refrigeration - nc state university · pdf file• refrigerator, air conditioner,...

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

Air flow from outside

Coolant Reservoir

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

Coolant High cp Low freezing pt.

5

Room (or Car) Air Conditioner

6

Household Refrigerator

HEAT

Extracted from food inside refrigerator

Extracted HEAT

Moved to the outside Can you cool the kitchen by keeping the refrigerator door open?

Are there parts in a refrigerator where you can get burnt?

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

Temperature (°C)

Pres

sure

(atm

)

0.006

1.0

0.01 100

Solid Liquid Gas

Melting point

Boiling point Triple point

Water

Temperature (°C)

Pres

sure

(atm

) 5.1

1.0

-56.6

Solid Liquid Gas

Triple point

CO2

-78.5

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”

Fan

Liquid Refrigerant

Gaseous Refrigerant

Warm Ambient Air

Cooled Air (After λvap of refrigerant is absorbed by the refrigerant from air)

Can boiling/evaporation of water serve as a refrigeration method? 12

Re-Utilization of Refrigerant

Fan

Liquid Refrigerant Gaseous Refrigerant

Warm Ambient Air

Cooled Air (after λvap of refrigerant is absorbed by refrigerant from air)

Low Pr. Gas

Compress the Gas

High Pr. Gas

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

Expand the Liquid

High Pr. Liq.

Low Pr. Liq.

13

Vapor Compression Refrigeration System Condenser

Expansion Valve Compressor

Evaporator

Liquid Vapor

VaporLiquid + Vapor

High Pressure Side

Low Pressure Side

a

b

d

e

Condenser

Evaporator


SUB-COOLEDLIQUID

SUPERHEATEDVAPORLIQUID + VAPOR

Enthalpy (kJ/kg)

Critical Point

Saturated Vapor LineSaturated Liquid Line

~ 30 °C

~ -15 °Ca

d bc

.

P2

P1

H2 H3H1

Energy Input

c

Energy Input

Energy Output

Condensing: Constant Pr. (P2) Expansion: Constant Enthalpy (H1) Evaporation: Constant Pr. (P1) 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

IDEAL CONDITIONS e

14



Evaporator

Liquid Vapor

VaporLiquid + Vapor

High Pressure Side

Low Pressure Side

a

b

d

e

Condenser

Evaporator


SUB-COOLEDLIQUID


Enthalpy (kJ/kg)

Critical Point


~ 30 °C

~ -15 °C

e a

d bc

.

P2

P1

H2 H3H1

c

d’

e’

b’

a’

Energy Input

Energy Input

Energy Output

Ideal: Solid line Real/Non-ideal: Dotted line (Super-heating in evaporator, sub-cooling in condenser)


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

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

Evaporator (5 °F)

Condenser

Compressor

Expansion valve

21

Industrial Refrigeration System

22

Pressure-Enthalpy Diagram for R-12 Constant Pressure Line Horizontal

Sub-Cooled Liquid

Superheated Vapor Liquid-Vapor Mixture

Specific Enthalpy (kJ/kg)

Abso

lute

Pre

ssur

e (b

ar)

23

Pressure-Enthalpy Diagram for R-12 Constant Enthalpy Line Vertical

Sub-Cooled Liquid



Abso

lute

Pre

ssur

e (b

ar)

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



Abso

lute

Pre

ssur

e (b

ar)

25

Pressure-Enthalpy Diagram for R-12 Constant Entropy Line ~60 °angled line: North-Northeast (superheated region)

Sub-Cooled Liquid



Abso

lute

Pre

ssur

e (b

ar)

26

Pressure-Enthalpy Diagram for R-12 Constant Dryness Fraction Curved (within dome)

Sub-Cooled Liquid


Dryness fraction (similar concept as steam quality) ranges from 0 on Saturated Liquid Line to 1 on Saturated Vapor Line


Abso

lute

Pre

ssur

e (b

ar)

27

Pressure-Enthalpy Diagram for R-12 Lines of Constant Values for Various Parameters

Sub-Cooled Liquid


Const. Pressure Const. Enthalpy Const. Temp. Const. Entropy Const. Dryness Fraction


Abso

lute

Pre

ssur

e (b

ar)

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

Superheated Vapor Liquid + Vapor Mixture

Constant Entropy Line

30

Pressure-Enthalpy Diagram for R-12

Condenser Pressure

Condensation Expansion

Evaporation

Evaporator Pressure

Compression

Ideal Conditions


Abso

lute

Pre

ssur

e (b

ar)

31


Evaporation

Evaporator Pressure

Condensation

Ideal Conditions

Condenser Pressure

Compression

H2 H3

Expansion

H1

Real/Non-Ideal Conditions Ideal Conditions

Qe = m (H2 – H1) Qw = m (H3 – H2) Qc = m (H3 – H1) Note: Qc = Qe + Qw C.O.P. = Qe/Qw = (H2 – H1)/(H3 – H2)

. . .


Abso

lute

Pre

ssur

e (b

ar)

Degree of super-heating

Degree of sub-cooling (Determination of Enthalpies)

Animated Slide (See next slide for static version of slide)

32


Evaporation

Evaporator Pressure

Condensation

Ideal Conditions

Condenser Pressure

Compression

H2 H3

Expansion

H1

Real/Non-Ideal Conditions

Qe = m (H2 – H1) Qw = m (H3 – H2) Qc = m (H3 – H1) Note: Qc = Qe + Qw C.O.P. = Qe/Qw = (H2 – H1)/(H3 – H2)

. . .


Abso

lute

Pre

ssur

e (b

ar)

Degree of sub-cooling

Degree of super-heating

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

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

Ideal Conditions

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



Evaporator

Liquid Vapor

VaporLiquid + Vapor

High Pressure Side

Low Pressure Side

a

b

d

e

Condenser

Evaporator


SUB-COOLEDLIQUID


Enthalpy (kJ/kg)

Critical Point


~ 30 °C

~ -15 °C

e a

bc

.

P2

P1

H2 H3H1

Qc

Qe

Qw

c

d

Qe = m (H2 – H1) Qw = m (H3 – H2) Qc = m (H3 – H1) Note: Qc = Qe + Qw (Energy gained by refrigerant in evaporator & compressor is lost in condenser) C.O.P. = Qe/Qw = (H2 – H1)/(H3 – H2)

Qe: Cooling load rate (kW) Qw: Work done by compressor (kW) C.O.P.: Coefficient of performance

. . .

Calculations

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) = 3516.8 Watts

λfusion (ice) (24 hr/day)*(60 min/hr)*(60 s/min) (2000 lb/ton)*(0.45359 kg/lb)

37

Household Refrigerator

HEAT

Extracted from food inside

Extracted HEAT

Moved to the outside Are there 2 vapor compression systems to maintain refrigerator and freezer at different temperatures?

38

Household Refrigerator as Room AC?

Can you cool the kitchen by keeping the refrigerator door open?

Condenser

Evaporator


SUB-COOLEDLIQUID


Enthalpy (kJ/kg)

Critical Point


~ 30 °C

~ -15 °C

e a

bc

.

P2

P1

H2 H3H1

d

Qe = m (H2 – H1) 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. Since Qc > Qe, the room will actually heat up by an amount, Q = Qc – Qe = Qw (Qw = power 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

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.

Cigarette lighter adapter USB adapter

Cooled Surface

Dissipated Heat

40

Heat Pump (Heating Cycle in Winter)

Condenser

Evaporator Duct

Qe

Qc

Qw

Compressor

Expansion Valve

65 °F

85 °F

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)

5 °F 32 °F

190 °F

Heat loss

41

Heat Pump (Cooling Cycle in Summer)

Evaporator

Condenser Duct

Qc

Qe

Qw

Compressor

Expansion Valve

85 °F

65 °F

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)

190 °F 100 °F

5 °F

Heat gain

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

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: 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)

)TT(cm)HH(mQ )in(cold)out(cold)watercold(pwatercold13trefrigerancondenser −=−=. .

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)+…..]

• 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? OR – 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

)TT(cm)HH(mQQ )out(product)in(product)product(pproduct12trefrigeraneevaporator −=−== 70 °C 20 °C

45 °C – 20 °C

48

Summary: Vapor Compression Refrig. System

Qe: Cooling load rate (kW) Qw: Work done by compressor (kW) C.O.P.: Coefficient of performance

Qe = m (H2 – H1) Qw = m (H3 – H2) Qc = m (H3 – H1) Note: Qc = Qe + Qw

. . .

Ideal: Solid line Real/Non-ideal: Dotted line (Sup. heat in evap., sub-cool in cond.)


Condenser


Evaporator

Liquid Vapor

VaporLiquid + Vapor

High Pressure Side

Low Pressure Side

a

b

d

e

Condenser

Evaporator

Expansion ValveCompressor

SUB-COOLEDLIQUID


Enthalpy (kJ/kg)

Critical Point


~ 30 °C

~ -15 °C

e a

d bc

.

P2

P1

H2 H3H1

Qc

Qe

Qw

c

d’

e’

b’

a’

H1 = hf based on temp. at ‘d’ (exit of cond.) H2 = hg based on temp. at ‘a’ (exit of evap.)

C.O.P. = Qe/Qw = (H2 – H1)/(H3 – H2)

From P-H Table (For Ideal Conditions)

Degree of superheating

Degree of sub-cooling

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

refrigeration - nc state university · pdf file• refrigerator, air conditioner,...

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