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Kinds of compressors
According to its manufacturing, technology or how they work, there are several types of compressors which are used for air conditioning units, here we have a brief summary of them.
Kinds of air conditioner compressors according to its manufacturing
Hermetic compressor
The hermetic compressor is used in small installations and those with a low power voltage because they cost less and use a lesser space inside the unit. This kind of compressor is directly cooled by the refrigerant itself and they don’t require engine-compressor transmissions. But when a hermetic compressor breaks down, its reparation is more expensive given it is difficult to gain access to the interior of it. In general, this compressor uses to be replaced and not repaired.
Partially hermetic compressor
The partially hermetic compressor is commonly used in average power voltage installations, these have both the engine and the compressor installed inside a pressure vessel which is accessible for reparation in case of breakdown. These can be subdivided in two types: the ones cooled by air and the ones cooled by aspiration.
Open compressor
This kind of air conditioner compressors are more accessible and are used in average and large power voltage. Open compressors are fully accessible for their reparation and the transmission is done on the outside through leashes which can have vibration problems and it is necessary to put them on the right place from time to time.
Kinds of air conditioner compressors according to their working/technology
Piston alternative compressor
These are based on the transformation of a rotational movement into an alternative similar to car engines but in an inverse way. They are made of a compression chamber in a shape of a cylinder and a piston which moves itself through the inside of this cylinder.
Rolling piston rotary compressor
In general, rotary air conditioning compressors are directly coupled to the engine and do not have intake valves, with the gas always circulating in the same direction.
They admit high compression ratios since the abundant oil of the same helps to lubricate and also helps to eliminate the heat produced by the compressor itself. They are bound to much less mechanical vibration than alternative compressors.
In rolling piston compressors, the motor shaft and stator shaft are concentric, while the rotor shaft is eccentric. When the rotor slides over the stator, contact is established between them, which in the stator takes place along each and every one of its generators, while in the rotor only along one, the one corresponding to the maximum distance from the motor shaft.
The steam is delivered through the inlet port and the discharge through the exhaust valve. The steam drawn into the compressor, which fills the space between the rotor and the stator, is compressed so that, as it rotates, it gradually decreases in physical space until it reaches the pressure in the exhaust valve, which then opens, and the steam is expelled or discharged.
Screw rotary compressor
The screw compressor uses a double set of rotors -male and female-. The male rotor usually has 4 lobes that engage within the 6 alveoli of the female rotor -4 + 6 arrangement-, with this rotor being driven by the former and the directions of rotation being opposite. Other special variants are for example the 5 + 7.
The gas, due to the rotation is trapped between the spaces of the rotors, being transported from one end to the other of the gearbox where the intake and outlet are located.
Each of the working chambers behaves as if the cylinder were a reciprocating compressor, where each tooth of the driving rotor acts as a piston, which first closes and then compresses the volume initially trapped, so that a helical compressor is nothing more than a six-cylinder reciprocating compressor, in which the crankshaft, the residual space and the intake and exhaust valves have been eliminated.
The gas is continuously compressed by the rotors until the lobes are fully engaged. This eliminates the undesirable condition existing in reciprocating compressors, where the gas in the clearance volume between the piston and the top of the cylinder is re-expanded inside the cylinder, resulting in a reduction in volumetric efficiency and an increase in power consumption.
An essential characteristic of screw air-conditioning compressors is the absence of any discharge or suction valve. Instead, they require a high degree of quality in the tolerances, which means high manufacturing costs. The internal compression ratio is determined by the shape of the suction and discharge holes.
Scroll or rotary spiral compressor
The scroll compressor uses two spiral-shaped parts, one fixed -top- and the other moving -bottom- driven by the motor shaft. The centre of rotation of the mobile spiral is offset from that of the fixed and upper spiral with an eccentricity ‘e’ called ‘orbital radius’ which allows the volumetric compression of the sucked vapours.
The lower spiral does not describe a rotary movement, but is a rotational translation movement. Between both parts -fixed and mobile spiral- they create from the inlet mouth and in a continuous way a compression chamber of decreasing volume so that the pressure increases. At the end of the gas path and when the volume of the compression chamber is minimal, it is expelled through the expulsion outlet. In the discharge there is a check valve that prevents the return of high-pressure gas to the low-pressure part when the machine is stopped.
Frequency converters are currently being incorporated into scroll air-conditioning compressors for capacity regulation, affecting the speed of the compressor (inverter).
Centrifugal compressor or turbocharger
The centrifugal compressor uses the centrifugal force caused by the high peripheral speed at which the fluid exits the rotor blades, a speed which, on passing through a diffuser with the consequent drop in speed, results in an increase in pressure. The normal motor speeds used in reciprocating, screw and spiral compressors are about 3,000 rpm. For some single-stage centrifugal compressors with a gearbox, speeds of 30,000 rpm are used. They are therefore high-speed machines capable of handling very large volumes of refrigerant gas with low compression ratios. The gearbox has two gears: the motor gear, which is larger, and the driven gear, which moves faster.
SIf the discharge pressure increases too much or the vaporization pressure drops too low, the compressor cannot withstand the pressure difference and stops pumping. The motor and compressor continue to turn, but the refrigerant stops moving from the low pressure side to the high pressure side of the system. The compressor and engine will not be damaged unless left in this overpressure state for a long time.
The refrigerating fluids used are almost always those of the halogenated type, although in the oil and chemical industries they are used very frequently, in centrifugal compressors, methane, propane, ethylene and propylene.
Its cooling capacity range extends from 350 kW to 12,000 kW and the vaporisation temperatures can vary from +10 C to -160ºC. They are used for air conditioning as water chillers.