Fast-acting valves, such as pulse valves on dust collectors, are often misapplied. The typical pulse time is 0.1 or 0.2 sec. If the valve discharges 1 std ft3 of air in 0.1 sec, the rate of flow would be 600 scfm. Some manufacturers insist that if this happens 10 times per minute at 1 std ft3 per cycle, the valve is using 10 scfm.

Another element of this particular sizing problem is the pressure. Most equipment manufacturers provide the initial pressure that they want as the pressure requirement. There are two issues here. One is: where are they specifying the pressure? Is it at the inlet to the tool or valve or is it at the inlet to the filter-regulator upstream of hose, disconnects, and fittings? In most cases, it is the latter. If the manufacturer assumes a 15- to 20-psig pressure loss through those components, the system may be operating at an unreasonably high pressure and cost. The second issue is that sizing is not based on initial pressure, but article pressure (the lowest pressure at the using equipment’s final fitting).

When averaging in evaluating the cumulative demand, a process flow diagram should be used, Figure 1. (This is normally done in a tabular format). List the air-using equipment in flow per minute. Use factors are applied and the percentage established is moved to the right hand column. Then, the factored volumes are added up, regardless of pressure — this is called the demand. The total factored volume is then expressed at the highest single use pressure of all components. The supply compressors are sized on this basis. All of the system components are then sized based on the rate of flow of the compressors, including dryers, filters, and piping. A receiver or receivers are also included in the components selected. Storage tanks and piping will increase the rate of flow based on their volume per unit pressure capacitance and how much the designer plans on allowing the pressure to fluctuate.

The highest rate of flow of the system from both the supply and demand perspectives must be carefully determined, in order to achieve the desired pressures at the point of use with the lowest energy needed at the supply end of the system.

Let’s say the compressors are capable of 1000 cfm at 110 psig. The total system capacitance for piping and tanks is 45 ft3 per psi. If the pressure is allowed to fluctuate 10 psig, the downstream components would have to be sized for:

1000 cfm + [(45 cfm/psi)×10 psi] = 1,450 cfm

Thus, 1,450 cfm is the highest supply rate of flow. The calculations should be done progressively outward from the individual compressors, in a process flow format, tracking rate of flow and pressure.

R. Scot Foss is president of Plant Air Technology, Charlotte, N.C., which specializes in air system auditing and design. This series of articles is based on his book, Compressed Air System Solution Series. To order a copy, click here.