This document discusses multi-pressure refrigeration systems. It explains that single-stage systems have limitations at very low evaporator or high condenser temperatures due to increased losses. Multi-stage systems address this by using multiple compression stages to reduce the temperature lift in each stage. Types of multi-stage systems include multi-compression, multi-evaporator, and cascade systems. Flash gas removal and intercooling can further improve the performance of multi-stage systems. Cascade systems use multiple refrigerants matched to different temperature ranges.

1. Refrigeration and Air-conditioning
Unit 3
Multi Pressure Systems
By,
Suyog S. Kadam

2.  A single stage vapor compression refrigeration system has one low side pressure
(evaporator pressure) and one high side pressure (condenser pressure).
 These systems are adequate as long as the temperature difference between
evaporator and condenser (temperature lift) is small.
 The temperature lift can become large either due to the requirement of very low
evaporator temperatures and/or due to the requirement of very high condensing
temperatures.
 In frozen food industries the required evaporator can be as low as –40oC, while
in chemical industries temperatures as low as –150oC may be required for
liquefaction of gases.
Introduction

3. As evaporator temperature decreases:
i. Throttling losses increase
ii. Superheat losses increase
iii. Compressor discharge temperature increases
iv. Quality of the vapor at the inlet to the evaporator increases
v. Specific volume at the inlet to the compressor increases.
As a result of this, the refrigeration effect decreases and work of compression
increases.
Effect of decreasing evaporator temperatures on
T s and P h

4.  The volumic refrigeration effect also decreases rapidly as the specific volume
increases with decreasing evaporator temperature.
 Similar effects will occur, though not in the same proportion when the
condenser temperature increases for a given evaporator temperature.
 Due to these drawbacks, single stage systems are not recommended when
the evaporator temperature becomes very low and/or when the condenser
temperature becomes high.
 In such cases multi-stage systems are used in practice. Generally, for
fluorocarbon and ammonia based refrigeration systems a single stage system
is used up to an evaporator temperature of –30oC.
 A two-stage system is used up to –60oC and a three-stage system is used for
temperatures below –60oC.

5.  Apart from high temperature lift applications, multi-stage systems are also
used in applications requiring refrigeration at different temperatures.
 For example, in a dairy plant refrigeration may be required at –30oC for
making ice cream and at 2oC for chilling milk.
 In such cases, it may be advantageous to use a multi-evaporator system with
the low temperature evaporator operating at –30oC and the high temperature
evaporator operating at 2oC.
 A multi-stage system is a refrigeration system with two or more low-side
pressures. Multi-stage systems can be classified into:
a) Multi-compression systems
b) Multi-evaporator systems
c) Cascade systems, etc.

6. 1. Flash gas removal using flash tank
 Problems with high temperature lift applications is the high quality of vapor
(flash gas) at the inlet to the evaporator.
 Flash gas does not contribute to the refrigeration effect as it is already in the
form of vapor, and it increases the pressure drop in the evaporator.
 It is possible to improve the COP of the system if the flash gas is removed as
soon as it is formed and re compressed to condenser pressure.
 However, continuous removal of flash gas as soon as it is formed and re
compressing it immediately is difficult in practice.
 One way of improving the performance of the system is to remove the flash
gas at an intermediate pressure using a flash tank.

7. 1. Flash gas removal using flash tank
The saturated liquid at point 8 is fed to the
evaporator after throttling it to the required
evaporator pressure, Pe (point 9) using an
expansion valve.
The saturated vapor in the flash tank (point 3)
is either compressed to the condenser pressure
or throttled to the evaporator pressure.
In flash tank the refrigerant
liquid and vapor are separated
at an intermediate pressure, Pi
using a low side float valve
(process 6-7). The float valve
also maintains a constant
liquid level in the flash tank.

8. Inter cooling In Multistage Compression
In stead of compressing the vapour in a single stage from state 1 to state 2’, if the
refrigerant is compressed from state 1 to an intermediate pressure, state 2, inter
cooled from 2 to 3 and then compressed to the required pressure (state 4), reduction
in work input results.
Intercooling not only reduces the work input but also reduces the compressor
discharge temperature leading to better lubrication and longer compressor life.

9. Inter cooling using external water cooled heat
exchanger
Depends on the availability of sufficiently cold water to which the refrigerant from
low stage compressor can reject heat.
The refrigerant at the inlet to the high stage compressor may not be saturated.

10. Inter cooling using liquid refrigerant in flash tank

11.  Inter cooling using in the flash tank may or may not reduce the power input to
the system, as it depends upon the nature of the refrigerant.
 The heat rejected by the refrigerant during inter cooling generates additional
vapor in the flash tank, which has to be compressed by the high stage
compressor.
 Thus the mass flow rate of refrigerant through the high stage compressor will be
more than that of the low stage compressor.
 Whether total power input to the system decreases or not depends on whether the
increased power consumption due to higher mass flow rate is compensated by
reduction in specific work of compression or not.
 For ammonia, the power input usually decreases, however, for refrigerants such
as R12, R22, the power input marginally increases.
Inter cooling using liquid refrigerant in flash tank

12.  One of the design issues in multi-stage compression is the selection of suitable
intermediate pressure.
 For air compressors with inter-cooling to the initial temperature, the theoretical
work input to the system will be minimum when the pressure ratios are equal
for all stages. Thus for a two-stage air compressor with inter-cooling,
 For refrigerants, correction factors to the above equation are suggested, for
example one such relation for refrigerants is given by:
 where Pe and Pc are the evaporator and condenser pressures, and Tc and Te are
condenser and evaporator temperatures (in K).

13. Multi-stage system with flash gas removal and inter
cooling
It is assumed that in this
process the superheated
refrigerant vapor gets
completely de-superheated and
emerges out as a saturated
vapor at state 4.
The superheated vapor from
the water cooled heat
exchanger bubbles through
the refrigerant liquid in the
flash tank.

14. COP of System can be Calculated as,
where mI
is the mass flow rate of refrigerant through Compressor-I and mII
is the
mass flow rate of refrigerant through Compressor-II.

15. Advantages
a) Quality of refrigerant entering the evaporator reduces thus giving rise to higher
refrigerating effect, lower pressure drop and better heat transfer in the evaporator
b) Throttling losses are reduced as vapor generated during throttling from Pc to Pi is
separated in the flash tank and re compressed by Compressor-II.
c) Volumetric efficiency of compressors will be high due to reduced pressure ratios.
d) Compressor discharge temperature is reduced considerably.
However, one disadvantage of the above system is that since refrigerant liquid in
the flash tank is saturated, there is a possibility of liquid flashing ahead of the
expansion valve due to pressure drop or heat transfer in the pipelines connecting
the flash tank to the expansion device.

16. Multi-Evaporator Systems
There are many applications where refrigeration is required at different
temperatures.
For example, in a typical food processing plant, cold air may be required at -30°C
for freezing and at +7 °C for cooling of food products or space cooling.
One simple alternative is to use different refrigeration systems to cater to these
different loads.
However, this may not be economically viable due to the high total initial cost.
Another alternative is to use a single refrigeration system with one compressor and
two evaporators

17. Multiple Evaporator System
A. Evaporator at Same TemperatureA. Evaporator at Same Temperature
1. Single Compressor
B. Evaporator at Different Temperatures with Individual ExpansionB. Evaporator at Different Temperatures with Individual Expansion
ValvesValves
1. Single Compressor.
2. Individual Compressors.
3. Compound Compression.

18. A. Evaporators at Same Temperature.
Multiple Evaporators At Same Temperature with Single Compressor &
Expansion Valve

19. B. Evaporators at Different Temperature
(Individual Expansion Valves)
Multiple Evaporators At Different Temperature with Single Compressor &
Individual Expansion Valve and Back Pressure Valves

20. B. Evaporators at Different Temperature
(Individual Expansion Valves)
Multiple Evaporators At Different Temperature with Individual Compressors &
Individual Expansion Valves

21. B. Evaporators at Different Temperature
(Individual Expansion Valves)
Multiple Evaporators At Different Temperature with COMPOUND
COMPRESSION & Individual Expansion Valves

22. B. Evaporators at Different Temperature (Multiple
Expansion Valves)
Multiple Evaporators At Different Temperature with COMPOUND
COMPRESSION & Multiple Expansion Valves & Flash Intercooler

23. Limitations of multi-stage systems
Since only one refrigerant is used throughout the system, the refrigerant used
should have high critical temperature and low freezing point.
The operating pressures with a single refrigerant may become too high or too
low. Generally only R12, R22 and NH3 systems have been used in multi-stage
systems as other conventional working fluids may operate in vacuum at very
low evaporator temperatures.
Possibility of migration of lubricating oil from one compressor to other,
leading to compressor break-down.
The above limitations can be overcome by using cascade systems.

24.  In a cascade system a series of refrigerants with progressively lower boiling
points are used in a series of single stage units.
 The condenser of lower stage system is coupled to the evaporator of the next
higher stage system and so on.
 The component where heat of condensation of lower stage refrigerant is
supplied for vaporization of next level refrigerant is called as cascade
condenser.
Cascade Systems

25. Cascade System

26. Cascade System
 This system employs two different refrigerants operating in two individual
cycles. They are thermally coupled in the cascade condenser. The refrigerants
selected should have suitable pressure-temperature characteristics.
 An example of refrigerant combination is Carbon dioxide (NBP = -78.4oC,
Tcr = 31.06oC) in low temperature cascade and ammonia (NBP = -33.33oC, Tcr
= 132.25oC) in high temperature cascade.
 It is possible to use more than two cascade stages, and it is also possible to
combine multi-stage systems with cascade systems.

27. Applications of cascade systems:
i.Liquefaction of petroleum vapors
ii.Liquefaction of industrial gases
iii.Manufacturing of dry ice
iv.Deep freezing etc.
Advantages of cascade systems:
i.Since each cascade uses a different refrigerant, it is possible to select a refrigerant
that is best suited for that particular temperature range. Very high or very low
pressures can be avoided
ii.Migration of lubricating oil from one compressor to the other is prevented

28. Cascade Refrigeration Systems

29. COP of Two Stage Cascade System
Work Done by L.T. Cascade System
Work Done by H.T. Cascade System
Work Done by the Cascade System
Power Required
Refrigerating Effect
COP of the system