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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.   

 

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