vapor compression refrigerator(cycle j-t principle) 2009

8/4/2019 Vapor Compression Refrigerator(Cycle J-T Principle) 2009

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Unexpected cooling effect by M. Faraday (1823)

(28:00 of 1st) !

High pressure gas

Liquefied at room temperature

Evaporation by low pressure

Cooling effect by evaporation of liquid


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Usual method to obtain low temperature Throttling process !

J-T coefficient > 0 forcooling effect

JTh

T

P


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JT = (T/p)h= - (T/h)p(h/p)Tif JT = (T/p)h > 0 cooling

JT = (T/p)h < 0 heatingEnthalpy, h = u + pvJT = (T/p)h= - (T/h)p(h/p)T= -1/Cp{(u/p)T + [(pv)/p]T}

Joule-Thomson effect


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Enthalpy, h = u + pvJT = (T/p)h= - (T/h)p(h/p)T= -1/Cp{(u/p)T + [(pv)/p]T}

(u/p)T : departure from Joules law, which states that the internalenergy of an ideal gas is f(T) only.always negative for real gases.relation with microscopic potential and kinetic energy(See figure of the intermolecular potential energy !)

[(pv)/p]T : departure from Boyles law, which states that the productof pressure and volume for an ideal gas is f(T) only.can be negative and positive for real gases.(-) At low P and T, gases are more compressibledue to attractive force near the saturated-vapor condition.(+) At high P and T, gases are less compressibledue to repulsive force.(See figure of the pv with P and T for a real gas !)


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intermolecular potential energy


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Pv with P and T for a real gas


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Fundamentals of refrigeration Work (W) transport of energy only

Heat (Q) transport of energy

and entropy

1st law of thermodynamics

2nd law of thermodynamics

COP (Coefficient Of Performance)

H LQ Q W

H L

H L

Q Q

T T

1

/ 1

L L L

H L H L H L

Q Q TCOP

W Q Q T T T T

TH ()

TL ()

QH

QL

W


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System Work

Heat

Work is transformed to elastic energy, or potential

energy, or kinetic energy, or magnetic energy,quantum energy, etc.

Internal energy is increased.

Heat is transferred to environment and entropy

transfer is also accompanied.

Principle of refrigeration


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Without internal entropy generation of the system, theentropy of the system is decreased due to heattransfer.

Returning to equilibrium state of the system causesentropy increaseof non-thermal entropy* componentas well as total system entropy.

If adiabatic, low temperature is obtained due to

reduced thermal entropy. If the internal energy change occurs by work transfer

only without generating entropy, the greatesttemperature decrease is obtained. Otherwise,

* Thermal entropy may be regarded as entropy due tolattice vibration (or temperature).

S (T,P) for fluid refrigeration method

or S(T,H) for magnetic refrigeration method


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As TL decreases, more work per unitrefrigeration (W/QL) is required.

Ref.

Temp

TL (K)

Work / RefrigerationW/QL (W/W)

Carnot

(minimum)Actual

270 0.11 0.3 ~ 0.5

100 2 10 ~ 20

20 14 100 ~ 200

4 74 700 ~ 1500

1 299 > 6000

As TL decreases, the Carnot efficiency goes down.


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General refrigerationcycle and its

components

P-h diagram ofrefrigeration cycle

Very important in cryogenic

refrigeration !


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TemperatureEntropy Diagram for

Cryogenic Fluid

vapor compression refrigerator(cycle j-t principle) 2009

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