working and types of evaporators

1. EVAPORATORS
REFRIGERATION AND AIR CONDITION
HRISHIKESH MAHADIK

2. EVAPOREATORS
•AT LOW PRESSURE
SIDE OF
REFRIGERATION
SYSTEM
•REFRIGERENT
CHANGES FROM LIQUID
PHASE TO VAPOUR
•HEAT ABSOPTION

3. CAPACITY OF AN EVAPORATOR
THE AMOUNT OF HEAT ABSORBED BY IT OVER A GIVEN
PERIOD OF TIME.
HEAT TRANSFER CAPACITY OF AN EVAPORATOR:
Q = UA(T2 - T1) ….. W or J/s
WHERE;
U = OVERALL HEAT TRANSFER COIFICIENT
A = AREA OF EVAPORATOR SURFACE
T2 = TEMPERATURE OF MEDIUM TO BR COOLED
T1 = SATURATION TEMPERATURE OF
REFRIGERENT AT EVAPORATOR PRESSURE

4. 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

5. 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

6.  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

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

8. Plate evaporator Shell and tube 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
•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

9. Shell and coil evaporator Tube in tube 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
•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

10. Natural convection
evaporators
Forced 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
•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

11. Frosting evaporators Non-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
•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

12. 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%

13. APPLICATIONS:
 EVAPORATORS ARE USED IN WIDE RANGE OF PROCESS
INDUSTRIES, PAPER AND PULP ONDUSTRIES, WINERY, BEVERGARES,
FOOD PRESERVATION, ICE PLANTS, CHEMICAL, POLYMERS AND
RESINS, INORGANIC SALTS, ACIDES AND VERITY OF OTHER
MATERIALS