Investment in screw compressor VFD
Nearly all rotary screw compressors utilize a slide vale
for unloading. Although this method of control is infinitely adjustable and
provides reasonable suction pressure control, there can be a substantial power
penalty associated with slide valve control. In general, part-load performance
degrades with deeper suction and higher discharge
pressure.
Example 1 . Three compressors have electrical loads 200 HP
at 100 % capacity. Operating conditions:
Condensing
pressure Suction
pressure
Compressor 1
150 psig
30 psig
Compressor 2
150 psig
15 psig
Compressor 3
150 psig
0 psig
At
50 % capacity electrical loads are:
Compressor 1
110 HP
Compressor 2
115 HP
Compressor 3
120 HP
Part-load performances are better for compressors
operating at higher suction pressure than for compressors operating at lower
suction pressure. An ideal compressor will have electrical load 100 HP at 50 %
capacity.
To
adjust the capacity of screw compressor, the most attractive alternative to
slide valve is a variable frequency drive (VFD). VFD will reduce the power
penalty associated with slide valve, however, it has two
drawbacks.
1.
Energy
losses for VFD operating, usually
2 -5 %.
2.
Most screw compressors can
operate down to 50 % speed. Below 50 % speed, the slide valve must be used for
further capacity reduction.
Example 2.
Compressor VFDs were installed for 3 compressors mentioned in example 1. VFD
loss is 2 % of full load (200 HP) or 4 HP.
Compressors with VFDs will have electrical load 104 HP at 50 % capacity.
Energy savings for compressors with VFDs will be
following:
Compressor 1
110 HP - 104 HP = 6 HP
Compressor 2
115 HP - 104 HP = 11 HP
Compressor 3
120 HP - 104 Hp = 16 HP
The best energy saving we can get for compressor 3. Different energy savings will
lead to the following paybacks on investment in compressor
VFDs.
Compressor 1 - 5 years
Compressor 2 - 3
years.
Compressor 3 - 2
years.
These examples show that return on investment (ROI) in
VFDs will be better for compressors with deeper suction pressure.