Control the flow to optimize compressor operation

Is an efficient compressed air system an oxymoron? Receivers and controls might be useful, if configured correctly.

By Hank van Ormer, Air Power USA

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During a recent audit, someone asked me a simple question - “What is the one thing I can add to every system that will ensure it always operates at optimum efficiency?” The answer is rather complex.

Let’s be clear about this. There’s no silver bullet that will ensure continuous, efficient operation in any air system, although many items of hardware have at one time or another been promoted as such. As often happens when promise doesn’t match performance, the negatives step in and imply that obviously, because the tool wasn’t perfect, it was no good. Reality lies somewhere in between. Like any piece of hardware, each has a place in some air systems. Each might be part of a good solution for achieving efficiency in an air system, depending on conditions working with some of the others. The compressed air system designer and operator must understand these products and techniques to take full advantage of the opportunities they present.

Here’s another point to be made before looking at these often-misunderstood and misapplied products: There are many things that dramatically affect the efficiency and productivity of a compressed air system, but they also might have to be adjusted before they have a positive impact. Such things as pipe sizing and configuration, air compressor selection, dryer and filter selection, automatic condensate drains selection and location, and unit capacity controls are part of success, or lack thereof.

So, what do we do about storage, pressure control, variable-speed operation, capacity control and central air management systems? How do we know what we need? How do they, or can they, work together?

Define the objective

When, exactly, is a compressed air system operating as its optimum efficiency? Some might call an efficient compressed air system an oxymoron. Evaluating a change in air supply requirements and assuming proper pipe sizing and configuration should always be verified to determine that they are acceptable.

Some might call an efficient compressed air system an oxymoron.

– Hank van Ormer, Air Power USA

An efficient system operates at optimum productivity and quality, which requires the system to be at the lowest effective discharge pressure, system header pressure and entry pressure to each process. Compressors with the best cfm/input kW ratio generally have only one unit at part load, all others at full load. There are circumstances, particularly with centrifugals, where this guideline might be modified. Air treatment equipment is at the lowest possible energy draw and pressure loss. For example, under many conditions in an oil-free system, heat-of-compression desiccant dryers can operate with no significant energy cost. Finally, the fewest possible number of air compressors are on — full load, part load or idle — as required to meet the demand. How do each of these products, with proper strategy and techniques, affect the ability to meet and hold the system at or near optimum efficiency?

Demand expander

Figure 1. Controlling unregulated air will provide a stable environment in which the compressor can operate efficiently.
Figure 1. Controlling unregulated air will provide a stable environment in which the compressor can operate efficiently.

This also is known as a flow controller, intermediate controller, isentropic flow controller and other names. It was introduced in the late 1970s as a special air system accessory and was generally recommended with a large air receiver installed after the dryer and filter. System air then goes through the pressure regulators or computer-controlled flow control valve.

The idea is to hold the system pressure within 1 psig, thus eliminating excess header pressure, which will overdrive the demand flow by about 1% per psig of unregulated uses, including leaks. Therefore, dropping the system pressure 10 psig should reduce the leak volume by a minimum of 10%.

Table 1 shows what can be expected. When no air was regulated, the operating band was 15 psig and system “float” (maximum/minimum) was also 15 psig. Control of unregulated air absolutely does work (Figure 1). Higher pressure always drives higher flows. When the flow controller was engaged, the header pressure in this three-story manufacturing facility dropped to 80 psig from 94 psig, and the flow dropped from 800 cfm to 615 cfm.

 

  Compressor outlet pressure (psig)
System entry pressure (psig) Auto-fixed demand (cfm)
Unregulated:      
Cut-in
90 90 910
Run
100 100 1000
Cut out
110 110 1054
       
Regulated:
     
Cut-in
90 90 910
Run
100 90 910
Cut out
110 90 910
Table 1. This data shows the stabilizing effect the regulation provides.

 

Unfortunately, these systems often were oversold with statements such as:

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