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INDUSTRIAL & COMMERCIAL COOLING CHILLER EFFICIENCY


Compressor absorbed power


65 55 45 35 25


40 50 with slide valve with inverter


9 8 7 6 5 4 3 2


60 70 Load (%)


Figure 1: The absorbed power of the inverter controlled compressor when installed with an inverter is lower, compared with a standard screw compressor with a slide valve


80 90 100 40 60 Load (%)


Figure 2: Performance of an example air cooled chiller with R134a refrigerant, with an inlet water temperature of 12C and an outlet water temperature of 7C


80


Ti


= 12C; To


= 7C; Tamb.max


= 35C, Tamb.min


= 15C


CoP EER


100


them by direct contact with the liquid refrigerant line. As the frequency of the supplied power


is altered (typically between 30 Hz and 70 Hz), the compressor speed will change proportionally. The main advantages of this technology


are: l The starting current is effectively equal to zero since the speed and power consumption is directly proportional to frequency;


Using a slide valve the gas flow is less controllable compared to the inverter controlled counterpart


l Cooling power can increase beyond the normal maximum as the screw compressor can rotate with higher speed with frequencies up to 70 Hz (compared to normal 50Hz supply);


l The electrical consumption at partial load between 30 and 50 Hz is reduced compared to a standard screw compressor with a slide valve capacity control. This results in a measured absorbed power being reduced by up to 15%;


l Superior control of water outlet temperature fluctuating less around the set point temperature. Typically tolerances of around +/- 0.5C are possible; and


l Reduced mechanical compressor wear, as the machine will rotate for most of the year with reduced speed.


The efficiency of the compression process can be maintained across the range of loads using both inverter technology and automatic volume index (Vi) control. (Vi is the ratio of the volume of gas in the compressor when the suction port closes to the volume of gas when the discharge port opens).


44 CIBSE Journal February 2012


Performance of an example chiller with inverter controlled compressors Figure 1 shows lower absorbed power of the screw compressor when installed with an inverter, compared with a standard screw compressor with a slide valve. Using a slide valve, the gas flow is less


controllable compared to the inverter controlled counterpart. With a standard compressor the capacity changes are in steps (eg, 100%, 75%, 50% and 25%); whereas in the inverter solution the speed decreases proportionally and the gas flow is modulated linearly. Figure 2 shows the real data performance


for an example air cooled chiller. The coefficient of performance (CoP) is the ratio of the chiller cooling capacity to the compressor power input. The energy efficiency ratio (EER) is the relationship between the cooling capacity of the chiller and the total power consumption of the refrigeration unit (compressors + fans). Figure 2 illustrates the beneficial effect


on the efficiency of the chiller unit as the ambient air temperature and the load on the chiller reduce from 100% load in a 35C ambient (worse case), to a situation where the load on the chiller is 50% and the ambient temperature is 15C. The EER for the unit at 50% load in a 15C ambient is


more than double (~7) the value compared to when at 100% load in a 35C ambient


temperature (~3) (and so qualifying for ‘Class A’ efficiency categorisation). These values of CoP and EER compete


well with other available technologies (eg, centrifugal compressors). Inverter


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Power (kW)


CoP – EER


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