(Solved) Function of an Evaporator

3 Evaporator:
The evaporator is one of the four basic and necessary hardware components of the
Refrigeration system. (The refrigerant may be considered as a fifth, most important, component)
Depending on the application the design of the evaporator will differ. The evaporator is an
important device used in the low pressure side of a refrigeration system. The liquid refrigerant
from the expansion valve enters into the evaporator where it boils and changes into vapour. The
function of an evaporator is to absorb heat from the surrounding location or medium which is to
be cooled, by means of a refrigerant. The temperature of the boiling refrigerant in the evaporator
must always be less than that of the surrounding medium so that the heat flows to the refrigerant.
The evaporator becomes cold and remains cold due to the following two reasons:
1. The temperature of the evaporator coil is low due to the low temperature of the refrigerant
inside the coil.
2. The low temperature of the refrigerant remains unchanged because any heat it absorbs is
converted to latent heat as boiling proceeds, Note
The evaporator is also known as a cooling coil, a chilling or a freezing coil. The
evaporator coons by using the refrigerant's latent heat of vapuorisation to absorb heat from the
medium being cooled
3.1 Working Principle:
The working of an evaporator may be best understood by considering the simple refrigerating
system, as shown in Fig3.1. The point 5 in the figure represents the entry of liquid refrigerant
into the expansion valve. Under proper operating conditions, the liquid refrigerant is sub-cooled
(i,e. cooled below its saturation temperature). The sub-cooling ensures that the expansion valve
receives pure liquid refrigerant with no vapour present to restrict the flow of refrigerant through
the expansion valve. The liquid refrigerant at low pressure enters the evaporator at point 6, as
shown in Fig. As the liquid refrigerant passes through the evaporator coil, it continually absorbs
heat through the coil walls, from the medium being cooled. During this, the refrigerant continues
to boil and evaporate. Finally at point 1', all the liquid refrigerant has evaporated and only
vapour refrigerant remains in the evaporator coil. The liquid refrigerant's ability to convert
absorbed heat to latent heat is now used up. Since the vapour refrigerant at point is still colder
than the medium being cooled, therefore the vapour refrigerant continues to absorb heat. This
heat absorption causes an increase in the sensible heat (or temperature) of the vapour refrigerant.
The vapour temperature continues to rise until the vapour leaves the evaporator to the suction
line at point 1. At this point, the temperature of the vapour is above the saturation temperature
and the vapour refrigerant is superheated.
Alinnauthri evaporator. The variation of refrigerant temperature (or sensible heat) and the
refrigerant heat content (or enthalpy) within the evaporator. We see that the temperature of the
refrigerant is constant during evaporation of the liquid refrigerant from point 6 to 1' and the
enthalpy increases steadily. It shows that the latent heat is absorbed by the evaporating liquid
with no change in temperature. Both the temperature and enthalpy of the refrigerant increases

from point 1' to 1. At point 1, all the liquid refrigerant has evaporated. The line shows the
increase in sensible heat of the vapour refrigerant.
Fig 3.1 Compression cycle
Capacity of an Evaporator:
“The capacity of an evaporator is defined as the amount of heat absorbed by it over
a given period of time.”
The heat absorbed or heat transfer capacity of an evaporator is given by
Q = CIA (T2— T1) W or Hs
Where U = Overall heat transfer coefficient in Wim2 °C
A = Area of evaporator surface in m2,
T2=Temperature of medium to be cooled (or temperature outside the evaporator) in °C
T1= Saturation temperature of refrigerant at evaporator pressure (or temperature inside the
evaporator) in C.

3.2 Classification of Evaporators used in regferigeration:
1- According to the type of construction:
(a) Bare tube coil evaporator
(b) Finned tube evaporator
(c) Plate evaporator
(d) Shell and tube evaporator
(e) Shell and coil evaporator
(f) Tube in tube evaporator
2- According to the manner in which liquid refrigerant is fed:
(a) Flooded evaporator,
(b) Dry expansion evaporator
3- According to the mode of heat transfer:
(a) Natural convection evaporator,
(b) Forced convection evaporator
4- According to operating conditions :
(a) Frosting evaporator,
(b) Non-frosting evaporator
(c) Defrosting evaporator
Followings are the some types of evaporators:
3.2.1 Finned Evaporators:
The finned evaporator, as shown in Fig3.1 , consists of bare tubes or coils over which
metal plates or fins are fastened. The metal fins are constructed of thin sheets of metal having
good thermal conductivity. The shape, size or spacing of the fins can be adapted to provide best
rate of heat transfer for a gives application_ Since the fins greatly increase the contact surfaces
for heat transfer, therefore the finned evaporators are also called extended surface evaporators,
Fig 3.2 Finned evaporator

The finned evaporators are primarily designed for air conditioning applications where the
refrigerator temperature is above O'C. Because of the rapid heat transfer of the finned evaporator.
it will defrost itself on the off cycle when the temperature of the coil is near O'C. A finned coil
should never be allowed to frost because the accumulation of frost between the fins reduces the
capacity_ The air conditioning coils, which operate at suction temperatures which are high
enough so that frosting never occurs, have fin spacing as small as 3 mm, The finned coils which
frost on the on cycle and defrost on the off cycle have wider fin spacing.
3.2.2 Plate Evaporators:
A common type of plate evaporator is shown in Fig3.3 In this type of evaporator, the
coils are welded on one side of a plate or between the two plates which are welded together at
the edges. The plate evaporators are generally used in household refrigerators, home freezers,
beverage coolers, ice cream cabinets, locker plants etc.
Fig 3.3 plate evaporator

3.2.3 Shell and Tube Evaporators:
The shell and tube evaporator, as shown in Fig3.4 , is similar to a shell and tube condenser. It
consists of a number of horizontal tubes enclosed in a cylindrical shell. The inlet and outlet
headers with perforated metal tube sheets are connected at each end of the tubes
Fig3.4 The shell and tube evaporator
3.2.4 Shell and Coil Evaporators:
The shell and coil evaporators, as shown in Fig.3.5, are generally dry expansion
evaporators to chill water. The cooling coil is a continuous tube that can be in the form of a
single or double spiral. The shell may be sealed or open. The sealed shells are usually found in
shell and coil evaporators used to cool drinking water. The evaporators having flanged shells are
often used to chill water in secondary refrigeration systems. Another type of shell and coil
evaporator is shown in Fig3.5.
Both types of evaporators are usually used where small capacity (2 to 10 TR) liquid
cooling is required. It may be noted that the shell and coil evaporator is restricted to operation
above 5°C in order to prevent the freezing problems. Liquid 4, t Suction inlet outlet coil

3.5 Fig the shell and coil evaporators
3.2.5 Bare Tube Evaporators:
The bare tube evaporators are made up of copper tubing or steel pipes. The copper tubing is
used for small evaporators where the refrigerant other than ammonia is used, while the steel
pipes are used with the large evaporators where ammonia is used as the refrigerant. The bare
tube evaporator comprises of several turns of the tubing, though most commonly flat zigzag and
oval trombone are the most common shapes. The bare tube evaporators are usually used for
liquid chilling. In the blast cooling and the freezing operations the atmospheric air flows over
the bare tube evaporator and the chilled air leaving it used for the cooling purposes. The bare
tube evaporators are used in very few applications, however the bare tube evaporators fitted with
the fins, called as finned evaporators are used very commonly. `
Fig. 3.6. Bare Tube Evaporators:

3.2.6 Natural Convection Evaporators:
The natural convection evaporators are used where low air velocity and minimum hydration of
the product is desired. The domestic refrigerators, water coolers and small freezers have natural
convection evaporators. The circulation of air in a domestic refrigerator by natural convection is
shown in Fig 3.7.The evaporator coil should be placed as high as possible in the refrigerator
because the cold air falls down as it leaves the evaporator.
Fig. 3.7. Circulation of air in natural convection evaporator
The velocity of air over the evaporator coil considerably affects the capacity. In natural
convection, the velocity of air depends upon the temperature difference between the evaporator
and the space to be cooled. When the temperature difference, using a natural convection
evaporator, is low (less than 8' C), the velocity of air is too low for satisfactory circulation. A
lower temperature difference than 8°C may even cause slime on certain products such as meat or
poultry. On the other hand, too great a temperature difference causes excessive dehydration.
3.2.7 Frosting Evaporators:
The frosting type evaporators always operate at temperatures below 0°C. This means that the
coil frosts continually when in use and it must be removed at regular intervals either manually or
automatically for most efficient operation. The frost which forms on the evaporator comes from
the moisture in the air. Some evaporators run at extremely low temperature in order to keep the
refrigeration mixture cool. This allows frost and ice to build up. It may be noted that as the frost
grows in thickness, the coil or cooling efficiency decreases until the ice and frost is removed. Mr
evaporators in household refrigerators, bare pipe coils in storage boxes and low tenver ame
evaporators fall under the frosting evaporators.

3.2.8 Non-Frosting Evaporators:
The non-frosting evaporators operate at a temperature above 0°C at all times. Therfore frost
does not form on the evaporator. Occasionally, the evaporator may build up a light coat of frost
just before the compressor shuts off. This frost immediately melts on the off cycle.
The big advantage of a non-frosting evaporator is its operation at a temperature close to freezing
(0.6°C to 1°C). At this temperature, it does not draw the moisture out of the air rapidly. This
permits to maintain a 'relative humidity from 75 to 85 per cent in the cabinet. This helps to keep
the food fresh and stops shrinking in weight. The use of non-frosting evaporators are limited to
high temperature work such as air conditioning, bakery refrigeration and candy storage. Note :
Since the non-frosting evaporators have greater area, therefore they are heavy than the frosting
evaporators. They also require baffles to direct the air flow in one direction past the coils. The
coil efficiency is increased as the baffles speed up the air cooling process.
3.3 Applications of Evaporators:
HVAC Industry:
An evaporator is used in an air-conditioning system to allow a compressed cooling chemical,
such as R-22 (Freon) or R-410A, to evaporate from liquid to gas while absorbing heat in the
process.[1] It can also be used to remove water or other liquids from mixtures
Food Industry:
Frequently in the food industry a raw material or a potential foodstuff contains more water than
is required in the final product. When the foodstuff is a liquid, the easiest method of removing
the water, in general, is to apply heat to evaporate it. Evaporation is thus a process that is often
used by the food technologist.
Chemical Industry:
The process of evaporation is widely used to concentrate foods and chemicals as well as
salvage solvents. In the concentration process, the goal of evaporation is to vaporize most of the
water from a solution which contains the desired product.
Water Treatment:
In the case of desalination of sea water or in Zero Liquid Discharge plants, the reverse purpose
applies; evaporation removes the desirable drinking water from the undesired product, salt.

Pharmaceutical Industry:
In the pharmaceutical industry, the evaporation process is used to eliminate excess moisture,
providing an easily handled product and improving product stability. Preservation of long-term
activity or stabilization of enzymes in laboratories are greatly assisted by the evaporation
process
Kraft Processes:
Another example of evaporation is in the recovery of sodium hydroxide in kraft pulping.
[2] Cutting down waste-handling cost is another major reason for large companies to use
evaporation applications. Legally, all producers of waste must dispose of waste using methods
compatible with environmental guidelines; these methods are costly. By removing moisture
through vaporization, industry can greatly reduce the amount of waste product that must be
processed.
Marine Industry:
Large ships usually carry evaporating plants to produce fresh water, thus reducing their reliance
on shore-based supplies. Steam ships must be able to produce high-quality distillate in order to
maintain boiler-water levels. Diesel-engine ships often utilize waste heat as an energy source for
producing fresh water. In this system, the engine-cooling water is passed through a heat
exchanger, where it is cooled by concentrated seawater (brine). Because the cooling water
(which is chemically treated fresh water) is at a temperature of 70–80 °C (158–176 °F), it would
not be possible to flash off any water vapour unless the pressure in the heat exchanger vessel
was dropped.