4.

HVAC and Refrigeration System

4. HVAC AND REFRIGERATION SYSTEM

Syllabus
HVAC and Refrigeration System: Vapor compression refrigeration cycle, Refrigerants,
Coefficient of performance, Capacity, Factors affecting Refrigeration and Air
conditioning system performance and savings opportunities.
Vapor absorption refrigeration system: Working principle, Types and comparison with
vapor compression system, Saving potential

4.1 Introduction

The Heating, Ventilation and Air Conditioning (HVAC) and refrigeration system
transfers the heat energy from or to the products, or building environment. Energy in form
of electricity or heat is used to power mechanical equipment designed to transfer heat
from a colder, low-energy level to a warmer, high-energy level.

Refrigeration deals with the transfer of heat from a low temperature level at the heat
source to a high temperature level at the heat sink by using a low boiling refrigerant.

There are several heat transfer loops in refrigeration system as described below:

Figure 4.1 Heat Transfer Loops In Refrigeration System

In the Figure 4.1, thermal energy moves from left to right as it is extracted from the space
and expelled into the outdoors through five loops of heat transfer:

− Indoor air loop. In the leftmost loop, indoor air is driven by the supply air fan
through a cooling coil, where it transfers its heat to chilled water. The cool air then
cools the building space.

− Chilled water loop. Driven by the chilled water pump, water returns from the cooling
coil to the chiller’s evaporator to be re-cooled.

− Refrigerant loop. Using a phase-change refrigerant, the chiller’s compressor pumps

heat from the chilled water to the condenser water.

− Condenser water loop. Water absorbs heat from the chiller’s condenser, and the
condenser water pump sends it to the cooling tower.

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− Cooling tower loop. The cooling tower’s fan drives air across an open flow of the hot
condenser water, transferring the heat to the outdoors.

Air-Conditioning Systems

Depending on applications, there are several options / combinations, which are available
for use as given below:
ƒ Air Conditioning (for comfort / machine)
ƒ Split air conditioners
ƒ Fan coil units in a larger system
ƒ Air handling units in a larger system
Refrigeration Systems (for processes)

ƒ Small capacity modular units of direct expansion type similar to domestic

refrigerators, small capacity refrigeration units.
ƒ Centralized chilled water plants with chilled water as a secondary coolant for
temperature range over 50C typically. They can also be used for ice bank formation.
ƒ Brine plants, which use brines as lower temperature, secondary coolant, for typically
sub zero temperature applications, which come as modular unit capacities as well as
large centralized plant capacities.
ƒ The plant capacities upto 50 TR are usually considered as small capacity, 50 – 250 TR
as medium capacity and over 250 TR as large capacity units.

A large industry may have a bank of such units, often with common chilled water pumps,
condenser water pumps, cooling towers, as an off site utility.

The same industry may also have two or three levels of refrigeration & air conditioning
such as:

ƒ Comfort air conditioning (200 – 250 C)

ƒ Chilled water system (80 – 100 C)
ƒ Brine system (sub-zero applications)
Two principle types of refrigeration plants found in industrial use are: Vapour
Compression Refrigeration (VCR) and Vapour Absorption Refrigeration (VAR). VCR
uses mechanical energy as the driving force for refrigeration, while VAR uses thermal
energy as the driving force for refrigeration.

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4.2 Types of Refrigeration System

Vapour Compression Refrigeration

Heat flows naturally from a hot to a colder body. In refrigeration system the opposite
must occur i.e. heat flows from a cold to a hotter body. This is achieved by using a
substance called a refrigerant, which absorbs heat and hence boils or evaporates at a low
pressure to form a gas. This gas is then compressed to a higher pressure, such that it
transfers the heat it has gained to ambient air or water and turns back (condenses) into a
liquid. In this way heat is absorbed, or removed, from a low temperature source and
transferred to a higher temperature source.

The refrigeration cycle can be broken down into the following stages (see Figure 4.2):

1 - 2 Low pressure liquid refrigerant in the evaporator absorbs heat from its surroundings,
usually air, water or some other process liquid. During this process it changes its state
from a liquid to a gas, and at the evaporator exit is slightly superheated.

2 - 3 The superheated vapour enters the compressor where its pressure is raised. There
will also be a big increase in temperature, because a proportion of the energy put into the
compression process is transferred to the refrigerant.

3 - 4 The high pressure superheated gas passes from the compressor into the condenser.
The initial part of the cooling process (3 - 3a) desuperheats the gas before it is then turned
back into liquid (3a - 3b). The cooling for this process is usually achieved by using air or
water. A further reduction in temperature happens in the pipe work and liquid receiver (3b
- 4), so that the refrigerant liquid is sub-cooled as it enters the expansion device.

4 - 1 The high-pressure sub-cooled liquid passes through the expansion device, which
both reduces its pressure and controls the flow into the evaporator.

Figure 4.2: Schematic of a Basic Vapor Compression Refrigeration System

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It can be seen that the condenser has to be capable of rejecting the combined heat inputs
of the evaporator and the compressor; i.e. (1 - 2) + (2 - 3) has to be the same as (3 - 4).
There is no heat loss or gain through the expansion device.

Alternative Refrigerants for Vapour Compression Systems

The use of CFCs is now beginning to be phased out due to their damaging impact on the
protective tropospheric ozone layer around the earth. The Montreal Protocol of 1987 and
the subsequent Copenhagen agreement of 1992 mandate a reduction in the production of
ozone depleting Chlorinated Fluorocarbon (CFC) refrigerants in a phased manner, with an
eventual stop to all production by the year 1996. In response, the refrigeration industry
has developed two alternative refrigerants; one based on Hydrochloro Fluorocarbon
(HCFC), and another based on Hydro Fluorocarbon (HFC). The HCFCs have a 2 to 10%
ozone depleting potential as compared to CFCs and also, they have an atmospheric
lifetime between 2 to 25 years as compared to 100 or more years for CFCs (Brandt,
1992). However, even HCFCs are mandated to be phased out by 2005, and only the
chlorine free (zero ozone depletion) HFCs would be acceptable.

Until now, only one HFC based refrigerant, HFC 134a, has been developed. HCFCs are
comparatively simpler to produce and the three refrigerants 22, 123, and 124 have been
developed. The use of HFCs and HCFCs results in slightly lower efficiencies as
compared to CFCs, but this may change with increasing efforts being made to replace
CFCs.

Absorption Refrigeration

The absorption chiller is a machine, which produces chilled water by using heat such as
steam, hot water, gas, oil etc. Chilled water is produced by the principle that liquid
(refrigerant), which evaporates at low temperature, absorbs heat from surrounding when it
evaporates. Pure water is used as refrigerant and lithium bromide solution is used as
absorbent

Heat for the vapour absorption refrigeration system can be provided by waste heat
extracted from process, diesel generator sets etc. Absorption systems require electricity to
run pumps only. Depending on the temperature required and the power cost, it may even
by economical to generate heat / steam to operate the absorption system.

Description of the absorption refrigeration concept is given below:

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The refrigerant (water)

evaporates at around 40C
under the high vacuum
condition of 754mmHg in
the evaporator. When the
refrigerant (water)
evaporates, the latent heat
of vaporization takes the
heat from incoming
chilled water.

This latent heat of

vaporization can cool the
chilled water which runs
into the heat exchanger
tubes in the evaporator by
transfer of heat to the
refrigerant (water).

In order to keep evaporating,

the refrigerant vapor must be
discharged from the
evaporator and refrigerant
(water) must be supplied. The
refrigerant vapor is absorbed
into lithium bromide solution
which is convenient to absorb
the refrigerant vapor in the
absorber. The heat generated
in the absorption process is
led out of system by cooling
water continually. The
absorption also maintains the
vacuum inside the
evaporator.

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As lithium bromide
solution is diluted, the
effect to absorb the
refrigerant vapor reduces.
In order to keep
absorption process, the
diluted lithium bromide
solution must be made
concentrated lithium
bromide.

Absorption chiller is
provided with the solution
concentrating system by
the heating media such as
steam, hot water, gas, oil,
which performs such
function is called
generator.
The concentrated solution
flows into the absorber
and absorbs the refrigerant
vapor again.

In order to carryout above

works continually and to
make complete cycle, the
following two functions are
required.
(1) To concentrate and
liquefy the evaporated
refrigerant vapor,
which is generated in
the high pressure
generator.
(2) To supply the
condensed water to the
evaporator as
refrigerant(water)
For these function,
condenser is installed.

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A typical schematic of the absorption refrigeration system is given in the Figure 4.3.

Figure 4.3 Schematic of Absorption Refrigeration System

Li-Br-water absorption refrigeration systems have a Coefficient of Performance (COP) in

the range of 0.65 - 0.70 and can provide chilled water at 6.7 oC with a cooling water
temperature of 30 oC. Systems capable of providing chilled water at 3 oC are also
available. Ammonia based systems operate at above atmospheric pressures and are
capable of low temperature operation (below 0oC). Absorption machines of capacities in
the range of 10-1500 tons are available. Although the initial cost of absorption system is
higher than compression system, operational cost is much lower-if waste heat is used.

Evaporative Cooling

There are occasions where air conditioning, which stipulates control of humidity up to 50
% for human comfort or for process, can be replaced by a much cheaper and less energy
intensive evaporative cooling.

The concept is very simple and is the same as that used in a cooling tower. Air is brought
in close contact with water to cool it to a temperature close to the wet bulb temperature.
The cool air can be used for comfort or process cooling. The disadvantage is that the air is
rich in moisture. Nevertheless, it is an extremely efficient means of cooling at very low
cost. Large commercial systems employ cellulose filled pads over which water is sprayed.
The temperature can be controlled by controlling the airflow and the water circulation
rate. The possibility of evaporative cooling is especially attractive for comfort cooling in
dry regions. This principle is practiced in textile industries for certain processes.

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4.3 Common Refrigerants and Properties

A variety of refrigerants are used in vapor compression systems. The choice of fluid is
determined largely by the cooling temperature required. Commonly used refrigerants are
in the family of chlorinated fluorocarbons (CFCs, also called Freons): R-11, R-12, R-21,
R-22 and R-502. The properties of these refrigerants are summarized in Table 4.1 and the
performance of these refrigerants is given in Table 4.2.

Table 4.1 Properties of Commonly used Refrigerants

Boiling Vapor Vapor Enthalpy *
Freezing
Refrigerant Point ** Pressure * Volume * Liquid (kJ Vapor (kJ
Point (oC)
(oC) (kPa) (m3 / kg) / kg) / kg)
R - 11 -23.82 -111.0 25.73 0.61170 191.40 385.43
R - 12 -29.79 -158.0 219.28 0.07702 190.72 347.96
R - 22 -40.76 -160.0 354.74 0.06513 188.55 400.83
R - 502 -45.40 --- 414.30 0.04234 188.87 342.31
R-7 -33.30 -77.7 289.93 0.41949 808.71 487.76
(Ammonia)
* At -10 oC
** At Standard Atmospheric Pressure (101.325 kPa)

Table 4.2 Performance of Commonly used Refrigerants*

Vapor
Evaporating Condensing Pressure
Refrigerant Enthalpy (kJ / COP**carnot
Press (kPa) Press (kPa) Ratio
kg)
R - 11 20.4 125.5 6.15 155.4 5.03
R - 12 182.7 744.6 4.08 116.3 4.70
R - 22 295.8 1192.1 4.03 162.8 4.66
R - 502 349.6 1308.6 3.74 106.2 4.37
R - 717 236.5 1166.5 4.93 103.4 4.78
o o
* At -15 C Evaporator Temperature, and 30 C Condenser Temperature
** COP carnot = Coefficient of Performance = Temp.Evap. / (Temp.Cond. -TempEvap.)

The choice of refrigerant and the required cooling temperature and load determine the
choice of compressor, as well as the design of the condenser, evaporator, and other
auxiliaries. Additional factors such as ease of maintenance, physical space requirements
and availability of utilities for auxiliaries (water, power, etc.) also influence component
selection.

4.4 Compressor Types and Application

For industrial use, open type systems (compressor and motor as separate units) are
normally used, though hermetic systems (motor and compressor in a sealed unit) also find
service in some low capacity applications. Hermetic systems are used in refrigerators, air
conditioners, and other low capacity applications. Industrial applications largely employ

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reciprocating, centrifugal and, more recently, screw compressors, and scroll compressors.
Water-cooled systems are more efficient than air-cooled alternatives because the
temperatures produced by refrigerant condensation are lower with water than with air.

Centrifugal Compressors

Centrifugal compressors are the most efficient type

(see Figure 4.4) when they are operating near full
load. Their efficiency advantage is greatest in large
sizes, and they offer considerable economy of scale,
so they dominate the market for large chillers. They
are able to use a wide range of refrigerants
efficiently, so they will probably continue to be the
dominant type in large sizes.

Centrifugal compressors have a single major

moving part - an impeller that compresses the Figure 4.4 Centrifugal Compressor
refrigerant gas by centrifugal force. The gas is given
kinetic energy as it flows through the impeller. This kinetic energy is not useful in itself,
so it must be converted to pressure energy. This is done by allowing the gas to slow down
smoothly in a stationary diffuser surrounding the impeller.

To minimize efficiency loss at reduced loads, centrifugal compressors typically throttle

output with inlet guide vanes located at the inlet to the impeller(s). This method is
efficient down to about 50% load, but the efficiency of this method decreases rapidly
below 50% load.

Older centrifugal machines are not able to reduce load much below 50%. This is because
of “surge” in the impeller. As the flow through the impeller is choked off, the gas does
not acquire enough energy to overcome the discharge pressure. Flow drops abruptly at
this point, and an oscillation begins as the gas flutters back and forth in the impeller.
Efficiency drops abruptly, and the resulting vibration can damage the machine. Many
older centrifugal machines deal with low loads by creating a false load on the system,
such as by using hot gas bypass. This wastes the portion of the cooling output that is not
required.

Another approach is to use variable-speed drives in combination with inlet guide vanes.
This may allow the compressor to throttle down to about 20% of full load, or less, without
false loading. Changing the impeller speed causes a departure from optimum
performance, so efficiency still declines badly at low loads. A compressor that uses a
variable-speed drive reduces its output in the range between full load and approximately
half load by slowing the impeller speed. At lower loads, the impeller cannot be slowed
further, because the discharge pressure would become too low to condense the refrigerant.
Below the minimum load provided by the variable-speed drive, inlet guide vanes are used
to provide further capacity reduction.

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Reciprocating Compressors

The maximum efficiency of reciprocating

compressors (see Figure 4.5) is lower than
that of centrifugal and screw compressors.
Efficiency is reduced by clearance volume
(the compressed gas volume that is left at
the top of the piston stroke), throttling losses
at the intake and discharge valves, abrupt
changes in gas flow, and friction. Lower
efficiency also results from the smaller sizes
of reciprocating units, because motor losses 4.5 Reciprocating Compressor
and friction account for a larger fraction of
energy input in smaller systems.

Reciprocating compressors suffer less efficiency loss at partial loads than other types, and
they may actually have a higher absolute efficiency at low loads than the other types.
Smaller reciprocating compressors control output by turning on and off. This eliminates all
part-load losses, except for a short period of inefficient operation when the machine starts.

Larger multi-cylinder reciprocating compressors commonly reduce output by disabling

(“unloading”) individual cylinders. When the load falls to the point that even one cylinder
provides too much capacity, the machine turns off. Several methods of cylinder unloading
are used, and they differ in efficiency. The most common is holding open the intake
valves of the unloaded cylinders. This eliminates most of the work of compression, but a
small amount of power is still wasted in pumping refrigerant gas to-and-fro through the
unloaded cylinders. Another method is blocking gas flow to the unloaded cylinders,
which is called “suction cutoff.”

Variable-speed drives can be used with reciprocating compressors, eliminating the

complications of cylinder unloading. This method is gaining popularity with the drastic
reduction in costs of variable speed drives.

Screw Compressors

Screw compressors, sometimes called “helical rotary”

compressors, compress refrigerant by trapping it in
the “threads” of a rotating screw-shaped rotor (see
Figure 4.6). Screw compressors have increasingly
taken over from reciprocating compressors of medium
sizes and large sizes, and they have even entered the
Figure 4.6 Screw Compressor
size domain of centrifugal machines. Screw
compressors are applicable to refrigerants that have higher condensing pressures, such as
HCFC-22 and ammonia. They are especially compact. A variety of methods are used to
control the output of screw compressors. There are major efficiency differences among
the different methods. The most common is a slide valve that forms a portion of the
housing that surrounds the screws.

Using a variable-speed drive is another method of capacity control. It is limited to oil-

injected compressors, because slowing the speed of a dry compressor would allow

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excessive internal leakage. There are other methods of reducing capacity, such as suction
throttling that are inherently less efficient than the previous two.

Scroll Compressors

The scroll compressor is an old invention that has finally

come to the market. The gas is compressed between two
scroll-shaped vanes. One of the vanes is fixed, and the
other moves within it. The moving vane does not rotate,
but its center revolves with respect to the center of the
fixed vane, as shown in Figure 4.7. This motion squeezes
the refrigerant gas along a spiral path, from the outside of
the vanes toward the center, where the discharge port is
located. The compressor has only two moving parts, the
moving vane and a shaft with an off-center crank to drive
the moving vane. Scroll compressors have only recently
become practical, because close machining tolerances are
needed to prevent leakage between the vanes, and
between the vanes and the casing.

The features of various refrigeration compressors and Figure 4.7 Scroll Compressor
application criteria is given in the Table 4.3.

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Table 4.3 Comparison of Different Types of Refrigeration Plants

(Source : Ashrae & Vendor Information)

Vapour Absorption Chiller
Vapour Compression Chillers
S. LiBr Ammonia
Parameters
No
Reciprocating Centrifugal Screw Single Effect Double Effect Half Effect Triple Effect Single Stage
Refrigeration Temp. Range (Brine /
1 +7 to -30oC +7 to -0oC +7 to -25oC Above 60C Upto –330C
Water)
Heat (Steam /
Heat (Steam / Heat (Steam
Hot Water / Heat (Hot Heat (Steam/ Hot
Hot Water / /Hot Oil
2 Energy Input Electricity Electricity Electricity Hot Water) Water / Hot Oil)
Hot Oil/ /Direct Fired
Oil/Direct
Direct Fired) )
Fired)
Heat Input Temp. Range -Maximum - Minimum Minimum Minimum Minimum
3 - - - Minimum 85oC
Minimum 85oC 130oC 55oC 190oC
Typical Energy to TR Ratio

4 Air Conditioning Temp. Range0.7-0.9 kW/TR 0.63kW/TR 0.65 kW/TR 5000 kcal/TR 2575 kcal/TR 7500 kcal/TR 2000 kcal/TR 4615 kcal/TR
Subzero Temp. Range1.25 to 2.5 ----- 1.25 to 2.5
----- ----- ----- ----- 6666 kcal/hr
kW/TR kW/TR
R11,R123,R134a R22, R134a Pure
5 Refrigerant R22, R12 Pure Water Pure Water Pure Water Pure Water
Ammonia Ammonia Ammonia
Water-LiBr Water-LiBr Water-LiBr Water-LiBr Ammonia-
6 Absorbent ------ ------ ------
solution solution solution solution LiBr solution
Typical single unit capacity range
7 30 TR & 30 TR & 50 TR &
Air Condition temp. range 1-150 TR 300 TR & above 50-200 TR 30 TR & above 30 TR & above
above above above
Subzero temp. range 10-50 TR ------ 50-200 TR ----- ----- ----- ----- 30 TR & above
Reduces at part Reduces at part Improves by
8 Typical COP at Part Load upto 50% Marginal Improvement at Part Load No variation
load load 15-20%
5-6 mm Hg 5-6 mm Hg 5-6 mm Hg
2.5-3.5 bar a 5-6 mm Hg 1.2kg/cm2(a)
Typical Internal Pressure Levels -Low 0.15-0.40 bar a 2-5.5 bar (abs) (abs) (abs)
11-12 bar (abs) 18 kg/cm2(a)
9 -High 1.20-1.50 bar a 18-20 bar 60-70 mm Hg 370-390 mm 60-70 mm Hg
-5 to 50oC 2 kg/cm2 (a) -25 to +150oC
Typical Internal Temp. Levels -25 to 50oC o
-25 to 50 C (abs) Hg (abs) (abs)
+4 to 160oC
+4 to 75oC +4 to +130oC +4 to 130oC

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Vapour Absorption Chiller

Vapour Compression Chillers
S. LiBr Ammonia
Parameters
No
Reciprocating Centrifugal Screw Single Effect Double Effect Half Effect Triple Effect Single Stage
Typical Cooling tower capacity range
per 100 TR of chillers
130 120 120 260 200 370 170 290
10 -Air conditioning Temperature Range
190 ----- 160 --- --- --- --- 290
- Subzero temp. range

Typical Make-up water quantity range

in Ltrs/Hr. 672 620 620 1345 1035 1914 880 1500
11
-Air Conditioning temperature range 983 --- 830 --- --- --- --- 1500
-Subzero temp. range
Material of construction
12 --- --- --- Cu-Ni or Stainless Steel Carbon Steel
-Generator
-Absorber --- --- --- Cu-Ni Carbon Steel
Copper / Carbon copper / Carbon Copper/
-Evaporator Cu-Ni Carbon Steel
steel steel Carbon steel
Copper / Carbon Copper / Carbon Copper / Cu-Ni
-Condenser Carbon Steel
steel steel Carbon steel
-Solution Heat Exchange --- --- --- Carbon Steel Carbon Steel
Cast Iron Cast Iron with
-Solution Pump --- --- ---
Hermatically Sealed (Canned motor type) Meh.Seal
Cast Iron
-Refrigerant pump --- --- --- Not needed
Hermatically Sealed (Canned motor type)
13 Expected Life 25-30 years 15-20 years 50 years
Normally Expected Repairs / Periodic Compressor Overhaul Tube Replacement Practically no
14
Maintenance Tube Replacement after 1-12 years due to Corrosion repairs
Factory
Assembled
Factory Assembled packaged Or Site
15 Factory Assembled upto 230 TR in
Assembled
A/C & subzero
range
Low cost Low cost Low cost a) Waste Heat
16 Beneficial Energy Sources
Electricity Electricity Electricity b) Low cost steam / Low cost fuels

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Vapour Absorption Chiller

Vapour Compression Chillers
S. LiBr Ammonia
Parameters
No Single Double Triple
Reciprocating Centrifugal Screw Half Effect Single Stage
Effect Effect Effect
Sudden Power
failure for 45-
60 min. or
more can
-Lubrication
a) Vacuum in Chiller disturb the
System
b) Purge System for Vacuum distillation
-Compressor
c) Corrosion Inhibitors in Absorbent column for
-Electricity Operation &
17 Critical Parameters ---- d) Surfactants in Absorbent continuous
supply Maintenance
e) Cooling Water Treatment operation.
-Electrical
f) Cooling Water Temperature Needs D.G.set
Power Panel
g) Heat Source Temperature if there is
Maintenance
frequent power
failure for
periods longer
than 30 min.

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4.5 Selection of a Suitable Refrigeration System

A clear understanding of the cooling load to be met is the first and most important part of
designing / selecting the components of a refrigeration system. Important factors to be
considered in quantifying the load are the actual cooling need, heat (cool) leaks, and
internal heat sources (from all heat generating equipment). Consideration should also be
given to process changes and / or changes in ambient conditions that might affect the load
in the future. Reducing the load, e.g. through better insulation, maintaining as high a
cooling temperature as practical, etc. is the first step toward minimizing electrical power
required to meet refrigeration needs. With a quantitative understanding of the required
temperatures and the maximum, minimum, and average expected cooling demands,
selection of appropriate refrigeration system (single-stage / multi-stage, economized
compression, compound / cascade operation, direct cooling / secondary coolants) and
equipment (type of refrigerant, compressor, evaporator, condenser, etc.) can be
undertaken.

4.6 Performance Assessment of Refrigeration Plants

• The cooling effect produced is quantified as tons of refrigeration.

1 ton of refrigeration = 3024 kCal/hr heat rejected.

• The refrigeration TR is assessed as TR = Q ⋅Cp ⋅ (Ti – To) / 3024

Where Q is mass flow rate of coolant in kg/hr

Cp is coolant specific heat in kCal /kg deg C
Ti is inlet, temperature of coolant to evaporator (chiller) in 0C
To is outlet temperature of coolant from evaporator (chiller) in 0C.

The above TR is also called as chiller tonnage.

• The specific power consumption kW/TR is a useful indicator of the

performance of refrigeration system. By measuring refrigeration duty
performed in TR and the kiloWatt inputs, kW/TR is used as a reference
energy performance indicator.

• In a centralized chilled water system, apart from the compressor unit, power is
also consumed by the chilled water (secondary) coolant pump as well
condenser water (for heat rejection to cooling tower) pump and cooling tower
fan in the cooling tower fan. Effectively, the overall energy consumption
would be towards:

− Compressor kW
− Chilled water pump kW
− Condenser water pump kW
− Cooling tower fan kW, for induced / forced draft towers
ƒ The specific power consumption for certain TR output would therefore have
to include:

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− Compressor kW/TR
− Chilled water pump kW/TR
− Condenser water pump kW/TR
− Cooling tower fan kW/TR
The overall kW/TR is the sum of the above.

The theoretical Coefficient of Performance (Carnot), COPCarnont - a standard measure of

refrigeration efficiency of an ideal refrigeration system- depends on two key system
temperatures, namely, evaporator temperature Te and condenser temperature Tc with COP
being given as:

COPCarnot = Te / Tc - Te

This expression also indicates that higher COPCarnot is achieved with higher evaporator
temperature and lower condenser temperature.

But COPCarnot is only a ratio of temperatures, and hence does not take into account the
type of compressor. Hence the COP normally used in the industry is given by

Cooling effect (kW)

COP =
Power input to compressor (kW)
where the cooling effect is the difference in enthalpy across the evaporator and expressed
as kW. The effect of evaporating and condensing temperatures are given in the Figure 4.8
and Figure 4.9 below:

Figure 4.8 Figure 4.9

In the field performance assessment, accurate instruments for inlet and outlet chilled
water temperature and condenser water temperature measurement are required, preferably
with a least count of 0.10C. Flow measurements of chilled water can be made by an
ultrasonic flow meter directly or inferred from pump duty parameters. Adequacy check
of chilled water is needed often and most units are designed for a typical 0.68 m3/hr per
TR (3 gpm/TR) chilled water flow. Condenser water flow measurement can also be made
by a non-contact flow meter directly or inferred from pump duty parameters. Adequacy

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check of condenser water is also needed often, and most units are designed for a typical
0.91 m3/hr per TR (4 gpm / TR) condenser water flow.

In case of air conditioning units, the airflow at the Fan Coil Units (FCU) or the Air
Handling Units (AHU) can be measured with an anemometer. Dry bulb and wet bulb
temperatures are measured at the inlet and outlet of AHU or the FCU and the refrigeration
load in TR is assessed as ;

Q × ρ × (h in − h out )
TR =
3024

Where, Q is the air flow in m3/h

ρ is density of air kg/m3

h in is enthalpy of inlet air kCal/kg
h out is enthalpy of outlet air kCal/kg

Use of psychometric charts can help to calculate hin and hout from dry bulb, wet bulb
temperature values which are, in-turn measured, during trials, by a whirling
psychrometer.

Power measurements at, compressor, pumps, AHU fans, cooling tower fans can be
accomplished by a portable load analyzer.

Estimation of air conditioning load is also possible by calculating various heat loads,
sensible and latent based on inlet and outlet air parameters, air ingress factors, air flow,
no. of people and type of materials stored.

An indicative TR load profile for air conditioning is presented as follows:

ƒ Small office cabins = 0.1 TR /m2

ƒ Medium size office i.e., = 0.06 TR/ m2

10 – 30 people occupancy
with central A/C

ƒ Large multistoried office = 0.04 TR/ m2

complexes with central A/C

Integrated Part Load Value (IPLV)

Although the kW/ TR can serve as an initial reference, it should not be taken as an
absolute since this value is derived from 100% of the equipment's capacity level and is
based on design conditions that are considered the most critical. These conditions occur
may be, for example, during only 1% of the total time the equipment is in operation
throughout the year. Consequently, it is essential to have data that reflects how the
equipment operates with partial loads or in conditions that demand less than 100% of its
capacity. To overcome this, an average of kW/TR with partial loads ie Integrated Part
Load Value (IPLV) have to be formulated.

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The IPLV is the most appropriate reference, although not considered the best, because it
only captures four points within the operational cycle: 100%, 75%, 50% and 25%.
Furthermore, it assigns the same weight to each value, and most equipment usually
operates at between 50 % and 75% of its capacity. This is why it is so important to
prepare specific analysis for each case that addresses the four points already mentioned,
as well as developing a profile of the heat exchanger's operations during the year.

4.7 Factors Affecting Performance & Energy Efficiency of

Refrigeration Plants
Design of Process Heat Exchangers

There is a tendency of the process group to operate with high safety margins which
influences the compressor suction pressure / evaporator set point. For instance, a process
cooling requirement of 150C would need chilled water at a lower temperature, but the
range can vary from 60C to say 100C. At 100C chilled water temperature, the refrigerant
side temperature has to be lower, say –50C to +50C. The refrigerant temperature, again
sets the corresponding suction pressure of refrigerant which decides the inlet duty
conditions for work of compression of the refrigerant compressor. Having the optimum /
minimum driving force (temperature difference) can, thus, help to achieve highest
possible suction pressure at the compressor, thereby leading to less energy requirement.
This requires proper sizing of heat transfer areas of process heat exchangers and
evaporators as well as rationalizing the temperature requirement to highest possible value.
A 10C raise in evaporator temperature can help to save almost 3 % on power
consumption. The TR capacity of the same machine will also increase with the evaporator
temperature, as given in Table 4.4.

Table 4.4 Effect of Variation in Evaporator Temperature on Compressor Power Consumption

Refrigeration
Evaporator Specific Power Increase in
Capacity*
Temperature (0C) Consumption kW/ton (%)
(tons)
5.0 67.58 0.81 -
0.0 56.07 0.94 16.0
-5.0 45.98 1.08 33.0
-10.0 37.20 1.25 54.0
-20.0 23.12 1.67 106.0
* 0
Condenser temperature 40 C

Towards rationalizing the heat transfer areas, the heat transfer coefficient on refrigerant
side can be considered to range from 1400 – 2800 watts /m2K.

The refrigerant side heat transfer areas provided are of the order of 0.5 Sqm./TR and
above in evaporators.

Condensers in a refrigeration plant are critical equipment that influence the TR capacity
and power consumption demands. Given a refrigerant, the condensing temperature and

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corresponding condenser pressure, depend upon the heat transfer area provided,
effectiveness of heat exchange and the type of cooling chosen. A lower condensing
temperature, pressure, in best of combinations would mean that the compressor has to
work between a lower pressure differential as the discharge pressure is fixed by design
and performance of the condenser. The choices of condensers in practice range from air
cooled, air cooled with water spray, and heat exchanger cooled. Generously sized shell
and tube heat exchangers as condensers, with good cooling tower operations help to
operate with low discharge pressure. values and the TR capacity of the refrigeration plant
also improves. With same refrigerant, R22, a discharge pressure of 15 kg/cm2 with water
cooled shall and tube condenser and 20 kg/cm2 with air cooled condenser indicate the kind
of additional work of compression duty and almost 30 % additional energy consumption
required by the plant. One the best option at design stage would be to select generously
sized (0.65 m2/TR and above) shell and tube condensers with water-cooling as against
cheaper alternatives like air cooled condensers or water spray atmospheric condenser
units.

The effect of condenser temperature on refrigeration plant energy requirements is given in

Table 4.5.

Table 4.5 Effect of Variation in Condenser Temperature on Compressor Power Consumption

Specific Power
Condensing Refrigeration Increase in
Consumption
Temperature (0C) Capacity (tons) kW/TR (%)
(kW / TR)

26.7 31.5 1.17 -

35.0 21.4 1.27 8.5

40.0 20.0 1.41 20.5

* Reciprocating compressor using R-22 refrigerant.
Evaporator temperature.-100C

Maintenance of Heat Exchanger Surfaces

After ensuring procurement, effective maintenance holds the key to optimizing power
consumption.

Heat transfer can also be improved by ensuring proper separation of the lubricating oil
and the refrigerant, timely defrosting of coils, and increasing the velocity of the secondary
coolant (air, water, etc.). However, increased velocity results in larger pressure drops in
the distribution system and higher power consumption in pumps / fans. Therefore, careful
analysis is required to determine the most effective and efficient option.

Fouled condenser tubes force the compressor to work harder to attain the desired
capacity. For example, a 0.8 mm scale build-up on condenser tubes can increase energy
consumption by as much as 35 %. Similarly, fouled evaporators (due to residual
lubricating oil or infiltration of air) result in increased power consumption. Equally
important is proper selection, sizing, and maintenance of cooling towers. A reduction of
0.550C temperature in water returning from the cooling tower reduces compressor power
consumption by 3.0 % (see Table 4.6).

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Table 4.6 Effect of Poor Maintenance on Compressor Power Consumption

Cond. Specific Power Increase in
Evap. Temp Refrigeration
Condition Temp Consumption kW/Ton
(0C) Capacity* (tons)
(0C) (kW/ton) (%)
Normal 7.2 40.5 17.0 0.69 -
Dirty condenser 7.2 46.1 15.6 0.84 20.4
Dirty evaporator 1.7 40.5 13.8 0.82 18.3
Dirty condenser 1.7 46.1 12.7 0.96 38.7
and evaporator
* 15 ton reciprocating compressor based system. The power consumption is lower than that
for systems typically available in India. However, the percentage change in power
consumption is indicative of the effect of poor maintenance.

Multi-Staging For Efficiency

Efficient compressor operation requires that the compression ratio be kept low, to reduce
discharge pressure and temperature. For low temperature applications involving high
compression ratios, and for wide temperature requirements, it is preferable (due to
equipment design limitations) and often economical to employ multi-stage reciprocating
machines or centrifugal / screw compressors.

Multi-staging systems are of two-types: compound and cascade – and are applicable to all
types of compressors. With reciprocating or rotary compressors, two-stage compressors
are preferable for load temperatures from –20 to –580C, and with centrifugal machines for
temperatures around –430C.
In multi-stage operation, a first-stage compressor, sized to meet the cooling load, feeds
into the suction of a second-stage compressor after inter-cooling of the gas. A part of the
high-pressure liquid from the condenser is flashed and used for liquid sub-cooling. The
second compressor, therefore, has to meet the load of the evaporator and the flash gas. A
single refrigerant is used in the system, and the work of compression is shared equally by
the two compressors. Therefore, two compressors with low compression ratios can in
combination provide a high compression ratio.
For temperatures in the range of –460C to –1010C, cascaded systems are preferable. In
this system, two separate systems using different refrigerants are connected such that one
provides the means of heat rejection to the other. The chief advantage of this system is
that a low temperature refrigerant which has a high suction temperature and low specific
volume can be selected for the low-stage to meet very low temperature requirements.

Matching Capacity to System Load

During part-load operation, the evaporator temperature rises and the condenser
temperature falls, effectively increasing the COP. But at the same time, deviation from
the design operation point and the fact that mechanical losses form a greater proportion of
the total power negate the effect of improved COP, resulting in lower part-load efficiency.

Therefore, consideration of part-load operation is important, because most refrigeration

applications have varying loads. The load may vary due to variations in temperature and
process cooling needs. Matching refrigeration capacity to the load is a difficult exercise,

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requiring knowledge of compressor performance, and variations in ambient conditions,

and detailed knowledge of the cooling load.

Capacity Control and Energy Efficiency

The capacity of compressors is controlled in a number of ways. Capacity control of

reciprocating compressors through cylinder unloading results in incremental (step-by-
step) modulation as against continuous capacity modulation of centrifugal through vane
control and screw compressors through sliding valves. Therefore, temperature control
requires careful system design. Usually, when using reciprocating compressors in
applications with widely varying loads, it is desirable to control the compressor by
monitoring the return water (or other secondary coolant) temperature rather than the
temperature of the water leaving the chiller. This prevents excessive on-off cycling or
unnecessary loading / unloading of the compressor. However, if load fluctuations are not
high, the temperature of the water leaving the chiller should be monitored. This has the
advantage of preventing operation at very low water temperatures, especially when flow
reduces at low loads. The leaving water temperature should be monitored for centrifugal
and screw chillers.

Capacity regulation through speed control is the most efficient option. However, when
employing speed control for reciprocating compressors, it should be ensured that the
lubrication system is not affected. In the case of centrifugal compressors, it is usually
desirable to restrict speed control to about 50 % of the capacity to prevent surging. Below
50 %, vane control or hot gas bypass can be used for capacity modulation.

The efficiency of screw compressors operating at part load is generally higher than either
centrifugal compressors or reciprocating compressors, which may make them attractive in
situations where part-load operation is common. Screw compressor performance can be
optimized by changing the volume ratio. In some cases, this may result in higher full-load
efficiencies as compared to reciprocating and centrifugal compressors. Also, the ability of
screw compressors to tolerate oil and liquid refrigerant slugs makes them preferred in
some situations.

Multi-level Refrigeration for Plant Needs

The selection of refrigeration systems also depends on the range of temperatures required
in the plant. For diverse applications requiring a wide range of temperatures, it is
generally more economical to provide several packaged units (several units distributed
throughout the plant) instead of one large central plant. Another advantage would be the
flexibility and reliability accorded. The selection of packaged units could also be made
depending on the distance at which cooling loads need to be met. Packaged units at load
centers reduce distribution losses in the system. Despite the advantages of packaged units,
central plants generally have lower power consumption since at reduced loads power
consumption can reduce significantly due to the large condenser and evaporator surfaces.

Many industries use a bank of compressors at a central location to meet the load.
Usually the chillers feed into a common header from which branch lines are taken to
different locations in the plant. In such situations, operation at part-load requires extreme
care. For efficient operation, the cooling load, and the load on each chiller must be
monitored closely. It is more efficient to operate a single chiller at full load than to

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operate two chillers at part-load. The distribution system should be designed such that
individual chillers can feed all branch lines. Isolation valves must be provided to ensure
that chilled water (or other coolant) does not flow through chillers not in operation.
Valves should also be provided on branch lines to isolate sections where cooling is not
required. This reduces pressure drops in the system and reduces power consumption in
the pumping system. Individual compressors should be loaded to their full capacity before
operating the second compressor. In some cases it is economical to provide a separate
smaller capacity chiller, which can be operated on an on-off control to meet peak
demands, with larger chillers meeting the base load.

Flow control is also commonly used to meet varying demands. In such cases the savings
in pumping at reduced flow should be weighed against the reduced heat transfer in coils
due to reduced velocity. In some cases, operation at normal flow rates, with subsequent
longer periods of no-load (or shut-off) operation of the compressor, may result in larger
savings.

Chilled Water Storage

Depending on the nature of the load, it is economical to provide a chilled water storage
facility with very good cold insulation. Also, the storage facility can be fully filled to
meet the process requirements so that chillers need not be operated continuously. This
system is usually economical if small variations in temperature are acceptable. This
system has the added advantage of allowing the chillers to be operated at periods of low
electricity demand to reduce peak demand charges - Low tariffs offered by some electric
utilities for operation at night time can also be taken advantage of by using a storage
facility. An added benefit is that lower ambient temperature at night lowers condenser
temperature and thereby increases the COP.

If temperature variations cannot be tolerated, it may not be economical to provide a

storage facility since the secondary coolant would have to be stored at a temperature
much lower than required to provide for heat gain. The additional cost of cooling to a
lower temperature may offset the benefits. The solutions are case specific. For example,
in some cases it may be possible to employ large heat exchangers, at a lower cost burden
than low temperature chiller operation, to take advantage of the storage facility even
when temperature variations are not acceptable. Ice bank system which store ice rather
than water are often economical.

System Design Features

In overall plant design, adoption of good practices improves the energy efficiency
significantly. Some areas for consideration are:
ƒ Design of cooling towers with FRP impellers and film fills, PVC drift
eliminators, etc.
ƒ Use of softened water for condensers in place of raw water.
ƒ Use of economic insulation thickness on cold lines, heat exchangers,
considering cost of heat gains and adopting practices like infrared
thermography for monitoring - applicable especially in large chemical /
fertilizer / process industry.

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ƒ Adoption of roof coatings / cooling systems, false ceilings / as applicable, to

minimize refrigeration load.
ƒ Adoption of energy efficient heat recovery devices like air to air heat
exchangers to pre-cool the fresh air by indirect heat exchange; control of
relative humidity through indirect heat exchange rather than use of duct
heaters after chilling.
ƒ Adopting of variable air volume systems; adopting of sun film application for
heat reflection; optimizing lighting loads in the air conditioned areas;
optimizing number of air changes in the air conditioned areas are few other
examples.

4.8 Energy Saving Opportunities

a) Cold Insulation

Insulate all cold lines / vessels using economic insulation thickness to minimize heat
gains; and choose appropriate (correct) insulation.

b) Building Envelop

Optimise air conditioning volumes by measures such as use of false ceiling and
segregation of critical areas for air conditioning by air curtains.

c) Building Heat Loads Minimisation

Minimise the air conditioning loads by measures such as roof cooling, roof painting,
efficient lighting, pre-cooling of fresh air by air- to-air heat exchangers, variable
volume air system, otpimal thermo-static setting of temperature of air conditioned
spaces, sun film applications, etc.

e) Process Heat Loads Minimisation

Minimize process heat loads in terms of TR capacity as well as refrigeration level,

i.e., temperature required, by way of:
i) Flow optimization
ii) Heat transfer area increase to accept higher temperature coolant
iii) Avoiding wastages like heat gains, loss of chilled water, idle flows.
iv) Frequent cleaning / de-scaling of all heat exchangers

f) At the Refrigeration A/C Plant Area

i) Ensure regular maintenance of all A/C plant components as per manufacturer

guidelines.

ii) Ensure adequate quantity of chilled water and cooling water flows, avoid bypass
flows by closing valves of idle equipment.

iii) Minimize part load operations by matching loads and plant capacity on line; adopt
variable speed drives for varying process load.

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iv) Make efforts to continuously optimize condenser and evaporator parameters for
minimizing specific energy consumption and maximizing capacity.

v) Adopt VAR system where economics permit as a non-CFC solution.

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QUESTIONS
List the types of refrigeration plants by operating principle and by
1. applications?

List the instruments necessary for performance assessment of a refrigeration

2. plant

3. What is the relation for TR duty in a chilled water system?

4. List the main components of a vapour compression refrigeration plant?

List the major energy efficiency improvement options in the condenser

5. system of a refrigeration plant.

List the major energy efficiency improvement options in the evaporator

6. system of a refrigeration plant

List the methods of capacity control in a vapour compression refrigeration

7. plant using a reciprocating compressor with multi cylinders?

List the design related improvement measures for energy efficiency

8. improvement in a vapour compression refrigeration plant?

What is the refrigeration load in TR when 15 m3/hr of water is needed to be

cooled from 210C to 150C. If the compressor motor draws 29 kW, chilled
9. water pump draws 4.6 kW, condenser water pump draws 6.1 kW and CT
fan draws 2.7 kW what is kW/TR overall?

10. List the energy efficiency parameters of a refrigeration plant.

11. Briefly explain methodology of refrigeration plant energy audit?

REFERENCES
1. Technology Menu on Energy Efficiency (NPC)
2. ASHRAE Hand Book
3. NPC Case Studies
4. PCRA Literature
5. Vendor Information

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