Air Compressor Control Gap Explained
The current trend in compressed air seems to be replacing fixed speed machines with variable speed machines. One of the biggest problems we are encountering with this phenomenon is educating end users of the issues which may arise from them not clearly understanding the control gap.
Uneducated compressed air sales representatives are flooding the market place with variable speed compressor technology and fail to educate the customer on the looming prospect of control gaps.
The following visuals and graphs indicate some simple guidelines regarding how to avoid a control gap. Our technical engineering department would be happy to assist you or preform a free of charge onsite survey to access the risk your system may face with the addition of a variable speed air compressor.
You always want to avoid the control gap.
The best way to demonstrate the effect of the “Control Gap”, is to imagine a fixed speed compressor with an output capacity of 494 CFM. If we had a variable speed/frequency driven compressor of the same size, then at speeds lower than the minimum Frequency Drive/ VFD, the VFD will “short cycle”, which is a disaster in terms of wear on the motor and increased maintenance costs.
The control range on the variable speed drive compressor is the difference between the maximum and minimum CFM. You should size your fixed speed compressor to fit within the control range of the variable speed drive compressor.
In this case, we have a 494 CFM compressor and in terms of KAESER SFC compressors, we would select a SFC 110 because of the control range of 546 CFM (692-146 = 546 control range at 125psi).
This allows for the flow demonstrated in the 2nd graphic and the system always running efficiently and effectively.