Are VSD-controlled compressors best for regulating air pressure?
Determine the best configuration to regulate compressed air pressure.
By Tom Taranto and Ron Marshall, Compressed Air Challenge
Figure 1. Airflow through an orifice as a function of pressure.
The idea of regulating the compressed air pressure of the whole plant from one location is a fairly new innovation. Operating a compressed air system at the lowest optimum pressure to meet the needs of production reduces energy cost. But there’s a controversy: Should compressed air pressure/flow control be used, or should modern compressor controls, especially VSD-controlled compressors, be used as an alternative to providing precise pressure regulation? Or is the best configuration using both?
What is artificial demand?
Artificial demand is defined as the excess volume of air that is required by unregulated end uses as a result of supplying higher pressure than necessary for applications. Compressed air flows from the compressed air system to atmosphere through an opening or hole in the system. That opening could be a leak, an air tool, a pneumatic cylinder, control valve or other production application. If there’s a hole or orifice in the air system, increasing compressed air pressure increases the airflow (Figure 1).
Flow rates for various orifice sizes and pressures are shown in Table 1. A 10-psig reduction in air pressure causes approximately 10% less compressed airflow.
Example system pressure profile
A typical compressed air system pressure profile (Figure 2) shows a lowest acceptable end use pressure of 75 psig. This is the pressure below which unacceptable end use performance results. Calculating backward, the corresponding system pressure differentials are 8 psid for end use pipe connections, 2 psid for the distribution piping and 5 psid for the treatment equipment pressure loss — main air dryer and filter. Therefore, the minimum acceptable operating pressure at the air compressor discharge is 95 psig. Operation at lower pressure results in low pressure at the end use during peak system demands.
Figure 2. A simple pressure profile illustrates how the overall system CONSUMES outlet pressure.
Where it goes
More efficient compressed air system operation can be achieved by reducing the system pressure to a point low enough for good energy savings, but not below the point where critical end uses experience low pressure during normal system events such as high flow transient demands and compressor switching operations.
Pressure reduction using only VSD control
“Compressors equipped with a variable speed drive offer the most efficient part-load capacity control available.”
- Tom Taranto and Ron Marshall, Compressed Air Challenge
Compressors equipped with a variable speed drive offer the most efficient part-load capacity control available. Suppliers of VSD compressors often argue that, because of the precise control at a constant setpoint, pressures can be reduced to the minimum acceptable pressure. So, why not just reduce the compressor discharge pressure to get the energy reduction benefits of both artificial demand reduction and reduced compressor power? It’s not that easy, the problem being that this strategy leaves the system without stored energy.
Compressed air energy storage
Compressed air energy available in a storage system depends on two factors: storage receiver size and the pressure differential between the storage pressure and the system’s minimum acceptable target pressure. The relationship for this is shown in Equation 1.
Vgas = Vrcvr x (Psto-Ptar)/Patm
Vgas = storage volume (scf)
Vrevr = receiver volume in cu. ft.
Psto = storage pressure (psig)
Ptar = target pressure (psig)
Patm = atmospheric pressure (psia)
Figure 3. Compressed air block diagram.