CHILLERS AND HEAT PUMPS

What type of compressor to choose and when?

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General information

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• Scroll Compressor:
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  • They are hermetic compressors, cheaper than screw compressors, for example, with limited capacity, so large equipment requires several compressors (typically up to 6, although there are equipment with more). The maximum capacity that can be found in a team is around 700 - 800 KW.

  • On the other hand, its corrections and repairs are limited and in most cases it requires the replacement of the compressor.

  • Fixed speed and inverter can be found on the market, and due to their "economic" nature they are usually assembled with plate heat exchangers.

  • Typical refrigerant gas, R410A, R32, R454b, etc. They work at high pressure and could find limitations with very high outside temperatures.

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• Screw Compressor:
   

  • They are semi-hermetic compressors, the minimum capacity is higher than scroll compressors and can be found in equipment from 300 KW to 3,600 KW (in water condensation).​

  • Because they are semi-hermetic, preventive and corrective maintenance can be carried out to extend their useful life and repairs.

  • According to the manufacturers, they are usually of two types, single-screw and twin-screw, and can be fixed speed, inverter with AC motor and frequency inverter, and inverter with EC motor.

  • The load regulation by means of a sliding valve (and a variator in the case of an inverter) allows them to have a very precise supply cold water temperature.

  • The latest trend in these compressors is to use a variable compression volume to improve efficiency at part loads.

  • In general, equipment with these compressors usually has tubular exchangers.

  • typical refrigerant gas,R134a, R513A, 1234ze

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• Centrifugal Compressor:
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  • In addition, centrifugal compressors can be classified by the number of stages, and can be single, double or triple stage. The stages are the number of turbines that the compressor has and its main advantage is stability at low load with high condensing temperature.

  • The operation map of a centrifugal compressor is like that of a pump, the capacity is related to the differential pressure at which it works.

  • When designing, it is very important to take into account that if the condensation conditions are higher than the design conditions, the turbine will not be able to project the refrigerant from the low side to the high side and a reverse effect of refrigerant known as "surge".

  • High speed compressors can be found in bothcondensation by air and by water. Special attention must be paid to the limitation by outdoor temperature in air-cooled.

  • Low speed compressors are only found in water condensation for high power, normally from 1,800 - 2,000 KW.

  • Typical refrigerant gas, R134a, R513A, HFO 1234ze, HFO 1233zd(E)

  • Due to the speed of rotation, they can be low speed (around 3,500 rpm) and larger turbines,  y high speed or magnetic levitation (around 40,000 rpm) with turbine diameter lower.

  • They are semi-hermetic compressors, and the main difference is the principle of operation. Unlike the previous ones that are of volumetric compression, the centrifugal ones are based on the speed at which the refrigerant leaves the turbine towards the condenser and therefore two factors are taken into account.factors, theradial velocity and tangential velocity,that is, turbine diameter and turbine rotation speed.

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• Alternative Compressor:
   

Positive displacement compressor, like scroll and screw, with little presence in current commercial equipment and more focused on very low temperature applications with CO2.

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• Summary:

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• Equipment with scroll compressors, focused on the low power sector and with lower investment and maintenance costs.

• The equipment with screw compressors, more robust with greater maintenance options to extend its useful life, are in the medium power range, from 300 KW to 1,800 KW in air-cooled and 3,600 in water-cooled units.

• High-speed centrifugal equipment, magnetic levitation, overlaps in power with the screws, they are more costly equipment and with very good efficiency at partial loads, mainly very low, and their main advantage is the absence of oil for lubrication, and noise level. .

• Low speed centrifugal equipment, only for water condensation, from 2 MW.

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📣 Note: Although the SEER measures efficiency at partial loads and two different technologies can have very similar SEERs, the point of maximum efficiency is at a different % load, so it is recommended to ask the manufacturers for the curves to select the equipment that best suits.