refrigeration - nc state university · pdf file• refrigerator, air conditioner,...
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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
Expansion Valve Compressor
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
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
Expansion Valve Compressor
SUB-COOLEDLIQUID
SUPERHEATEDVAPORLIQUID + VAPOR
Enthalpy (kJ/kg)
Critical Point
Saturated Vapor LineSaturated Liquid Line
~ 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)
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
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
Superheated Vapor Liquid-Vapor Mixture
Specific Enthalpy (kJ/kg)
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
Superheated Vapor Liquid-Vapor Mixture
Specific Enthalpy (kJ/kg)
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
Superheated Vapor Liquid-Vapor Mixture
Specific Enthalpy (kJ/kg)
Abso
lute
Pre
ssur
e (b
ar)
26
Pressure-Enthalpy Diagram for R-12 Constant Dryness Fraction Curved (within dome)
Sub-Cooled Liquid
Superheated Vapor Liquid-Vapor Mixture
Dryness fraction (similar concept as steam quality) ranges from 0 on Saturated Liquid Line to 1 on Saturated Vapor Line
Specific Enthalpy (kJ/kg)
Abso
lute
Pre
ssur
e (b
ar)
27
Pressure-Enthalpy Diagram for R-12 Lines of Constant Values for Various Parameters
Sub-Cooled Liquid
Superheated Vapor Liquid-Vapor Mixture
Const. Pressure Const. Enthalpy Const. Temp. Const. Entropy Const. Dryness Fraction
Specific Enthalpy (kJ/kg)
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
Specific Enthalpy (kJ/kg)
Abso
lute
Pre
ssur
e (b
ar)
31
Pressure-Enthalpy Diagram for R-12
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)
. . .
Specific Enthalpy (kJ/kg)
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
Pressure-Enthalpy Diagram for R-12
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)
. . .
Specific Enthalpy (kJ/kg)
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
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
Expansion Valve Compressor
SUB-COOLEDLIQUID
SUPERHEATEDVAPORLIQUID + VAPOR
Enthalpy (kJ/kg)
Critical Point
Saturated Vapor LineSaturated Liquid Line
~ 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
Expansion Valve Compressor
SUB-COOLEDLIQUID
SUPERHEATEDVAPORLIQUID + VAPOR
Enthalpy (kJ/kg)
Critical Point
Saturated Vapor LineSaturated Liquid Line
~ 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.)
Condensing: Constant Pr. (P2) Expansion: Constant Enthalpy (H1) Evaporation: Constant Pr. (P1) Compression: Constant Entropy (S)
Condenser
Expansion Valve Compressor
Evaporator
Liquid Vapor
VaporLiquid + Vapor
High Pressure Side
Low Pressure Side
a
b
d
e
Condenser
Evaporator
Expansion ValveCompressor
SUB-COOLEDLIQUID
SUPERHEATEDVAPORLIQUID + VAPOR
Enthalpy (kJ/kg)
Critical Point
Saturated Vapor LineSaturated Liquid Line
~ 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