Introduction
It is becoming a “best practice” to install a variable speed drive (VFD) air compressor whenever one is replacing an old air compressor. As a result, real systems have fixed-speed and VFD air compressors, mixed. I have observed several VFD compressor sizing methods. In my last article, I referred to a common method: size one VFD compressor for the whole system. This can work. However, if it doesn’t meet a higher peak demand, one or more of the old compressors will be started, and a mixed system results. Another method is to replace a compressor with the same size, but with a VFD. If the compressor that was replaced is large, a big VFD is installed. If small, a small one. Yet another method is to segment the system. In all these cases, there is a VFD compressor in a system that has not been really designed. It just happened. Many of these systems aren’t operating efficiently or reliably, because system issues and VFD compressor sizing wasn’t well thought out. The goal of this article is to give some pointers for selecting and installing the right size VFD air compressor in a multiple compressor system.
Summary
In a nutshell, I recommend the following when a VFD air compressor is added to a multiple compressor system:
Develop a Solid Flow Profile
Determine Flow Ranges for “Trim” Air Compressor(s)
Size VFD & Base-load Air Compressor(s) From Variance & Base-load Size(s)
The complete article to determine VFD sizing