Page 367 - Boiler_Operators_Handbook,_Second_Edition

352 Boiler Operator’s Handbook

leaving the container as we press more in but the two temperature have to be absolute values, add 15 to gage
flows do not have to match. A control air compressor may pressure to get absolute pressure and add 460 to tem-
run five minutes to fill a compressed air storage tank with perature to get absolute temperature. For volume you
enough air to supply the system after that tank for a half could use cubic feet or cubic inches, it doesn’t matter.
hour or longer. That should help explain why most com- Comparable metric units work just as well because it’s
pressor operations are on-off. The fluid stored under high the relationship, not the units that is determined; all that
pressure will expand to produce flow for the system, the counts is using the same units on both sides of the equa-
fluid flows out of the container as it is used and fewer and tion. To eliminate any consideration of algebra, here are
fewer pounds remain in the container. The container or the solutions for each factor in the equation. To learn the
storage tank serves as a reservoir for the fluid required by second condition of any one of them perform the math
the system and the compressor refills the reservoir when on the right of the equals sign
the fluid level drops to a preset value.
Specific compressor operations require special P = P × V × T ÷ T ÷ V
2 1 1 2 1 2
consideration because the fluid being compressed may
contain other fluids or contaminants that interfere with V = P × V × T ÷ T ÷ P
2 1 1 2 1 2
or require consideration in the process. When compress-
ing air we also pack in the moisture that’s in the air, T = P × V × T ÷ P ÷ V
2 2 2 1 1 1
the humidity. Since we’re packing molecules of air into
smaller and tighter spaces the water vapor in that air is Sometimes this formula is referred to as the ideal
subjected to higher pressures so it condenses to form liq- gas law. It would be ideal if it worked perfectly but it
uid water. Since compressors don’t run well on liquids doesn’t. For what we have to deal with as operators it’s
we have to remove that water. more than adequate. It not only applies to compression
We also don’t want the water in our system be- but any change in the pressure, volume, or tempera-
cause the combination of air and water is very corrosive. ture of gases. It’s most accurate with common diatomic
Water must be drained where it forms and collects in the gases, O , N etc.
2 2,
compressed air system, in between compressor stages Since you know that it’s the inefficiency of the
and in the storage tank. It also has to be drained at low compressor that produces the heat you can understand
points in the piping system, especially where the piping why you burned your arm on the piping or that com-
goes through a colder area (as in outdoors during the pressor head the last time you got too close. It’s a good
winter) where the water would be condensed by heat thing to measure to monitor the health of your compres-
loss. sor system. The temperature will vary with load so you
Despite what some people think, coolers on com- have to relate the temperature you’re reading with one
pressors aren’t there to condense the water. As long as at a similar load at an earlier time to identify any pend-
the water remains a vapor it acts just like the air and ing problems.
does little harm to the compressed air system. The cool- There’s a lot of confusion associated with compres-
ers are required because the compression is not efficient. sor application that I want to make sure you don’t get
Some of the energy that’s used by the compressor does involved in. Unless otherwise indicated the capacity
the work to compress the fluid. The inefficiency of the of a compressor is always described in scfm (standard
compressor is associated with simply heating the fluid cubic feet per minute) equal to air at 70°F and one atmo-
and since there is little mass in the fluid the temperature sphere. Sometimes it’s called ‘atmospheric cubic feet per
of the fluid increases dramatically. minute’ and abbreviated acfm which many engineers,
Now is probably the best time to say that there’s including me, interpret as ‘actual cubic feet per minute’
a simple formula for compression that says P × V ÷ T with unpleasant consequences.
1 1 1
= P × V ÷ T which means that the pressure (P), vol- If I’m looking at an application, such as air atom-
2 2 2
ume (V), and temperature (T) are all related before and izing for a burner, and the burner manufacturer’s table
after compression. Pressure times volume divided by indicates I need 30 cubic feet per minute of air at 80 psig
temperature at one condition for a gas will be equal to I’ll call that 30 acfm. It’s actually 190 scfm or a compres-
the pressure times temperature divided by the volume sor salesman’s atmospheric acfm (30 × 95 ÷ 15 = 190).
at another condition. If we double the pressure and the I know of several occasions where that confusion has
temperature remains the same then the volume has to be resulted in attempts to change steam atomizing burners
half as much. It’s important to note that the pressure and to air atomizing because the engineer didn’t realize the