Refrigeration Cycles

MOHAMMAD FAISAL HAIDERLECTURERLECTURER

Department of Mechanical EngineeringDepartment of Mechanical EngineeringBangladesh University of Engineering and Technology

ObjectivesjIntroduce the concepts of refrigerators and heat pumps and the measure oftheir performance.Analyze the ideal and actual vapor-compression refrigeration cycles.Discuss the operation of refrigeration and heat pump systems.E l t th f f i ti i f i tiEvaluate the performance of innovative vapor-compression refrigerationsystems.

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Refrigeration

Refrigeration is the action of cooling, and in practice this requires removal of heat anddiscarding it at a higher temperature. Refrigeration is therefore the science of movingheat from low temperature to high temperature. In addition to chilling and freezingapplications, refrigeration technology is applied in air conditioning and heat pumps.

Refrigerators and Heat Pumps Refrigerators Take Heat from cooler place

Transfer heat to hotter place Transfer heat to hotter place Objective: Keeping low temp. at the cooler place Used in Tropical Countries/Deserts for Space Cooling Heat Pumps Heat Pumps Take Heat from cooler place Transfer heat to hotter place Objective: Keeping high temp. at the hotter place

3Object e eep g g te p at t e otte p ace

Used in Cold Countries for space heating

Sensible heat and latent heatSensible heat and latent heat

If a change of enthalpy can be sensed as a change of temperature, it is called sensibleheat . This is expressed as specific heat capacity, i.e. the change in enthalpy per degreeeat s s e p essed as spec c ea capac y, e e c a ge e a py pe deg eeof temperature change, in kJ/(kg K). If there is no change of temperature but a changeof state (solid to liquid, liquid to gas, or vice versa) it is called latent heat. This isexpressed as kJ/kg but it varies with the boiling temperature, and so is usually qualifiedby this condition. The resulting total changes can be shown on a temperatureenthalpydiagram.

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Refrigerators And Heat PumpsThe transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators.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.p p pp y ( )Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only.

Performance:

5for fixed values of QL and QH

COP = ( Desired work)/ ( Net work done)COP = ( Desired work)/ ( Net work done)

=QL/ (QH - QL) = (TL/ (TH - TL)

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For fixed values of QL and QH This relation implies that COPHP > 1 since COPR is a positiveFor fixed values of QL and QH. This relation implies that COPHP > 1 since COPR is a positivequantity. That is, a heat pump functions, at worst, as a resistance heater, supplying as muchenergy to the house as it consumes.

I lit h t f QH i l t t th t id i th h i i d th d i dIn reality, however, part of QH is lost to the outside air through piping and other devices, andCOPHP may drop below unity when the outside air temperature is too low. When this happens,the system normally switches to the fuel (natural gas, propane, oil, etc.) or resistance-heatingmode.ode

The cooling capacity of a refrigeration systemthat is, the rate of heat removal from therefrigerated spaceis often expressed in terms of tons of refrigeration.g p p g

The capacity of a refrigeration system that can freeze 1 ton (2000 lbm) of liquid water at0C (32F) into ice at 0C in 24 h is said to be 1 ton. One ton of refrigeration is equivalent to211 kJ/min or 200 Btu/min211 kJ/min or 200 Btu/min.

The cooling load of a typical 200-m2 residence is in the 3-ton (10-kW) range.

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The Reversed Carnot C l

The most efficient refrigeration cycle operatingbetween TL and TH. But not a suitable model for Cyclebetween TL and TH. But not a suitable model forrefrigeration cycles because: (i) process 2-3involves compression of a liquidvapor mixture -requires a compressor that will handle two phases,(ii) process 4-1 involves expansion of high-moisture-content refrigerant in a turbine.

Both COPs increase as the difference between the two temperatures decreases

Schematic of a Carnot

temperatures decreases, i.e. as TL rises or TH falls.

refrigerator and T-s diagram of the reversed Carnot cycle.

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Ideal Vapor-compression Refrigeration CycleIs the ideal model for refrigeration systems. The refrigerant is vaporized completely before it is compressed and the turbine is replaced with a throttling device.

Th t id l The most widely used cycle for refrigerators, A-C systems, and h t heat pumps.

9Schematic and T-s diagram for the 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.p y

Steady-flow energy balance

An ordinary household refrigerator.

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The P-h diagram of an ideal vapor-compression refrigeration cycle.

Actual Vapor-Compression Refrigeration CycleAn actual vapor compression refrigeration cycle involves irreversibilities in variousAn actual vapor-compression refrigeration cycle involves irreversibilities in variouscomponents - mainly due to fluid friction (causes pressure drops) and heat transfer toor from the surroundings. As a result, the COP decreases.

Differences Non-isentropic

compression;compression; Superheated vapor at

evaporator exit; Sub-cooled liquid at Sub cooled liquid at

condenser exit; Pressure drops in

condenser and tevaporator.

11Schematic and T-s diagram for the actual vapor-compression refrigeration cycle.

Selecting the Right RefrigerantSeveral refrigerants may be used in refrigeration systems such as chlorofluorocarbons(CFCs), ammonia, hydrocarbons (propane, ethane, ethylene, etc.), carbon dioxide, air (inthe 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 marketR 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 is used in domestic refrigerators and freezers, as well as automotive airR 134a is used in domestic refrigerators and freezers, as well as automotive airconditioners.R-22 is used in window air conditioners, heat pumps, air conditioners of commercialbuildings, and large industrial refrigeration systems, and offers strong competition to

iammonia.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 theCFCs allow more ultraviolet radiation into the earth s atmosphere by destroying theprotective ozone layer and thus contributing to the greenhouse effect that causes globalwarming. Refrigerants that are friendly to the ozone layer have been developed.Two important parameters to be considered - the temperatures of the refrigerated space

f 12and the environment with which the refrigerant exchanges heat.

Refrigerantsg

Refrigerants are chemicals that act as the working fluid in Refrigeration cycles The names of the Refrigerants were initially developed by a company namedDuPont for VCRS

Chemical Formula

R (Carbon atoms-1) (Hydrogen atoms+1) (Flourine atoms)R-11 = R011 = CCl3F and R-22= R022 = CHF2Cl

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Innovative Vapor-compression Refrigeration SystemsThe simple vapor-compression refrigeration cycle is the most widely usedrefrigeration cycle, and is adequate for most refrigeration applications.The ordinary vapor-compression refrigeration systems are simple, inexpensive,The ordinary vapor compression refrigeration systems are simple, inexpensive,reliable, and practically maintenance-free.However, for large industrial applications, efficiency (not simplicity) is the majorconcern.concern.For some applications the simple vapor-compression refrigeration cycle isinadequate and needs to be modified.For moderately and very low temperature applications some innovativeFor moderately and very low temperature applications, some innovativerefrigeration systems are used. The following cycles will be discussed:

Cascade refrigeration systemsMultistage compression refrigeration systems Multistage compression refrigeration systems

Multipurpose refrigeration systems with a single compressor Liquefaction of gases

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Cascade Refrigeration Systems Some industrial applications require moderately low temperatures, and the

temperature range they involve may be too large for a single vapor compressionrefrigeration cycle to be practical.

A large temperature range also means a large pressure range in the cycle and apoor performance for a reciprocating compressor.

One way of dealing with such situations is to perform the refrigeration process iny g gstages, that is, to have two or more refrigeration cycles that operate in series.

Such refrigeration cycles are called cascade refrigeration cycles.

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Cascade Refrigeration Systems

A two-stage cascade refrigeration cycle isshown The two cycles are connected throughSystems shown. The two cycles are connected throughthe heat exchanger in the middle, which servesas the evaporator for the topping cycle and thecondenser for the bottoming cycle.condenser for the bottoming cycle.

16A two-stage compression refrigeration system with a flash chamber.

A two-stage cascade refrigeration system with the same refrigerant in both stages.

Cascade Refrigeration Systems Assuming the heat exchanger is well insulated and the kinetic and potential

energies are negligible, the heat transfer from the fluid in the bottoming cycleshould be equal to the heat transfer to the fluid in the topping cycle.

Thus, the ratio of mass flow rates through each cycle should be

The coefficient of performance of the cascade system is

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Multistage Compression Refrigeration Systems

When the fluid used throughout the cascaderefrigeration system is the same the heatRefrigeration Systems refrigeration system is the same, the heatexchanger between the stages can be replacedby a mixing chamber (called a flash chamber)since it has better heat transfer characteristics.since it has better heat transfer characteristics.

18A two-stage compression refrigeration system with a flash chamber.

Multipurpose Refrigeration Systems with a Single Compressor

Some applications require refrigeration at more than one temperature This could beSome applications require refrigeration at more than one temperature. This could beaccomplished by using a separate throttling valve and a separate compressor for eachevaporator operating at different temperatures.

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Absorption Refrigeration Systems

When there is a source of inexpensive thermal energy at a temperature of 100 to 200C is absorption refrigeration. Some examples include geothermal energy, g gy,solar energy, and waste heat from cogeneration or process steam plants, and even natural p ,gas when it is at a relatively low price.

20Ammonia absorption refrigeration cycle.

This system looks very much like the vapor-compression system, except that the compressor hasbeen replaced by a complex absorption mechanism consisting of an absorber, a pump, agenerator, a regenerator, a valve, and a rectifier.

Ammonia vapor leaves the evaporator and enters the absorber, where it dissolves and reacts withwater to form NH3 H2O This is an exothermic reaction; thus heat is released during this processwater to form NH3 H2O. This is an exothermic reaction; thus heat is released during this process.The amount of NH3 that can be dissolved in H2O is inversely proportional to the temperature.Therefore, it is necessary to cool the absorber to maintain its temperature as low as possible, henceto maximize the amount of NH3 dissolved in water. The liquid NH3 H2O solution, which is rich inNH3 i h d h H i f d h l i f iNH3, is then pumped to the generator. Heat is transferred to the solution from a source to vaporizesome of the solution. The vapor, which is rich in NH3, passes through a rectifier, which separates thewater and returns it to the generator. The high-pressure pure NH3 vapor then continues its journeythrough the rest of the cycle.through the rest of the cycle.

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Absorption refrigeration systems (ARS) involve the absorption of a refrigerant by atransport medium. The most widely used system is the ammoniawater system,transport medium. The most widely used system is the ammonia water system,where ammonia (NH3) serves as the refrigerant and water (H2O) as the transportmedium.Other systems include waterlithium bromide and waterlithium chloride systemsOther systems include water lithium bromide and water lithium chloride systems,where water serves as the refrigerant. These systems are limited to applicationssuch as A-C where the minimum temperature is above the freezing point of water.Compared with vapor-compression systems ARS have one major advantage: ACompared with vapor compression systems, ARS have one major advantage: Aliquid is compressed instead of a vapor and as a result the work input is very small(on the order of one percent of the heat supplied to the generator) and oftenneglected in the cycle analysis.g y yARS are much more expensive than the vapor-compression refrigeration systems.They are more complex and occupy more space, they are much less efficient thusrequiring much larger cooling towers to reject the waste heat, and they are moreq g g g j , ydifficult to service since they are less common.Therefore, ARS should be considered only when the unit cost of thermal energy islow and is projected to remain low relative to electricity.

22o a d s p ojected to e a o e at e to e ect c tyARS are primarily used in large commercial and industrial installations.

The COP of actual absorption refri-geration systems is usually less thanunityunity.Air-conditioning systems based onabsorption refrigeration, called theabsorption chillers perform bestabsorption chillers, perform bestwhen the heat source can supplyheat at a high temperature with littletemperature drop

The maximum COP of an

temperature drop.

23absorption refrigeration system.