Purpose of Refrigeration

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Godfrey Pearson
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1 Refrigeration

2 Outline Purpose of refrigeration Examples and applications Choice of coolant and refrigerants Phase diagram of water and CO 2 Vapor compression refrigeration system Pressure-enthalpy diagram for refrigerants Refrigerator, air conditioner, thermoelectric cooler, heat pump Designation, choice, criteria for selection, and characteristics of refrigerants Alternatives to vapor compression refrigeration system Heat transfer in refrigeration applications 2

3 Purpose of Refrigeration To slow down rates of detrimental reactions Microbial spoilage Enzyme activity Nutrient loss Sensorial changes Guideline: Generally, rates of reactions double for every 10 C rise in temperature 3

4 Examples/Applications of Cooling Cooling engine of a car Coolant/water Cooling food/beverage during prolonged period of transportation in a car (vacation trip) Ice, dry ice in an insulated container Cooling interior of car Car AC unit Cooling interior of room/house Window AC unit Whole house unit (can it be used for heating also???) Cooling food in a refrigerator/freezer 4

heat; fins on radiator results in high surface area (A); as car moves, air flow and hence h increases due to forced convection

5 Cooling of Engine of Car HOT Engine Head Coolant High c p Low freezing pt. Coolant Reservoir Finned Radiator Air flow from outside Coolant/water is pumped through pipes to hot engine; coolant absorbs heat; fins on radiator results in high surface area (A); as car moves, air flow and hence h increases due to forced convection Q = h A ( T); high A and high h results in high Q or heat loss from engine to outside air Note: During prolonged idling of car, engine can overheat due to low h by free convection 5

6 Room (or Car) Air Conditioner 6

7 Household Refrigerator HEAT Extracted from food inside refrigerator Are there parts in a refrigerator where you can get burnt? Can you cool the kitchen by keeping the refrigerator door open? Extracted HEAT Moved to the outside 7

8 Evaporative (Swamp) Cooler Water 8

9 Refrigerants/Coolants Cold water (at say, 0 C) Heat extracted from product is used as sensible heat and increases water temperature Ice (at 0 C) Heat extracted from product is used as latent heat and melts ice (λ fusion = kj/kg at 1 atm, 0 C); it can then additionally extract heat from product and use it as sensible heat to increase the temperature of water Dry ice (Solid CO 2 ) Heat extracted from product is used as latent heat and sublimates dry ice (λ sublimation = 571 kj/kg at 1 atm, C) Liquid nitrogen Heat extracted from product is used as latent heat and evaporates liquid N 2 (λ vaporization = 199 kj/kg at 1 atm, C) Why does dry ice sublimate while regular ice melt under ambient conditions? 9

10 Phase Diagram Water CO 2 Solid Liquid Gas Solid Liquid Gas Pressure (atm) Melting point Triple point Boiling point Pressure (atm) 5.1 Triple point Temperature ( C) Temperature ( C) 10

11 Drawback of Ice/Dry-Ice as Refrigerant Neither can be re-used Ice melts Dry-ice sublimates Expensive and cumbersome technique 11

12 Alternatives to Ice/Dry-Ice Blue ice or gel packs (cellulose, silica gel etc) Low freezing point Though it isn t lost, it has to be re-frozen Endothermic reaction (Ammonium nitrate/chloride and water) Evaporation of refrigerant Cooled Air (After λ vap of refrigerant is absorbed by the refrigerant from air) Liquid Refrigerant Gaseous Refrigerant Fan Warm Ambient Air Can boiling/evaporation of water serve as a refrigeration method? 12

13 Re-Utilization of Refrigerant High Pr. Liq. Condense the Gas High Pr. Gas High Pr. Liq. Expand the Liquid Low Pr. Liq. Cooled Air (after λ vap of refrigerant is absorbed by refrigerant from air) High Pr. Gas Compress the Gas Low Pr. Gas Liquid Refrigerant Gaseous Refrigerant Fan Warm Ambient Air 13

14 Vapor Compression Refrigeration System Liquid Expansion Valve Liquid + Vapor P 2 P 1 d e Saturated Liquid Line SUB-COOLED LIQUID Condenser High Pressure Side Low Pressure Side Evaporator Energy Input d Expansion Valve e a Condenser Evaporator LIQUID + VAPOR H 1 Energy Output Enthalpy (kj/kg) c Critical Point ~ 30 C ~ -15 C. Vapor Vapor a b Compressor c Saturated Vapor Line b Compressor SUPERHEATED VAPOR H 2 H 3 Energy Input IDEAL CONDITIONS Condensing: Constant Pr. (P 2 ) Expansion: Constant Enthalpy (H 1 ) Evaporation: Constant Pr. (P 1 ) Compression: Constant Entropy (S) Constant Temperature Line Left of dome: Vertical Within dome: Horizontal Right of dome: Curved down IDEAL CONDITIONS Refrigerant is 100% vapor at end of evap. AND Refrigerant is 100% liquid at end of condenser 14

15 Vapor Compression Refrigeration System Liquid Expansion Valve Liquid + Vapor P 2 P 1 d e Saturated Liquid Line SUB-COOLED LIQUID d Condenser High Pressure Side Low Pressure Side Evaporator Expansion Valve e d e a Condenser Evaporator LIQUID + VAPOR H 1 Energy Output Energy Input Enthalpy (kj/kg) c Critical Point ~ 30 C ~ -15 C. Vapor Vapor a b Compressor c Saturated Vapor Line b Compressor a SUPERHEATED VAPOR H 2 H 3 b Energy Input Condensing: Constant Pr. (P 2 ) Expansion: Constant Enthalpy (H 1 ) Evaporation: Constant Pr. (P 1 ) Compression: Constant Entropy (S) Constant Temperature Line Left of dome: Vertical Within dome: Horizontal Right of dome: Curved down Ideal: Solid line Real/Non-ideal: Dotted line (Super-heating in evaporator, sub-cooling in condenser) 15

16 Functions of Components of a Vapor Compression Refrigeration System Evaporator Extract heat from the product/air and use it as the latent heat of vaporization of the refrigerant Compressor Raise temperature of refrigerant to well above that of surroundings to facilitate transfer of energy to surroundings in condenser Condenser Transfer energy from the refrigerant to the surroundings (air/water) Slightly sub-cool the refrigerant to minimize amount of vapor generated as it passes through the expansion valve Expansion valve Serve as metering device for flow of refrigerant Expand the liquid refrigerant from the compressor pressure to the evaporator pressure (with minimal conversion to vapor) 16

17 Evaporator Types: Plate (coil brazed onto plate) Flooded (coil) 17

18 Compressor Types: Positive disp. (piston, screw, scroll/spiral) Centrifugal 18

19 Condenser Types: Air-cooled, water-cooled, evaporative 19

20 Expansion Valve Types: Manual, automatic const. pr. (AXV), thermostatic (TXV) For nearly constant load, AXV is used; else, TXV is used 20

21 Vapor Compression Refrigeration System Condenser Evaporator (5 F) Expansion valve Compressor 21

22 Industrial Refrigeration System 22

23 Pressure-Enthalpy Diagram for R-12 Constant Pressure Line Horizontal Absolute Pressure (bar) Sub-Cooled Liquid Liquid-Vapor Mixture Superheated Vapor Specific Enthalpy (kj/kg) 23

24 Pressure-Enthalpy Diagram for R-12 Constant Enthalpy Line Vertical Absolute Pressure (bar) Sub-Cooled Liquid Liquid-Vapor Mixture Superheated Vapor Specific Enthalpy (kj/kg) 24

25 Pressure-Enthalpy Diagram for R-12 Constant Temperature Line Left of dome: Vertical Within dome: Horizontal Right of dome: Curved down Absolute Pressure (bar) Sub-Cooled Liquid Liquid-Vapor Mixture Superheated Vapor Specific Enthalpy (kj/kg) 25

26 Pressure-Enthalpy Diagram for R-12 Constant Entropy Line ~60 angled line: North-Northeast (superheated region) Absolute Pressure (bar) Sub-Cooled Liquid Liquid-Vapor Mixture Superheated Vapor Specific Enthalpy (kj/kg) 26

27 Pressure-Enthalpy Diagram for R-12 Constant Dryness Fraction Curved (within dome) Absolute Pressure (bar) Sub-Cooled Liquid Liquid-Vapor Mixture Superheated Vapor Dryness fraction (similar concept as steam quality) ranges from 0 on Saturated Liquid Line to 1 on Saturated Vapor Line Specific Enthalpy (kj/kg) 27

28 Pressure-Enthalpy Diagram for R-12 Lines of Constant Values for Various Parameters Absolute Pressure (bar) Sub-Cooled Liquid Liquid-Vapor Mixture Superheated Vapor Const. Pressure Const. Enthalpy Const. Temp. Const. Entropy Const. Dryness Fraction Specific Enthalpy (kj/kg) 28

Pressure-Enthalpy Table for R-12 P-H Diagram for Ideal Conditions e H 1 = h f based on temperature at d (exit of condenser) H 2 = h g based on temperature at a (exit of evaporator) Note 1: If there

29 Pressure-Enthalpy Table for R-12 P-H Diagram for Ideal Conditions e H 1 = h f based on temperature at d (exit of condenser) H 2 = h g based on temperature at a (exit of evaporator) Note 1: If there is super-heating in the evaporator, H 2 can not be obtained from P-H table Note 2: If there is sub-cooling in the condenser, H 1 can not be obtained from P-H table Note 3: For ideal or non-ideal conditions, H 3 can not be obtained from P-H table (For the above 3 conditions, use the P-H Diagram to determine the enthalpy value) 29

30 P-H Diagram for Superheated R-12 Saturated Vapor Line Liquid + Vapor Mixture Superheated Vapor Constant Entropy Line 30

31 Pressure-Enthalpy Diagram for R-12 Ideal Conditions Condenser Pressure Absolute Pressure (bar) Expansion Condensation Evaporation Evaporator Pressure Compression Specific Enthalpy (kj/kg) 31

32 Pressure-Enthalpy Diagram for R-12 Degree of sub-cooling Real/Non-Ideal Conditions Conditions (Determination of Enthalpies) Absolute Pressure (bar) Condenser Pressure Animated Slide Condensation (See next Expansion slide for static Compression version of slide) Evaporation Evaporator Pressure Degree of super-heating. Q e = m. (H 2 H 1 ) Q w = m (H 3 H 2 ). Q c = m (H 3 H 1 ) Note: Q c = Q e + Q w H 1 H 2 H 3 C.O.P. = Q e /Q w = (H 2 H 1 )/(H 3 H 2 ) Specific Enthalpy (kj/kg) 32

33 Pressure-Enthalpy Diagram for R-12 Degree of sub-cooling Real/Non-Ideal Conditions Conditions Absolute Pressure (bar) Expansion Condenser Pressure Condensation Evaporation Evaporator Pressure Compression Degree of super-heating. Q e = m (H 2 H 1 ). Q w = m (H 3 H 2 ). Q c = m (H 3 H 1 ) Note: Q c = Q e + Q w H 1 H 2 H 3 C.O.P. = Q e /Q w = (H 2 H 1 )/(H 3 H 2 ) Specific Enthalpy (kj/kg) 33

34 Processes undergone by Refrigerant Evaporation Constant pressure process Liquid + Vapor => Vapor Compression Constant entropy process Vapor => Vapor Condensation Constant pressure process Vapor => Liquid Expansion Constant enthalpy process (adiabatic process; Q transfer = 0) Liquid => Liquid + Vapor 34

35 P, T, H, and Phase changes in a Vapor Compression Refrigeration Cycle Ideal Conditions Component Pressure Temperature Enthalpy Phase of Refrigerant Inlet Outlet Evaporator Constant Constant Increases Liquid + Vapor Vapor (On Dome) Compressor Increases Increases Increases Vapor (On Dome) Vapor (Sup. Heat) Condenser Constant Decreases Decreases Vapor (Sup. Heat) Liquid (On Dome) Expansion Valve Decreases Decreases Constant Liquid (On Dome) Liquid + Vapor Real Conditions (Super-heating in Evaporator, Sub-cooling in condenser) Component Pressure Temperature Enthalpy Phase of Refrigerant Inlet Outlet Evaporator Constant Increases Increases Liquid + Vapor Vapor (Sup. Heat) Compressor Increases Increases Increases Vapor (Sup. Heat) Vapor (Sup. Heat) Condenser Constant Decreases Decreases Vapor (Sup. Heat) Liquid (Sub-Cool) Expansion Valve Decreases Decreases Constant Liquid (Sub-Cool) Liquid + Vapor 35

36 Vapor Compression Refrigeration System Q c Liquid Expansion Valve Liquid + Vapor P 2 P 1 d e Saturated Liquid Line SUB-COOLED LIQUID Condenser High Pressure Side Low Pressure Side Evaporator d Expansion Valve e Q e c a Condenser Evaporator LIQUID + VAPOR Critical Point ~ 30 C ~ -15 C. Vapor Vapor a b Compressor c Saturated Vapor Line b Compressor SUPERHEATED VAPOR Q w Calculations. Q e = m (H 2 H 1 ). Q w = m (H 3 H 2 ). Q c = m (H 3 H 1 ) Note: Q c = Q e + Q w (Energy gained by refrigerant in evaporator & compressor is lost in condenser) C.O.P. = Q e /Q w = (H 2 H 1 )/(H 3 H 2 ) Q e : Cooling load rate (kw) Q w : Work done by compressor (kw) C.O.P.: Coefficient of performance H 1 Enthalpy (kj/kg) H 2 H 3 36

37 Cooling Load Rate (Q e ) Useful cooling effect takes place in evaporator Units of Q e : kw or tons 1 ton refrigerant = Power required to melt 1 ton (2000 lbs) of ice in 1 day = (2000* kg) ( x 10 3 J/kg) / (24 x 60 x 60 s) (2000 lb/ton)*( kg/lb) λ fusion (ice) (24 hr/day)*(60 min/hr)*(60 s/min) = Watts 37

38 Household Refrigerator HEAT Extracted from food inside Are there 2 vapor compression systems to maintain refrigerator and freezer at different temperatures? Extracted HEAT Moved to the outside 38

Household Refrigerator as Room AC? Saturated Liquid Line Critical Point. Saturated Vapor Line P 2 P 1 SUB-COOLED LIQUID d Expansion Valve e Condenser Evaporator LIQUID + VAPOR H 1 Enthalpy (kj/kg).

39 Household Refrigerator as Room AC? Saturated Liquid Line Critical Point. Saturated Vapor Line P 2 P 1 SUB-COOLED LIQUID d Expansion Valve e Condenser Evaporator LIQUID + VAPOR H 1 Enthalpy (kj/kg). ~ 30 C ~ -15 C Q e = m (H 2 H 1 ). Q w = m (H 3 H 2 ). Q c = m (H 3 H 1 ) a c b Compressor SUPERHEATED VAPOR H 2 H 3 Can you cool the kitchen by keeping the refrigerator door open? If you leave the refrigerator door open, Q e will be the energy the system will remove from the room/air and Q c will be the energy the system will release into the room/air. Since Q c > Q e, the room will actually heat up by an amount, Q = Q c Q e = Q w (Q w = power from AC mains) instead of cooling down. 39

Principle Thermoelectric Cooling Peltier effect (converse of

of two dissimilar metals, a current flows through it, and heat

Can be used for heating too Cooled Surface Dissipated Heat USB

Thermocouples): When two dissimilar metals are joined in a

potential difference is created between the ends, and a

40 Principle Thermoelectric Cooling Peltier effect (converse of Seebeck effect) When a voltage is applied across the junctions of two dissimilar metals, a current flows through it, and heat is absorbed at one end and heat is generated at the other end Can be used for heating too Cooled Surface Dissipated Heat USB adapter Cigarette lighter adapter Seebeck effect (in Thermocouples): When two dissimilar metals are joined in a loop and their junctions are kept at different temperatures, a potential difference is created between the ends, and a current flows through the loop. This can be used to generate energy from waste heat. 40

41 Heat Pump (Heating Cycle in Winter) Q: When does the heat pump become ineffective in heating the house? A: When the outside temp. becomes so low that not much transfer of energy can take place from outside air to the refrigerant in the evaporator (Q = h A T; if T between outside air and refrigerant in evaporator is low, Q is low) Evaporator 85 F Duct 32 F 5 F Expansion Valve 190 F Heat loss Q c Q e Q w 65 F Condenser Compressor 41

42 Heat Pump (Cooling Cycle in Summer) Q: When does the heat pump become ineffective in cooling the house? A: When the outside temp. becomes so high that not much transfer of energy can take place from the refrigerant in the condenser to outside air (Q = h A T; if T between refrigerant in condenser and outside air is low, Q is low) Condenser 65 F Duct 100 F 190 F Expansion Valve 5 F Heat gain Q e Q c Q w 85 F Evaporator Compressor 42

43 Designation and Choice of Refrigerants Designation of a refrigerant derived from a hydrocarbon C m H n F p Cl q is R(m-1)(n+1)(p) Choices of refrigerants R-11 (CCl 3 F), R-12 (CCl 2 F 2 ), R-13 (CClF 3 ), R-14 (CF 4 ), R-22 (CHClF 2 ), R-30 (CH 2 Cl 2 ), R-113 (C 2 Cl 3 F 3 ), R-114, R-115, R-116, R-123, R-134a (CF 3 CH 2 F), R-401A, R- 404A, R-408A, R-409A, R-500, R-502, R-717 (NH 3 ), R- 718 (water), R-729 (air), R-744 (CO 2 ), R-764 (SO 2 ) Suffix: a, b, c indicate increasingly unsymmetric isomers R-400 Series: Zeotropic blends (Boiling point of constituent compounds are quite different) R-500 Series: Azeotropic blends 43

44 Criteria for Selection of Refrigerant High latent heat of vaporization High critical temperature High chemical stability High miscibility with lubricant (except when oil separator is used) Low vaporization temperature Low condensing pressure Low freezing temperature Low toxicity Low flammability Low corrosiveness Low cost Low environmental impact (ozone depletion potential, global warming potential) Easy to detect leaks Easy separability from water 44

45 Characteristics of Refrigerants NH 3 R-12 R-22 R-134a λ vap at -15 C (kj/kg) Boiling point at 1 atm ( C) Freezing point at 1 atm ( C) Compression ratio (-15 to 30 C) Flammability Yes No No At high pr. Pr. to inc. b.p. to 0 C (kpa) Corrosiveness Use steel Not Cu Low No Low Toxicity High No No No Environmental impact (Ozone Depletion Potential, Global Warming Potential) ODP: 0 GWP: 0 ODP: 1 GWP: 8100 ODP: 0.05 GWP: 1700 ODP: 0 GWP:

46 Alternative to Vapor Compression Refrigeration Absorption refrigeration Evaporation Same as in vapor compression refrigeration system Absorption Refrigerant dissolves in absorbent (eg. NH 3 in H 2 O with H 2 for pr.) Regeneration Separation of refrigerant by heat No compressor (no moving parts), no power needed Used where electricity is expensive, unavailable or unreliable (rural areas, recreational vehicles) Variation: Water spray absorption refrigeration system 46

47 Water Cooled Condenser A water cooled condenser is a double tube heat exchanger (co- or counter-current) with the refrigerant in the inside tube and cold water in the outer annulus It is used when Temperature of refrigerant in condenser is not much higher than the ambient air temperature (In this case, refrigerant can not lose much energy to outside air) OR Additional cooling of refrigerant is desired (beyond cooling capacity of ambient air) Q condenser ) = m. refrigerant (H3 H 1) = m. cold watercp(cold water) (Tcold(out) Tcold(in ) 47

48 Heat Transfer in Refrigeration Applications What should be the rating of a room AC unit to maintain room at 20 C when it is 45 C outside? Q e = T/[( x 1 /k 1 A) + ( x 2 /k 2 A)+(1/h i A i )+(1/(h o A o )+..] 45 C 20 C What should be the rating of a refrigeration system to cool a product from 70 C to 20 C when it is flowing at a certain rate in a double tube heat exchanger? = Q = m (H H ) = m T Q evaporator e refrigerant 2 1 product p(product) product(in) product(out) OR Q e = U A lm T lm with 1/(UA lm ) = 1/(h i A i ) + r/(ka lm ) + 1/(h o A o ) How long will it take to cool an object of mass m from an initial temperature of T i to a final temperature of T f? Q e = {m c p ( T)}/{Time} with T = T i - T f c (T 70 C 20 C ) 48

49 Summary: Vapor Compression Refrig. System Liquid Expansion Valve Liquid + Vapor Condenser Evaporator Ideal: Solid line Q e Real/Non-ideal: Dotted line (Sup. heat in evap., sub-cool in cond.) P 2 P 1 d e Saturated Liquid Line Degree of sub-cooling SUB-COOLED LIQUID d High Pressure Side Low Pressure Side Expansion Valve e d e Q c c a Condenser Evaporator LIQUID + VAPOR Critical Point ~ 30 C ~ -15 C. Vapor Vapor a b Compressor c Saturated Vapor Line b Compressor a b SUPERHEATED VAPOR Condensing: Constant Pr. (P 2 ) Expansion: Constant Enthalpy (H 1 ) Evaporation: Constant Pr. (P 1 ) Compression: Constant Entropy (S) Q w. Q e = m (H 2 H 1 ). Q w = m (H 3 H 2 ). Q c = m (H 3 H 1 ) Note: Q c = Q e + Q w From P-H Table (For Ideal Conditions) H 1 = h f based on temp. at d (exit of cond.) H 2 = h g based on temp. at a (exit of evap.) C.O.P. = Q e /Q w = (H 2 H 1 )/(H 3 H 2 ) Q e : Cooling load rate (kw) Q w : Work done by compressor (kw) C.O.P.: Coefficient of performance Degree of superheating H 1 Enthalpy (kj/kg) H 2 H 3 49

50 How, Will, Why, What, When, and Where? How are we able to maintain different temperatures in the freezer and refrigerator compartments if you have only 1 refrigeration system? Will a regular refrigerator work well in the garage During winter? During summer? Why does the temperature change when you turn the knob of the AC unit in a car or room? What happens when the heat pump is set to Emergency/Auxiliary Heat? When/why does ice build up on the outdoor coils (evaporator) of a heat pump during heating in winter? Dehumidification occurs on heating or cooling? Why? Where and in what state is the refrigerant when the compressor is not running? 50

FS 231: Final Exam (5-6-05) Part A (Closed Book): 60 points

Name: Start time: End time: FS 231: Final Exam (5-6-05) Part A (Closed Book): 60 points 1. What are the units of the following quantities? (10 points) a. Enthalpy of a refrigerant b. Dryness fraction of