Lecture 8 ChE v2 [Compatibility Mode]

8/8/2019 Lecture 8 ChE v2 [Compatibility Mode]

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e r gera on an que ac on

MLP Dalida


2/13

Refrigeration and liquefaction

Refrigeration maintenance of a temperature belowthat of surroundings

ont nuous a sorpt on o eat at a ow temperaturelevel, usually accomplished by evaporation of liquidin a steady-state flow process.


3/13

The Carnot Refrigerator

The ideal refrigerator, like the ideal heat engine,operates on a Carnot cycle, consisting of two

isothermal steps in which heat |QC| is absorbed at

t e ower temperature C an eat H s re ecteat the higher temperature TH and two adiabatic

steps.

The coefficient of performance:

CH

C

CH

CC

TT

T

QQ

Q

W

Q

worknet

etemperaturlowertheatabsorbedheat

=

==

||||

||||


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The vapor-compression cycle

12: liquid (absorb heat) evaporating at constantpressure

23: isentropic compression to a higher pressure

34: cooled and condensed with rejection of heatat a higher temperature level

41: expansion throttling process


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6/13


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On the basis of a unit mass of fluid

12

|| HHQC

=

The heat absorbed in the evaporator:43

|| HHQH =The heat rejected in the condenser:23

12||

HH

HH

W

QC

==

23HHW =The work of compression:

Define the rate of circulation of refrigerant:12

||

HH

Qm C

=&

&


8/13

The choice of refrigerant

Dependence? The efficiency of a Carnot heat engine is independent

of the working medium of the engine.

e coe c ent o per ormance o a arnot re r geratoris independent of the refrigerant.

Irreversibilities invapor-compression cycle cause the

coefficient of performance to dependent to some extent

on the refrigerant.

Other factors:

toxicity, flammability, cost, corrosion properties, vapor

pressure in relation to temperature, etc.


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Two requirements:

The vapor pressure of the refrigerant at the evaporator

temperature should be greater than atmospheric

pressure to avoid air leaking.

The vapor pressure at the condenser temperature shouldnot be unduly high, because of the initial cost and

operating expense of high-pressure equipment.

Ammonia, methyl chloride, carbon dioxide, propane

and other hydrocarbons

Halogenated hydrocarbons

common in 1930s (e.g. CCl3F, CCl2F2) and now mostly end

stable molecules causing severe ozone depletion

replacements are certain hydrochlorofluorocarbons, less than

fully halogenated hydrocarbons, and hydrofluorocarbons

which contains no chlorine (e.g., CHCl2CF3, CF3CH2F).


10/13

A standard refrigeration cycle is to be used in the production of acontinuous supply of chilled water at a temperature of 10oC andat a rate of 25 kg/s. The process is shown. The refrigerant isH2O. Saturated steam at 0.008 bar at point 2 enters an adiabatic

compressor of 80% efficiency based on isentropic operation andis compressed to 0.07 bar at point 3, from which it enters acondenser. It emerges at point 1 as a saturated liquid , heat Qbeing discharged to the surroundings at 27oC. The liquid flashes

t roug a va ve w c acts as a t rott e to re uce pressure to0.008 bar. The remaining liquid is vaporized through theevaporator and by heat exhange with the evaporating refrigerantis cooled from its feed temp of 10oC to 27oC. Determine theflow rate of refrigerant, power requirement of compressor, rateof heat transfer in the condenser and COP.


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Heat Pump


12/13

The heat pump

for heating houses in winter: Refrigerant evaporates in coils placed underground or

in the outside air; vapor compression is followed by

, ,

which is used to heat the building.

and cooling them in summer:

The flow of refrigerant is reversed, and heat is absorbed

from the building and rejected through underground

coils or to the outside air.


13/13

A house has a winter heating requirement of 30 kJ/s and a summer cooling

requirement of 60 kJ/s. Consider a heat-pump installation to maintain the house

temperature at 20C in winter and 25C in summer. This requires circulation of therefrigerant through interior exchanger coils at 30C in winter and 5C in summer.

Underground coils provide the heat source in winter and the heat sink in summer.

For a year-round ground temperature of 15C, the heat-transfer characteristics of the

coils necessitate refrigerant temperatures of 10C in winter and 25

C in summer.What are the minimum power requirements for winter heating and summer cooling?

The minimum power requirements are provided by a Carnot heat pump:

For winter heating, the heat absorbed in the ground coils:

s

kJ

T

TQQ

H

CHC 02.28

15.27330

15.2731030|||| =

+

+==

The power requirement:

s

kJQQW CH 98.102.2830|||| ===

For summer cooling, the house coils are at the lower temperature TC :

The power requirement:

s

kJ

T

TTQW

C

CH

C

13.415.2735

)15.2735()15.27325(60|| =

+

++=

=

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Lecture 8 ChE v2 [Compatibility Mode]