Multi Pressure Refrigeration Cycles

7/22/2019 Multi Pressure Refrigeration Cycles

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Multi -pressure Systems


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Industrial refrigeration plants operate with a large

difference between evaporating and condensing

temperatures

50 ~ 80 C

Opportunities: Multi-stage compression, Flash gas,

intercooling, number of evaporators at different

temperatures

Multipressure systems - classified as compound

systemsor cascade systems

Multi-stage compression used when ratio of suction to

discharge pressure is high enough to cause a serious

drop in efficiency or an unacceptably high discharge

temperature

Introduction


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A multi-pressure system is a refrigeration system that

has two or more low side pressures

The low side pressure is the pressure of the refrigerantbetween the expansion valve and the intake of

compressor

Different than the single pressure system which has but

one low side pressure

In Dairy where one evaporator operates at -35 C to

harden ice cream while another evaporator operates at 2

C to cool milk

Two functions often integral to multi-pressure systems

are; Removal of flash gas

Inter-cooling

Several combinations of multiple evaporators and

compressors May have same or different refrigerants

Introduction


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Flash gas - the mass of vaporized refrigerant per kg of

total refrigerant which is just leaving the throttling valve Flash chamber/tank is that portion of the expansion

valve system where liquid-vapour mixture of refrigerant

gets accumulated before entering into the evaporator

Saving in the power requirement results if the flash gasthat develops in the throttling process between the

condenser and evaporator is removed and recompressed

before complete expansion

When saturated liquid expands through an expansion

valve, the fraction of vapour or flash gas progressively

increases

Removal of F lash Gas


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The expansion process shown on the pressure enthalpy

diagram takes place from 1 to 2

The state point as the expansion proceeds moves into aregion of a greater fraction of vapour

The end point of expansion, 2, could have been achieved

by interrupting the expansion at 3 and separating the

liquid vapour phases which are 4 and 6 respectively The expansion could then continued by expanding the

liquid at 4 and the vapour at 6 to the final pressure, giving

5 and 7 respectively

The combination of refrigerant at 5 and 7 gives point 2

The expansion process from 6 to 7 is wasteful because of

the following two reasons;

The refrigerant at 7 can do no refrigerating

Work will be required to compress the vapour back to

the pressure it had at 6



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Why not perform the part of expansion, separate the liquid

from the vapour, continue expanding the liquid, and

recompress the vapour without further expansion The equipment to achieve this separation is called a flash

tank



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The expansion from 1 to 3 takes place through a float

valve, which serves the further purpose of

maintaining a constant level in the flash tank

To recompress the vapour at 6, a compressor must be

available with a suction pressure of 6

Thus two compressors are needed in the system



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Intercooling between two stages of compression reduces

the work of compression per kg of vapour

Intercooling

In two stage compression of air, for example an

intercooler from point 2 to 4 on PV diagram saves some

work


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This figure shows how compression with Intercooling

appears on the ph diagram of a refrigerant Processes 1- 2 - 3 & 4 - 5 are on lines of constant entropy

Between the same two pressures, therefore, process 4 5

shows a smaller increase in enthalpy, which indicates that

less work is required than in 2

3

Intercooling


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Between the two given pressures, the work of

compression is proportional to the specific volume ofentering gas

Sp. Volume at 2 is greater than it is at 4 so work required

for compressing from 2 to 3 is greater than in

compressing from 4 to 5

Intercooling in a refrigerating system can be

accomplished by two ways:

Using a water cooled heat exchanger

Using a refrigerant

The water-cooled intercooler may be satisfactory for two-

stage air compression, but for refrigerant compression,

the water is usually not cold enough

Multipurpose Refr igeration Systems


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Another way of Intercooling uses liquid refrigerant fromthe condenser to do the Intercooling

Discharge gas from the low-stage compressor bubbles

through the liquid in the intercooler

Refrigerant leaves the intercooler at 4 as saturated vapour

IntercoolingMultipurpose Refr igeration Systems


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Intercooling


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Intercooling

Example: Calculate the power needed to compress 1.2 kg/s

of ammonia from saturated vapour at 80 kPa to 1000 kPa (a)

by single-stage compression and (b) by two stage

compression with intercooling by liquid refrigerant at 300

kPa.


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Heat & Mass Balance Around Intercooler

&


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Heat & Mass Balance Around Intercooler

O E t & O C


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One Evaporator & One Compressor

With one compressor and one evaporator the flash tank may function as shown below

A pressure reducing valve throttles the flash gas from the intermediate pressure to the

evaporator pressure

Throttling is necessary because there is no compressor available with a high suction

pressure

Calculations show that the flash tank does not improve the performance of the system and

the system is used infrequently


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T E t & O C


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Two Evaporators & One Compressor

A pressure reducing valve is installed after the high-temperature evaporator

regulates the pressure and maintains a temperature in the air-conditioning

evaporator of 5 C

T C A d O E t


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Two Compressors And One Evaporator

Two stage compression with Intercooling and removal of flash gas is often the ideal

way to serve one low-temperature evaporator

The system requires less power than with a single compressor

T C A d O E t


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Two Compressors And One Evaporator

T C & T E t


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Two Compressors & Two Evaporators

The system which has two evaporators operating at

different temperatures is common in industrial

refrigeration Example of dairy cooling milk & manufacturing ice cream

Frozen food plant requires two evaporators at different

temperatures

Evaporators at two different temperatures can be handled

efficiently by a two-stage system which employs

Intercooling and removal of flash gas

Example

In an ammonia system one evaporator is to provide 180 kW ofrefrigeration at -300C and another evaporator is to provide 200kW

at 50C. The system uses two-stage compression with intercooling

and is arranged as in Fig. Below. The condensing temperature is

400C. Calculate the power required by the compressors.

T C & T E t


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Press re Enthalp Diagram


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Pressure Enthalpy Diagram

Solution: Sketch the pressure-enthalpy diagram as shown


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Two Compressors & Two EvaporatorsThe discharge pressure of the low-stage compressor and the suction pressure of the

high-stage compressor are the same as the pressure in the 5 C evaporator.

Next determine the enthalpies at the state points.

The simplest way to calculate the mass rate of flow handled by the high-stage

compressor is to make a heat and mass balance about the high-temperatureevaporator and the intercooler, as shown



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Multi Stage Systems


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a. Compound

Two low compression ratios = 1 high

First stage compressor meets cooling load

Second stage compressor meets load evaporator and

flash gas

Single refrigerant

b. Cascade

Preferred for -46 C to -101 C

Two systems with different refrigerants

Multi -Stage Systems

Cascade Refr igeration Systems


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Very low temperatures can be achieved by operating two or more vapor-compression

systems in series, called cascading.


65,21,

inin

LR

WW

QCOP

Advantages:Different refrigerants, equipment, and oils can

be used for the higher and the lower systems. Also, COP ofthe system increases.

Disadvantage:High first cost; More complicated.



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Consider a two-stage cascade refrigeration system operating between the pressure

limits of 0.8 and 0.14 MPa. Each stage operates on an ideal vapor compression cycle

with R134a as the working fluid. Heat rejection from the lower cycle to the upper

cycle takes place in an adiabatic counterflow heat exchanger where both streamsenter at about 0.32 MPa. If the mass flow rate of the refrigerant is 0.05kg/s,

determine

(a) The mass flow rate of the refrigerant through the lower cycle,

(b) The rate of heat removal from the refrigerated space and the power input to the

compressor,

(c) The COP of the refrigerator.


Multistage Compression with F lash Chamber


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I ntercooling (Compound System)

Optimum Intermediate pressure?



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The optimum intermediate pressure is given by:

The optimum intermediate pressure in a two-stage refrigeration system is quite close,

but not equal, to that shown in above equation. The reason for the deviation is that the

cooling process in air compression is without cost, but in the refrigeration system there

is a power cost associated. with compressing the refrigerant vapor that performs thecooling.

Interstage pressure is usually set so that the compression ratio at each stage is nearly

the same for higher COPs.



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Consider a two-stage cascade refrigeration system operating between the pressure

limits of 0.8 and 0.14 MPa as before but, this time, with a flash chamber operating at

0.32 MPa. Determine

(a) The fraction of the refrigerant that evaporates as it is throttled to the flash

chamber,

(b) The amount of heat removed from the refrigerated space and the compressor

work per unit mass of refrigerant flowing through the condenser,

(c) The COP of the refrigerator.





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A refrigerator with a single compressor can provide refrigeration at several

temperatures by throttling the refrigerant in stages.


Assignment:Whats the COP and heat transfer amounts for conditions used in

the previous problem (Check for both paths)? Assume QL,Ris a third of QL,F.

Also called

pressure-regulating

valve

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Multi Pressure Refrigeration Cycles