Pressure drop is proportional to the square of the velocity. Any high-volume, intermittent demand produces dramatic pressure drop during peak periods. Ignoring this fact affects every process connected to the header. For more detail, see “The compressed air receiver: The endless question,” page 49, Plant Services, May 1997 and Appendix 1, Tables and Outline from DOE/CAC Air Master Training Manual. For a given size pipe:
- At constant pressure, the greater the flow, the greater the loss per foot of pipe.
- At constant flow rate, the lower the inlet pressure, the greater the loss per foot of pipe.
- At any condition, smooth-bore pipe (copper, stainless steel) exhibits lower friction losses.
The most overlooked idea in piping layout and design is air velocity. Excessive velocity can be a root cause of backpressure, erratic control signals, turbulence and turbulence-driven pressure drop.
The British Compressed Air Society suggests that a velocity of 20 fps or less prevents carrying moisture and debris past drain legs and into controls. A velocity greater than 30 fps is sufficient to transport any water and debris in the air stream. Thus, the recommended design pipeline velocity for interconnecting piping and main headers is 20 fps or less, and never to exceed 30 fps. Field testing reveals that, under these conditions, air stream turbulence is generally negligible. Line drops, feed lines or branch lines less than 50 ft. long work well at a velocity of 30 fps, but velocity here should not exceed 50 fps.
First, look at the velocity at maximum anticipated flow conditions using the following equation:
V = 3.056 * Q/D[+]2[+] (Eqn 1)
Where V = air velocity (in ft/sec)
Q = volumetric flow rate (in cfm)
D = conduit inside diameter (in inches)
Although this method of determining the minimum pipe size on the basis of air velocity is easy, you also must consider that the compressed air volume is expressed in cubic feet per minute of free air, which is the air volume at ambient atmospheric conditions, not the compressed volume.
To adjust the inlet air volumetric flow rate to actual pipeline conditions, you’ll need to divide the volume of free air by the compression ratio using the following equation:
CR = (P+P[-]a[-])/P[-]a[-] (Eqn 2)
Where P = line pressure (in psig)
P[-]a[-] = average atmospheric pressure at your elevation (in psi)
Table 1 shows the compression ratio as a function of gauge pressure for a location at sea level, where the atmospheric pressure is 14.7 psi. At higher elevations, the average atmospheric pressure drops and the compression ratio rises. For example, Flagstaff, Ariz., at a 7,000-ft. elevation, has an average atmospheric pressure of about 11 psi. At 100 psig, the compression ratio is equal to 10 (i.e. 111/11).
To determine the pipeline velocity at conditions, merely divide the velocity given in Equation 1 by the compression ratio given in Equation 2. After selecting the minimum pipe size on the basis of velocity, check any long runs for excessive pressure drop using an appropriate drop chart. For example, a velocity of 25 fps in black iron pipe represents about 0.25 psi loss per 100 ft. of run. Although this is a little above the recommended minimum of 20 fps and, depending on the layout, would probably be acceptable from a turbulence standpoint, a high total frictional loss may dictate using a larger pipe.
This might seem to be somewhat complicated at first, but it’s the most accurate way to avoid problems in sizing compressed air piping. Once you get the hang of it, it’s easy to use.
After carefully selecting a conduit size that eliminates unnecessary loss, be sure to pay the same attention to downstream items, such as quick disconnects, regulators, filters, controls, fittings, number of drops from a given header and number of connections per header, so as not to offset the gains made with the pipe. Good piping performance is not an accident -â€“ it takes planning.
Hank van Ormer, Don van Ormer and Scott van Ormer are owners of AirPower USA, Pickerington, Ohio. Contact them at HankvanOrmer@aol.com and (740) 862-4112.