Ref. Cycles

8/13/2019 Ref. Cycles

1/39

Refrigeration Cycles /


2/39


3/39

3

The above figure shows the objectives of refrigeratorsand heat pumps. The purpose of a refrigerator is the

removal of heat, called the cooling load, from a low-temperature medium. The purpose of a heat pump isthe transfer of heat to a high-temperature medium,called the heating load. When we are interested in theheat energy removed from a low-temperature space,the device is called a refrigerator. When we areinterested in the heat energy supplied to the high-temperature space, the device is called a heat pump.In general, the term heat pump is used to describe thecycle as heat energy is removed from the low-temperature space and rejected to the high-

temperature space.


4/39

4

The performance of refrigerators and heat pumps isexpressed in terms of coefficient of performance(COP), defined as

COP Q

W

COP QW

R L

net in

HP H

net in

Desired outputRequired input

Cooling effectWork input

Desired outputRequired input

Heating effectWork input

,

,

Both COP R and COP HP can be larger than 1. Under thesame operating conditions, the COPs are related by

COP COP HP R 1


5/39

5

Can you show this to be true?

Refrigerators, air conditioners, and heat pumps are ratedwith a SEER number or seasonal adjusted energyefficiency ratio. The SEER is defined as the Btu/hr ofheat transferred per watt of work energy input. The Btuis the British thermal unit and is equivalent to 778 ft-lbf ofwork (1 W = 3.4122 Btu/hr). An SEER of 10 yields aCOP of 2.9.

SEER = COP * 3.4122Refrigeration systems are also rated in terms of tons ofrefrigeration. One ton of refrigeration is equivalent to12,000 Btu/hr or 211 kJ/min. How did the term ton ofcooling originate?

1 TR = 12000 Btu/hr = 3.517 kW


6/39


7/39

7


8/39

8

The standard of comparison for refrigerationcycles is the reversed Carnot cycle. A

refrigerator or heat pump that operates on thereversed Carnot cycle is called a Carnotrefrigerator or a Carnot heat pump, and theirCOPs are

COP T T

T T T

COP T T

T T T

R Carnot H L

L

H L

HP Carnot L H

H

H L

,

,

/

/

11

11


9/39


10/39

10

The Vapor-Compression Refrigeration Cycle

The vapor-compression refrigeration cycle hasfour components: evaporator, compressor,condenser, and expansion (or throttle) valve.The most widely used refrigeration cycle is the

vapor-compression refrigeration cycle. In anideal vapor-compression refrigeration cycle, therefrigerant enters the compressor as asaturated vapor and is cooled to the saturatedliquid state in the condenser. It is then throttledto the evaporator pressure and vaporizes as itabsorbs heat from the refrigerated space.


11/39

11

The Vapor-Compression Refrigeration Cycle

The ideal vapor-compression cycle consists of fourprocesses.

Ideal Vapor-Compression Refrigeration Cycle

Process Description1-2 Isentropic compression2-3 Constant pressure heat rejection in the

condenser

3-4 Throttling in an expansion valve4-1 Constant pressure heat addition in the

evaporator


12/39

12

The following diagrams, show the flow and T-s diagram illustrates the refrigeration cycle .


13/39

13

The P-h diagram is another convenient diagramoften used to illustrate the refrigeration cycle .


14/39

14

The ordinary household refrigerator is a good exampleof the application of this cycle.


15/39

15

COP Q

W h hh h

COP Q

W

h h

h h

R L

net in

HP H

net in

,

,

1 4

2 1

2 3

2 1

Thermodynamic analysis of ideal vapor-compressioncycle SSVCC simple saturation vaporcompression refrigeration cycle


16/39

16

Relative efficiency

Refrigerator relative efficiency is defined as the

ratio between the actual refrigeration cyclecoefficient of performance and that for Carnotcycle at same heat source and heat sink

temperatures .

h R =


17/39

17

DefinitionsRefrigeration capacity: is the amount of heat transfer to theevaporator in kW; for steady operations the refrigerationcapacity equal to the refrigeration load Refrigeration effect: the amount of heat transfer to theevaporator per one kg of refrigerant mass flow rate through it

kJ/kg" Condenser duty: is the amount of heat transfer fromcondenser to surroundings in kW Compressor indicated power: is power added to the

refrigerant during compression process ;kW Compressor brake power: the power required to brake thecompressor kW; compressor brake power greater than thecompressor indicate power" defined the compressormechanical efficiency"


18/39


19/39

19

12

2 2

11 2

2 11

3

3

21

238.41278.23

900200.9456 43.79

0.94561.0

3

900

0

s

soo

s s

StateState kJ

kJ h Compressor exit hCompressor inlet kg kg P P kPakJ T C s T C kJ

kg K s s xkg K

State

Condenser exit

P kPa x

3 4

44 13

4 3

4101.61 0.358

0.4053200.3738.0

o

StatekJ h x

Throttle exit kg kJ

skJ T T C s kg K kg K h h


20/39

20

1 4 1 4

, 2 1 2 1

( )( )

(238.41 101.61)

(278.23 238.41)

3.44

L R

net in

Q m h h h hCOP

W m h h h h

kJ kg kJ kg

The tons of refrigeration, often called the cooling load or refrigeration effect, are

1 4( )1

3 (238.41 101.61)min 211

min1.94

LQ m h hkg kJ Ton

kJ kg

Ton

,

( 20 273)(43.79 ( 20))

3.97

L R Carnot

H L

T COP T T

K K


21/39

21

Another measure of the effectiveness of therefrigeration cycle is how much input power to

the compressor, in horsepower, is required foreach ton of cooling.The unit conversion is 4.715 hp per ton ofcooling.Find the system SEER?

, 4.715

4.7153.44

1.37

net in

L R

W

Q COP

hpTon

hp

Ton


22/39

22

ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE

An actual vapor-compressionrefrigeration cycle differs from the idealone in several ways, owing mostly to theirreversibilities that occur in variouscomponents, mainly due to fluid friction(causes pressure drops) andheat transfer to or from thesurroundings.The COP decreases as a result ofirreversibilities .


23/39

23

Schematic and T -s diagram for the actual

vapor-compression refrigeration cycle .



24/39

24

DIFFERENCE :The difference between actual and simplerefrigeration cycles are:1- Non-isentropic compression2- Superheated vapor at evaporator exit3- Sub-cooled liquid at condenser exit4- Pressure drops in condenser and evaporator5- pressure drop in suction and discharge valves.6- refrigerant heating during suction stroke.



25/39

25


Compressor isentropic efficiencyPolytrophic powerPressure drop in suction and discharge valvesRefrigerant heating during suction process.Actual volumetric efficiency


26/39

Reciprocating compressors


27/39


28/39

28


29/39

Single acting

Clearance volumetric efficiency=


30/39

Apparent clearance volumetricefficiency

30

Clearance ratio C = clearance volume/swept volume

Clearance volume = volume above the topdead center = V c

Swept volume = volume of the cylinderbetween top and bottom dead centers =volume above the bottom dead center -volume above the top dead center


31/39

PP d

P S

VC V1 V2

31


32/39

Actual volume = volume above bottom deadcenter volume of residual gages afterexpansion from discharge to suctionpressure = V 2 V1

Theoretical volume = swept volume =volume above bottom dead center - volumeabove top dead center = V 2 - V c

Clearance ratio C = clearance volume /swept volume = V c / (V 2 V1)

32


33/39

Volumetric efficiency =

33


34/39

34


35/39

Compressor power

Compressors are classified into the followingcategories from Th.D. point of view1- isentropic compressors Adiabatic + reversible

Q =0 and no internal and external

irreversiblenessS = constant IW isent = m ref Dh isent

2- Adiabatic compressors, Q= 0 and DS 0

IW adiab = m ref Dh act.Compressor efficiency = compressor isentropic

efficiency = IW isent / IW adiab = Dh isent / Dh act


36/39

Compressor power

3- poly-tropic compressors During the compression process the

refrigerant follows the relationPv n = constant = C

Since the indicated work is given as

36


37/39

Compressor power

Since,

37


38/39

Heat transfer to poly-tropiccompressor

Find the rate of heat transfer to the poly-tropic compressor?

38

H t P S t


39/39

Heat Pump Systems

Documents

Ref. Cycles