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View Subscription Details: Letter ( 07/29/2011 ) - HTML Format

Design conditions and energy savings


I read many articles and technical papers where authors estimated energy savings at design conditions. Did they get real life estimation of the potential energy savings? I do not think so.


A refrigeration plant was designed for condensing temperature of 95°F or corresponding condensing pressure of 181 psig. It has five 500 HP high stage screw compressors. Suction pressure of these compressors is 30 psig. During operation, several compressors run at full load and one (trimming) compressor load and unload to accommodate fluctuation of the refrigeration load. Average inefficiency of trimming compressor operating at design conditions is 15%. One engineer proposed to install VFD for the trimming compressor. VFD itself uses 3% of compressor energy. It means that benefit of using VFD is 15% - 3% = 12%

This engineer has estimated that mentioned compressor VFD will save

500 x 0.12 x 0.7457 x 8760 = 391,936 KWh

However, I think that this estimation is wrong, because energy savings were estimate at condensing pressure of 181 psig and operating time was 8760 Hrs. This is nonstop operation for full year. Typically, a refrigeration plant operates at design conditions 1% of operating time or 87.6 Hrs per year. Why did this engineer estimate energy savings for the period of 8760 Hrs? Does he suggest operating plant at condensing pressure of 181 psig all year around?

If yes, reduction of condensing pressure from 181 psig to 151 psig will save 15% of compressor energy. Each operating compressor will save 15%. Assume that average number of operating compressors is three. Total energy savings will be 15% x 3 = 45%. This is significantly greater then 12% of energy that can be saved by compressor VFD.  

In energy saving process we save KWh. It means how many KW will be saved for how long. To get a correct energy savings, estimation should be done at average condensing pressure. For mentioned refrigeration plant average condensing pressure is 120 psig. At this pressure compressor VFD will save 11% of compressor energy. This is less then 15% estimated at condensing pressure of 181 psig.

To get better numbers, many engineers do estimation of potential energy savings at design conditions. However, to get real life energy savings, estimation should be done at average operating conditions. 



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