearings. This is called liquid slugging.

Normally TXV's are set to maintain 10degrees of superheat. That means that thegas returning to the compressor is at least 10degrees away from the risk of having anyliquid.

8/3/2019 Refrigeration 2ab

12/20

12

The Evaporator

The evaporator is where the heat is

removed from your house , business orrefrigeration box.

Low pressure liquid leaves the meteringdevice and enters the evaporator.

Usually, a fan will move warm air fromthe conditioned space across theevaporator finned coils.

The cooler refrigerant in the evaporatortubes, absorb the warm room air. Thechange of temperature causes therefrigerant to flash or boil, andchanges from a low pressure liquid to alow pressure cold vapor.

The low pressure vapor is pulled into the

compressor and the cycle starts over.

The amount of heat added to the liquidto make it saturated and change states iscalled Super Heat.

One way to charge a system withrefrigerant is by super heat.

8/3/2019 Refrigeration 2ab

13/20

13

8/3/2019 Refrigeration 2ab

14/20

14

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.

8/3/2019 Refrigeration 2ab

15/20

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 (in

applications above the freezing point). R-11, R-12, R-22, R-134a, and R-502 account for over 90 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 domesticrefrigerators and freezers, as well as automotive air conditioners.

R-22 is used in window air conditioners, heat pumps, air conditioners of commercialbuildings, 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 the

protective 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.

8/3/2019 Refrigeration 2ab

16/20

HEAT PUMP SYSTEMS

A heat pump can beused to heat a housein winter and to coolit in summer.

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

Water-source systems usually use

well 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 in

areas 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.

8/3/2019 Refrigeration 2ab

17/20

INNOVATIVE VAPOR-COMPRESSIONREFRIGERATION SYSTEMS

The simple vapor-compression refrigeration cycle is the most widely

used refrigeration cycle, and it is adequate for most refrigerationapplications.

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 someinnovative 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

8/3/2019 Refrigeration 2ab

18/20

18

Gas for Heat Pumps

Heat pumps fired by natural gas have been commercially

developed. One type uses the absorption cycle, where the energy for refrigerant

compression 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. During

operation, 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 efficient

conversion of natural gas and reduced emissions from electricpower plants as they do not use electricity to drive the heat pump.

8/3/2019 Refrigeration 2ab

19/20

19

Gas engine driver

8/3/2019 Refrigeration 2ab

20/20

20

District heating and heat pump