REFRIGERATION SYSTEMBy: Engr. Yuri G. Melliza

Refrigeration : Is that area of engineering that deals with the different mechanism involved in maintaining a temperature of a space or material below that of the immediate surroundings.

Uses of Refrigeration:1. Ice making2. Cold Storage3. Air conditioning4. Food preservation5. Other industrial processes that uses refrigeration

Carnot Cycle:A.Carnot EngineProcesses:

1 to 2 - Heat Addition (T = C)2 to 3 - Expansion (S = C)3 to 4 - Heat Rejection (T = C)4 to 1 - Compression (S = C)

T

S

1 2

34

TH

TL

QA

QR

Heat Added (T = C)

QA = TH(S2 - S1) 1S2 - S1 = S3 – S4 = SQA = TH S 2Heat Rejected (T =

C)QR = TL(S3 – S4) 3S2 - S1 = S3 – S4 = SQR = TL S 4

Net WorkW = Q ; W = QA - QR W = S (TH – TL) 4

Where:TH – high temperature, KTL – low temperature, K

Thermal Efficiency

100% x QW

eA

100% x Q

Q-Qe

A

RA

100% x QQ

eA

R

1

100% x T

T-Te

H

LH

100% x TT

eH

L

1

5

6

7

9

8

B. Carnot RefrigeratorProcesses

1 to 2 - Compression (S = C) 2 to 3 - Heat Rejection (T = C)3 to 4 - Expansion (S = C)4 to 1 - Heat Addition (T = C)TH

T

TL

S1

23

4QA

QR

Heat Added (T = C)

QA = TL(S1 – S4) 10S2 – S3 = S1 – S4 = SQA = TL S 11Heat Rejected (T =

C)QR = TH(S2 – S3) 12S2 – S3 = S1 – S4 = SQR = TH S 13

Net WorkW = Q ; W = QR - QA W = S (TH – TL) 14

Coefficient of Performance: It is the ratio of the refrigerating capacity to the net cycle work.

WQ

COP A

Q-Q

QCOP

AR

A

T-T

TCOP

LH

L

15

16

17

C. Carnot Heat Pump: A heat pump uses the same components as the refrigerator but its main purpose is to reject heat at high thermal energy level.Heat Added (T = C)

QA = TL(S1 – S4) 20S2 – S3 = S1 – S4 = SQA = TL S 21Heat Rejected (T =

C)QR = TH(S2 – S3) 22S2 – S3 = S1 – S4 = SQR = TH S 23

Performance Factor

Net Work

W = Q ; W = QR - QA W = S (TH – TL) 24

WQ

PF R

Q-Q

QPF

AR

R

T-T

TPF

LH

H

1COPPF

25

26

27

28

Vapor Compression Cycle

Processes:1 to 2 - Compression (S = C) 2 to 3 - Heat Rejection (P = C)3 to 4 - Expansion (h = C)4 to 1 - Heat Addition (P = C)

Basic Components:1. Gas Compressor 2. Condenser3. Expansion Valve4. Evaporator

Schematic Diagram

Evaporator

Condenser

4 1

23

QA (Heat Added)

QR (Heat Rejected)

W (Work)

Compressor

Expansion Valve

Ph and TS Diagram

1

23

4

S = C

P

h

1

2

3

4

h = C

T

S

CompressorW = m(h2 – h1) KWFor Isentropic Compression (PVk = C)

k1k

1

2

1

2

PP

TT

1PP

1kkmRT1

Wk

1k

1

2

P1V1’ = mRT1

Where:m – mass flow rate in kg/secV1’ – volume flow rate in m3/secP1 – suction pressure in KPaP2 – discharge pressure in KPaT1 – suction temp. in KT2 – discharge temp. in K

100% x PP

c- c 1η

100% x VV

η

k1k

1

2v

D

1'v

Volumetric Efficiency:

Where:V1’ - volume flow rate measured at intake,m3/secVD -displacement volume, m3/secDisplacement Volume:a. For Single acting

m3/sec 4(60)

Nn'LD=V2

Dπ

b. For Double acting (without considering piston rod)

m3/sec 4(60)

Nn'LD2=V2

Dπ

c. For Double acting (considering piston rod)

[ ]m3/sec d-2D 4(60)LNn'=V 22

Dπ

Piston Speed:PS = 2LN m/min

Where:L - length of stroke, m

D - diameter of bore, md - piston rod diameter, mN - no. of RPMn’ no. of cylinders

Compressor Efficiencies:

100% x Work Indicated

Work Ideal cn

a. Compression Efficiency

100% x WorkShaft or Brake

Work Indicated m

b. Mechanical Efficiency

100% x WorkShaft or Brake

Work Ideal c

mcnc

c. Compressor Efficiency

Condenser: QR = m(h2 – h3) KJ/sec

For an air cooled condenserQR = m(h2 – h3) = mCPa(ta2 – ta1) KJ/sec

For water cooled condenserQR = m(h2 – h3) = mwCPw(tw2 – tw1)

KJ/secWhere:

a – refers to airw – refers to water1 – inlet condition2 – exit condtitionCpa = 1.0045 KJ/kg-°CCPw = 4.187 KJ/kg- -°C

Expansion Valve:

h3 = h4

% 100 x h

hhx

fg4

f444

Where: x - quality

Evaporator:QA = m(h1 – h4) KJ/sec or KW

QA = 60 m(h1 – h4) KJ/min

1 TR = 211 KJ/minTR – tons of refrigeration

Coefficient of Performance

WQ

COP A

whereQA – refrigerating effect or

Refrigerating capacity, KW

W – compressor work, KW

Wet compression

11

2

2

33

44

P

h S

T

Subcooling the refrigerant

11

2

2

33

44

P

h S

T

Superheating the suction vapor

11

2

2

33

44

P

h S

T

Effects of Operating ConditionsEffects of Increasing the vaporizing temperature:

a. The refrigerating effect per unit mass increases.

b. The mass flow rate per ton decreases

c. The volume flow rate per ton decreases.

d. The COP increases.e. The work per ton decreases.f. The heat rejected at the condenser

per ton decreases.

Effects of Increasing the condensing temperature:

a. The refrigerating effect per unit mass

decreases.b. The mass flow rate per ton increasesc. The volume flow rate per ton

increases.d. The COP decreases.e. The work per ton increases.f. The heat rejected at the condenser

per ton increases.

Effects of superheating the suction vaporA. When superheating produces useful

cooling: a. The refrigerating effect per unit mass

increases.

b. The mass flow rate per ton decreasesc. The volume flow rate per ton decreases.d. The COP increases.e. The work per ton decreases.

B. When superheating occurs without useful cooling:

a. The refrigerating effect per unit mass

remains the same.b. The mass flow rate per ton remains the same.c. The volume flow rate per ton increases.d. The COP decreases.

e. The work per ton decreases.f. The heat rejected at the condenser

per ton increases.Effects of subcooling the liquid:

a. The refrigerating effect per unit mass increases.

b. The mass flow rate per ton decreases

c. The volume flow rate per ton decreases.

d. The COP increases.e. The work per ton decreases.f. The heat rejected at the condenser

per ton decreases.

Liquid – Suction Heat Exchanger

The function of the heat exchanger are:1. To ensure that no liquid enter the compressor2. To subcool the liquid from the condenser to prevent bubbles of vapor from impeding the flow of refrigerant through the expansion valve.

Actual vapor compression cycle:As the refrigerant flows through the

system there will be pressure drops in the condenser, evaporator and piping. Heat loses or heat gains will occur depending on the temperature difference between the refrigerant and the surroundings. Compression will be polytropic with friction and heat transfer instead of isentropic.

Condenser

Compressor

Evaporator

Heat exchanger

Multipressure System

A multipressure system is a refrigeration system thathas two or more low-side pressure. The low-side Pressure is the pressure of the refrigerant between the expansion valve and the intake of the compressor.

Removal of Flash gas:The flash gas that develops during the throttling process between the condenser and evaporator wasremoved and recompressed before complete expansion.With flash gas removal a savings in power requirementwill occur.

IntercoolingIntercooling between two stages of compression redu-ces the work of compression per kg of vapor. Intercoo-ling in a refrigeration system can be accomplished witha watercooled heat exchanger or by using refrigerant.The watercooled intercooler may be satisfactory for twostage air compression, but for refrigerant compressionThe water is not cold enough. The alternate methoduses liquid refrigerant from the condenser to do theintercooling. Discharge gas from the low stage com-pressor bubbles through the liquid in the intercooler.Refrigerant leaves the intercooler as saturated vaporat the intercooler pressure.

Two evaporators and one compressor

1

23

4 5 6

7 8

compressor

Pressure-reducing valve

condenser

HP evaporator

LP evaporator

condenser

evaporator

Flash tank and Intercooler

LP compressor

HP compressor

Two compressors and one evaporator

condenser

LP evaporator

Flash tank and Intercooler

LP compressor

HP evaporator

Two compressors and two evaporators

HP compressor

Optimum Intercooler or Inter-stage pressure

41i PPP Where:

Pi – optimum interstage or intercooler pressure in KPa

P1 – suction pressure of LP compressor, KPaP4 – discharge pressure of HP compressor, KPa

Cascade System

Condenser

CascadeCondenser

Evaporator

HP Compressor

LP Compressor

A. Closed cascade condenser

Condenser

CascadeCondenser

Evaporator

HP Compressor

LP Compressor

B. Direct Contact type cascade condenser

Air Cycle RefrigerationA. Closed or Dense - Air System

Cooler

Expander Compressor

Refrigerator

Cooler

Expander Compressor

Refrigerator

B. Open - Air SystemP

V

1

23

4

T

S

1

2

3

4

Compressor Work:

111

k1k

1

21C

mRTVP

1PP

1kkmRT

W

Cooler:

)T(TmCQ 32pR Expander:

333

k1k

3

43E

mRTVP

1PP

1kkmRT

W

Refrigerator:

)T(TmCQ 41pA

Network

W = Wc – WE

W = QR - QA

PRODUCT LOADProduct Load – is the total amount of heat removed from a product in a refrigerated space.

m m m

t1 t2tf

Q1 Q2

Q3

Q = Q1 + Q2 + Q3 + Q4

CP1 CP2

Where:Q1 – sensible heat in cooling the

productfrom t1 to tf

Q2 – latent heat of fusion (freezing) of the

product at tf

Q3 – sensible heat in cooling further the

product from tf to the final temperature t2

Q4 – heat losses or other heat gains from the products Q1 = mCP1(t1 – tf) KJ/hr

Q2 = m(hL) KJ/hrQ3 = mCP2(tf – t2)Q4 = Q – (Q1 + Q2 + Q3)

Where:m – mass of product, kg/hrCp1 – specific heat of product below

freezing, KJ/kg-C or KJ/kg-KCp2 - specific heat of product above

freezing, KJ/kg- °C or KJ/kg- °Kt1 – initial temperature, °Ctf – freezing point temperature, °Ct2 – final temperature, °ChL – latent heat of freezing, KJ/kg