Problem: Size an Air Compressor and Receiver
A company has a machine that uses 50 cfm at 100 psi. Size an air compressor in SCFM that will have a duty cycle of 40%. Then size an air receiver in gallons that would supply that SCFM for three (3) minutes with the pressure dropping from 100 psi to 80 psi.
Find Out The Solution
I have two equations that I use for sizing an air compressor with a duty cycle:
SCFM = CFM X C.R./Duty Cycle
CFM = SCFM/C.R./Duty Cycle
Note: (C.R.) is the compression ratio. These two equations cannot be transposed into the other equation.
Step 1: Find C.R.
(100 + 14.7) / 14.7 = 7.8 : 1 C.R.
We are looking for SCFM, so let’s use the first equation and substitute in our numbers.
Step 2: SCFM = 50 X 7.8 / 0.40 = 975 SCFM per machine.
We multiply the CFM by the C.R. to get SCFM and then we divide by 0.4 to oversize the compressor so that it will only run 40% of the time, allowing for expansion of our system in the future.
Sizing of a Receiver Tank
Again I have two equations, one for sizing the tank in cubic feet and one for sizing in gallons:
T(min.) = V(cu.ft.) X (P2-P1)
14.7 X SCFM
T(min.)=V(gal.) X (P1-P2)
110 X SCFM
We are looking for the receiver size in gallons, so let’s use the second equations.
3 X 110 X 975 / 20 = 16,087.5 gallon receiver per machine
3 is time in minutes, 110 is a constant, 975 is the SCFM, 20 is the pressure drop (P2-P1)
The answers are 975 SCFM and 16,087.5 gallon receiver per machine.
Deadline past. Not available for submissions.
Jeremy Palm, CFPS, Hennepin Technical College, Eden Prairie, MN
Richard Throop, CFPAI, CFPMT, CFPMM, CFPS, Neff Engineering, Flint, Michigan
By Ernie Parker, AI, AJPP, AJPPCC, S, MT, MM, MIH, MIP, MMH,
Fluid Power Instructor, Hennepin Technical College, EParker@Hennepintech.edu
This teaser is printed in the Fluid Power Journal. Those who submit the correct answer before the deadline will have their names printed in the Society Page newsletter and in Fluid Power Journal. The winners will also be entered into a drawing for a special gift.