thermodynamic 2-topic 5 : refrigerant

7/30/2019 THERMODYNAMIC 2-TOPIC 5 : REFRIGERANT

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REFRIGERATIONREFRIGERATION ((svsv: Kylteknik): Kylteknik) 424503 E 2010 #2424503 E 2010 #2 -- rzrz

2.2. Va ourVa our--com ressioncom ression refri erationrefri eration

processesprocesses

Ron Zevenhovenbo Akademi UniversityThermal and Flow Engineering Laboratory / Vrme- och strmningsteknik

tel. (02 215)3223 ; [email protected]

2.11.2010 bo Akademi Univ - Thermal and Flow EngineeringPiispankatu 8, 20500 Turku

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2.1 The ideal2.1 The idealvapourvapour--compression cyclecompression cycle


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Reversed Carnot c cleReversed Carnot c cle /1/11-2 and 3-4:reversible and

qu -vapoursaturation

dome

isothermal2-3 and 4-1:isentropic

maximum thermalefficiency = 1 Q /Q

Picture: B98

if reversibleth = 1-TH/TL

Condensation / evaporation of a fluid can be done at almost anytemperature/pressure combination, unlike freezing / melting, andnvo ves grea er ea e ec s vaporisation melting , orexample: water

The Carnot ower c cle can be executed in a reverse within the


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saturation dome of a refrigerant fluid


Reversed Carnot c cleReversed Carnot c cle /2/21-2 and 3-4:reversible and

qu -vapoursaturation

dome

isothermal2-3 and 4-1:isentropic

maximum thermalefficiencyth = 1 QH/QL

Picture: B98

if reversibleth = 1-TH/TL

The (reversed) Carnot cycle is the most efficient cycle operatingbetween two tem erature levels. But:

process 2-3 involves compression of a two-phase mixture,and


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process 4-1 involves expansion of wet refrigerant


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Ideal va ourIdeal va our--com ression c clecom ression c cle /1/1 Operating the Picture: B98

Carnot cycle

outside the no isothermalconditions, /for heatabsorption andre ec on

-

QH = 23 Tds

QL = 41

Tdsusing a throttling valve (or a capillary tube)

This results in a rocess with 3 reversible ste s, and


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1 irreversible step


T s dia ramT s dia ram here for Hhere for H OO

LINES OF CONSTANT ENTHALPY IN THE SATURATION REGION

isenthalpic


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Ideal va ourIdeal va our--com ression c clecom ression c cle /2/2 Step 4-1: boiling of

Picture: B98T

Step 1-2: compressionof saturated vapour tohigh p and T

-superheated gas iscooled to saturated

,

Step 3-4: expansion to

low p, also T down Note: sub-cooling a bit(due to someevaporation)

beyond (3) reduces

the risk of flashing

in the eva orator

(Qin - Qout) +

(Win -Wout) +


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mrefr(hin-hout) = 0.


Pressure levelsPressure levelsA freezer at -18C in

Operation pressures forevaporator and

R-134a

condensor are thevapour pressures forco ot

refrigerantReversible if cold

reservoir Tlow = Tcold ,hot reservoir Thigh = Thot

0F = -18C, 70F = 21C, 250F = 121C

Reversible:or - a, psat

1.44 atm @ -18C,

Trefrigerant = TreservoirThigh = 21C, Tlow = -18CCOP = 1 / T /T -1 = 6.6


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.Picture: T06


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ea vapourea vapour--compress on cyc ecompress on cyc e A vapour-compression refrigeration

c cle uses refri erant R-134a atSource & picture: B98

pressure levels p1 = 1.4 bar and p2 =

8 bar, respectively, with mass flow= . . Calculate:

The rate of heat removal QL andcompressor power input in

The rate of heat rejection QH andthe COPR of the refrigerator

Answer: data for R-134a ives T = -18.8C, T = 31.3C,for (1) h1 = hg = 236.0 kJ/kg; s1 = sg = 0.932 kJ/(kg.K); for (2) s2 =s1 gives h2 = 272.1 kJ/kg, for (3) h3 = hf= 93.42 kJ/kg, s3 = 0.346


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kJ/(kg.K); for (4) h3 h4,


R134a data:R134a data: saturation ressuresaturation ressure

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R134a data:R134a data: saturation tem eraturesaturation tem erature

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Source

Test/s

100C


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R134a data:R134a data: su erheated va oursu erheated va our

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PC/super

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1.6 MPa


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Exam le:Exam le: ideal va ourideal va our--com ression c clecom ression c cle /2/2Answer (cont.): Source & picture: B98

QL = m(h1-h4) = 7.13 kW

Win = m(h2-h1) = 1.80 kW.

.

QH = QL + Win = 8.93 kW

COP = / W = 3.96 =

.

.

(h1-h4)/(h2-h1)

Replacing the throttling valve (34) by an isentropic turbine (34s)

gives, with h4s = 86.92 kJ/kg a turbine power output of 0.34 kW,reducing the net power input Win to 1.46 kW.

The removal of heat from the refrigerated space QL increases from7.13 kW to m h h = 7.46 kW.

.


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COPR increases from 3.96 to 5.11, an increase of 29%.


2.2 Household refrigerators2.2 Household refrigerators


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Household refri eratorHousehold refri erator /1/1Four Main Components:

Compressor, which increases the

htm

,pushing it through the system, and

increasing the vapour's temperature

edu/A3/A3..

Condenser, usually behind therefrigerator, where the refrigerant

.geo4va.vt..

Expansion valve, which causes asudden drop in refrigerant pressure,

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ht

tp://www

valve, since it passes only as much liquidas can be completely vaporised in the

ure&text:.

evaporator, where the latent heat ofrefrigerant vaporisation is absorbed


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Pic

.


Household refri eratorHousehold refri erator /2/2


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Picture: T06Picture: B98


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Irreversible heat transferIrreversible heat transferA freezer at -18C in

Tcold1C or Thot1C

Heat transfer TO therefrigerant in evaporator

R-134a

-

and FROM the refrigerantin condensor requires a

o

Tcold

TsurrThot Tcold space

T, say T = 10C

0F = -18C, 70F = 21C, 250F = 121C

Irreversible:

cold

cold = - psat = .bar),Thot = + 31C (psat =7.93 bar for the

Trefrigerant Treservoir; ifT =10C Tcold = -28C, Thot = +31C

= - =


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refrigerant Picture: T06R hot cold .


2.3 Pressure2.3 Pressure -- enthalpy diagramsenthalpy diagrams


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Pressure enthal dia ramsPressure enthal dia ramsIn a p, h diagram

also. t e vapour-compress on

refrigeration cycle gives

possible

steps, and

2. the heat transferredQL) is proportional to thelength of the lines

s s

==

12

41

hh

hh

W

QCOP

in

LR

p@hhandp@hh 3f31g1 ==

==

12

32

hhWCOP

in

HHP The

correspondingCarnot cycle


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caseidealthefor


h dia ram Rh dia ram R--134a134a


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Picture: B98


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h dia ram Rh dia ram R--134a134a


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Picture:96


h dia ram Rh dia ram R--717 NH717 NH


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Picture:96


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h dia ram Rh dia ram R--2222


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Picture:96


h dia ram Rh dia ram R--1212


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Picture:96


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h dia ram Rh dia ram R--744 CO744 CO


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Picture:http://refrigerant.itri.org.tw/thi.htm


2.4 The real2.4 The realvapourvapour--compression cyclecompression cycle


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-- In a real refrigerator

man irreversibilitiesreduce the

efficiency: u r ct on(gives heat /)

with thesurroundings

The real process differs a bit from the ideal process:

Picture: B98

o ensure comp e e vapor sa on, e re r geran s s g yoverheated at the evaporator inlet (8)

A lon line between eva orator and com ressor ives fluid


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friction and heat exchange with surroundings (81)


--More differencescompared to theideal process:

The compression is

s > 0 (12) or

by cooling,decreasing the

volume ! c ure:

There will be some pressure drop between compressor andcon ensor, n t e con ensor, etween con ensor an

throttling device (2/245) and in the evaporator -


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throttling device, located near the evaporator.


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Exam le:Exam le: real va ourreal va our--com ression c cle /1com ression c cle /1 A vapour-compression refrigeration

c cle uses refri erant R-134a with Picture: B98mass flow = 0.05 kg/s.

Vapour enters the compressor at -, . ar an eaves t at ,

bar.

The vapour enters the condenser at7.2 bar and is cooled to 26C.

The throttling valve reduces the. .

Calculate:

The heat removal QL and thecompressor power Win The adiabatic efficiency of the

compressor Neglect the heat tranfer and


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The COPR valuepressure rops n connec ng nes


Exam le:Exam le: real va ourreal va our--com ression c cle /2com ression c cle /2 At p1,T1: h1 = 243.4 kJ/kg

Picture: B98p2, 2: 2 . g At p3,T3: h3 hf= 85.75 kJ/kg h h

QL = (h1-h4) = 7.88 kW Win = (h2-h1) = 2.05 kW Adiabatic eff. of compressor

c = 2s 1 2- 1p2s = 8 bar, s2s = s1,

=s

gives c = 0.919


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Finally, COPR = QL/Win = 7.88 kW / 2.05 kW = 3.84


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2.5 Refrigerants2.5 Refrigerants--


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Refri erants freezin mixturesRefri erants freezin mixtures In a refrigeration process, energy is converted into

id=841

, . The heat carrier medium will take up the heat at a low

il.asp?article

temperature (and pressure) at another location

A refri erant sv: kldmedie k lmedel artici ates in the /article_de

t

process by a phase transition and/or pressure changes. Itcan also be electricity !

brication.co

A cooling or freezing mixture(sv: kldblandning) can carry or store

.machinerylu

eat, w ic can invo ve aphase transition, but little or no pressure

e:http://www


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. not a refrigerant..... Pi

ctur


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Refri erants for va ourRefri erants for va our--compression (vcompression (v--c) systemsc) systems /1/1

< 1 bar

c12ausa.co

ww.housene

that expensive pressure vessels and tubing elements areneeded in practice below 20 bar.h

ttp://www.

ture:http://w


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Pi


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Refri erants for vRefri erants for v--c s stemsc s stems /3/3 Used / found in refrigeration systems (see also D03, TW00):

CFCs (chloro fluoro carbons), HCFCs (hydro chloro fluoro carbons), mperature.gif

HFCs (hydro fluoro carbons) mostly CFCs: R-11 in water

chillers in building air conditioning, R-12 in domestic refrigerators, inautomotive air conditionin R-22 in air conditionin in industrial au

tion-Low-te

refrigeration, R-134a replaces R-12, R-502 (R-115 / R-22 mix) in

supermarket refrigeration

ductimages/C

- Hydrocarbons (C3, C2, C2= ...) (R-6xx) (Non-)Azeotropic mixtures (R-4xx, R-5xx)

ns.uk.com/pro

CO2 (R-744) making a return; used in aircraft Air also used in aircraft; and also: Water

alth-safety-sig

, , 2 Halogenated hydrocarbon R-code: rightmost digit = no. of F, 10-digit = 1+no. of

H, 100-digit = -1+no. of C, 1000-digit = no. of double bonds, a indicates isomerhttp://www.he


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unbalance, the rest is Cl, B = no. of Br.Picture:


Refri erant va our ressureRefri erant va our ressure


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Vapour pressures of gases and refrigerants Picture: S90


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Some refri erant dataSome refri erant dataGas Refrigerant T boil C * Gas Refrigerant T boil C *

2 5 2 - 3 -

SO2 R-762 -10 CCl2F2 R-12 -30CH3Cl R-40 -24 CHClF2 R-22 -41

CH2Cl2 R-30 +40 C2Cl3F3 R-113 +48

NH3 R-717 -34 C2Cl2F4 R-114 +4

CO2 R-744 -78 C2ClF5 R-115 -38

CH4 R-850 -162 CF3CH2F R-134a -26

CHClF2 +2 6 - - C2ClF5 ** - -

i-C4H10 R-600a -12hydrocarbon

mix HC-12a -33


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* for pressure = 1 bar ** azeotrope


--Boiling temperatures for 1 bar and 20 bar

ers.jpg

Ammonia: -33C and +50C R12: -30C and +70C

ormal_Cylin

R11: +25C and +140C R114: +5C and +120C 954

7/

_DOT_

Or_

N

R134a: -26C and + 68C

m/photo/113

efrigerant_In

Heat of vaporisation and density at 0C: Ammonia: 1260 k /k , 3.45 k /m3 4350 k /m3 g.a

libaba.co

w_

R134a_

R

R22: 207 kJ/kg, 21.23 kg/m3 4400 kJ/m3volumetric heat o va orisation tur

e:http://i

re_

Brand_

N


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Pi

Pu


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Greenhouse asesGreenhouse ases GHGsGHGs Greenhouse gases (GHGs), most importantly carbon

dioxide CO methane CH and nitrous oxide(N2O) trap the outgoing solar radiation that is

00.html

which leads to global warming

Note that water causes of the reenhouse ahd/g/g0258

effect; the changing amounts of other GHGscause an enhanced greenhouse effect

tionary.com/

t er s an t eir g o a warmingpotential (GWP, CO

2

= 1 by definition)www.yourdi

4 ~ , 2 ~

HFCs (hydro fluoro carbons) (140-11700)

PFCs er fluoro carbons 7400 icture:http:/


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SF6 (23900) Source: ZK01


Ozone de letin substancesOzone de letin substances ODSODS ODS substances do not have a direct global warming

tropospheric/ stratospheric ozone

, , -(volatile organic compounds)

du/~tbw/

tm zone ep e ng o en a , . . Carbon tetrachloride, methyl chloroform, halons CnFxClyBrz CFCs are re laced b non-ODS but un

ter.cuny.e

depletion.2.

GHG!) compounds: HFCs, PFCs, SF6

Class II ODS (ODP


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Refi erant use in FinlandRefi erant use in Finland Most important: CFCs R11, R12; HFC R134a

(R-22 belongs to HCFC group)

6

innis ecision : use o or i en exceptin special cases

EU le islation: roduction and im ort/ex ert of ?Item

Id=768

CFCs forbidden as of 1995, as a well as putting CFCcontaining products on the market

- ontPage.asp

out during 2000-2015

Alternatives should be found for HFCs also (mainlym.f

i/main/Fr

- - - CFCs, HCFCs and HFCs are hazardous wastes

Special regulations as to the handling of CFC- End-of-life ://www.e

kok

containing coolers, freezers, and isolation materials(R-11 in poly urethane foam !)

In the future more use of iso-butane R-600a

refrigeratorhandlingat Ekokem

Pictures:http


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propane, propene, CO2 and ammonia Sources: 96, D03, SKL06


2.6 Special vapour2.6 Special vapour--compressioncompressionrefrigeration systemsrefrigeration systems


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--Picture: B98

In industry, efficiencymay be more importantthan simplicity

Sometimes thewide for a single v-ccycle use a cascade cycle

One figure if the same

Two cycles, a bottoming cycle and a topping cycle are

connected via a heat exchanger

For the heat exchanger without heat losses or kinetic /potential energy effects, and mass streams mA, mB :

&&

..


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)hh(m)hh(mWhhm)hh(m)hh(m

BA

B

in,net

L

B

ABA

1256

41

85

323285

+==

==

&&&&

&&RCOP;


-- -- Consider the system in the Figure: a

cascade v-c refrigerator operatingPicture: B98

etween . an ar w t - a asrefrigerant. The heat exchangeroperates at 3.2 bar for both streams. (In

practice p and T are a bit higher in thebottom cycle.) Mass stream mA = 0.05kg/s. Calculate

.

mass stream mB, the heat stream QL taken from the

refri erated s ace

.

compressor power Win the COPR for the process

.

kW1.60)()(WWWkW;7.13)(Q

kg/s;0.039)()(

1256bottomin,topin,in41L

32

3285

=+=+===

=

==

hhmhhmhhm

mhh

mhhmhhm

BAB

ABBA

&

&&&&&&

&&&&


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4.46kW1.60

.

)()(COP

1256

41

,R ==

+

==

hhmhhmW BA

B

innet

L

&&&


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22--sta e com ression refri erationsta e com ression refri eration In a cascade

system using one Picture: B98

refrigerant, a

mixing chambercan be usedinstead of a heatexchanger

Referred to as multistage compression refrigerationsys ems Saturated vapour from the flash chamber is fed to the high


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pressure expansion valve


-- Consider the system in the Figure:

a cascade c-v refrigeratoroperating between 1.4 and 8 barwith R-134a as refrigerant. The

saturated liquid and is throttled toa flash chamber at 3.2 bar. Thevapour product is mixed with therefrigerant leaving the low

Assuming that both compressorsare isentropic and that therefrigerant leaves the evaporatoras saturated vapour: (continues)


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Picture: B98


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--Calculate

, ,of the refrigerant that is

evaporated when throttled to the

The amount of heat that isremoved from the refrigeratedspace and the compressor workper unit mass refrigerant flowingthrou h the condenser, and w,and

The COPR

for the system;

using the given T,s plot


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Picture: B98


-- The mass fraction, x, of refrigerant

evaporated as it is throttled to theflash chamber equals x6 = (h6-hf)/(hg-hf) =(h6-h7)/(h3-h7) = 0.205

e amoun o ea remove romthe refrigerated space per unit massequals qL = QL / m = (1-x6) (h1-h8)= 145.3 kJ/kg

Enthalpy h9 follows fromh9 = x6 h3 +(1-x6) h2 = 251.9 kJ/kg

= =.

tables for R-134a that h4 = 271.1 kJ/kg Compressor work win = (1-x6) (h2-h1)+(h4-h9) = 31.8 kJ/kg


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COPR = qL/win = 145.3 / 31.8 = 4.56 Picture: B98


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MultiMulti-- ur ose refri eration with aur ose refri eration with asingle compressorsingle compressor

Picture: B98

Refrigeration at more than one temperature (as in an ordinaryouse o re r gerator reezer can e accomp s e w t onecompressor by throttling in two steps

Usin one throttle valve and one cold tem erature would ive ice


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in the refrigerator section.


2.72.7 Real vapourReal vapour--compressioncompressioncycles and p,h diagramscycles and p,h diagrams


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Real vReal v--c refri eration rocessc refri eration rocess A real vapour-

compression

refrigeration process ina p, agram: 1s = throttle valve in

2i = evaporator in = 2k = compressor in

1k = com ressor out

Includes pressure drop over

connection lines 2u-2k and 1k-1i; 1i = condenser in 1u = condenser out

heat exchange with surroundingsand in the compressor


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Picture: 96


A commercial vA commercial v--c refri eratorc refri erator

Using a water-cooled condensor and a heat exchanger


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Temperature, pressure and heat of vaporisation can be optimised


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Va ourVa our--com ression refri erationcom ression refri erationprocess with superheat / subcoolingprocess with superheat / subcooling

Heat exchange between evaporator outlet and condensor outletcan improve the COP value.

Superheating by increased compressor pressure gives no improvedefficiency but only results in larger condensor equipment

Subcooling also ensures 100% liquid to the throttling valve and giveseither more heat extracted from the refrigerated space, or a smallerrequired refrigerant mass flow

Less attractive if the suction line to the compressor is long, especially


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w en us ng ammon a as re r geran


--Picture: 96

ssor

compr

refrigerant at acceptable vapour pressures (a one-stage +10C/-30C unitcan reach -65C with two stages or -100C with three)

With minimum and maximum pressures p1 and p2 it can be shown that

the optimum intermediate pressure level pm = (p1p2) Disadvanta es are lower efficienc , hi her ower in ut, increased


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temperature of refrigerant vapor from first compressor


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Cascade vCascade v--c s stemsc s stems /1/1 A two-stage

Picture: D03

differentrefrigerants andheat exchange

Allows for a lowerempera ure an

with a single-stagesystem

Typically -150C

can be reached Compressor workdecreases COP

Condenser B of system I is cooled


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by evaporator C of system 2


Cascade vCascade v--c s stemsc s stems /2/2Cascade systems arecommonly used for Pictures: D03

CO2 or

natura gas liquefaction

Linde-Hampsonsystem


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Picture: B98compression


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2.8 Final remarks2.8 Final remarks


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Defrostin ur in airDefrostin ur in air Defrosting is

necessary from timePicture: D03

to time to removeice (from airhumidit

An effective methodis to use hot

compressor;otherwise warm air,wa er or e ec r c ycan be used

immiscible with the refrigerant it acts as an insulator at heattransfer surfaces, making the condensor smaller)


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Manual or automatic purging methods can remove this air


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Tons of refri erationTons of refri eration For refrigerators used for producing ice, one way to

26.jpg

1 ton of refrigeration = heat needed to freeze 1 short= = ood/c

h16fig1

24 hours

1 ton of refri eration docs/wwii/bl

= 211 kJ/min = 200 BTU/min= 3.52 kW heat removal from

my.m

il/book

the refrigerated space

ory

.amedd.ar

re:http://hist


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Pictu


Heat exchan er irreversibilitiesHeat exchan er irreversibilities vS91vS91 A simple steady-state

heat transfer rocess; Thermodynamic analysis

balanceEner

heat is transported frommedium 1 to medium 2

= 21

balanceEntropy

QQ &&

by conduction through amaterial that separates =+2

2

1

1

TQS

TQ

gen&&&

. Temperature T1 > T2

0>

=

1

1=

21

211

12

1 TT

TTQ

TTQSgen

&&&

This shows that Sgen islar e for lar e tem erature

.

differences (T1-T2) and lowtemperatures T1 and T2Q1.

Q2.

T = T T = T


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2.9 Vapour2.9 Vapour--compression cyclecompression cycleheat pumpsheat pumps


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Heat um s usin vHeat um s usin v--c c clec c cle

A heat pump vapour-compression system with reversing valve forsummer / coolin a or winter / heatin o eration b

NOTE:

HP=

COPR+1


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Pictures: KJ05


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Heat um s in FinlandHeat um s in Finland

Total capacity (2004)Total capacity (2004)

Waste heat

Air heat

Geothermal

~~ ~~ ~~


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Source / picture: http://www.sulpu.fi/index.phpHeat pumps: to be continued


Sources #2Sources #2 CB98: Y.A. engel, M.A. Boles Thermodynamics. An Engineering Approach, McGraw-

Hill (1998) D03: . Diner Refri eration s stems and a lications Wile 2003 KJ05: D. Kaminski, M. Jensen Introduction to Thermal and Fluids Engineering, Wiley

(2005) SEHB06: P.S. Schmidt, O. Ezekoye, J. R Howell, D. Baker Thermodynamics: An oa

t09.jpg

Integrated Learning System (Text + Web) Wiley (2006) S90: A.L. Stolk Koudetechniek A1, Delft University of Technology (1990) SKL06: Suomen Kylmliikkeiden Liitto (2006) http://www.skll.fi/

3/0809bee

rfl

T06: S.R. Turns Thermal Fluid Sciences, Cambridge Univ. Press (2006) TW00: A.R. Trott, T.C. Welsh Refrigeration and Air-Conditioning

3rd Ed. Butterworths-Heineman (2000)l.com/ent/gif

ZK01: R. Zevenhoven, P. Kilpinen Control of pollutants in fluegases and fuel gases Picaset (Espoo), 2001 (Chapter 9)

96: G. hman Kylteknik, bo Akademi University (1996)ww.azcentra

See also: Martinez, I. Lectures on Thermodynamics lecture 18 (English or Spanish)http://webserver.dmt.upm.es/~isidoro/bk3/index.htmlupdated and based on re

:http://w

thermodynamic 2-topic 5 : refrigerant

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