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

Refrigeration cycle

4 basic component in refrigeration system

1. Evaporator2. Compressor3. Condenser 4. Expansion valve/metering

device/flow control device

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)

Evaporator

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.

Dry expansion evaporator(DX)

•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)

Flooded Evaporator

•A liquid pool of refrigerant is maintained

Diagram

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.

Compressor

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)

1. Reciprocating Compressor

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.

Operation1. TDCBDCSuction pressure created,opening of inlet valve,atmospheric air enter cylinder2. CompressionCompressed with both inlet closed.Compression continued until the pressure of air inside becomes sufficient to cause deflection in exit valve 3. ExhaustExit valve get lifted, compressed air exhaust. Piston reaches TDC

TDC(Top dead Centre)

BDC(Bottom

dead Centre)

4. ContinuousCycle repeat continuously

Diagram

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

2. Rotary Compressor

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.

Diagram

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

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

3. Scroll Compressor

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.

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.

Operation

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.

4. Screw Compressor

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.

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.

Operation

Advantages• Higher specific output.

• Less moving parts

• Reduced Vibration

• Less refrigerant loss

Disadvantages• Debris • bearings

5. Centrifugal Compressor

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.

Diagram

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.

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.

Capacity Control

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.