l2-ref hp systems ssr

7/28/2019 L2-Ref Hp Systems Ssr

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REFRIGERATION AND HEAT PUMP SYSTEMSObjective: Study Common Types of Refrigeration and Heat Pump Systems

Vapour Compression Refrigeration Systems (VCRS)

Carnot refrigeration cycle and departures from it

Analysing of VCRS

Refrigerant properties and selecting refrigerants

Improving performance using

ME 306 Applied Thermodynamics

Multistage with inter-cooling

Absorption Systems

Heat Pumps

Carnot heat pump cycle

Gas Refrigeration Systems:

Brayton Refrigeration Cycle (reverse Brayton cycle) 1


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CARNOT REFRIGERATION CYCLE

Moran and Shapiro (2006) Fig 10.1

ME 306 Applied Thermodynamics 2

Coefficient ofperformance

Moran and Shapiro (2006)


3/20

DEPARTURE FROM THE CARNOT CYCLE


Wet compression should be avoided Turbine work is very less because of lower efficiencies; replacedby throttle valve Vapor compression systems


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VAPOR COMPRESSION REFRIGERATION SYSTEM


Process 12s: Isentropic compression of the refrigerant from state 1 to the

condenser pressure at state 2s.Process 2s3: Heat transfer from the refrigerant as it flows at constantpressure through the condenser. The refrigerant exits as a liquid at state 3.Process 34: Throttling process from state 3 to a two-phase liquidvapormixture at 4.

Process 41: Heat transfer to the refrigerant as it flows at constant pressurethrough the evaporator to complete the cycle.

( )( )

1 4

2 1

/

/

in

sc

Q m h h

h hW m

= =


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V C R: ACTUAL CYCLE

( )

( )2 1

2 1

/

/

c s sc

c

W m h h

h hW m

= =

Isentropic efficiencyof the Compressor


Temperature differences in the condenser and the evaporator

Non-isentropic compression work

Usually state 1 will be superheated and state 3 will be subcooled

Pressure drops in condenser, evaporator and piping systems; but ignored here


6/20

Numerical ProblemRefrigerant 134a is the working fluid in an ideal vapor-compression refrigeration

cycle that communicates thermally with a cold region at 0C and a warm region at26C. Saturated vapor enters the compressor at 0C and saturated liquid leaves thecondenser at 26C. The mass flow rate of the refrigerant is 0.08 kg/s. Determine (a)the compressor power, in kW, (b) the refrigeration capacity, in tons, (c) the coefficient

of performance, and (d) the coefficient of performance of a Carnot refrigeration cycleoperating between warm and cold regions at 26 and 0C, respectively.

ME 306 Applied Thermodynamics 6Moran and Shapiro 2006


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Numerical ProblemModify this problem to allow for temperature differences between the refrigerant and

the warm and cold regions as follows.Saturated vapor enters the compressor at 10C. Saturated liquid leaves thecondenser at a pressure of 9 bar. Determine for the modified vapor-compressionrefrigeration cycle (a) the compressor power, in kW, (b) the refrigeration capacity, in

tons, (c) the coefficient of performance. Compare results.



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Numerical Problem

Reconsider the vapor-compression refrigeration cycle as in previous problem, butinclude in the analysis that the compressor has an efficiency of 80%. Also, let thetemperature of the liquid leaving the condenser be 30C. Determine for the modifiedcycle (a) the compressor power, in kW, (b) the refrigeration capacity, in tons, (c) thecoefficient of performance



9/20

Refrigerants

Chlorofluorocarbons (CFCs): CCl2F2 R12 affects ozone layer

Hydrogen in place of chlorine: HFCs: R134a (CF3CH2F): tetrafluro-ethane

R22 (CHClF2): mostly used

NH3: Mainly used in vapor absorption system

Moving towards hydro-carbons: Methane: CH4 Propane: C3H8


p-h chart Avoid very low pressures in theevaporator and very high pressures inthe condenser

Centrifugal compressors: low

evaporator pressures; refrigerants withhigh sp.vol

Reciprocating compressors: largepressure range; low sp.vol. refrigerants


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CASCADE CYCLES

To achieve very low temperature, pressuredifference becomes more; compressorefficiency reduces

Two vapor compression systems arearranged in series

Counterflow heat exchanger to link them


in

cA cB

Q

W W =

+

B

A



11/20

MULTISTAGE COMPRESSION WITH

INTERCOOLING

ME 306 Applied Thermodynamics 11Moran & Shapiro (2006)



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AMMONIA-WATER ABSORPTION

REFRIGERATION SYSTEM

Compressor is replaced by

absorber, pump andgenerator.


mmon a vapor sso ves nthe absorber; exothermic

process

Usually waste heat issupplied to generator to

separate the vapor



13/20

AMMONIA-WATER ABSORPTION

REFRIGERATION SYSTEM (Modified)

Rectifier is added to remove any

water particles entering thecondenser

Hot weak ammonia solution


preheats the mixture going to thegenerator, there by saving someheat input



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HEAT PUMP SYSTEMS

Carnot heat pump system Hhp

H C

TcopT T

=

Vapor compression heat pump system


never beless than 1



15/20

AIR SOURCE REVERSING HEAT PUMP

ME 306 Applied Thermodynamics 15Moran and Shapiro (2006)


16/20

GAS REFRIGERATION SYSTEM

ME 306 Applied Thermodynamics16

1 4

2 1 3 4( ) ( )

h h

h h h h

=



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NUMERICAL PROBLEM

Air enters the compressor of an ideal Brayton refrigeration cycle at 1 bar, 270K, with avolumetric flow rate of 1.4 m3/s. If the compressor pressure ratio is 3 and the turbine inlettemperature is 300K, determine (a) the net power input, in kW, (b) the refrigeration capacity, inkW, (c) the coefficient of performance.

Assumptions

Each component of the cycle isanalyzed as a control volume at steadystate. The control volumes areindicated by dashed lines on the


.

The turbine and compressorprocesses are isentropic.

There are no pressure drops throughthe heat exchangers.

Kinetic and potential energy effectsare negligible.

The working fluid is air modeled as anideal gas.



18/20

NUMERICAL PROBLEM

Reconsider previous example, but include in the analysis that the compressor and turbineeach have an isentropic efficiency of 80%. Determine for the modified cycle (a) the net powerinput, in kW, (b) the refrigeration capacity, in kW, (c) the coefficient of performance, andinterpret its value.

ME 306 Applied Thermodynamics18

Irreversibilities in the compressor and turbinehave a significant effect on the performanceof gas refrigeration systems


19/20

BRAYTON CYCLE WITH REGENERATIVE

HEAT EXCHANGER

Higher pressure and

higher volume flow ratesare required to achievehigher refrigeration effect


,

turbine inlet temperatureis brought below the hottemperature

Refrigeration effect takes

place from 4 to b at alower temperature



20/20

AIRCRAFT CABIN COOLING

ME 306 Applied Thermodynamics 20Moran and Shapiro (2006)

l2-ref hp systems ssr

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