Types of compressor
The compressor is the component at the heart of the a refrigerant circuit when based on the so called “vapour compression cycle”.
This thermodynamic cycle exploits the evaporation of a refrigerant inside a closed loop piping circuit.
Specifically, the evaporation occurs in a heat exchanger called the evaporator, which absorbs energy from the surrounding air; this is then delivered to the food storage compartment or air-conditioned space by natural or fan-forced convection.
The same also applies when using water as the medium, which is pumped through the heat exchanger and then flows into the storage tank for use by the terminal units.
Once having evaporated, the refrigerant can no longer absorb considerable amounts of energy, and consequently it needs to be returned to the liquid state by condensation.
The problem thus arises of having an environment that’s “cold” enough to absorb energy from the refrigerant, which naturally cannot be the same compartment or space that’s just been cooled.
The compressor is then used to compress the refrigerant to a pressure that’s higher than in the evaporator (up to 8-10 times!) so that the condensation process can take place at a temperature that’s compatible with a readily available “cold” source, typically the outside air.
Condensation thus occurs at a high temperature (usually 35-55°C) inside a heat exchanger where the two fluids are outside air and refrigerant. The latter condenses and returns to the liquid state, while the outside air will be heated.
The liquid refrigerant is still at high pressure when it leaves the condenser. An expansion device is thus needed to expand the liquid refrigerant and reduce its pressure to the value at which evaporation occurs.
The refrigerant has now returned to its initial state (liquid at low pressure and temperature) and can once again absorb energy from the air or water.
Therefore, the compressor has the function of circulating refrigerant inside the circuit, drawing it in as a gas from the evaporator and then compressing it and delivering it at higher pressure to the condenser.
It provides a volumetric compression, i.e. a progressive reduction in volume, using rotating or reciprocating systems. This mechanical work implies a significant increase in the temperature of the gas (at times above 100°C) as well as power consumption. Compressor power consumption depends on the difference between the two operating pressures. The refrigerant entering the compressor must be in the gaseous state, as liquids are notoriously incompressible. The compressor starts working when the unit needs to provide cooling, and is usually activated via temperature control systems.
Not all air-conditioning and refrigeration applications have the same requirements in terms of capacity, noise, efficiency and operating range, and as a result there are different types of compressor.
These essentially differ as regards the way compression is achieved, with reciprocating compressors in featuring a reciprocating movement to create compression, and rotary compressors, including rotary vane, scroll, screw and centrifugal compressors, featuring a rotational movement to bring about compression.
Reciprocating compressors, or piston compressors, divided into hermetic, semi-hermetic and open, are used above all for applications with very high cooling capacity requirements. In the past these were also used in applications with very low cooling capacities due to their low costs, however they've progressively been replaced by rotary compressors.
These types of compressors adopt a volumetric compression system using pistons and work much like an internal combustion engine. The pistons run up and down inside cylinders, producing suction and compression of the refrigerant gas. Each cylinder has a suction valve for the gas refrigerant and a discharge valve to deliver the gas to the condenser after having been compressed.
Obviously in this case there's no combustion stage as in the case of vehicle engines.
In general these are noisy compressors and create considerable vibrations that are felt around the entire circuit.
Their construction technology is however quite simple and well established, making these compressors the best choice in the past for low cost and low capacity home or commercial applications. They've now been replaced by rotary compressors, which offer better performance and lower costs.
They can however reach very high capacities, and for applications exceeding one megawatt they're currently the only solution available.
Reciprocating compressors are defined as open when one end of the crankshaft protrudes outside of the sump (the casing that houses the pistons and mechanisms inside the compressor) and can be opened. Semi-hermetic means the compressor itself and the motor are housed in the same casing, which is designed to be opened for inspection and servicing. In this case, the motor shaft and crankshaft are one single piece. Semi-hermetic compressors are made in such a way as to avoid air or dust from entering the mechanisms.
Reciprocating compressors are called hermetic when the casing is welded and sealed and the cylinder heads cannot be accessed for inspection or maintenance.
Rotary compressors, which include rotary vane, scroll, screw and centrifugal compressors, are hermetic compressors used in many applications and over different operating ranges.
Rotary vane compressors consist of a cylindrical casing, two openings - one suction and one discharge - and a rotor positioned eccentrically with respect to the casing.
Compression occurs by refrigerant flowing into the chamber where, due to eccentric rotation, there is a reduction in the desired volume.
The advantages of rotary vane compressors are low cost and compact dimensions, making these the best choice for low capacity home applications (fridges, freezers, air-conditioners).
A more advanced version is the "twin" compressor, featuring two rotors revolving in counter rotating directions; these have even lower noise and generate less vibrations, making these compressors often a preferable alternative to scroll compressors, which tend to be more expensive and bulky.
Twin rotary compressors also have increased energy efficiency and the same low cost as the traditional version. This makes them suitable for higher cooling capacity applications (up to 50 kW), moreover with good results in industrial contexts.
The top in rotary technology involves compressors fitted with inverters and brushless DC motors (see "EFFICIENCY IN REFRIGERANT CIRCUITS").
Currently, these are the compressors with the best capacity/dimensions ratio, being able to reach 7800 rpm against the 5400-6600 typically achieved by scroll compressors.
Scroll compressors use two scrolls, one fixed and the other moving and coupled to the motor.
The scrolls are interleaved so as to be in contact with one another in various places and thus form a series of gradually thinner pockets of air towards the centre. The fluid is compressed by rotation of the orbiting scroll around the fixed scroll until being discharged in the centre.
Scroll compressors are widely used in home and commercial air-conditioning systems, in heat pumps and in air-conditioning systems for telephone applications (shelter, close control units). Capacity ranges from 3 to 120 kW, and consequently they have limited use in refrigeration applications with low cooling capacity, where piston and rotary compressors are preferred, and high capacity air-conditioning applications, where screw or centrifugal compressors are preferred.
Advantages of scroll compressors include relative small dimensions and much lower weight than medium capacity reciprocating compressors, albeit higher than rotary compressors. They also feature excellent efficiency at a defined compression ratio, which nonetheless decreases with variations in operating conditions.
Other benefits are low noise and reduced vibrations. High efficiency versions of these compressors are also available fitted with inverters and brushless DC motors (see "EFFICIENCY IN REFRIGERANT CIRCUITS").
Screw compressors are based on a mechanism made up of two intermeshing screws, called rotors. As the rotors revolve, the fluid is drawn in through the inlet port, filling the volume between two lobes.
When the spaces between the lobes are full of fluid, suction ceases, the fluid between the lobes is then forced due to rotation into an increasingly small space, causing compression. When the lobes reach a certain position, the compressed fluid is completely discharged through the outlet port. These compressors require abundant lubrication due to the meshing of the two rotors. Abundant lubrication and subsequent cooling of the oil using a special heat exchanger guarantees higher compression ratios than reciprocating compressors, meaning wide use in both refrigeration and air-conditioning (large chillers operating on R410A).
Finally, a sliding valve mechanism that opens a passageway between the inlet and the compressor casing in a position where the rotors have already partially compressed the fluid allows a continuous reduction in refrigerant flow, down to 25% of the maximum, thus guaranteeing greater compressor efficiency in part load conditions.
Centrifugal compressors feature a cast iron casing, a steel shaft and a cast aluminium alloy impeller.
The fluid is drawn in by the impeller near its axis and due to centrifugal force is pushed to the edge of the compressor casing. The fluid leaves the impeller with significant kinetic energy, which is then converted into pressure energy in the diffuser.
Use is limited to high cooling capacities and low compression ratios (usually large chillers running on R134A)
Cooling capacity can be varied using fins that change the angle at which fluid enters the impeller.