1.0 TITLE

Compressor in Refrigerant System

2.0 INTRODUCTION

The transfer of heat from a low temperature medium to a high temperature one

requires special devices call refrigerants. A refrigerant is a compound used in a heat cycle

that undergoes a phase change from a gas to a liquid and back.

The refrigerant enters the compressor as a vapor and compressed to the condenser

pressure. Refrigerant leaves the compressor at a relatively high temperature, cools down

and condenses as flows through the coils of the condenser by rejecting heat to the

surrounding medium. Then, enters a capillary tube where the pressure and temperature

drop drastically due to the throttling effect. The low temperature refrigerant then enters

the evaporator, where evaporates by absorbing heat from the refrigerated space. The

cycle is completed as the refrigerant leaves the evaporator and reenters the compressor.

The refrigerant can be applied in Industrial refrigeration systems present in an

ethylene plant or an ammonia plant is designed based on the demand of refrigerant in the

process. Once the function of the heat exchangers and the temperatures of the refrigerant

are specified, the refrigeration system consisting of compressors, condensers and flash

drums is designed. The refrigeration system is design by using feed forward or feed back

controllers to determine required refrigerant flow rates and the resulting compressor

capacities.

3.0 DISCUSSION

3.1 Types of Compressor

The compressor is dividing into two types which are positive displacement and

dynamic. For positive displacement is contains reciprocating and rotary type. For

dynamic is dividing into centrifugal and axial.

Figure 1: Types of Compressor

3.1.2 Positive Displacement

A reciprocating compressor is a compressor that use pistons driven by a

crankshaft to deliver gases at high pressure. The intake gas enters the suction manifold,

then flows into the compression cylinder where gets compressed by a piston driven in a

reciprocating motion via a crankshaft, and then discharged. This equipment is used in a

great many industries, including oil refineries, gas pipelines, chemical plants, natural gas

processing plants and refrigeration plants.

Figure 2: The six-cylinder reciprocating compressor

Rotary air compressors are positive displacement compressors. The most common

rotary air compressor is the single stage helical or spiral lobe oil flooded screw air

compressor. These compressors consist of two rotors within a casing where the rotors

compress the air internally. There are no valves. These units are basically oil cooled (with

air cooled or water cooled oil coolers) where the oil seals the internal clearances.

Rotary screw air compressors are easy to maintain and operate. Capacity control

for these compressors is accomplished by variable speed and variable compressor

displacement. For the latter control technique, a slide valve is positioned in the casing. As

the compressor capacity is reduced, the slide valve opens, bypassing a portion of the

compressed air back to the suction. Advantages of the rotary screw compressor include

smooth, pulse-free air output in a compact size with high output volume over a long life.

The oil free rotary screw air compressor utilizes specially designed air ends to compress

air without oil in the compression chamber yielding true oil free air. Oil free rotary screw

air compressors are available air cooled and water cooled and provides the same

flexibility as oil flooded rotaries when oil free air is required.

3.1.3 Dynamic Compressor

The axial compressor are rotating, aerofoil based compressors in which the

working fluid principally flows parallel to the axis of rotation. This is in contrast with

centrifugal and axi-centrifugal compressors where the air enters axially but will have a

significant radial component on exit.

Axial flow compressors produce a continuous flow of compressed gas, and have

the benefits of high efficiencies and large mass flow capacity, particularly in relation to

their cross-section. However, require several rows of aerofoils to achieve large pressure

rises making them complex and expensive relative to other designs (centrifugal

compressor).

Axial compressors are widely used in industrial applications such as large volume

air separation plants, blast furnace air, fluid catalytic cracking air, and propane

dehydrogenation.

Figure 3: The Axial Compressor

The centrifugal air compressor is a dynamic compressor which depends on

transfer of energy from a rotating impeller to the air. Centrifugal compressors produce

high pressure discharge by converting angular momentum imparted by the rotating

impeller (dynamic displacement). In order to do this efficiently, centrifugal compressors

rotate at higher speeds than the other types of compressors. These types of compressors

are also designed for higher capacity because flow through the compressor is continuous.

Adjusting the inlet guide vanes is the most common method to control capacity of

a centrifugal compressor. By closing the guide vanes, volumetric flows and capacity are

reduced. The centrifugal air compressor is an oil free compressor by design. The oil

lubricated running gear is separated from the air by shaft seals and atmospheric vents

3.2 Properties of Compressor

A compressor is a machine for raising a gas to compressible fluid at a higher level of

pressure.

3.3 Sample Calculation for Compressor

The calculation is done for compressor equipment part only in refrigerant system from

the PFD.

3.4 Process Flow Diagram (PFD)

Figure 5: The Single-level Refrigerant System

Figure 5 shows the basic elements of a single level refrigeration system. The

vapor is compressed in a compressor C1 from pressure P1 to P2 while temperature rises

from T1 to T2. The compressed ethylene is cooled to temperature T3 and condensed to

liquid in an exchanger E1. The liquid ethylene is expanded adiabatically in an expansion

valve V1 to P3 while the temperature is further lowered to T4 and the liquid is partially

vaporized. The vapor is separated in a flash drum from the liquid which is used as a

refrigerant in a process exchanger E2. The process stream is cooled and the liquid

refrigerant absorbs the process duty Q and is vaporized. The vapor ethylene returns to the

suction head of the compressor hence completing the cycle.

The size of the compressor C1, exchanger E1, expansion valve V1 and the flash

drum F1 depends on the temperature level of the refrigerant multi-level industrial

refrigeration system is comprised of a combination of several single level refrigeration

system. There are four-level ethylene refrigeration system which has four refrigeration

loops each consisting of a compressor, a flash drum and several process exchangers

demanding refrigerants.

4.0 CONCLUSION

4.1 Advantages Very mature technology.

Relatively inexpensive.

Can be driven directly using mechanical energy (water) or with electrical energy.

4.2 Disadvantages

Many refrigeration systems use compressors having lower efficiencies of between 40-

55%, since the 60% efficient ones cost almost twice as much as the lower efficiency

ones.

4.3 Applications

Partial lists of compressor applications are including:

In pipeline transport of natural gas to move the gas from the production site to the

consumer.

In oil refineries, natural gas processing plants, petrochemical and chemical plants

In air separation plants to manufacture purified end product gases.

In refrigeration and air conditioner equipment refrigerant cycles: see Vapor-

compression refrigeration.

In industry and manufacturing to supply compressed air for all types of pneumatic

tools.

In oil field re-injection of high pressure natural gas to improve oil recovery

5.0 REFERENCES

1. http://en.wikipedia.org/wiki/Axial_compressor

2. http://www.engineeringtoolbox.com/air-compressor-types