> Also, after investigating the airflow ratings on several different
> compressors, I've noticed that, between manufacturers, there doesn't seem
> to
> be any industry standard pressure at which cfm ratings are measured. This
> complicates the process of "scientifically" comparing units, and it makes
> me
> just want to ask the list: "what really works and what doesn't"?
>
The industry standard for compressed air flow is to define it at standard
conditions, which are 14.7 psia (0 psig) and 60 degF. This is referred to
as scfm. scfm is much different from actual delivered cfm (acfm), because
the volume of the air changes after it is compressed, so acfm will be
different depending upon the pressure at which it is delivered. For
example, I just looked at the Sears website and they have a 5.5 HP
compressor, running at 175 psig, that produces 17.4 scfm at 100 psig. This
is equivalent to about 2.3 acfm at that pressure. The rule of thumb for
larger industrial compressors (say, 25 HP and above), which usually deliver
air at around 100 psig, is that a compressor passes about 4 scfm/HP. In the
smaller sizes, this ratio drops off because of inefficiencies. Thus, when
comparing/sizing compressors, always compare in the same units, i.e. scfm to
scfm, and when sizing a compressor to serve a particular air tool, calculate
the acfm consumed by the tool at its operating pressure and then choose a
compressor that produces a greater acfm at that pressure. I have attached
an scfm/acfm converter for Steve (others will not see it because the list
does not support attachments).
The other issue with compressors is the HP rating. There have been many
discussions on this list about the inflated HP ratings assigned to
compressors. I recommend that you search the digest for those discussions!
Michael Marr
1960 TR3A
Plainfield, IL
[demime 1.01d removed an attachment of type application/vnd]
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