What to consider in sizing a Breathing Air Compressor?

One of the more common questions we run into from our customers is in regard to how large of a compressor is actually needed. “Large” in this sense refers to the capacity of the compressor, in breathing air produced, measured in cubic feet per minute (CFM).  It should be noted here that there is no “plug-in” formula to get this answer. However, by analyzing your department’s needs and consulting with a breathing air specialist the right combination of storage and compressor can be matched to meet your requirements. A significant consideration in compressor selection deals with the customer’s available electrical power. This aspect of compressor selection will be discussed in our next article.

Bauer Compressor - Breathing Air CompressorHigh pressure breathing air compressors, like those typically used in the fire service, range in size from less than 10 to more than 25cfm. Determining what size you actually need depends upon several factors. The first, action that should be taken to answer this question should involve looking at your normal use or need for SCBA fills. What does your routine incident tend to require?  Obviously, your routine use will not cover every situation that you may ever run into. It is those situations “outside the norm” that are the foundation of good “mutual aid” programs. No department ever has everything that they need, as resources are typically too expensive and good planning can avoid excessive expenditures.

OK, I know what my department typically requires on an event so how do I take this information to get to the size compressor I need? Consider a standard 30 minute 4500 psi SCBA cylinder holds 45 cubic feet of air when full. Cylinders on the fire ground typically come to the fill station with approximately 500 psi of air still in them. Let’s figure that the 500 psi represents approximately 5 cubic feet of air. Now I have a bottle that needs approximately 40 cubic feet of air to be fully refilled. If I have two such bottles (2 x 40 cubic/ft.) and I place them in a two bottle fill station which is attached to a 10 CFM compressor, it will take approximately  8 minutes to refill the cylinders when filling directly from the compressor (8 minutes x 10 CFM = 80 cubic/ft.).  At this point, most of you are thinking I don’t have eight minutes to fill 2 bottles when I will likely have many bottles that will need filling.

This leads us to the second consideration, do you have or do you plan to add storage capability? Adding sufficient storage is often the least inexpensive method to increase operational capacity. Storage typically consists of large DOT cylinders which are piped together and are pre-filled with breathing air that will be used to supplement the work of the compressor.  Consider that 2 – 6000 psi DOT cylinders can be interconnected to “cascade fill” and alone can fill 10 – 45 cubic /ft., 4500 psi SCBA cylinders. The 10 cylinders we are talking about represent only the capability of the storage system and do not include the compressor’s capability. The typical cascade system consists of 4 such DOT bottles and at 6000 psi can fill 31, 45 cubic feet SCBA cylinders, before needing to be refilled themselves.  If you filled the 31 SCBA cylinders, two at a time, and took 3 minutes to fill each pair it would take approximately 48 minutes to refill them all from the storage system. During this same period of time a 10 CFM compressor would add 450 CFM back to the storage cylinders and be available for continued filling. It should be noted in this scenario that the first hour of refilling would be much better than the second hour. The rationale for this statement is being derived from the fact that you have consumed the majority of your stored air and at some point your compressor will not be able to keep the pace of the original hour because of the diminished storage. If in this same scenario you had a 25 CFM compressor, connected to the storage cylinders, you could fill at this same rate literally indefinitely because the compressor would replenish the storage cylinders nearly as fast as you were pulling it down.

A general statement in regard to compressor size would be, the smaller the storage system then the larger the compressor needs to be.  For example, if you purchased a 25 CFM compressor and had no storage system, you could fill two 45 cu/ft. SCBA cylinders, that were completely empty, every 3.6 minutes literally non-stop. The trade-off is expense and at times the physical size of the compressor.

So, while we may not have been able to answer the original question as a mathematical equation, we have provided insight into what some of the considerations are and how the compressor works in conjunction with the storage to meet your needs. As always, Breathing Air Systems OR Safe Air Systems can assist you in answering this question and your next breathing air system.

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