Question: I’m a compressor salesperson. Lately, the large building tenders we’ve seen have been spec’d for two variable speed screw compressors to run together sharing the load. This would make sense to me if one was a 100% backup and they never ran together, but that does not seem to be the case. Everything I’ve ever learned about multiple compressor systems indicates that two variable speed units running together will cause issues.
At the time of our quote, there was no way to get back to the actual decision maker and if we quoted any other configuration, it would be up to the contractor to sell it … and we know that “ain’t gonna” happen …
Clearly, we need to do a better job of getting with the local engineering firms so we can be doing the spec’ing, but I’m just wondering about your thoughts on this situation that we’ve kind of been forced into.
Answer: Yes, any compressed air system really needs only one properly sized VSD compressor, but it drives maintenance guys crazy if one compressor gets all the hours — and the other minimal run time — which needs to be the case with a mix of VSD and fixed speed.
So, the result is often the purchase of two or more VSDs. It is possible to run efficiently with two variable compressors, but the settings need to be adjusted so the control points are stacked; this way you don’t have two VSDs running together at part load, which can happen (for example: two units running at 25% instead of one at 50%). VSDs running at low speeds is inefficient and can cause internal condensation problems and high oil carryover.
But, even with this, the compressor with the highest setpoint will get the majority of hours. Some manufacturers have two sets of pressure setpoints that can be programmed into the compressor controllers, and even set on time schedules. This can be used to easily to swap settings, and still maintain compressor coordination, if operators want to change which compressor is in the lead position.
I’m unsure what can be done about the poor specifications, I can only hope your tender writers are reading this.
