Modulating Technologies

  • There are several ways to modulate the cooling capacity in refrigeration or air conditioning and heating systems. The most famous common in air conditioning are: on-off cycling, hot gas bypass, manifold configurations of multiple compressors, mechanical modulation (also called digital) and inverter technology. Each has advantages and drawbacks.

    • On-off cycling: results in switching off the fixed-speed compressor under light load conditions and could lead to short cycling and the reduction in compressor lifetime. Efficiency of the unit is reduced by pressure cycling and transient losses. The turndown capacity is 100% or 0%
    • Hot gas bypass: involves bypassing a quantity of gas from discharge to the suction side. The compressor will keep operating at the same speed but thanks to the bypass, the refrigerant mass flow circulating with the system is reduced and thus the cooling capacity. This naturally causes the compressor to run uselessly during the periods where the bypass is operating. The turndown capacity varies between 0 and 100%.
    • Manifold configurations: several compressors can be installed in the system to provide the peak cooling capacity. Each compressor can run or not in order to stage the cooling capacity of the unit. The turndown capacity is either 0/33/66 or 100% for an even trio configuration and either 0/50 or 100% for a tandem.
    • Mechanically modulated compressor: this internal mechanical capacity modulation is based on periodic compression process with a control valve, the 2 scroll set move apart stopping the compression for a given time period. This method varies refrigerant flow by changing the average time of compression, but not the actual speed of the motor. Despite an excellent turndown ratio – from 10 to 100% of the cooling capacity, mechanically modulated scrolls have high energy consumption as the motor continuously runs.
    • Inverter compressor: uses a variable frequency drive – also known as inverter drive– to slow down or speed up the motor that rotates the compressor. This method varies refrigerant flow by actually changing the speed of the compressor. The turndown ratio depends on the system configuration and manufacturer. It modulates from 10% (depending on the compressor model) up to 100% at full capacity with a single inverter.

     

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