Refrigeration Outline • Purpose of refrigeration • Examples and applications • Choice of coolant and refrigerants

• Phase diagram of water and CO2 • 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 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 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 Cooling of Engine of Car

HOT Engine Head

Finned Radiator Coolant

High cp Low freezing pt.

Coolant Reservoir

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 Room (or Car) Air Conditioner

6 Household Refrigerator

HEAT Are there parts Extracted from in a refrigerator food inside where you can refrigerator get burnt?

Can you cool the kitchen by keeping Extracted HEAT the refrigerator door open? Moved to the outside 7 Evaporative (Swamp) Cooler

Water

8 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 (λfusion = 334.94 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 CO2) – Heat extracted from product is used as latent heat and sublimates dry ice (λsublimation = 571 kJ/kg at 1 atm, -78.5 °C) • Liquid nitrogen – Heat extracted from product is used as latent heat and evaporates liquid N2 (λvaporization = 199 kJ/kg at 1 atm, -195.8 °C)

Why does dry ice sublimate while “regular” ice melt under ambient conditions? 9 Phase Diagram

Water CO2

Solid Liquid Gas Solid Liquid Gas atm ) atm ) Melting point 1.0

Pressure ( Pressure Triple point ( Pressure Triple point Boiling point 0.006 5.1

1.0

0.01 100 -78.5 -56.6 Temperature (°C) Temperature (°C) 10 Drawback of Ice/Dry-Ice as Refrigerant

• Neither can be re-used – Ice melts – Dry-ice sublimates • Expensive and cumbersome technique

11 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 λvap of refrigerant is absorbed by the refrigerant from air)

Liquid Gaseous Refrigerant Refrigerant

Warm Ambient Air Fan

Can boiling/evaporation of water serve as a refrigeration method? 12 Re-Utilization of Refrigerant

High Pr. Liq. High Pr. Gas Condense the Gas

High Pr. Liq. High Pr. Gas Cooled Air (after λvap of refrigerant is Expand the Liquid absorbed by refrigerant from air) Compress the Gas Low Pr. Liq. Low Pr. Gas

Liquid Refrigerant Gaseous Refrigerant

Warm Ambient Air Fan

13 Vapor Compression Refrigeration System Energy Output d Liquid c Vapor Condenser IDEAL CONDITIONS

b High Pressure Side Expansion Valve Compressor Low Pressure Side Energy Input

Evaporator Condensing: Constant Pr. (P2) Liquid + Vapor e Vapor a Expansion: Constant Enthalpy (H1) Energy Input Evaporation: Constant Pr. (P1) Compression: Constant Entropy (S) Critical Point

Saturated Liquid Line . Saturated Vapor Line Constant Temperature Line Left of dome: Vertical d Condenser c b P 2 ~ 30 °C Within dome: Horizontal Expansion Valve Compressor Right of dome: Curved down ~ -15 °C P1 e Evaporator a IDEAL CONDITIONS SUB-COOLED LIQUID + VAPOR SUPERHEATED LIQUID VAPOR Refrigerant is 100% vapor at end of evap. AND

H1 H2 H3 Refrigerant is 100% liquid at end of condenser Enthalpy (kJ/kg) 14 Vapor Compression Refrigeration System Energy Output d Liquid c Vapor Condenser

b High Pressure Side Expansion Valve Compressor Low Pressure Side Energy Input

Evaporator Condensing: Constant Pr. (P2) Liquid + Vapor e Vapor a Expansion: Constant Enthalpy (H1) Energy Input Evaporation: Constant Pr. (P1) Compression: Constant Entropy (S) Critical Point

Saturated Liquid Line . Saturated Vapor Line Constant Temperature Line Left of dome: Vertical d’ d Condenser c b b’ P 2 ~ 30 °C Within dome: Horizontal Expansion Valve Compressor Right of dome: Curved down ~ -15 °C P1 e’ e Evaporator a a’ Ideal: Solid line SUB-COOLED Real/Non-ideal: Dotted line LIQUID + VAPOR SUPERHEATED LIQUID VAPOR (Super-heating in evaporator, H1 H2 H3 sub-cooling in condenser) Enthalpy (kJ/kg) 15 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 Evaporator

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

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

18 Condenser

Types: Air-cooled, water-cooled, evaporative

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

20 Vapor Compression Refrigeration System

Condenser

Evaporator (5 °F)

Expansion valve Compressor

21 Industrial Refrigeration System

22 Pressure-Enthalpy Diagram for R-12

Constant Pressure Line Horizontal

Sub-Cooled Liquid

Liquid-Vapor Mixture Superheated Vapor Absolute Pressure (bar) Pressure Absolute

Specific Enthalpy (kJ/kg) 23 Pressure-Enthalpy Diagram for R-12

Constant Enthalpy Line Vertical

Sub-Cooled Liquid

Liquid-Vapor Mixture Superheated Vapor Absolute Pressure (bar) Pressure Absolute

Specific Enthalpy (kJ/kg) 24 Pressure-Enthalpy Diagram for R-12

Constant Temperature Line Left of dome: Vertical Within dome: Horizontal Right of dome: Curved down

Sub-Cooled Liquid

Liquid-Vapor Mixture Superheated Vapor Absolute Pressure (bar) Pressure Absolute

Specific Enthalpy (kJ/kg) 25 Pressure-Enthalpy Diagram for R-12

Constant Entropy Line ~60 °angled line: North-Northeast (superheated region)

Sub-Cooled Liquid

Liquid-Vapor Mixture Superheated Vapor Absolute Pressure (bar) Pressure Absolute

Specific Enthalpy (kJ/kg) 26 Pressure-Enthalpy Diagram for R-12

Constant Dryness Fraction Curved (within dome)

Sub-Cooled Liquid

Liquid-Vapor Mixture Superheated Vapor Absolute Pressure (bar) Pressure Absolute

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 Pressure-Enthalpy Diagram for R-12

Lines of Constant Values for Various Parameters

Sub-Cooled Liquid

Liquid-Vapor Mixture Superheated Vapor Absolute Pressure (bar) Pressure Absolute

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

H1 = hf based on temperature at ‘d’ (exit of condenser) H2 = hg based on temperature at ‘a’ (exit of evaporator)

Note 1: If there is super-heating in the evaporator, H2 can not be obtained from P-H table Note 2: If there is sub-cooling in the condenser, H1 can not be obtained from P-H table Note 3: For ideal or non-ideal conditions, H3 can not be obtained from P-H table (For the above 3 conditions, use the P-H Diagram to determine the enthalpy value)

29 P-H Diagram for Superheated R-12

Saturated Vapor Line

Liquid + Vapor Mixture Superheated Vapor

Constant Entropy Line

30 Pressure-Enthalpy Diagram for R-12

Ideal Conditions

Condenser Pressure

Condensation Expansion Compression Evaporation

Evaporator Pressure Absolute Pressure (bar) Pressure Absolute

Specific Enthalpy (kJ/kg) 31 Pressure-Enthalpy Diagram for R-12

Real/NonIdeal- IdealConditions Conditions (Determination of Enthalpies) Degree of sub-cooling

Condenser Pressure

AnimatedCondensation Slide Expansion Compression (See next slideEvaporation for static version of slide) Evaporator Pressure . Qe = m. (H2 – H1) Qw = m. (H3 – H2) Absolute Pressure (bar) Pressure Absolute Degree of super-heating Qc = m (H3 – H1)

Note: Qc = Qe + Qw

C.O.P. = Qe/Qw H H = (H – H )/(H – H ) H1 2 3 2 1 3 2 Specific Enthalpy (kJ/kg) 32 Pressure-Enthalpy Diagram for R-12

Real/NonIdeal- IdealConditions Conditions Degree of sub-cooling

Condenser Pressure

Condensation Expansion Compression Evaporation . Evaporator Pressure Q = m (H – H ) e . 2 1 Qw = m. (H3 – H2) Absolute Pressure (bar) Pressure Absolute Degree of super-heating Qc = m (H3 – H1)

Note: Qc = Qe + Qw

C.O.P. = Qe/Qw

H1 H2 H3 = (H2 – H1)/(H3 – H2)

Specific Enthalpy (kJ/kg) 33 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; Qtransfer = 0) • Liquid => Liquid + Vapor

34 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 Vapor Compression Refrigeration System Qc d Liquid c Vapor Calculations Condenser . Q = m (H – H ) b e . 2 1 High Pressure Side Qw = m (H3 – H2) Expansion Valve Compressor . Low Pressure Side Qc = m (H3 – H1) Qw Note: Q = Q + Q Evaporator c e w Liquid + Vapor Vapor e a (Energy gained by refrigerant in evaporator & Q e compressor is lost in Critical Point condenser)

Saturated Liquid Line . Saturated Vapor Line C.O.P. = Qe/Qw = (H – H )/(H – H ) d Condenser c b 2 1 3 2 P 2 ~ 30 °C Expansion Valve Compressor Qe: Cooling load rate (kW) ~ -15 °C P1 Q : Work done by compressor (kW) e a w Evaporator C.O.P.: Coefficient of performance SUB-COOLED LIQUID + VAPOR SUPERHEATED LIQUID VAPOR

H1 H2 H3 Enthalpy (kJ/kg) 36 Cooling Load Rate (Qe)

• Useful cooling effect takes place in evaporator

• Units of Qe: kW or tons

• 1 ton refrigerant = Power required to melt 1 ton (2000 lbs) of ice in 1 day = (2000*0.45359 kg) (334.94 x 103 J/kg) / (24 x 60 x 60 s)

λ (24 hr/day)*(60 min/hr)*(60 s/min) (2000 lb/ton)*(0.45359 kg/lb) fusion (ice) = 3516.8 Watts 37 Household Refrigerator

HEAT Extracted from food inside

Are there 2 vapor compression systems to maintain Extracted HEAT refrigerator and freezer at different temperatures? Moved to the outside 38 Household Refrigerator as Room AC? Critical Point

Saturated Liquid Line . Saturated Vapor Line

d Condenser c b P 2 ~ 30 °C Expansion Valve Compressor ~ -15 °C P1 e Evaporator a SUB-COOLED LIQUID + VAPOR SUPERHEATED LIQUID VAPOR

H1 H2 H3 Enthalpy (kJ/kg) . Q = m (H – H ) e . 2 1 Qw = m. (H3 – H2) Qc = m (H3 – H1)

If you leave the refrigerator door open, Qe will be the energy the system will remove

from the room/air and Qc will be the energy the system will release into the room/air. Can you cool the kitchen by keeping Since Qc > Qe, the room will actually heat up by an amount, Q = Qc – Qe = Qw (Qw = power the refrigerator door open? from AC mains) instead of cooling down. 39 Thermoelectric Cooling • Principle – 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 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

Heat loss 32 °F Expansion 5 °F Valve 190 °F

Qc

Q 65 °F e Condenser Qw Compressor 41 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

Heat gain 100 °F Expansion 190 °F Valve 5 °F

Qe

Qc 85 °F Evaporator Qw Compressor 42 Designation and Choice of Refrigerants • Designation of a refrigerant derived from a hydrocarbon CmHnFpClq is R(m-1)(n+1)(p)

• Choices of refrigerants

– R-11 (CCl3F), R-12 (CCl2F2), R-13 (CClF3), R-14 (CF4), R-22 (CHClF2), R-30 (CH2Cl2), R-113 (C2Cl3F3), R-114, R-115, R-116, R-123, R-134a (CF3CH2F), R-401A, R- 404A, R-408A, R-409A, R-500, R-502, R-717 (NH3), R- 718 (water), R-729 (air), R-744 (CO2), R-764 (SO2)

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 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 Characteristics of Refrigerants

NH3 R-12 R-22 R-134a

λvap at -15 °C (kJ/kg) 1314.2 161.7 217.7 209.5 Boiling point at 1 atm (°C) -33.3 -29.8 -40.8 -26.16 Freezing point at 1 atm (°C) -77.8 -157.8 -160.0 -96.6 Compression ratio (-15 to 30 °C) 4.94 4.07 5.06 4.65 Flammability Yes No No At high pr. Pr. to inc. b.p. to 0 °C (kPa) 430.43 308.61 498.11 292.769 Corrosiveness Use steel Low No Low Not Cu Toxicity High No No No Environmental impact ODP: 0 ODP: 1 ODP: 0.05 ODP: 0 (Ozone Depletion Potential, Global Warming Potential) GWP: 0 GWP: 8100 GWP: 1700 GWP: 1300

45 Alternative to Vapor Compression Refrigeration • Absorption refrigeration – Evaporation • Same as in vapor compression refrigeration system – Absorption

• Refrigerant dissolves in absorbent (eg. NH3 in H2O with H2 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 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)

. . Qcondenser = mrefrigerant (H3 − H1) = mcold watercp(cold water) (Tcold(out) − Tcold(in) ) 47 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?

– Qe = ∆T/[(∆x1/k1A) + (∆x2/k2A)+(1/hiAi)+(1/(hoAo)+…..] 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? = =  − =  − Qevaporator Qe mrefrigerant (H2 H1) mproductcp(product) (Tproduct(in) Tproduct(out) ) 20 °C OR 70 °C

– Qe = U Alm ∆Tlm with 1/(UAlm) = 1/(hiAi) + ∆r/(kAlm) + 1/(hoAo) • How long will it take to cool an object of mass ‘m’ from an initial temperature of Ti to a final temperature of Tf? – Qe = {m cp (∆T)}/{Time} with ∆T = Ti - Tf

48 Summary: Vapor Compression Refrig. System

Qc Condensing: Constant Pr. (P2) d Liquid c Vapor Expansion: Constant Enthalpy (H1) Condenser Evaporation: Constant Pr. (P1) b Compression: Constant Entropy (S) High Pressure Side . Expansion Valve Compressor Q = m (H – H ) Low Pressure Side e 2 1 Qw . Qw = .m (H3 – H2) Q = m (H – H ) Evaporator c 3 1 Vapor Liquid + Vapor e a Note: Qc = Qe + Qw

Ideal: Solid line Qe From P-H Table (For Ideal Conditions) H = h based on temp. at ‘d’ (exit of cond.) Real/Non-ideal: Dotted line (Sup. Critical Point 1 f heat in evap., sub-cool in cond.) H2 = hg based on temp. at ‘a’ (exit of evap.) Saturated Liquid Line . Saturated Vapor Line C.O.P. = Q /Q Degree of e w sub-cooling = (H2 – H1)/(H3 – H2) d’ d Condenser c b b’ P Qe: Cooling load rate (kW) 2 ~ 30 °C Expansion Valve Q : Work done by compressor (kW) Compressor w ~ -15 °C C.O.P.: Coefficient of performance P1 e’ e Evaporator a a’ SUB-COOLED LIQUID + VAPOR SUPERHEATED Degree of superheating LIQUID VAPOR

H1 H2 H3 Enthalpy (kJ/kg) 49 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