CHE/MATE 151- CHE/MATE 151- THERMODYNAMICSTHERMODYNAMICS

REFRIGERATION AND LIQUIFACTION SYSTEMS

GENERAL REFRIGERATION CONCEPTS

• REFRIGERATION ≡ TRANSFER OF HEAT FROM A COLD TO HOT TEMPERATURE REGION

• CIRCULATING FLUID ≡ REFRIGERANT• HEAT PUMPS ARE A FORM OF REFRIGERATION• COEFFICIENT OF PERFORMANCE DEFINES EFFICIENCY:

• VALUES ARE TYPICALLY > 1.0• 1 TON OF REFRIGERATION = 200 BTU/min OR 211 kJ/min

(AMOUNT TO FREEZE 1 TON H2O AT 0C IN 24 HOURS)

NET

HHTPMP

NET

CREF

W

Q

POWER

RATEHEATINGCOP

W

Q

POWER

RATECOOLINGCOP

CARNOT REFRIGERATOR

• REVERSED CARNOT CYCLE IS CALLED A CARNOT REFRIGERATOR

• REQUIRES WORK INPUT TO FUNCTION

http://coolingdevice.net/images/8.jpg

CARNOT REFRIGERATOR η

• WORK FOR THE CARNOT REFRIGERATOR WOULD BE: W = |QH| - |QC|

• USING THE COEFFICIENT OF PERFORMANCE DEFINITION: ω = |QC|/W

• FOR THE CARNOT ENGINE:

• COMBINING THESE YIELDS:

C

H

C

H

T

T

Q

Q

CH

C

TT

T

VAPOR-COMPRESSION

• TYPICAL PROCESS USED FOR MOST REFRIGERATION SYSTEMS

VAPOR COMPRESSION CYCLE1. SATURATED LIQUID AT LOW PRESSURE, PL,

VAPORIZES TO SATURATED VAPOR AT TC. THIS IS QC, HEAT ABSORPTION IN THE EVAPORATOR.

2. SATURATED VAPOR IS COMPRESSED TO A HIGH PRESSURE PH AND ELEVATED TEMPERATURE, TH.

3. THE VAPOR IS COOLED TO A SATURATED LIQUID AT TH AND PH. THIS IS QH, HEAT REJECTION IN THE CONDENSER.

4. LIQUID EXPANDS FROM PH TO PL, WITH A DECREASE IN TEMPERATURE TO TC AND PARTIAL VAPORIZATION. THIS IS A THROTTLING PROCESS.

TYPE OF REFRIGERANT• PRIMARY FACTORS ARE

– REFRIGERANT VAPOR PRESSURE CURVE– REFRIGERANT JOULE-THOMSON COEFFICIENT

TYPICAL REFRIGERANTS• 1850s – ETHYL ETHER

• LIGHT HYDROCARBONS C2 – C4, AMMONIA,

• SO2, CO2, METHYL CHLORIDE, CHLOROFLUOROCARBONS

http://www.iifiir.org/en/doc/1015.pdf

REFRIGERANT HISTORY

• CONSUMPTION RATES INDICATE LEAKS OF UP TO 30% PER YEAR (1).

• ANNUAL WORLD CONSUMPTION (1991) WAS 484 TONS (1) .

• CFCs ARE OZONE SCAVENGERS AND WERE TARGETED FOR ELIMINATION FROM PRODUCTION BY THE MONTREAL PROTOCOL IN 1987

1 http://www.usgbc.org/Docs/LEED_tsac/Energy/ASHRAE%20Journal%2009-00%20Refrigerant%20Use%20in%20Europe.pdf

CFC PHASEOUT TARGETS• TARGETED OZONE DEPLETION CHEMICALS INCLUDE

CFCs, HALONS, CCl4, METHYL CHLOROFORM, HCFCs,

HBFCs, AND METHYL BROMIDE.  

http://www.afeas.org/montreal_protocol.html

CFC REPLACEMENTS

• TARGETS NEED TO BE NON-TOXIC• TARGETS SHOULD NOT REQUIRE HIGHER

ENERGY CONSUMPTION AND TRADE OZONE DEPLETION WITH CO2 GENERATION

• CHANGING CHEMICALS WITHOUT COSTLY EQUIPMENT MODIFICATION IS PREFERRED

• MINIMIZING THE QUANTITY OF REFRIGERANT IN SYSTEMS MAY ALSO BE PRACTICAL – INDIRECT SYSTEMS WITH “LAYERS” OF REFRIGERANTS

http://www.usgbc.org/Docs/LEED_tsac/Energy/ASHRAE%20Journal%2009-00%20Refrigerant%20Use%20in%20Europe.pdf

ABSORPTION REFRIGERATION• USES HEAT FOR THE COMPRESSION PROCESS IN

VAPOR-COMPRESSION

LITHIUM BROMIDE SALT PROCESS• TWO PRIMARY TYPES OF ABSORPTION

REFRIGERATION, LiBr SALT AND NH3

Marina Donate, Luis Rodriguez, Antonio De Lucas and Juan F. Rodríguez, Thermodynamic evaluation of new absorbent mixtures of lithium bromide and organic salts for absorption refrigeration machines, International Journal of Refrigeration ,Volume 29, Issue 1, January 2006, Pages 30-35

LiBr SALT PROCESS• REFERENCING PREVIOUS DIAGRAM – WATER AS

REFRIGERANT– LOW PRESSURE WATER VAPOR (2) COMES FROM THE

EVAPORATOR– WATER IS ABSORBED IN STRONG LiBr SOLUTION – WEAK SOLUTION IS PUMPED TO (RE)GENERATOR

THROUGH HEAT EXCHANGER (3,4,6)– HEAT FROM EXTERNAL SOURCE VAPORIZES WATER

WHICH GOES TO CONDENSER (7)– STRONG LiBr SOLUTION RETURNS THROUGH

EXCHANGER AND THROTTLING VALVE TO ABSORBER (5, 8)

– WATER VAPOR IS COOLED IN CONDENSER THEN FLOWS THROUGH A J-T VALVE TO THE EVAPORATOR (9, 1)

AMMONIA ABSORPTION PROCESS• FOR

TEMPERATURES BELOW 0 C, ALTERNATE REFRIGERANTS ARE USED

• RHX ≡ REFRIGERANT HX

• SHX ≡ SOLUTION HX

R.D. Misra, P.K. Sahoo, , and A. Gupta, Thermoeconomic evaluation and optimization of an aqua-ammonia vapour-absorption refrigeration system, International Journal of Refrigeration Volume 29, Issue 1, January 2006, Pages 47-59

ABSORPTION SYSTEM OPTIONS• ALTERNATE REFRIGERANTS

– SALT SOLUTIONS WITH WATER– ALCOHOLS AND ETHERS– AMMONIA

• ALTERNATE CONFIGURATIONS (SEE REFERENCE BELOW)– ENERGY CONSERVATION THROUGH ADDITIONAL HEAT EXCHANGES– GAX PROCESS USES GENERATOR/ABSORBERHEAT EXCHANGER

Pongsid Srikhirin, Satha Aphornratana and Supachart Chungpaibulpatana, A review of absorption refrigeration technologies, Renewable and Sustainable Energy Reviews Volume 5, Issue 4, December 2001, Pages 343-372

HEAT PUMP FOR HEATING• REVERSED HEAT ENGINE• TAKES HEAT FROM LOW TEMPERATURE AND

EXHAUSTS TO HIGH TEMPERATURE

http://www.nccc.gov.sg/building/heat_p2.jpg

HEAT PUMP FOR COOLING• TAKES HEAT FROM LOW TEMPERATURE AND

EXHAUSTS TO HIGH TEMPERATURE

http://www.nccc.gov.sg/building/heat_pump.shtm

LIQUIFACTION PROCESSES

• USES JOULE-THOMSON EFFECT THROUGH VALVE

LIQUIFACTION PROCESSES

• USES JOULE-THOMSON EFFECT THROUGH EXPANDER

AIR SEPARATION PROCESS

• USES SIGNIFICANT HEAT EXCHANGE AND JOULE-THOMSON EFFECTS

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