Air Consumption of Cylinders
FLUID POWER  Design Data Sheet 58
To calculate the HP load to be placed on an air compressor by a
reciprocating air cylinder, the power to operate the cylinder with
its circuit losses should first be calculated. Then, the additional
HP needed for compression losses caused by compressing to too high
a pressure and reducing through a pressure regulator should be
estimated and added.
Air consumption for the cylinder and its circuit can be
calculated from Table 1 below. Then the air
consumption can be converted to compressor HP. Finally, any
additional losses can be estimated and added.
The procedure described in this sheet involves a certain amount
of estimation of flow losses because an accurate calculation which
includes all pertinent data is difficult.
How To Use Table 1
First, calculate the pressure behind the cylinder piston to just
balance the load resistance. Then add the estimated pressure loss
due to flow resistance in the circuit, through valves and plumbing.
For most air circuits, unless the application is highly special, an
additional 25% of pressure, above the load balance pressure will be
sufficient, and the pressure regulator should be set for this
pressure. This extra pressure is quite necessary. It pushes the
volume of air required for the cylinder cycle rate through the flow
resistance of valves and plumbing. It is consumed in the circuit
and is not effective against the load while the cylinder is in
motion, but does add to the HP load on the compressor.
Pressures along the top of the table represent the outlet
setting of the pressure regulator. By using the regulator outlet
pressure instead of load balance pressure, all circuit flow losses
are included in the HP calculation.
Calculate load balance pressure, then add 25%. Use this pressure
column in the table. With a specified cylinder bore diameter, take
the figure in the table which is air consumption for a 1inch
stroke. Multiply times the number of inches stroke and by the
number of cycles, forward and return which the cylinder will make
in one minute. This gives the SCFM air consumption for this
application.
Table 1 was calculated using compression ratios
in the table on the back side of this sheet.
Consumption is given for cylinders with standard diameter piston
rods. The saving of air for cylinders with larger rods is
negligible for most calculations.
Air consumption is calculated assuming the cylinder piston will
be allowed to stall, at least momentarily, at each end of its
stroke, giving it time to fill with air up to the pressure
regulator setting. If reversed at either end before full stall
occurs, air consumption will be less than shown.
Calculating Cylinder Horsepower
After the SCFM air consumption has been calculated, convert into
horsepower using the condensed table on the back side of this
sheet, or for other than a 2stage, pistontype compressor, see
table on Data Sheet 72.
System Losses
Flow losses downstream of the pressure regulator outlet have
already been included. There are several small additional losses
which should be considered.
Compression of air is an inefficient process because a
substantial part of the energy is lost as the heat of compression
is radiated to atmosphere. By overcompressing the air and then
reducing its pressure through a pressure regulator, these
compression losses are increased. Also there is a small flow loss
through the pressure regulator. Calculation of these losses is very
difficult and we suggest the addition of an extra 5 to 10% HP above
the calculated level to take care of these losses.
Table 1.
Cylinder air consumption per 1inch stroke, forward and
return.
Cylinder
Bore
Inches 
PSI on
Pressure Regulator Outlet 
40
PSI 
50
PSI 
60
PSI 
70
PSI 
80
PSI 
90
PSI 
100
PSI 
110
PSI 
120
PSI 
130
PSI 
140
PSI 
150
PSI 
160
PSI 
1.50 
0.007 
0.008 
0.010 
0.011 
0.012 
0.013 
0.015 
0.016 
0.017 
0.018 
0.020 
0.021 
0.022 
2.00 
0.013 
0.015 
0.018 
0.020 
0.022 
0.025 
0.027 
0.029 
0.032 
0.034 
0.036 
0.039 
0.041 
2.50 
0.021 
0.024 
0.028 
0.032 
0.035 
0.039 
0.043 
0.047 
0.050 
0.054 
0.058 
0.062 
0.065 
3.00 
0.030 
0.035 
0.040 
0.046 
0.051 
0.056 
0.062 
0.067 
0.071 
0.076 
0.081 
0.087 
0.092 
3.25 
0.034 
0.040 
0.047 
0.053 
0.059 
0.065 
0.071 
0.077 
0.084 
0.090 
0.096 
0.102 
0.109 
4.00 
0.052 
0.062 
0.072 
0.081 
0.091 
0.100 
0.110 
0.120 
0.129 
0.139 
0.148 
0.158 
0.168 
5.00 
0.083 
0.098 
0.113 
0.128 
0.143 
0.159 
0.174 
0.189 
0.204 
0.219 
0.233 
0.249 
0.265 
6.00 
0.119 
0.140 
0.162 
0.184 
0.205 
0.227 
0.249 
0.270 
0.292 
0.313 
0.334 
0.357 
0.379 
8.00 
0.213 
0.252 
0.291 
0.330 
0.369 
0.408 
0.447 
0.486 
0.525 
0.564 
0.602 
0.642 
0.682 
10.00 
0.334 
0.394 
0.455 
0.516 
0.576 
0.637 
0.698 
0.759 
0.820 
0.881 
0.940 
1.00 
1.07 
12.00 
0.480 
0.568 
0.656 
0.744 
0.831 
0.919 
1.01 
1.09 
1.18 
1.27 
1.36 
1.45 
1.54 
14.00 
0.652 
0.771 
0.891 
1.01 
1.13 
1.25 
1.37 
1.49 
1.61 
1.74 
1.85 
1.98 
2.10 
Calculation Example
Estimate the compressor HP capacity to cycle an air cylinder
having a 4inch bore and 28inch stroke, 11 cycles per minute.
Assume the load against the cylinder is 800 lbs., and that a
2stage pistontype air compressor is used.
Solution: Calculate the pressure to balance the
load:
800 lbs. ÷ 12.57 sq. in. (on
a 4in. bore cylinder) = 64 PSI
Add 25% more pressure to cause air to flow:
64 + (0.25 × 64) = 64 + 16 =
80 PSI
This is the suggested setting of the pressure regulator. Use the
80 PSI column in Table 1. The table shows a
requirement of 0.091 SCF per inch of stroke. Calculate the total
air requirement:
SCFM = 0.091 × 28 × 11 =
17.9
Use Table 2 below to convert this into
compressor HP. The table shows a figure of 0.148 HP for each SCFM.
The total HP then is:
HP = 0.748 × 17.9 =
2.65
To take care of the extra losses due to overcompression and
flow, add 10%:
2.65 + (0.1 × 2.65) = 2.92
or approximately 3 HP total
Table 2. HP
required to compress air.
PSI 
HP* 

PSI 
HP* 

PSI 
HP* 

PSI 
HP* 
50 
0.116 
80 
0.148 
110 
0.171 
140 
0.190 
60 
0.128 
90 
0.155 
120 
0.178 
150 
0.196 
70 
0.138 
100 
0.164 
130 
0.185 
160 
0.201 
*HP to compress 1 SCFM from 0 PSI to the values shown.
Compression between adiabatic and isothermal is assumed, with a
2stage pistontype compressor working at 85% efficiency. For a
more complete chart, see Data Sheet 72.
Power Losses in a Pressure Regulator
Power supplied to the inlet of a pressure regulator is delivered
through to the outlet at a similar power level. Outlet power is at
a lower pressure level but at a higher flow rate. The inlet would
produce a higher force on a cylinder piston but with a slower
movement. The outlet pressure would produce less force on the same
cylinder but at a higher travel speed. The result would be the same
HP except for certain energy losses or gains which occur inside the
regulator.
Air expanding inside the regulator will absorb energy from the
atmosphere if its temperature drops below ambient. On the other
hand, flow loss produces heat which is an energy loss to
atmosphere. Calculation of energy gains and losses through the
regulator is impractical. As previously stated, we resort to
approximations and rulesofthumb which we have found give
satisfactory practical results.
Increasing the Efficiency of an Air
System
Any change which will put more of the available pressure across
the cylinder ports while the cylinder is moving, and less across
the regulator, piping, speed control valves, 4way valve, and other
restrictions, will increase efficiency.
 Increase the diameter of all plumbing lines.
 Increase the flow size of the 4way valve.
 Set the cutout pressure of the compressor as low as
practical.
 Use the smallest bore cylinder which will produce the desired
results. It will use more of the available pressure, leaving less
to be wasted in flow losses.
 Use a second regulator to reduce return pressure to just
sufficient for return speed and force desired. Remove the return
speed control valve.
Pressure Conversion Chart
Air volume calculations must be based on absolute zero pressure,
PSIA. "Free Air" is defined as air at standard conditions of
atmospheric pressure and temperature, and for the chart is taken as
14.7 PSIA or 0 PSIG. Compression ratio in the third column of the
chart, is the number of times the pressure is greater than normal
atmospheric pressure, again taken as 14.7 PSIA.
Pressure
Conversions
Calculated from formula shown underneath the chart.
Pressure
PSIG 
Pressure
PSIA 
No.
of
Atmos.* 
No.
of
Bars 
0 
14.7 
1.00 
1.01 
5 
19.7 
1.34 
1.36 
10 
24.7 
1.68 
1.70 
15 
29.7 
2.02 
2.01 
20 
34.7 
2.36 
2.40 
25 
39.7 
2.70 
2.74 
30 
44.7 
3.04 
3.08 
35 
49.7 
3.38 
3.43 
40 
54.7 
3.72 
3.77 
45 
59.7 
4.06 
4.12 
50 
64.7 
4.40 
4.46 
55 
69.7 
4.74 
4.81 
60 
74.7 
5.08 
5.15 
65 
79.7 
5.42 
5.50 
70 
84.7 
5.76 
5.84 
75 
89.7 
6.10 
6.19 
80 
94.7 
6.44 
6.53 
85 
99.7 
6.78 
6.88 
90 
104.7 
7.12 
7.22 
95 
109.7 
7.46 
7.57 
100 
114.7 
7.80 
7.91 
105 
119.7 
8.14 
8.26 
110 
124.7 
8.48 
8.60 
115 
129.7 
8.82 
8.94 
120 
134.7 
9.16 
9.29 
125 
139.7 
9.50 
9.63 
130 
144.7 
9.84 
9.98 
135 
149.7 
10.2 
10.3 
140 
154.7 
10.5 
10.7 
145 
159.7 
10.9 
11.0 
150 
164.7 
11.2 
11.4 
155 
169.7 
11.5 
11.7 
160 
174.7 
11.9 
12.0 
165 
179.7 
12.2 
12.4 
170 
184.7 
12.6 
12.7 
175 
189.7 
12.9 
13.1 
*This is also the compression ratio used to convert a given
volume of compressed air (cubic feet, or CF) to an equivalent
volume of free air (standard cubic feet, or SCF).
For values of gauge pressure not shown, calculate as
follows:
PSIA = PSIG
Atmos. = (PSIG + 14.7) ÷
14.7
Bars = (PSIG + 14.7) ÷
14.5
Download a PDF of Fluid Power Design
Data Sheet 58  Air Consumption of Cylinders.
© 1990 by Womack Machine Supply Co. This
company assumes no liability for errors in data nor in safe and/or
satisfactory operation of equipment designed from this
information.
