Explaination of compressor used in refrigeration system

1. Chapter 4 (Part 1)
4.1 Principles
4.2 Equipment
4.3 Evaporators
4.4 Types of compressors
4.5 Reciprocating compressors
4.6 Rotary compressors
4.7 Screw (Helical rotary) compressor
4.8 Scroll compressor
4.9 Centrifugal compressor
4.10 Capacity control of compressors

2. Refrigeration cycle
4 basic component in refrigeration
system
1. Evaporator
2. Compressor
3. Condenser
4. Expansion valve/metering
device/flow control device

3. Refrigerant
absorb heat
from warm air
and reach low
boiling point.
Refrigerant
vaporize and
absorb
maximum heat
Pressure has been reduced,
Dissipating heat
(cooler than fan air flowing around)
Low temperature and low
pressure refrigerants enter
compressor.
A liquid at a high pressure is
needed to achieve
refrigerating effect
Refrigerant at high
pressure and
temperature but in the
state of gas. Heat from
refrigerant is
transferred to the
cooling fluid.
Refrigerant
condenses to liquid
Refrigerant is high temperature and high
pressure but loses pressure when go through
expansion valve.some refrigerant vaporize
and absorb heat from refrigerant that
doesn’t vaporize(cooling effect)

4. Evaporator

5. Evaporator
 refrigerant liquid is converted to gas, absorbing heat from the air in the compartment.
 A larger area is cooled, which requires that large volumes of air be passed through the evaporator coil for
heat exchange.
 A blower becomes a necessary part of the evaporator in the air conditioning system.
 The blower fans must not only draw heat-laden air into the evaporator, but must also force this air over the
evaporator fins and coils where it surrenders its heat to the refrigerant and then forces the cooled air out of
the evaporator into the space being cooled.
• Refrigerant flows through tubing
• No liquid storage or refrigerant in the
evaporator
• Exists in 2 types-DX cooling coils(air) ,DX
chillers(water or other liquids)
Dry expansion
evaporator(DX)
• A liquid pool of refrigerant is maintained
Flooded
Evaporator

6. Diagram

7. Technical Problems
 Changing the state of the refrigerant in the evaporator coils is as
important as the air flow over the coils. Liquid refrigerant supplied to
the coils by the expansion valve expands to a vapor as it absorbs heat
from the air. Some liquid refrigerant must be supplied throughout the
total length of the evaporator coils for full capacity.
 A starved evaporator coil is a condition in which not enough
refrigerant has been supplied through the total coil length. Therefore,
expansion of the refrigerant has not occurred through the whole coil
length, resulting in poor coil operation and too-low heat exchange.
 A flooded evaporator is the opposite of the starved coil. Too much
refrigerant is passed through the evaporator coils, resulting in
unexpanded liquid passing onto the suction line and into the
compressor.

8. Compressor

9. Compressor
Positive Displacement
(increase P of air by reducing V)
Reciprocating Rotary Scroll Screw
Dynamic
(transfer of energy from a rotating
impeller to the air)
Centrifugal
Type of Compressor
(based on type of mechanism used for compression)

10. 1. Reciprocating Compressor

11. 1. Reciprocating Compressor
 Reciprocating air compressors are positive displacement machines, meaning
that they increase the pressure of the air by reducing its volume. This means
they are taking in successive volumes of air which is confined within a closed
space and elevating this air to a higher pressure.
 Construction is similar to the reciprocating engine of a vehicle with pistons,
cylinder, valves, connecting rods and crankshaft
 The compressor functions by drawing gas into the cylinder, compressing it,
and sending it out into a holding tank or supply line. This cycle is repeated
continuously producing a constant supply of compressed gas.
 available either as air-cooled or water-cooled in lubricated and non-
lubricated configurations and provide a wide range of pressure and capacity
selections.

12. Operation
1. TDCBDC
Suction pressure created,opening of
inlet valve,atmospheric air enter
cylinder
2. Compression
Compressed with both inlet closed.
Compression continued until the pressure
of air inside becomes sufficient to cause
deflection in exit valve
3. Exhaust
Exit valve get lifted, compressed air
exhaust. Piston reaches TDC
TDC
(Top dead
Centre)
BDC
(Bottom dead
Centre)
4. Continuous
Cycle repeat continuously

13. Diagram

14. Advantages
•the initial cost of installing
them is typically lower.
•able to provide continuous
flow
•able to produce both high
power and high pressure
•do not suffer from the
problem of oil carry over
Disadvantages
•high cost of maintaining it
because of the many
moving parts
•reciprocation of the
cylinder causes vibration

15. 2. Rotary Compressor

16. 2. Rotary Compressor
 The rotary compressors have two rotating elements, like gears, between which the
refrigerant is compressed. These compressors can pump the refrigerant to lower
or moderate condensing pressures.
 can handle small volume of the gas and produce lesser pressure
 used in fewer applications. Typically used to power small cooling devices, such as
window air conditioners, packaged terminal air conditioners and heat pumps up to
five tons.
 Two main types of rotary compressors: stationary blade and a rotating blade.
 Blade (scapula) on a rotating blade rotary compressor rotates with the shaft.
 Stationary blade, which remains fixed, and part of the body assembly. In both
types, the blade provides continuous printing for refrigerant vapour.
 Rotating blades of compressors are often used as a "booster compressor cascade
systems. This is the name commonly given to the first compressor in a cascade
system.

17. Diagram

18. 1. Low vapour pressure of the suction line is drawn into the hole
2. A pair of elbow space for the blade as it spins.
3. As the blades rotate, the steam trap in the space in front of the blade is
compressed until it can be moved into the exhaust line of the capacitor.
Operation

19. Advantages
• They can deliver liquid to high
pressures.
• Give a relatively smooth
output, (especially at high
speed).
• Positive Acting.
• Can pump viscous liquids.
• Small size compressor can
produces high flow rate.
• Less vibration.
• Low leakage levels.
Disadvantages
• More expensive than
centrifugal pumps.
• Should not be used for fluids
containing suspended solids.
• Excessive wear if not pumping
viscous material.
• Must never be used with the
discharge closed.
• Not suitable for dirty
environment.
• Short lifetime

20. 3. Scroll Compressor

21. 3. Scroll Compressor
 Compared to reciprocating compressors, scroll compressors expel smaller
portions of refrigerant more frequently. This leads to smaller pulsations. For
installers, this means that mufflers to dampen pulsations do not need to be
used as often.
 Systems featuring scroll compressors are also much less likely to suffer
problems with noise or malfunctions in pressure switches caused by
pulsations.
 When a compressor is operating, one of the two spirals is pressed against the
other with the help of a medium pressure from a “pocket” in the scroll set
where full compression has not yet been achieved. This results in the two
spirals “engaging” with one another. However, it takes a maximum of 72 hours
of operation before this “engagement phase” is complete.

22. Diagram
In scroll compressors, the crankshaft is arranged
vertically. The scroll set is located above it. This scroll
set comprises one fixed and one orbiting spiral. These
two spirals mesh with one another, compressing the
refrigerant through an orbital motion from the outer part
of the scroll set towards the middle. As a result of this
principle, there are various stages of compression
(differently sized “pockets” in which compression is
currently occurring) at any point in the compression
process.

23. Operation

24. Advantages
• reduced energy costs.(100% volumetric
efficiency due to no piston)
• Re-expansion losses, a typical feature of each
piston stroke encountered in reciprocating
models, are eliminated. In addition, valve
(ports) losses are eliminated, since suction
and discharge valves (ports) do not exist.
• due to the absence of several moving parts,
scroll compressors are considerably quieter in
operation compared to other types of
compressors
• Their weight and footprint are considerably
smaller
• Gas pulsation is also minimized, if not
eliminated and consequently, they can
operate with less vibration.
Disadvantages
• they are generally not easily repairable. They
cannot be disassembled for maintenance.
• Many reciprocating compressors are tolerant
on rotating in both directions. This is usually
not the case for scroll compressors.
• Incremental capacity control on systems with
several scroll compressors has proven several
times to be problematic as well.

25. 4. Screw Compressor

26. 4. Screw Compressor
 A rotary screw compressor is a type of gas compressor which uses a rotary type
positive displacement mechanism. They are commonly used to replace piston
compressors where large volumes of high pressure air are needed, either for large
industrial applications or to operate high-power air tools.
 The gas compression process of a rotary screw is a continuous sweeping motion, so
there is very little pulsation or surging of flow, as occurs with piston compressors.
 The effectiveness of this mechanism is dependent on precisely fitting clearances
between the helical rotors, and between the rotors and the chamber for sealing of the
compression cavities.

27. Diagram
 Rotary screw compressors use two
meshing helical screws, known as rotors,
to compress the gas.
 In a dry running rotary screw compressor,
timing gears ensure that the male and
female rotors maintain precise alignment.
 In an oil-flooded rotary screw compressor,
lubricating oil bridges the space between
the rotors, both providing a hydraulic seal
and transferring mechanical energy
between the driving and driven rotor.
 Gas enters at the suction side and moves
through the threads as the screws rotate.
The meshing rotors force the gas through
the compressor, and the gas exits at the
end of the screws.

28. Operation

29. Advantages
•Higher specific
output.
•Less moving parts
•Reduced Vibration
•Less refrigerant
loss
Disadvantages
•Debris
•bearings

30. 5. Centrifugal Compressor

31. 5. Centrifugal Compressor
 The centrifugal compressor uses the principle of dynamic compression, which
involves converting energy from one form to another, to increase the pressure
and temperature of the refrigerant. It converts kinetic energy (velocity) to
static energy (pressure). The core component of a centrifugal compressor is
the rotating impeller.

32. Diagram

33. Operation
 Centrifugal compressor's fluid enters axially (in the same direction as the axis of
the rotating shaft) in the center of the pump, but is discharged radially (or
tangentially) along the outer radius of the pump casing.
 Centrifugal compressor is based on that the action of high speed airflow and
working impeller or fixed blade.
 The mechanical energy exist is increased by centrifugal action when the gas enters
the suction eye of a high speed rotary element called impeller. These impeller
caries radial canes integrally cast in it. It function is to pick up and accelerate air
outward to the diffuser.
 As the gas was force to outward from the centre the impeller to the outer rim, the
increase in velocity of the gas creates a flow pressure way at the eye of the
impeller.
 In diffuser, the velocity of the gas was decreases in the pressure of the gas.
 Centrifugal compressors can be use a variety of blade orientation including both
forward and backward curves as well as other designs. There may be several stages
to a centrifugal compressor and the result would be the same; a higher pressure
would be produce.

34. Advantages
•High reliability, eliminating the need for multiple
compressors and installed standby capacity.
•For the same operating conditions, machine prices
are lower for high volume flow rates.
•Less plot area for installation for a given flow rate.
•Machine is small and light weight with respect to its
flow rate capacity.
•Installation costs are lower due to smaller size
•Low total maintenance costs
•When a turbine is selected as a driver, the centrifugal
compressor’s speed level allows direct drive (no gear
unit), thereby minimizing equipment cost, reducing
power requirements, and increasing unit reliability.
•Flow control is simple, continuous, and efficient over
a relatively wide flow range.
•No lube (or seal) oil contamination of process gas.
•Absence of any pressure pulsation above surge point.
Disadvantages
•Lower efficiency than most positive displacement
types for the same flow rate and pressure ratio,
especially for pressure ratios over 2.
•Due to recycle not efficient below the surge point.
•Very sensitive to changes in gas properties, especially
molecular weight
•Not effective for low molecular weight gases. The
pressure ratio capability per stage is low, tending to
require a large number of machine stages, hence
mechanical complexity.

35. Capacity Control

36.  On refrigeration and air conditioning applications where the load may vary over a wide range,
due to lighting, occupancy, product loading, ambient weather variations, or other factors,
some means of compressor capacity control is desirable for optimum system performance.
 Capacity modulation can reduce power and energy consumption, provide better
dehumidification, reduce compressor cycling, decrease the starting load, and provide good oil
return if properly piped.
 The simplest form of capacity control is on-off cycling of the compressor. Under light load
conditions, this could lead to short cycling and could reduce the life of the compressor.
 On systems where ice formation is not a problem, users will sometimes lower the low
pressure cut out setting beyond the design limits in order to prevent short cycling. As a
result, the compressor may operate for long periods at extremely low evaporator
temperatures.
 Compressor capacity decreases as suction pressure decreases. Refrigerant velocity is
inadequate to return oil to the compressor also resulting in a high compressor superheat,
which causes the compressor to overheat. All of these conditions can cause premature
compressor failure.
 Capacity control allows more continuous operation of the compressor, minimizing electrical
problems and improving lubrication.
 There are many ways to achieve capacity control. Variable speed compressors, hot gas bypass
with or without liquid injection, unloading, Moduload, Digital control for scrolls, and simple
on/off compressor operation on multiple compressor setups. Some applications will use two
or more methods for smoother switching and better control such as unloading in conjunction
with hot gas bypass. This article will deal with the most common methods today which are
unloading and hot gas bypass.