Configure your compressor system for optimum efficiency

Figure 1. Most of the energy input to a compressor is lost and unable to perform useful work.

Also, in a typical plant, production uses only about 50% of the pneumatic output the compressors generate, with the rest lost to inefficiencies, waste and inappropriate uses. Compressed air is often the principal, if not the greatest, production cost component. As such, it offers substantial savings opportunities from reducing consumption and using the air more efficiently (Figure 2). A key factor for realizing the available savings is properly applying storage, which, in this context, refers to the stored energy, as indicated by the pressure contained in the fixed volume of your air system. Volume alone, however, doesn’t equal storage. There also must be a change in the pressure within that fixed volume to produce useable storage. Take, for example, a large receiver installed in the compressor room. If the pressure at the tank inlet is the same as the pressure at the outlet, the useable stored energy contained in the tank is zero. Figure 2

This is a typical distribution of the air consumption in a manufacturing operation.

The volume adds to the overall system capacitance, but it can’t be applied to manipulating the energy balance between the system’s supply side and the demand side unless it’s accompanied by a controlled change in pressure. The simplified storage relationship for fixed volume vessels like air receivers is:

Vs = ΔP x Vf/Pa

where Vs = the stored volume
ΔP = change in pressure
Pa = atmospheric pressure
Vf = the vessel’s fixed volume

Pressure changes in the storage vessel are based on the flow of air into and out of the receiver. If more air flows out than in, the air expands into the piping distribution system and the internal pressure decreases. Conversely, if the air flow into the receiver exceeds the outflow, the delivered air increases the pressure in the piping distribution system. Monitoring the outlet pressure and controlling the release of the air from storage by means of an intermediate pressure/flow control produces a stable reference point for balancing the system. The results:

• A reliable, stable compressed-air source for production purposes.
• Fewer compressed air-related problems and work stoppages.
• Energy savings from optimizing system efficiency.

An unanticipated compressor shutdown usually dictates the minimum amount of storage. To prevent serious production interruptions while the standby compressor starts, comes on line and begins to contribute air, storage must sustain the system. The required amount of storage, therefore, depends on the compressor capacity and the how much pressure degradation can be tolerated before production begins to shut down.

If, for example, the standby compressor takes 30 seconds to begin filling in for a failed 100 hp compressor rated at 500 scfm, and atmospheric pressure is 14.5 psia, the storage requirement is 250 cf (500 scfm x 1 min./60 sec. x 30 sec.). If the storage pressure is allowed to degrade 15 psi during those 30 seconds, the receiver capacity would be:

Vf = (Vs x Pa / ΔP) x 7.481 gal/cf

Where Vf = Receiver volume
Vs = 250 cf
Pa = 14.5 psia
ΔP = 15 psi

In other words, a fixed volume of 1,870 gal (250 x 14.5/15 x 7.481) will release 250 scf while dropping 15 psi in pressure. If the storage pressure is allowed to degrade only 5 psi during the 30 seconds, the receiver capacity would be 5,500 gal (250 x 14.5/5 x 7.481). These examples clearly demonstrate the relevance of the allowable pressure change.

Demand surges that cause flow spikes also influence the storage requirement. Evaluate them to determine if additional storage is needed either in the compressor room or at the local workstations to mitigate unacceptable pressure fluctuations.

The energy required to keep a system in balance comes from a combination of storage and available excess compressor capacity. Running a compressor partially loaded provides the unused reserve that can be drawn upon as needed. Partially loading a compressor, however, can be inefficient and costly, particularly if the compressor is oversized.

In fact, an unloaded, fixed-speed compressor can consume 20% to 25% of its rated horsepower even when idling. Storage, therefore, must be sufficient to allow an unloaded compressor to time out and shut down, even during short-duration events. The compressor would come online only when the operating compressor(s) plus storage could no longer sustain the minimum system pressure.

Using motor horsepower as an energy reserve is costly. The better approach is to take advantage of the storage-based reserve in advance of the intermittent peak demands and keep unloaded compressors shut down.

Speed factor

Compressors with variable-speed drive (VSD) change the storage variables in the balance equation because they have no consequential cost penalty from operating partially loaded. Horsepower is essentially matched to the demand load. Oversizing a VSD compressor to provide an energy reserve is an acceptable practice. For example, if a system needs 100 hp in reserve, a 200 hp VSD compressor could run at 50% load without introducing appreciable waste and inefficiency. In systems with VSD compressors, the energy required to maintain the system balance can rely more on a reserve of rotating motor horsepower and less on storage than a system configured with only fixed speed machines (Figure 3).

Figure 3

This schematic illustrates a typical application of a VSD with intermediate control. (Block diagram courtesy of Tom Taranto and ConservAIR)

The arbitrary substitution of storage with the reserve energy of motor horsepower from a VSD compressor must be weighed carefully. There’s still a time lag between the demand event and the compressor response that you must take into account. Insufficient storage puts the compressor in a catch-up mode, in which it constantly chases system demand, never taking maximum advantage of its inherent highly efficient part-load performance. In general, you can realize an additional 7% to 10% savings by trimming the system using the VSD compressor through controlled storage produced by the pressure/flow control.

The storage required to swing a trim compressor into the base position without interrupting production in the event of an unanticipated compressor failure must still be addressed. Also, VSD compressors frequently are networked with fixed-speed machines and centrifugal units. Proper application of storage ensures the VSD always can trim the system and not cause other networked compressors to load up or blow off.

Applying storage to control the system balance is essential for optimizing the energy efficiency, regardless of the compressor configuration. Storage ensures that a stable, reliable source of compressed air always is available for production. Effective compressor sequencing can be automated in a balanced system. The reduced air consumption from leak management and the elimination of wasteful practices and inappropriate uses will fully translate into real energy savings back at the compressor motors. You can profile the system, design the storage response, and then control the energy balance of the system at the optimum level.

Bob Wilson is the products manager at Pneumatech LLP in Kenosha, Wis. Contact him at [email protected] and (727) 866-8118.

Figures 1 and 2 courtesy of ConservAIR Energy Management