types of compressors

Compressors
The compressor has two functions in the refrigeration cycle: firstly it sucks refrigerant vapor and reduces the pressure in the evaporator at a point where it can be maintained the desired evaporation temperature. Secondly, the compressor raises the pressure of refrigerant vapor at a high enough level so that the saturation temperature exceeds the temperature of the available cooling medium for condensing the refrigerant vapor.
There are three basic types of compressors: Reciprocating, Rotary and Centrifugal.
Centrifugal compressors are widely used in large central systems and air conditioning compressors are used in the rotating field of domestic refrigerators. However, most of compressors used in sizes less horsepower for commercial applications, domestic and industrial are reciprocating.
Reciprocating Compressors.


The design of this type of compressors is similar to a modern automobile engine with a piston driven by a crankshaft runs performed alternating suction and compression in a cylinder provided with suction and discharge valves. Because the compressor is a reciprocating positive displacement pump is appropriate for small displacement volume, and is very effective at high condensing pressures and at high compression ratios.
Advantages:
· Adaptability to different refrigerants
· Ease with which allows the movement of liquid through the pipe due to the high pressure created by the compressor.
· Durability
· Simplicity of design
· Relatively low cost





Open Type Compressors
The first models of refrigeration compressors were of this type. With the pistons and cylinders sealed on the inside of a sump and a crankshaft extending through the body to be driven outwards by some external force. It has a seal around the crankshaft to prevent loss of refrigerant and oil comprsor.
Disadvantages:
· More weight
· Higher cost
· Increased size
· Vulnerability to failure of the seals
· Difficult alignment of the crankshaft
· Excessive noise
· Short life of the bands or components of direct action
This compressor has been replaced by the motor-compressor semi-hermetic and hermetic type, and its use continues to decline except for specialized applications such as car air conditioning.



Semi-hermetic motor compressor
This type of compressor was started by Copeland and is widely used in popular models Copelametic. The compressor is driven by an electric motor mounted directly on the crankshaft of the compressor, with all its parts, both the engine and the compressor, hermetically sealed inside a common housing.
Disorders are eliminated seal, the motors can be calculated specifically for the burden they have to operate, and the resulting design is compact, economical, efficient and basically maintenance free. Stator covered heads, bottom plates and are removable covers Carter allowing easy access for repair in case of deterioration in the compressor.



Moto-hermetic compressor.
It was developed in an effort to achieve a reduction in size and cost and is widely used in low power equipment unit. As in the case of semi-hermetic motor-compressor, the electric motor is mounted directly on the crankshaft of the compressor, but the body is a metal housing sealed with solder. In esti type compressors can not be carried out interior repairs since the only way is to cut open the casing of the compressor.
Compressor speed.
The first models of compressors designed to operate at relatively low speeds, well below 1000 rpm. To use the electric motors standard four-pole introduced the operation of the hermetic motor compressor and semi-hermetic at 1750 rpm (1450 rpm at 50 cycles).
The growing demand for air conditioning equipment more compact and lighter weight has forced the development of motor-hermetic compressors with two-pole motor operating at 3500 rpm (2900 rpm at 50 cycles).
Specialized applications for air conditioning in aircraft, automobiles and military equipment, use of higher speed compressors, although commercial application for normal domestic electricity supply 60 cycles existing generally limits the speed of the compressor to the currently available 1750 and 3500 rpm.
Higher speeds produce lubrication problems and duration. And these factors, as well as cost, size and weight must be considered in the design and implementation of the compressor.
Basic Operation
When the piston moves down in the suction stroke the pressure is reduced in the cylinder. And when the cylinder pressure is lower than the compressor suction line pressure difference drives the opening of the suction valves and forces the refrigerant vapor to flow into the cylinder.
When the piston reaches the end of his career and starts sucking up (compression stroke) creates a pressure in the cylinder forcing the closure of the suction valves. The pressure in the cylinder continues to rise as the cylinder moves upwards compressing the vapor trapped in the cylinder. Once the cylinder pressure is greater than the pressure in the discharge line of the compressor, the discharge valves are opened and the compressed gas flows into the discharge pipe and the condenser.
When the piston begins its downward stroke the pressure reduction allows to close the discharge valve, given the high condenser pressure and the discharge conduit, and the cycle repeats.
During each revolution of the crankshaft occurs a suction stroke and a compression of each piston. So that the motor-compressors of 1750 to 1750 rpm occur complete cycles of suction and compression in each cylinder during each minute. At 3500 rpm compressors have 3500 full cycles per minute.
Valves in the compressor
Most of the reciprocating compressor valves are of the tongue and must be properly positioned to prevent leakage. The smallest fragment of foreign material or corrosion under the valve result in leaks and extreme care should be taken to protect the compressor against contamination.
Compressor displacement
Displacement of a reciprocating compressor is the volume displaced by the pistons. The extent of displacement depends on the manufacturer, for example, Copeland is published in cubic meters per hour and cubic feet per hour but some manufacturers published in cubic inches per revolution or cubic feet per minute.
The displacement of the compressor can be calculated using the following formulas:

MCH = cubic meters per hour
MCM = cubic meters per minute
Cm3/rev = cubic centimeters per revolution
D = diameter of the cylinder (cm)
L = Length of run (cm)
N = number of cylinders
RPM = revolutions per minute
1000 = cubic centimeters per meter.
Volume of free space
The efficiency of a compressor depends on their design. If the valves is well positioned, the most important factor is the volume of free space. After completion of the compression stroke that still some free space which is essential for the piston does not hit against the valve plate. There is another space in the holes of the discharge valves since they are in the upper plate.
This residual space and not displaced by the piston at the end of his career, is called free space volume. Remains filled with compressed gas and hot at the end of the compression stroke. When the piston begins its descent in the suction stroke, expands the residual gas of high pressure and reduced pressure. In the cylinder steam can not penetrate the suction line until the pressure is reduced to its lower value than the suction line. The first part of the suction stroke is lost under a capacity point of view, since it increases as the compression ratio, a higher percentage of the suction stroke is occupied by the waste gas.
Lubrication
You should always keep an adequate supply of oil in the crankcase, to ensure continuous lubrication. In some compressors lubrication is effected by means of an oil pump, positive displacement.
Charge of dry air.
Some compressors are shipped with a load of dry air. The internal pressure of a compressor treated on site warrant that you have a seal and the inside is completely dry. When installing the compressor must be evacuated to remove this burden from air.
Compressor cooling
Air cooled compressors require an adequate flow of air over the compressor casing to prevent overheating. The airflow from the fan must be discharged directly onto the motor-compressor.
Water-cooled compressors are equipped with a jacket for circulating the water or are wrapped with a copper coil. The water must flow through the cooling circuit when the compressor is in operation.
The motor compressor cooled refrigerant are designed so that the suction gas flow around and through the motor for cooling. The evaporation temperature below -18 ° C or 0 ° F is required additional cooling by air flow since the decreasing density of the refrigerant reduces the cooling property.
Compressor capacity
The data capacity provided by the manufacturer for each model of compressor for the refrigerant which can be used. These data can be provided in the form of curves or tables, indicates the capacity in Btu / h at various temperatures of suction and discharge.
Two-stage compressors
Have developed two-stage compressors to increase the efficiency of evaporation when temperatures are in the range of -35 ° C to -62 ° C.
These compressors are divided internally into low or high. Three-cylinder engine having two cylinder in a first stage and the second, whereas models of six cylinders are four in the first and two in the second.