2.

REFRIGERATION EQUIPMENT

Basic Components of Vapour Compression Refrigeration System

Basic components parts of a mechanical refrigeration system are common both for refrigeration
and air-conditioning systems. They are;

• Compressor
• Condenser
• Expansion device
• Evaporator.

1. COMPRESSOR
Is a mechanical device it compresses the refrigerant. The compressor receives low pressure gas
from the evaporator and converts it to high pressure gas. So that the corresponding temperature
is higher than that of the cooling medium.

There are two different pressures in the refrigeration cycle. The evaporator / low pressure is
being maintained by the expansion valve. The condenser / high pressure being maintained
by the compressor. Pipe from the outlet of evaporator to inlet of compressor is “Suction Line”.
The pipe at the outlet of the compressor is called the “Discharge Line”. Compressor consists
of an arrangement in which an electric motor drive it.

There are two types of machines:

(i) Positive displacement machines (reciprocating, rotary, scroll and screw

compressors).
(ii) Non-positive displacement machines (centrifugal compressors).

Positive displacement machines ensure positive admission and delivery preventing undesired
reversal of flow within the machine as achieved by the use of valves in the case of reciprocating
compressors. They have intermittent operation, subjecting the fluid to non-flow processes, and
work is transferred by virtue of a hydrostatic force on the moving boundary.

Non-positive displacement machines, i.e., centrifugal compressors have no means to prevent

the reversal of flow. The fluid is subject to flow processes and the work is transferred by virtue
of the change of momentum of a stream of fluid flowing at a high speed over blades or vanes
attached to a rotor.
 i Reciprocating Compressor.

Are one of the most widely used types of compressors for refrigeration and air conditioning
applications. The reciprocating compressors comprise of the piston and the cylinder
arrangement similar to the automotive engine. While the engine generates power after
consuming fuel, the reciprocating compressor consumes electricity to compress the refrigerant.
Inside the cylinder the piston performs reciprocating motion which enables the compression of
refrigerant inside it.

These compressors are used for refrigerants which have comparatively low volume per kg and
a large differential pressure, such as ammonia, R-12, R-22.

The reciprocating compressors are classified according to their enclosing pattern as

hermetically sealed, semi-sealed and open-type. The hermetically sealed and semi-hermetic
compressors have their motor enclosed along with the cylinder and crank-case inside a dome.
The motor windings are cooled by incoming suction vapours. These have the advantage of no
leakage, less noise and compactness.

Task: Discuss the hermetically sealed, semi-sealed and open-type compressor.

Principle of Working of the Reciprocating Compressors

A reciprocating compressor has a piston that moves downwards thus reducing pressure in its
cylinder by creating a vacuum. This difference in pressure forces the suction chamber valves
to open and bring gas or air in. When the cylinder goes up, it increases pressure thus forcing
the gas or air out of the cylinder through a discharge chamber.
Apart from the piston and the cylinder arrangement, the reciprocating compressor also
comprises of the crankshaft, connecting rod and other small connecting elements. The
crankshaft is connected to the electric motor directly by coupling or by belt and driven by the
pulley arrangement. The rotary motion of the crankshaft is converted into the reciprocating
motion of the piston inside the cylinder via the connecting rod.

ii Rotary Compressor

Rotary vane compressors are positive displacement, direct-drive machines. consist of a

cylindrical casing, two openings - one suction and one discharge - and a rotor positioned
eccentrically with respect to the casing. Compression occurs by refrigerant flowing into the
chamber where, due to eccentric rotation, there is a reduction in the desired volume.

Recently, rotary vane compressor technology (generically called “rotary”) has evolved through
the introduction of “twin” compressors with two rotors revolving in counter rotating directions.
These feature lower noise and generate less vibrations to a level that makes twin rotary
compressors a good alternative.

There are essentially two designs of this compressor:

(i) Rolling piston type. In the rolling piston type, shown in Fig. (a) the roller is mounted
on an eccentric shaft with a single blade, which is always in contact with the roller
by means of a spring.
 (ii) Rotating vane type. In the rotating vane type, as shown in Fig. (b) with four vanes,
the rotor is concentric with the shaft. The vanes slide within the rotor but keep
contact with the cylinder. The assembly of rotor and the vanes is off-centre with
respect to the cylinder. In both designs, the whole assembly is enclosed in a housing
(not shown in the figures), filled with oil and remains submerged in oil. An oil film
forms the seal between the high-pressure and the low-pressure sides. When the
compressor stops, this seal is lost and the pressure equalizes.

iii Scroll compressor

Scroll compressors use two scrolls, one fixed and the other moving and coupled to the
motor. The scrolls are interleaved so as to be in contact with one another in various
places and thus form a series of gradually thinner pockets of air towards the center. The
fluid is compressed by rotation of the orbiting scroll around the fixed scroll until being
discharged in the center.
Figure shows the scroll compression process in a sequence of suction, compression and
discharge.
Fig.(a) The outermost pockets are sealed off. Suction gas is trapped just before
compression begins.
Fig.(b) At stage (b), orbiting motion moves the gas to the centre. Pocket volume
progressively reduces, and pressure rises, as shown in (c).
Fig.(d) At (d), the gas reaches the discharge port in the centre, and discharge takes place.
Simultaneously, suction begins at the outer periphery.
 Advantages of scroll compressors include smaller dimensions and lower weight than
reciprocating compressors of the same capacity, even if higher than rotary compressors.
They also have very high efficiency at the design compressor ratio, which however
decreases at different working conditions.
The compressor gives high efficiency, and has low noise level.

iv Screw compressors
Rotary screw compressors also belong to the category of positive displacement
compressors. The machine essentially consists of two helically-grooved rotors as
illustrated in Figure below which rotate in a housing.
Are based on a mechanism made up of two intermeshing screws, called rotors. As the
rotors revolve, the fluid is drawn in through the inlet port, filling the volume between
two lobes. When the spaces between the lobes are full of fluid, suction ceases, the fluid
between the lobes is then forced due to rotation into an increasingly small space,
causing compression. When the lobes reach a certain position, the compressed fluid is
completely discharged through the outlet port. These compressors require abundant
lubrication due to the meshing of the two rotors.
v Centrifugal compressors
A single-stage centrifugal compressor mainly consists of the following four
components as shown in Figure below.
(i) An inlet casing to accelerate the fluid to the impeller inlet.
(ii) An impeller to transfer energy to the fluid in the form of increased static
pressure (enthalpy) and kinetic energy.
(iii) A diffuser to convert the kinetic energy at the impeller outlet into enthalpy
resulting in pressure rise.
(iv) A volute casing to collect the fluid and to further convert the remaining kinetic
energy into enthalpy resulting in further pressure rise.
 2. CONDENSER

The high-pressure high temperature refrigerant enters the condenser through the discharge
pipe. Heat from the hot refrigerant gas is rejected in the condenser to the condensing
medium-air or water. Its purpose is to covert all the vapour refrigerant to liquid refrigerant
delivered by the compressor.

Types of Condensers.

There are three types of condensers,

i Air-Cooled Condensers
Air-cooled condensers use air as the cooling medium. Heat transfer takes place air
convection around the condenser surface. There are two types of air-cooled condensers
are used.
(a) Natural air-cooled condensers - Air movement over the surface of condenser tubes
is by natural convection. These are cooled by natural air or atmospheric air. As air
comes in contact with the warm-condenser tubes, it absorbs heat from the refrigerant.
This type of condensers is used in domestic refrigerator, deep freezer, etc.
(b) Forced air cooled condensers – Air is circulated by means of a fan or blower. For
circulating air over the condenser, fans are mounted on the shaft/pulley of the
compressor motor. This type of condensers is compact in design and are relatively more
efficient. This type of condensers is used in air conditioner, water cooler etc.
Merits of Air-Cooled Condensers
1. Cheap due to simplicity of construction
2. Low maintenance cost.
3. No piping work involved.
4. Negligible corrosion effect.
5. Disposal of air is easier in comparison to disposal of water.
6. High flexibility.
Demerits of Air-Cooled Condensers
1. It is restricted to small capacity refrigeration units.
2. Rate of heat transfer is low.
3. Distribution of air on the condenser surface area is uneven.
4. Increase in ambient temperature causes reduction in the capacity of the condensers.
ii Water Cooled Condenser
The condenser which cooled by water, is called water cooled condenser. These
condensers are used in large capacity refrigerant plant such as, cold storage, ice plants
and central air-conditioners.
There are three types of water-cooled condenser
(a) Shell and tube condensers - A shell and tube condenser consist of a cylindrical shell,
in which a large number of parallel tubes are fitted and connected with tube sheets
on both ends of these tubes. These tubes are made of steel or copper tubes. These
tubes are generally finned to increase their surface area. The diameter of these tubes
is 19mm,25mm,30mm etc. The lengths are in between 1.5 m and 5.5m. These
condensers have the capacity ranging from 2 ton to 1000 ton. Cooling water enters
through the heads which are baffled to make one or more passes through the tubes.
Refrigerant vapour from the compressor enters shell at the top and gives its heat to
cooling water. After the completion of condensation, the liquid refrigerant flows
down and remain in the lower part of the shell. Which act as a liquid receiver. Is the
most commonly used condenser.

(b) Shell and coil condensers - Shell and coil type condenser is used for small tonnage
low pressure units. It consists of shell that contains a coil for circulating the water.
The refrigerant gas within the shell condenses on the outside of the water coils. The
liquid refrigerant collects at the bottom of the shell where it is then removed through
a bottom outlet. The shell ends are not removable and the water side of the coil may
be cleaned with chemicals. In case of a coil leakage the entire coil must be replaced.
 These are preferable where clean water is available because its coil cannot be
cleaned easily.

(c) Tube in tube type condensers - In this type of condenser, water flows through the
inner tube while the refrigerant flows through the circular space between these two
tubes. This type of condenser is also known as a Double-tube condenser. Water
flows in the opposite direction to the refrigerant. The refrigerant vapour from the
compressor enters at the top and flows downwards in the clearance space between
the two tubes. As result of heat transfer from refrigerant to water, the refrigerant
gets condensed. The liquid refrigerant leaves the condenser at the bottom. Since
copper tubes cannot be used with ammonia. These are made of steel pipes for
ammonia.
iii Evaporative Condenser
The Evaporative condenser combines the function of the condenser and cooling tower.
The refrigerant first rejects its heat to water and then water rejects its heat to air, mainly
in the form of evaporated water. Air leaves with high humidity as in a cooling tower.
Thus, an evaporative condenser combines the functions of condenser and cooling tower.
Its works on the principle of Evaporative cooling. The water evaporation when it comes
in contact with the refrigerant flowing in the condensing coil. In this, both air and water
are used as the cooling media. The condensing coil are encased in galvanized steel
cabinet. Water is sprayed over the condensing coil through which hot refrigerant vapour
flowing. A pump is utilized for this purpose. An exhaust fan is fitted at the top of the
condenser. It sucks air from side opening. Eliminators are provided in the condenser to
prevent the escaping of water particles with air.
3. EVAPORATOR

It is part of the system in which the refrigerant evaporates or boils; It is the cooling unit and
some times called the cooling coil or freezing coil. the refrigerant boils or evaporates and
in doing so absorbs heat from the substance being refrigerated. Evaporators are mainly
classified as flooded or direct-expansion.

(a) Dry – Expansion Evaporation - In this, the liquid refrigerant is fed by expansion
valve. The refrigerant entered in to one end of the tube and the other end of tube is
connected to suction line. The evaporator is filled with mixture of liquid and vapour
refrigerant. As refrigerant passes through the evaporator, more and more liquid is
vapourised. Thus, the refrigerant leaves the evaporator in dry state. A Feeder bulb
is provided to control the flow of refrigerant in to the evaporator. Are used in
domestic and commercial refrigeration units.

(b) Flooded evaporators - In a Flooded type evaporator a constant refrigerant liquid

level is maintained. A float valve is used as throttling device which maintains a
constant liquid level in the evaporator. It consists of a shell in to which the
refrigerant liquid is fed through the float valve. The shell is connected to the top and
bottom of the coil. The liquid flows from the bottom of the shell by gravity to coil
tubes in which it evaporates by absorbing heat from the surrounding. The liquid
vapour mixture from the coil returns to the shell. In the shell, the liquid and vapour
 are separated. The vapour is collected at the top of the shell (Flash chamber) from
there it enters into the compressor through suction line.

4. EXPANSION DEVICE

Expansion device is another basic component of a refrigeration system. The basic functions
of an expansion device used in refrigeration systems are to:

• Reduce pressure from condenser pressure to evaporator pressure, and

• Regulate the refrigerant flow from the high-pressure liquid line into the evaporator
at a rate equal to the evaporation rate in the evaporator.

There are different types of expansion or throttling devices. The most commonly used are:

Basically, there are two types of expansion devices. These are:

(a) Variable-restriction type - In the variable-restriction type, the extent of opening or

area of flow keeps on changing depending on the type of control. There are two
common types of such control devices;
i The automatic expansion valve - The basic function of an automatic expansion valve is
to maintain constant pressure in the evaporator.
ii The thermostatic expansion valve. A thermostatic-expansion or T-X valve is a throttling
device which works automatically, maintaining proper and correct liquid flow as per
the dictates of the load on the evaporator.
 The functions of the thermostatic-expansion valve are:

• To reduce the pressure of the liquid from the condenser pressure to evaporator
pressure,
• To keep the evaporator fully active and
• To modulate the flow of liquid to the evaporator according to the load requirements
of the evaporator so as to prevent flood back of liquid refrigerant to the compressor.

iii

In addition, there are the float-valves which are also variable restriction type
devices. The float valves again are of two types: The high-side float and the low-
side float. The high-side float maintains the liquid at a constant level in the
condenser and the low-side float maintains the liquid at a constant level in the
evaporator.
(b) Constant-restriction type - The constant-restriction type device is the capillary
tube which is merely a long tube with a narrow diameter bore.
Capillary tube - It is a long and narrow tube connecting the condenser directly to
the evaporator. Has a very small orifice usually 0.5 to 2.25mm inside diameter and
length is about 1 to 6 m. The capillary tube reduces the pressure of the refrigerant
from high-side pressure to low-side pressure. Liquid refrigerant from condenser
enters in to the capillary tubes and as it flows through the tube pressure drops due
to friction.
The capillary tube is used for small units such as Domestic Refrigerator, Room Air-Conditioner
and deep Freezers.

The advantages of capillary tubes are

• It is simple in construction and requires no maintenance

• When the compressor stops, the refrigerant continues to flow from high
• pressure side to low pressure side unless pressure is equalized.
• This requires low starting torque to start the compressor so a low
• starting torque motor can be used with these unit.
• No receiver is required for systems using this device.
• Its cost is much less as compared to other devices.

Disadvantages

• The refrigerant must be free from moisture and dirt otherwise it will choke the tube and
stop the flow of refrigerant.
• It cannot be used with high fluctuating loads.
SOLENOID VALVE

The solenoid valve is shut off valve that is actuated by an electromagnetic coil when the coil is
energised.

An armature or plunger placed inside the coil. A stem or pin attached to plunger open or close.
When the coil energised by passing current through it, the magnetic field attracts the plunger
up into the core of the coil which causes the opening of the valve. When the coil is deenergised
due to break in the circuit, the plunger fall of its own weight and pin closes the valve. These
valves fitted vertical and allows the fluid flow in one direction.