Good and bad investments in energy efficiency
(2)
Screw compressor VFD
Capacity of compressors should be changed based on the
variation of refrigeration loads. Screw compressor has a slide valve to increase
and decrease the capacity of this compressor. Unfortunately, unloading of screw
compressor reduces its energy efficiency. It means that compressor uses more
energy to produce one ton of refrigeration compare to 100% loaded compressor.
Look at energy use of partly unloaded screw compressor.
Capacity, % 100 90 80 70 60 50 40 30 20 10
Energy use, %
100
94
88
82 76 70 65 61 58
56
As
you can see, this compressor is very inefficient at capacities below 50%. Ideally, at 50% capacity screw
compressor should consume 50% of energy. This ratio is possible to achieve by
using compressor VFD. This VFD will gradually change the speed of electrical
motor and capacity of the compressor will change accordingly. However, in real
life VFD itself consumes 3% of the energy. It means that at 50% capacity
compressor with VFD will consume 53% of energy. Slide valve unloading to 50%
capacity will give us 70% of the energy use. The difference is 17% and this
energy can be saved by installing screw compressor VFD.
Magnitude of energy that can be saved by compressor VFD
depends on the average capacity of this compressor and compression ratio.
Compression ratio is the ratio of absolute discharge pressure to the absolute
suction pressure. Energy efficiency of the screw compressor is lower if it is
operating at a lower capacity and at higher compression ratio. It means that
screw compressor VFD can recover (save) more energy if it is installed on
compressor that operates at low capacity and high compression ratio. Operating
hours of the compressor is important as well. However, typically screw
compressor with VFD operates as trimming compressor and runs 8760 hours per
year.
To
estimate real life energy savings and payback on investments in screw compressor
VFD, real life average capacity of this compressor and real life compression
ratio should be chosen.