thermo3 topic5 final 2011 without deletions print

MECE09010 TOPIC 5 1

Engineering Thermodynamics 3

Topic 5

RefrigerationHeat pumps

MECE09010 TOPIC 5 2

The performance of refrigerators and heat pumps is expressed in terms of coefficient of performance (COP)

COP QW

COP QW

RL

net in

HPH

net in

Desired outputRequired input

Cooling effectWork input


Heating effectWork input

,

,

Both COPR and COPHP can be larger than 1. Under the same operating conditions, the COPs are related by

COP COPHP R 1

Slide based on http://highered.mcgraw-hill.com/sites/007352932x/student_view0/chapter11/student_study_guide_ppts.html

MECE09010 TOPIC 5 3

The ideal gas Carnot cycle may be reversed, in which case it operates as a refrigerator.But the Carnot cycle is almost impossible to implement as with a gas as the working fluid it has to combines compression and expansion with heat transfer to get isothermal processes.

But the Carnot refrigeration cycle is an example of what a reversible (i.e. the most efficient) refrigerator or heat pump could do!

P-v diagram for Carnot heat engine P-v diagram for Carnot refrigerator

Figures from http://highered.mcgraw-hill.com/sites/007352932x/student_view0/chapter03/student_study_guide_ppts.html

|QL | /QH = TL / TH

Gas Carnot refrigeration systemP

MECE09010 TOPIC 5 4

Practical Gas Refrigeration Systems The power cycles can be used as refrigeration cycles by simply reversing them. Of these, the reversed Brayton cycle, which is also known as the gas refrigeration cycle, is used to cool aircraft and to obtain very low (cryogenic) temperatures after it is modified with regeneration. The work output of the turbine can be used to reduce the work input requirements to the compressor. Thus, the COP of a gas refrigeration cycle is:

COP qw

qw wR

L

net in

L

comp in turb out

, , ,


actual expansion

lines

Work into drivethe refrigerator

MECE09010 TOPIC 5 5

Reversed Carnot Refrigerator and Heat Pump using a condensable fluidShown below are a cyclic refrigeration device operating between two constant temperature reservoirs and the T-s diagram for the working fluid when a reversed Carnot cycle is used. In the Carnot cycle heat transfers take place at constant temperature and during compression and expansion processes. For a condensable fluid these isothermal processes are also constant pressure processes, with compression and expansion due only to the phase change. Phase changes also occur during the adiabatic compression and expansion processes.


MECE09010 TOPIC 5 6

The Vapour-Compression Refrigeration CycleThe vapour-compression refrigeration cycle has four components: evaporator, compressor, condenser, and expansion (or throttle) valve. The most widely used refrigeration cycle is the vapour-compression refriger-ation cycle. In an ideal vapour-compression refrigeration cycle, the refrigerant enters the compressor as a saturated vapour and is cooled to the saturated liquid state in the condenser. It is then throttled to the evaporator pressure and vapourises as it absorbs heat from the refrigerated space.The ideal vapour-compression cycle consists of four processes.

Ideal Vapour-Compression Refrigeration Cycle Process Description 1-2 Isentropic compression 2-3 Constant pressure heat rejection in the condenser3-4 Throttling in an expansion valve4-1 Constant pressure heat addition in the evaporator

Advantages over the reversed Carnot refrigeration cycle Easier to compress vapour only and not liquid-vapor mixture. Cheaper to have irreversible expansion through an expansion valve. Hard to get an efficient small expander and use the work from it.


MECE09010 TOPIC 5 7

Ideal vapour-compression cycle


MECE09010 TOPIC 5 8

The P-h diagram is another convenient diagram often used to illustrate the vapour compression refrigeration cycle.


MECE09010 TOPIC 5 9

The ordinary household refrigerator is a good example of the application of the vapour compression cycle.

COP QW

h hh h

COP QW

h hh h

RL

net in

HPH

net in

,

,

1 4

2 1

2 3

2 1


MECE09010 TOPIC 5 10

Actual Vapor-Compression Refrigeration Cycle



Vapour Compression Heat Pump Systems



Cascade refrigeration systems Very low temperatures can be achieved by operating two or more vapor-compression systems in series, called cascading. The COP of a refrigeration system also increases as a result of cascading.



Multistage compression refrigeration systems



Multipurpose refrigeration systems A refrigerator with a single compressor can provide refrigeration at several temperatures by throttling the refrigerant in stages.



COP QQ W

QQR

L

gen pump in

L

gen


Cooling effectWork input ,

Absorption Refrigeration SystemsAnother form of refrigeration that becomes economically attractive when there is a source of inexpensive heat energy at a temperature of 100 to 200oC is absorption refrigeration, where the refrigerant is absorbed by a transport medium and compressed in liquid form. The most widely used absorption refrigeration system is the ammonia-water system, where ammonia serves as the refrigerant and water as the transport medium. The work input to the pump is usually very small, and the COP of absorption refrigeration systems is defined as:Slide based on http://highered.mcgraw-hill.com/sites/007352932x/student_view0/chapter11/student_study_guide_ppts.html


Thermoelectric Refrigeration Systems A refrigeration effect can also be achieved without using any moving parts by simply passing a small current through a closed circuit made up of two dissimilar materials. This effect is called the Peltier effect, and a refrig-erator that works on this principle is called a thermoelectric refrigerator.



ExampleRefrigerant-134a is the working fluid in an ideal compression refrigeration cycle. The refrigerant leaves the evaporator at -20oC and has a condenser pressure of 10 bar (0.1 Mpa). The mass flow rate is 5 kg/min. Find COPR and COPR, Carnot for the same Tmax and Tmin .

http://www2.dupont.com/Refrigerants/en_US/products/Suva/Suva134a.html

h4 255.6kJkg

h4 h3hg4 386.8kJkg

hf4 173.7kJkg

saturated liquid values

h3 255.6kJkg

T3 39.35 273.15( ) K

h2 428.954kJkg

h2 ha hb ha s1 sa

sb sa

T2 273.15K 47.959KT2 Ta Tb Ta s1 sa

sb sa

hb 431.2kJkg

sb 1.7491kJ

kg KTb 50 273.15( ) K

ha 425.7kJkg

sa 1.7322kJ

kg KTa 45 273.15( ) K

h1 386.8kJkg

s1 1.7422kJ

kg KT1 20 273.15( ) K x4

h4 hf4

hg4 hf4 x4 0.384

w12 h2 h1 w12 42.154kJkg

q41 h1 h4 q41 131.2kJkg

COPRq41w12

COPR 3.112

COPR_revT1

T2 T1 COPR_rev 3.725

the bulk of the heat is rejected at T3

T1T3 T1

4.265


1

23

4


Properties of some common refrigerants

based on http://www.engineeringtoolbox.com/refrigerants-properties-d_145.html

NotesODP - The ODP or Ozone Depletion Potential. The potential for a single molecule of the refrigerant to destroy the Ozone Layer. All refrigerants use R11 as a datum reference where R11 has an ODP = 1.0. The less the value of the ODP - the better the refrigerant is for the ozone layer and the environment.

GWP - The GWP, or Global Warming Potential. A measurement (usually measured over a 100-year period) of how much effect a refrigerant will have on Global Warming in relation to Carbon Dioxide. CO2 has a GWP = 1. The lower the value of GWP - the better the refrigerant is for the environment.

Currently there are no restrictions on the use of R134A, R407C, R410A, and R417A.


Properties of some common refrigerantsbased on http://www.engineeringtoolbox.com/refrigerants-properties-d_145.html

Refrigerant Formula Boiling temp. (oC)

Critical temp. (oC)

Properties Applications

Ammonia NH3 -33 133 Ammonia -An efficient refrigerant used successfully in industrial applications through many Highly toxic. Careful consideration must be given to any design or application. Penetrating odour, soluble in water. harmless in concentration up to 1/30%, non flammable, explosive

Large industrial plants

R11 CCl3F 8.9 198 R11 is a single chlorofluorocarbon or CFC compound. High chlorine content and Ozone Depletion Potential; ODP = 1. High Global Warming Potential; GWP = 4000 The use and manufacture of R11 and similar CFC refrigerants is now banned within the European Union even for servicing. Non flammable, non corrosive non toxic, stable

Commercial plants with centrifugal compressors.

R12 Dichlorodi-omethane

CCl2F2 -29.8 112 Little odour, colourless gas or liquid, non flammable, non corrosive of ordinary metals, stable

Small plants with reciprocating compressors. Automotive, Medium Temperature Refrigeration

R22 Chlorodi-omethane

CHClF2 -40.8 96 R22 is a single hydrochlorofluorocarbon or HCFC compound. Low chlorine content and ozone depletion potential, ODP = 0.05 Modest global warming potential GWP = 1700. R22 can still be used in small heat pump systems, but new systems cannot be manufactured for use in the EU after 2003. From 2010 only recycled or saved stocks of R22 can be used. It will no longer be manufactured. Little odour, colourless as gas or liquid, non toxic, non irritating, non flammable, non corrosive, stable

Packaged air-conditioning units where size of equipment and economy are important. Air Conditioning, Low and Medium Temperature Refrigeration

R-134a 1,1,1,2-tetra-luoroethane

CH2FCF3 -26.0 101 R134a is a single hydrofluorocarbon or HFC compound. No chlorine content, no ozone depletion potential, ODP = 0; Modest global warming potential GWP = 1300

Automotive replacement for R-12, Stationary A/C, Medium Temp Refrigeration

R290 C3H8 -42.1 96.6 R290 is pure propane, a hydrocarbon and efficient naturally occurring refrigerant with similar properties to R22 No ozone depletion potential, ODP = 0. Extremely low global warming potential GWP = 3 Environmentally safe but highly flammable. Used only after careful consideration to safety.


http://www2.dupont.com/Refrigerants/en_US/products/Suva/Suva134a.html




Tutorial questions1. Refrigerant-134a is the working fluid in an ideal compression refrigeration cycle.

The refrigerant leaves the evaporator at -30oC and has a condenser pressure of 9 bar (900 kPa). The mass flow rate is 10 kg/min. Find COPR and COPR, Carnot for the same Tmax and Tmin , also the power input and the amount of heat removed.

[2.65 , 3.16 , 8.2kW , 21.8 kW]

2. A closed Brayton cycle refrigerator using air as the working fluid (similar to the cycle shown here) has a pressure ratio of 1.5. The cycle is pressurised, with a minimum air pressure of 10 bar. The air can be treated as an ideal gas with Cp=1.1 kJ/kg.K and R=0.287 kJ/kg.Ka) What is the maximum cold air temperature, just before the recuperator?

What is the COP for an ideal cycle (no pressure drops, 100% isentropic efficiencies, best possible performance in the regenerator) for a minimum air temperature in the cycle of -10C and a maximum air temperature of 100C?

[19.4C , 3.628]

b) What is the COP if there is a temperature difference of 10K in the recuperator, and the isentropic efficiencies of the turbine and compressor are 80% and 75% respectively? [0.512]


3. A heat pump using R134a takes heat from a thermal store at 15C to use for space heating (using hot water) in a house. The design heating load is to be 30 kW at 45C. There needs to be a 5K temperature difference between the boiling fluid and the lake and a minimum 10K difference between the condensing fluid and the hot water.

a) What is the best theoretical COP for this heat pump application, both without and with the required temperature differences? [10.6 , 7.3]

b) Draw the cycle on a p-h diagram. Select the nearest pressure from tables that will just exceed the required temperature in the condenser. What is the COP for an ideal Rankine refrigeration cycle with an expansion valve, the electric power input at 30 kW output and the peak fluid temperature in the cycle? [5.7 , 5.3kW , 59.6C]

c) If the compressor has an isentropic efficiency of 70% but all other components are ideal, what is the peak fluid temperature in the cycle and the COP of the heat pump? [69C , 4.3]

thermo3 topic5 final 2011 without deletions print

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