The purpose of demand control is to control the maximum pressure that the using side of the system can remove. In addition:

• it is the primary system control. All parts respond to it, including the compressor control. It is always set lower than the lowest compression pressure.
• It guarantees that the production user will receive an accurate, consistent pressure at variable volume; more accurately, it will maintain constant density at variable mass.
• It will limit the pressure and therefore the volume of all users (including leaks in the system). By limiting demand, supply energy can be unloaded as actual demand drops.
• It creates storage on the supply side of the system. This storage is potential energy, which will immediately cascade to the downstream side of the demand controller whenever demand exceeds supply.
• It does not require operator adjustment and leak management to maintain balance in the system.

The demand controller essentially expands the stored gas from a higher pressure to a lower controlled pressure at which it will be used. The analogy in electricity would be a transformer, with the regulator at the point of use being a circuit breaker. Without the transformer, the voltage would fluctuate as a function of amperage draw.

Demand control only allows displacement of the exact amount of mass or work energy which has been consumed at the user end of the system. Expanding the mass to a lower pressure maintains mass which has the same energy, which has increased in volume, while reducing to a lower control pressure. Do not confuse a demand controller with a pressure regulator, which restricts mass to control pressure — the function is similar in terms of throttling, but the way it is sized and controlled can be very different.

Normally, base-load compressors are operated based on their higher pressure rating. With demand controls, the lowest pressure-rated compressor is the base, provided the demand control is set at a higher pressure than other demand controllers in the system. The compressors can be operated independently from the demand requirements, optimizing the system.

This may seem to be a lot of effort to design and manage the system correctly. I have heard people say that every 1 psig increase in system pressure only requires increasing the connected compressor brake horsepower by ½%. The problem with this statement is that depending on the amount of unregulated demand (including leaks) as a percent of the total demand, the elevation of the pressure will increase the unregulated demand linearly to the rise in pressure.

Example 1 — A system has a total demand of 2000 scfm of equivalent supply energy at 100 psig at various use pressures. Of the total amount of demand, 400 scfm are leaks and 800 scfm are used in production, but there is no regulation or the regulators are wide open. It it were possible to increase the supply pressure to 110 psig, the leaks and unregulated demand would increase by about 120 scfm - this in turn would increase the entire demand to 2120 scfm at 110 psig. If there was only capacity for for 2000 scfm at 100 psig, the demand would exceed supply and the system pressure would drop back to 100 psig or less. If the energy were available, the total energy would increase 5% from the initial value of 480 bhp to 504 bhp - plus the energy to supply the artificial demand created by operating the demand at 110 psig. That was 120 scfm at 100 psig, which would take about 35 bhp of compressor. The total increase, assuming the power was there to waste, would be about 59 bhp, or 12% of the original horsepower.

This, of course, assumes that the amount of added horsepower was available from one compressor which was trimming or could be discretely added. Unfortunately, most compressors work together in such a way that the problem in example 2 will more than likely occur.

Example 2 — Four compressors, either rotary-screw- or centrifugal-type, are all operating in the throttling mode. As the compressors reduce in displacement, the pressure rises. Without changing the dead band for each compressor, elevating the pressure on the compensators would reduce the total displacement and cause the pressure to drop. If an attempt is made to raise the pressure, the capacity of the compressors will be reduced and the pressure will fall. It will appear that the supply is insufficient. If another compressor is added to the mix, the same load will be spread over five units, each doing a little less; the pressure will increase by only a small amount. The proper solution would have been to adjust the dead bands on the compressors and the arrangement throttling band.


Principles of demand-side air system management

1. Limit air usage based on applicability and economic alternatives.

2. Specify using equipment based on minimum possible pressure, never exceeding the maximum agreed standard for initial or article pressure.

3. Equipment selected for the system will include volume at the standard pressure or less.

4. Do not alter the system for the sake of one or a few users. OEMs can accommodate slightly lower pressures, although it may cost a little more.

5. Carefully select the P5 installation components to control the differential pressure. Be particularly careful of the regulator selection based on initial pressure required to hold the pilot seat pressure. Don't forget to allow for increased cycles, filter dirt loading, and light leaks. The total differential should meet the maximum standard differential.

6. Protect users from being affected by other users by carefully designing storage between the demand controller set pressure and the lowest allowable initial pressure serving the highest regulator set pressure. Size for the largest demand of event, speed of response, and length of transmission. The intent of general storage is to control the maximum change in overhead piping until control storage or supply energy can stop the decay.

7. Control 100% of demand including leaks at a set pressure which is lower than the lowest compression pressure. This will not only limit demand and allow for control of potential energy expansion, but it allows you to control the compressors at their optimum operating pressure.

8. Waste cannot be created and usage in demand cannot be elevated when it is controlled. Only the precise amount of air that has been removed from the system can be supplied. A demand controlled system will unload supply in a linear fashion with demand requirements.

9. Maintaining potential energy in the system will avoid the need to service demand with direct energy. The more stored energy, the fewer peaks and valleys, and the lower peak power.

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.