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
Penyejat ialah satu sistem pemindahan haba,
sebahagian daripada kitaran penyejukan di
mana bahan penyejuk menyejat untuk
memindahkan haba dari ruang dalam peti
sejuk atau peralatan.
Evaporators are manufactured in a wide
variety of types, shapes, sizes and designs, and
are classified in the following way:-
type of construction
method of refrigerant feed
method of air (or liquid) circulation
type of controls
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.
Keupayaan mana-mana gegelung penyejat
atau penyejukan adalah pada kadar haba
yang akan melalui dinding penyejat dari ruang
dalam peti sejuk dalam bentuk cecair dan
This type consists of a bank of tubes, copper or
They are used for cold rooms with natural air
circulation over the tubes.
Maintenance consists of keeping the internal
walls of the tubes free from oil and external
surfaces clear of ice.
Copper tubes are used with the halogenated
Steel pipes are used with ammonia R717
On some commercial plants,
pipes containing refrigerant
are sealed between two dole-
The gap between the plates
is filled with a low
temperature freezing point
solution to improve heat
Alternatively, the gap is
(DIPINDAHKAN) to form a
vacuum. This vacuum type
relies on heat transfer being
conducted through metal
contact only. (See Figure 1)
The refrigerant is
contained between two
plates to give a large
and prefromed in a ‘U’
This type of evaporator is
widely used in domestic
The only maintenance
requirement is to keep the
plate free from ice (See
High velocity air over a closely finned coil can
increase heat transfer.
The use of forced draught can reduce the
overall size and weight of the evaporator
which can reduce purchasing and installation
Forced draught also gives:-
more rapid cooling
good humidity control
The fins on a forced draught
evaporator are very closely
spaced which gives a high
Defrosting must be carried
out more frequently.
It is possible to operate these
coils at about 60% flooded
conditions (High velocity air
is forced through the finned
coils with powerful fans.
The fan motors can be single
or three phase).
Dry expansion evaporators are so called because
liquid refrigerant enters the evaporator by an
expansion valve and vaporizes by the time it
reaches the end of the evaporator coil.
There is always at least 20% vapor present within
the evaporator pipe work.
The amount of liquid present in dry expansion
evaporators will depend upon the fixture load.
At light load conditions, the amount of liquid will
be small, and in high load conditions, the amount
of liquid will be large.
The larger the wetted surface, the greater the
efficiency. (See Figure 3)
A flooded evaporator is completely filled with
liquid refrigerant, and the level of the refrigerant is
maintained in an accumulator by a float-valve.
Evaporators are connected in parallel to the
One pipe from the bottom of the vessel supplies
The return pipe containing liquid refrigerant and
vapour enters the vessel near the top.
The liquid from the accumulator is quite often
pumped around the evaporators.
The disadvantage of the flooded evaporator is
that it requires a considerable charge of
refrigerant and is bulky.
The main advantage is that the inside of the
pipes are completely wetted with refrigerant
and this produces a very high rate of heat
Evaporators used to cool liquids can be divided
into five general types:-
double pipe coolers
shell and coil coolers
shell and tube coolers
tank type coolers
This cooler consists of one pipe inside another.
The fluid to be cooled flows in the inside and
the refrigerant flows in the outer space between
the inner and outer tube in the opposite
direction to the fluid to obtain a high heat
transfer rate – counter-flow action
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
They have thermal capacity for application for
high but infrequent loads.
They are used for chilling of drinking water
and other aspects where cleanliness is a factor,
e.g. Bakeries and Photographic Laboratories.
Shell and Tube evaporators consist of a
cylindrical shell in which there are a number of
straight tubes arranged in parallel, held in
place by end plates.
When in operation, refrigerant is expanded in
the tubes with chilled liquid in the shell.
Shell diameters range from 150 mm to 1.5 m,
numbers of tubes from less than fifty to several
thousand. Tube diameters range from 16 mm
to 50 mm with tube lengths 1.5 m to 6 m.
Steel tubes are used for ammonia and copper tubes
used for other refrigerants.
Copper tubes give high heat transfer coefficients.
Evaporators using Freon refrigerants often have
fins on the refrigerant side, i.e. internally (because
of low film conductance of these refrigerants).
The shell and tube evaporators are very common,
highly efficient, compact and readily available.
They are used for both Dry and Flooded
The Baudelot Cooler consists of a series of
horizontal pipes, one under another, connected to
form a circuit.
They can be either dry or flooded.
Refrigerant flows through the tubes, while chilled
liquid flows in a thin film over the outside, (in a
counter flow direction).
This is then collected in a trough at the bottom of
They were used for cooling milk (prior to the
introduction of the E.E.C. Regulations) wine and
water, since it is possible to chill to very nearly
freezing point without damaging equipment
The Tank type cooler consists of bare tube refrigerant coils
installed in the centre or at one end of a large tank
containing chilled liquid.
The tubes are submerged in the liquid but separated from
the main body by a baffle.
An agitator circulates liquid over the coils at a velocity of
Coil forms are spiral or race-way.
The coils are operated flooded. Applications involve chilling
water, brine or other liquids used as a secondary refrigerant.
They are particularly suitable for applications having
frequent and severe fluctuations in load.
Large storage tanks are used to minimize the rise in
temperature of chilled liquid during peak demand. (See
To summarise some of the points about coolers, we can
classify them as follows:-
Sub-classified as to whether refrigerant, either (a) inside
tubes or (b) outside tubes with a corresponding
classification of dry or flooded.
Has a thermostatic expansion valve metering just the correct
amount of liquid refrigerant to maintain predetermined
degree of superheat
Has a float valve maintaining liquidate constant level, ie, as
fast as refrigerant evaporates, more liquid is admitted by the