Abstract

This report covers the types of compressors used in refrigeration and air conditioning and the selection criteria of a compressor for a particular case. This report covers the thermo dynamical as well as mechanical working of a compressor, their various features and how do they perform the compression action. With this working we can better understand the basis on which we select a compressor for some refrigeration process

Introduction

Compressors are widely used in industries to transport fluids. It is a mechanical device that compresses a gas. There are many types of compressors, thus a proper selection is needed to fulfil the typical necessity of each industry. Generally, the compression of gases may be accomplished in device with rotating blades or in cylinders with reciprocating pistons. Rotary equipment is used for high volume flow where the discharge pressure is not too high, while the reciprocating compressors are required for high pressures. Besides volumetric flow rate, there are also many parameters to be considered including the valid standards to be used.

Types of Compressors

A. Dynamic Compressor

Dynamic compressor is a continuous-flow compressor which includes centrifugal compressor and axial flow compressor. The dynamic compressor is characterized by rotating impeller to add velocity and pressure to fluid. Compare to positive displacement type compressor, dynamic compressor are much smaller in size and produce much less vibration.

a. Centrifugal Compressor

In a centrifugal compressor, energy is transferred from a set of rotating impeller blades to the gas. The designation “centrifugal” implies that the gas flow is radial, and the energy transfer is caused from a change in the centrifugal forces acting on the gas.it is made up of one or more stages. Each stage consists of an impeller as the rotating element and the stationary element, i.e. diffuser. There are two types of diffuser:

i. Vaneless diffusers ii. Vaned diffusers

Vaneless diffuser is widely used in wide operating range applications, while the vanelessdiffuser is used in applications where a high pressure ratio or high efficiency is required.

The figure below shows the centrifugal compressor

Compression Action:In centrifugal compressor, the fluid flow enters the impeller in an axial direction anddischarged from an impeller radially at a right angle to the axis of rotation. The gas fluid isforced through the impeller by rapidly rotating impeller blades. The gas next flows througha circular chamber (diffuser), following a spiral path where it loses velocity and increasespressure.As there is a limit to maximum achievable pressure rise therefore multi stage compression may be required where multiple rotors are used in succession to achieve higher pressure.

Simple Centrifugal Compressor

Single Stage Compressor Multi Stage Compressor

a) Axial Flow CompressorAxial flow compressors are used mainly as compressors for gas turbines. The component of axial flow compressor consist of the rotating element that construct from a single drum to which are attached several rows of decreasing-height blades having airfoil cross sections. Between each rotating blade row is a stationary blade row. All blade angles and areas are designed precisely for a given performance and high efficiency.

Compression Action:The fluid flow is accelerated by a row of rotating airfoils (blades) called the rotor, and thendiffused in a row of stationary blades (the stator). Similar to the centrifugal compressor, thestator then converts the velocity energy gained in the rotor to pressure energy. One rotorand one stator make up a stage in a compressor. The axial flow compressor produces lowpressure increase, thus the multiple stages are generally used for higher pressures for some industrial applications.

B. Positive Displacement CompressorPositive displacement compressors deliver a fixed volume of air at high pressures. In all positive displacement machines, a certain inlet volume of gas is confined in a given space and subsequently compressed by reducing this confined space or volume. At this elevated pressure, the gas is next expelled into the discharge piping or vessel system. It can be divided in two types:

a. Reciprocating CompressorThe reciprocating, or piston compressor, is a positive displacement compressor that uses the movement of a piston within a cylinder to move gas from one pressure level to another higher pressure level. Reciprocating compressors might be considered as single acting, when the compressing is accomplished using only one side of the piston, or double acting when it is using both sides of the piston.

i. Single ActingThere is an inlet or suction valve and a discharge valve. Both are one way valves and both are spring loaded so it takes a certain amount of force to open them. The inlet valve will only let fluid come into the cylinder and the discharge valve will only let fluid out. As the piston moves downward through the cylinder, fluid is sucked in the inlet

Axial Flow Compressor

valve. When the piston moves up through the cylinder the inlet valve closes and the discharge won't open until a certain amount of force is applied. This causes the fluid to be trapped inside while the volume is reduced, increasing the pressure. When the pressure is enough to open the discharge valve, then the fluid goes out at the higher pressure.

ii. Double Acting Double-acting compressors are similar to single-acting, but they have inlet and discharge valves on both sides of the cylinder. This gives you two compression cycles for every turn of the crankshaft

Single Acting Reciprocating Compressor

Double Acting Reciprocating Compressor

iii. Diaphragm TypeThe diaphragm compressor is similar to the single-acting recip. The difference is that instead of a piston moving inside of a cylinder, the piston moves a diaphragm that contracts and expands

Features of Compressors

Refrigeration compressors and air conditioning compressors can incorporate a number of features which may be important for certain applications.

Thermal shut off - compressor features controls which turn the compressor off at high temperatures to prevent it from overheating. They also can provide restart once the compressor has cooled down below a certain temperature.

Sealing - describes how the compressor and motor drive are situated in relation to the gas or vapor being compressed. Sealed compressors do not allow the gas a route to leak out of the system. Compressors may be one of three types:

Open hermetic semi-hermetic.

Open types have a separate housing for the compressor and the motor. They rely on lubricant in the system to splash on pump components and seals. If not operated frequently, the system can leak its

operating gases. Open compressors can be driven by non-electric power sources such as combustion engines.

Hermetic types seal the compressor and motor together in the same housing. These compressors are leak-free and can sit for long periods unused, but cannot be maintained or repaired.

Semi-hermetic types also contain the motor and compressor in one housing, but instead of a one-piece housing they incorporate gasketed/bolted covers. These can be removed for maintenance and repair of the compressor or motor.

Low noise - compressor operation generates less noise for applications where a quiet environment is desired.

Light weight - compressor is compactly built or constructed with low-density materials for cooling systems which require low weight components.

Variable speed - compressor has speed adjustment for running at various operating flow rates and conditions.

http://www.globalspec.com/learnmore/building_construction/hvac/ventilation/refrigeration_compressors_air_conditioning_compressors

COMPRESSOR SELECTION AND USE

The selection of a compressor should be made according to the following system cooling characteristics and where it will be installed:

1. Minimum Evaporator Temperature

The minimum evaporating temperature and the condensing temperature allows for the identification of the compressor application (LBP, MBP, or HBP). Low Back Pressure systems such as freezers have evaporator temperatures below -20ºC (-4ºF). Medium Back Pressure systems such as food coolers or beverage dispensers have evaporator temperatures higher than -20ºC (-4ºF). High Back Pressure systems such as chillers and air conditioners have evaporator temperatures higher than -5ºC (+23ºF).

2. Refrigeration Capacity or Thermal Load

This is usually defined as the specified cooling capacity at normal working conditions of the system in watts or kcal/h, according to evaporating and condensing temperatures. The capacity is determined by the mass flow rate of refrigerant, which depends on the compressor’s displacement, RPM, and volumetric efficiency.

3. Refrigerant Type

Refrigerant selection can be made on the basis of availability, performance, and ecological considerations such as ozone depletion potential (ODP) and global warming potential (GWP). Aspen

rotary compressors have been verified for use with R134a and R404a. Other low-pressure refrigerants or refrigerant blends may perform satisfactorily, but have not been verified.

4. Ambient Temperature

The compressor must be selected in order to ensure it’s suitable to operate at the highest expected ambient temperature. Compressors are usually intended for indoor use, and a minimum of +5ºC is required in order for proper lubrication.

5. Electrical Power Available

Generally, DC compressors are used in mobile or portable refrigeration systems. DC power is produced by batteries, fuel cells, vehicle alternators, solar panels, or from an AC inverter power supply. The compressor must be selected for use according to the DC voltage available. Aspen compressors are available in 12V, 24V, and 48V versions.

6. Compressor and Drive Cooling

Compressors with small motors can often be cooled in a static environment, with refrigerant removing most of the heat generated. Nonetheless, Aspen compressors due to their small size and high power density should not be placed within a sealed enclosure.

Aspen recommends a small volume of air for cooling by forced convection for both the compressor and drive. If the compressor case temperature reaches 275ºF (135ºC), permanent damage could result in the motor magnets.

http://www.termotek-ag.com/fileadmin/data/general/PDF/Oper_Installation_Instruction.pdf

http://www.daveycompressor.com/differenttype.html

http://kolmetz.com/pdf/EDG/ENGINEERING%20DESIGN%20GUIDELINES%20-%20Compressors%20REV02.pdf

http://www.air-compressor-guide.com/buy/air-compressor-buying-guide/

Advantages Disadvantages

Dynamic Compressors

Centrifugal • Wide operating range

• High reliability

• Low Maintenance

• Instability at reduced flow

• Sensitive to gas composition

change

Axial • High Capacity for given size

• High efficiency

• Heavy duty

• Low maintenance

• Low Compression ratios

• Limited turndown

Advantages Disadvantages

Positive displacement compressor

Reciprocating • Wide pressure ratios

• High efficiency

• Heavy foundation required

• Flow pulsation

• High maintenance

Diaphragm • Very high pressure

• Low flow

• No moving seal

• Limited capacity range

• Periodic replacement of

diaphragm

Screw • Wide application

• High efficiency

• High pressure ratio

• Expensive

• Unsuitable for corrosive or

dirty gases

Safety

a. Limiting gas properties (e.g., decomposition, flammability, toxicity).

b. Compatibility of process gas with materials of construction

c. Over-pressure protection

Economics

a. Life-cycle cost.

b. User and vendor capabilities and facilities for maintaining equipment.

c. Expected equipment reliability.

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