What type of compressors are
Without divulging too much information, the main use of compressed air will be our "burnout" units, which fluctuate on time and BTU's with each different pattern. The units are capable of requiring up to 150cfm each (there may be up to 14 total in the future) but our process right now runs them between 80-100cfm (we currently have 3). The short term we are upgrading to a total of 9 burnouts so our estimated maximum cfm is 100x9 = 900cfm.The other users of compressed air are 3 coating robots, hand chiseling tools, dust collector (air valves) and our machining area (several Mazaks and Mori Seiki machines, as well as some festo cylinders on our leak test equipment). We really don't have a good time study to say what is being used when and for how long,etc as we don't have all this equipment in our facility yet. The way I look at it, is the casting process is always shuffling, and what I mean by that is Chemical pump we will likely never have 9 burnout stations running at the exact same pace, they will be starting and stopping at different points meaning our usage will fluctuate.It is this fluctuation in our usage on the burnout stations that leads me to belive that a variable speed may be better as we may end up running at 70% or lower capacity most of the time.
What type of compressors are being considered, reciprocating, screw, centrifugal, ...? What is the required system pressure?Most likely, you would be wise to pay more attention to receiver sizing to cover reasonable demand variations. Remember that the amount of work required varies with the type of compressor chosen. The minimum work is achieved by best approximating an isothermal (constant temperature) process. For a typical compressed air system operating at 100 psig, a 2-stage reciprocating compressor with decent intercooling and aftercooling will cost significantly more than a single stage compressor, but it will use much less energy, and it will last much longer and be more reliable because the maximum temperatures are much lower. Operating a single stage compressor at variable speed will still require more energy than a 2-stage machine. Also, the compressor may not tolerate variable speed operation very well if the speeds go too low or too high.Based on my experience and observations, it is very difficult to beat a big, slow running multi-stage reciprocating compressor for energy efficiency and reliability. Since you seem to be needing to supply a nominally constant pressure system, varing the speed of a centrifugal compressor would make no sense because its discharge pressure is highly speed dependent.Rather than worrying about having one compressor cover the performance range needed, I would look very seriously at the probable actual duty cycles required. It may make much more sense to consider three 50% compressors or even four 35% compressors.
2011-08-29
