evaporators and its types

1. Definition :
The evaporator is a heat transfer system, and is that part of a
refrigeration cycle in which liquid refrigerant is evaporated for the
purpose of removing heat from the refrigerated space or product.

2. Responsible for absorbing heat into the refrigeration system
The evaporator is maintained at a temperature that is lower than the
medium being cooled
The capacity of any evaporator or cooling coil is the rate at which
heat will pass through the evaporator walls from the refrigerated
space to the vaporizing liquid
Removes both latent and sensible heat from the air in the
refrigerated box
Latent heat is in the form of moisture
Sensible heat reduces air and product temperature.

3. Types Of Evaporators
Based on their construction the various types of
evaporators are:
1. Bare Tube Evaporators :
a) The bare tube
pipes
evaporators are made up of copper tubing or steel
b) The copper tubing is used for small evaporators
refrigerant other than ammonia is used
where the
c) the steel pipes are used with the large
ammonia is used as the refrigerant.
evaporators where
d) the atmospheric air flows over the bare tubeevaporator
chilled air leaving it used for the cooling purposes..
and the
e) The bare tube evaporators are usually used for liquid chilling

4. 2) Plate Type of Evaporators
a) the coil usually made up of copper or
aluminum is embedded in the plate so as so
to form a flat looking surface.
b) the plate type of evaporator looks like a
single plate, but inside it there are several
turns of the metal tubing through which the
refrigerant flows.
c) advantage of the plate type
of evaporators is that they are more rigid as
the external plate provides lots of safety.
d) This type of evaporator is widely used in
domestic refrigeration.
e) The plates can also be welded together
forming the bank of the plate type of
evaporators that can be used the
larger evaporators of higher capacities.

5. 3) Finned Evaporators
a) Are the bare tube type
of evaporators covered with the fins
b) he fluid tends to move between the
open spaces of the tubing and does
not come in contact with the surface
of the coil, thus the bare
tube evaporators are less effective.
c) the finned evaporators are more
effective than the bare
tube evaporators.
d) are most commonly used in the air
conditioners of almost all types like
window, split, packaged and the
central air conditioning systems.

6. 4) Shell and Tube types of Evaporators
a) Are used in the large refrigeration and central air conditioning
systems.
b) comprise of large number of the tubes that are inserted inside the
drum or the shell.
c) Depending on the direction of the flow of the refrigerant in the
shell and tube type of evaporator, they are classified into two
types: dry expansion type and flooded type of evaporator.
1. Dry expansion evaporator: In dry expansion evaporators the
refrigerant flows along the tube side and the fluid to be chilled
flows along the shell side.
2. Flooded evaporator :In case of the flooded type
of evaporators the refrigerant flows along the shell side and fluid
to be chilled flows along the tube.

8. 5) SHELL AND COIL EVAPORATOR
1. Shell and Coil evaporators have one or more spiral shaped, bare
tube coils enclosed in a welded steel shell.
They are generally dry expansion type with refrigerant in coils
and chilled liquid in the shell.
2.
3. They have thermal capacity for application for high but
infrequent loads.
4. They are used for chilling of drinking water and other aspects
where cleanliness is a factor, e.g. Bakeries and Photographic
Laboratories.

9. According to mode of heat transfer
 Natural Convection Evaporators :
a) are often used in applicationswhere low speed air desirable or
damage dehydration product possible.
b) The greater the temperature difference will produce higher rates
of air circulation, because the difference between the density of
warm and cold air increases.
c) Cold air is denser than warm air.Therefore, the natural circulation
evaporators, usually mounted high above the floor die refrigerated
Cabinet.

10. Forced convection evaporator :
a) High velocity air over a
transfer.
closely finned coil can increase heat
b) The use of forced draught can reduce the overall size and weightof
the evaporator which can reduce purchasing and installation costs.
c) Forced draught also gives:-
 more rapid cooling
 even temperatures
 good humidity control
 easy defrosting.
The fins on a forced draught evaporator are very closely spaced which
gives a high capacity rating.

12. FACTORS AFFECTING THE HEAT
TRANSFER CAPACITY OF AN EVAPORATOR
 MATERIAL
 TEMPERATURE DIFFERENCE
 VELOCITY OF REFRIGERENT
 THICKNESS OF THE EVAPORATOR COIL WALL
 CONTACT OF SURFACE AREA

13. TYPES OF EVAPORATORS
 According to type of construction
1. Bare tube coil evaporator
2. Finned tube evaporator
3. Plate evaporator
4. Shell and tube evaporator
5. Shell and coil evaporator
6. Tube in tube evaporator
 According to the manner in which liquid refrigerant is fed
1. Flooded evaporator
2. Dry expansion evaporator

14.  According to the mode of heat transfer
1. Natural convection evaporator
2. Forced convection evaporator
 According to operating condition
1. Frosting evaporator
2. Non-frosting evaporator
3. Defrosting evaporator

15. Bare tube coil evaporator
•Prime surface evaporator
•Easy to clean and defrost
•s/f contact area is less
•Limited applications
Finned tube evaporator
•Over the bare tube metal fins
are fastened
•Shape, size, spacing can be
adapted for better rate of heat
transfer
•Extended surface evaporators

16. Plate evaporator
•The bare coils are either welded
on the plate or between the two
plates which are welded together
•Used in household refrigerators,
beverage cooler, ice cream
cabinets
Shell and tube evaporator
•Contraction is same as shell and
tube type of condenser
•Available in flooded as well as
dry expansion type
•Baffle plates are provided for
good turbulence of liquid
•Capacity 2TR to 250TR

17. Shell and coil evaporator
•Generally dry expansion
evaporators for chilling water
•Used for small capacity
2TR to 10TR
•Restricted to operate above 5
degree calicoes to prevent
freezing problems
Tube in tube evaporator
•Double tube evaporator
•Refrigerant can flow in outer pipe
and liquid to be cooled can flow in
inner pipe
•The flow of refrigerant can be
parallel or counter

18. Natural convection
evaporators
•Low velocity and min. hydration is
require
•Velocity of air depends upon temp.
difference
•Circulation of air around coil
depends upon its size shape and
location
•The coil should occupy 2/3rd of
width of the path & 3/4th the length
of the box
Forced convection
evaporators
•Air is forced over refrigerant coils
•Fins are provided to increase heat
transfer rate
•More efficient than natural
convection evaporators
•Require less cooling surface and
high evaporator pressure can be
used which save power input to the
compressor

19. Frosting evaporators
•Operates below 0°C
•The frost forms on the evaporator
comes from the moisture of the air
•Cooling efficiency is decreases
until the ice and frost is removed
Non-frosting evaporators
•Operates above 0°C therefore
frost does not forms on
evaporators
•Temp. close to cooling 0.6°C to 1°C
•RH from 75-80 % in the cabinet
•This keeps the food fresh & stops
shrinking in weight

20. DEFROSTING EVAPORATORS
•Frost creates on the coils on the
coils when the compressor is
running & melts after the
compressor shuts off
•Temp. of about -7°C to -6°C
•It also keeps high RH of about
90% to 95%