K J SOMAIYA COLLEGE OF ENGINEERING, MUMBAI-77

(CONSTITUENT COLLEGE OF SOMAIYA VIDYAVIHAR UNIVERSITY)

Condenser and Evaporator

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 Air cooled condenser

• As the name implies, in air-cooled condensers

• Air is the external fluid, i.e., the refrigerant rejects heat to air flowing
over the condenser.
• Air-cooled condensers can be further classified into natural convection
type or forced convection type.
• Natural convection type:
• Forced convection type:

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 Natural convection type

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 Natural convection type
• In natural convection type, heat transfer from the condenser is by buoyancy induced natural
convection and radiation.
• Since the flow rate of air is small and the radiation heat transfer is also not very high, the
combined heat transfer coefficient in these condensers is small.
• As a result a relatively large condensing surface is required to reject a given amount of heat.
• Hence these condensers are used for small capacity refrigeration systems like household
refrigerators and freezers.
• The natural convection type condensers are either plate surface type or finned tube type.
• In plate surface type condensers used in small refrigerators and freezers, the refrigerant
carrying tubes are attached to the outer walls of the refrigerator.
• The whole body of the refrigerator (except the door) acts like a fin.
• Insulation is provided between the outer cover that acts like fin and the inner plastic cover of
the refrigerator.
• It is for this reason that outer body of the refrigerator is always warm. Since the surface is
warm, the problem of moisture condensation on the walls of the refrigerator does not arise in
these systems. These condensers are sometimes called as flat back condensers.

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 Forced convection type

• Plate fin type condenser

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 Forced convection type
• In forced convection type condensers, the circulation of air over the condenser surface is
maintained by using a fan or a blower.
• These condensers normally use fins on air-side for good heat transfer. The fins can be either plate
type or annular type.
• Forced convection type condensers are commonly used in window air conditioners, water
coolers and packaged air conditioning plants.
• These are either chassis mounted or remote mounted. In chassis mounted type, the compressor,
induction motor, condenser with condenser fan, accumulator, HP/LP cut- out switch and pressure
gauges are mounted on a single chassis. It is called condensing unit of rated capacity.
• The components are matched to condense the required mass flow rate of refrigerant to meet the
rated cooling capacity. The remote mounted type, is either vertical or roof mounted horizontal
type.
• Typically the air velocity varies between 2 m/s to 3.5 m/s for economic design with airflow
rates of 12 to 20 cmm per ton of refrigeration (TR). The air specific heat is 1.005 kJ/kg-K and
density is 1.2 kg/m3.
• Therefore for 1 TR the temperature rise Δta = 3.5167/(1.2x1.005 x 16/60) = 10.9°C for average
air flow rate of 16 cmm. Hence, the air temperature rises by 10 to 15°C as compared to 3 to
6°C for water in water cooled condensers

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 Water cooled condenser

• Double pipe or tube-in-tube type

• Shell-and-coil type
• Shell-and-tube type

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 Double pipe or tube-in-tube type

•Double pipe condensers are normally used up to

10 TR capacity.
•The cold water flows through the inner tube,
while the refrigerant flows through the annulus
in counter flow.
•The refrigerant in the annulus rejects a part of its
heat to the surroundings by free convection and
radiation.

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 Shell-and-coil type:

• These condensers are used in systems up to 50

TR capacity.
• The water flows through multiple coils, which
may have fins to increase the heat transfer
coefficient.
• The refrigerant flows through the shell. In
smaller capacity condensers, refrigerant flows
through coils while water flows through the
shell.
• Figure shows a shell-and-coil type condenser.
When water flows through the coils, cleaning is
done by circulating suitable chemicals through
the coils.

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 Shell-and-tube type:
• This is the most common type of condenser used in systems
from 2 TR upto thousands of TR capacity.
• In these condensers the refrigerant flows through the shell
while water flows through the tubes in single to four passes.
• The condensed refrigerant collects at the bottom of the shell.
The coldest water contacts the liquid refrigerant so that some
sub-cooling can also be obtained.
• The liquid refrigerant is drained from the bottom to the
receiver. There might be a vent connecting the receiver to the
condenser for smooth drainage of liquid refrigerant.
• The shell also acts as a receiver. Further the refrigerant also
rejects heat to the surroundings from the shell. The most
common type is horizontal shell type.
• Vertical shell-and-tube type condensers are usually used with
ammonia in large capacity systems so that cleaning of the
tubes is possible from top while the plant is running.

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 Evaporative condenser

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 Evaporative condenser (contd…)
• In evaporative condensers, both air and water are used to extract heat from the
condensing refrigerant.
• Evaporative condensers combine the features of a cooling tower and water-
cooled condenser in a single unit.
• In these condensers, the water is sprayed from top part on a bank of tubes carrying the
refrigerant and air is induced upwards. There is a thin water film around the
condenser tubes from which evaporative cooling takes place.
• The heat transfer coefficient for evaporative cooling is very large. Hence, the
refrigeration system can be operated at low condensing temperatures (about 11 to 13
K above the wet bulb temperature of air).
• The water spray countercurrent to the airflow acts as cooling tower. The role of air is
primarily to increase the rate of evaporation of water. The required air flow rates are
in the range of 350 to 500 m3/h per TR of refrigeration capacity.

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 Evaporative condenser (contd…)
• Evaporative condensers are used in medium to large capacity systems.
• These are normally cheaper compared to water cooled condensers, which
require a separate cooling tower.
• Evaporative condensers are used in places where water is scarce. Since water is
used in a closed loop, only a small part of the water evaporates. Make-up water
is supplied to take care of the evaporative loss.
• The water consumption is typically very low, about 5 percent of an equivalent
water cooled condenser with a cooling tower.
• However, since condenser has to be kept outside, this type of condenser
requires a longer length of refrigerant tubing, which calls for larger refrigerant
inventory and higher pressure drops.
• Since the condenser is kept outside, to prevent the water from freezing, when
outside temperatures are very low, a heater is placed in the water tank. When
outside temperatures are very low it is possible to switch-off the water pump
and run only the blowers, so that the condenser acts as an air cooled condenser.

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 Air cooled Vs water cooled condensers:
• Air-cooled condensers are simple in construction since no pipes are required
for air.
• Further, the disposal of warm air is not a problem and it is available in plenty.
• The fouling of condenser is small and maintenance cost is low. However, since
the specific heat of air is one fourth of that of water and density is one
thousandth of that of water, volume flow rates required are very large.
• The thermal conductivity is small; hence heat transfer coefficient is also very
small. Also, air is available at dry-bulb temperature while water is available at
a lower temperature, which is 2 to 3 °C above the wet-bulb temperature.
• The temperature rise of air is much larger than that of water, therefore the
condenser temperature becomes large and COP reduces. Its use is normally
restricted to 10 TR although blower power goes up beyond 5 TR.
• In systems up to 3 TR with open compressors it is mounted on the same
chassis as the compressor and the compressor motor drives the condenser fan
also. In middle-east countries where is shortage of fresh water these are used
up to 100 TR or more.

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 Air cooled Vs water cooled condensers:

• The air-cooled condensers cost two to

three times more than water-cooled
condensers.
• The water-cooled condenser requires
cooling tower since water is scarce in
municipality areas and has to be
recycled. Water from lakes and rivers
cannot be thrown back in warm state
since it affects the marine life
adversely.
• Increased first cost and maintenance
cost of cooling tower offsets the cost
advantage of water-cooled condenser.
Fouling of heat exchange surface is a
big problem in use of water.

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 Factors affecting condenser capacity

• Material
• Amount of contact
• Temperature difference

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 Evaporator
• The purpose of the evaporator is to receive low-pressure, low
temperature fluid from the expansion valve and to bring it in close
thermal contact with the load.
• The refrigerant takes up its latent heat from the load and leaves the
evaporator as a dry gas.
• The function of the evaporator will be to cool gas, liquid or other product
loads.
• In most cases air or a liquid is first cooled, and this is then used to cool
the load. e.g., in a cold-room air is cooled and this air cools the stored
produce and carries away heat leaking through the structure; in a water
chiller, water is circulated to cool the load, etc

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 Types of evaporator

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 Flooded evaporator

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 Flooded evaporator (contd…)

• This is typically used in large ammonia systems. The refrigerant enters a surge drum
through a float type expansion valve.
• The compressor directly draws the flash vapour formed during expansion.
• This vapour does not take part in refrigeration hence its removal makes the
evaporator more compact and pressured drop due to this is also avoided.
• The liquid refrigerant enters the evaporator from the bottom of the surge drum. This
boils inside the tubes as heat is absorbed. The mixture of liquid and vapour bubbles
rises up along the evaporator tubes. The vapour is separated as it enters the surge
drum.
• The remaining un-evaporated liquid circulates again in the tubes along with the
constant supply of liquid refrigerant from the expansion valve.
• The mass flow rate in the evaporator tubes is f*m , where m is the mass flow rate
through the expansion valve and to the compressor. The term f is called recirculation
factor.

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 Flooded type shell and tube evaporator

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 Flooded type shell and tube evaporator (contd…)
• The liquid (usually brine or water) to be chilled flows through the tubes in double pass just
like that in shell and tube condenser.
• The refrigerant is fed through a float valve, which maintains a constant level of liquid
refrigerant in the shell. The shell is not filled entirely with tubes.
• This is done to maintain liquid refrigerant level below the top of the shell so that liquid
droplets settle down due to gravity and are not carried by the vapour leaving the shell.
• If the shell is completely filled with tubes, then a surge drum is provided after the evaporator
to collect the liquid refrigerant. Shell-and-tube evaporators can be either single pass type or
multipass type. In multipass type, the chilled liquid changes direction in the heads.
• Shell and-tube evaporators are available in vertical design also. Compared to horizontal type,
vertical shell-and-tube type evaporators require less floor area. The chilled water enters from
the top and flows downwards due to gravity and is then taken to a pump, which circulates it
to the refrigeration load.
• At the inlet to tubes at the top a special arrangement introduces swirling action to increase the
heat transfer coefficient
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