Most people aren’t aware that compressed air is one of the most costly utilities in a plant. Typical wire-to-work efficiency of a 100-psi system is only about 10%–15% at best. That is, if you put 10 hp into an air compressor, you only can get about 1 hp of work out of the other end when operating a compressed-air-powered device or tool.
Worse yet, this efficiency ratio is for a perfect system — for the compressor running at 100 psi and at full load, its most efficient operating point. However, many compressors don’t operate in this way. Also, the ratio doesn’t include leakage or waste in a system and doesn’t consider pressure differentials across system components. This ratio is kind of like the gas mileage number that appears on the sticker of a new car, a value only attainable in the laboratory — not in real life.
There isn’t a lot you can do about most of this loss — it’s simple physics: when you compress air, the biggest product of the operation is heat. If you can use this heat, don’t let it go to waste. Heat recovery is the most effective way of making a compressed air system more efficient. (For more on the topic, see “Recover Heat from Air Compression.")
What’s left for your attention is to ensure your system is as close to perfect as possible.
So, let’s look at 10 issues, mostly related to screw compressors, that frequently reduce the efficiency of compressed air systems.
Inefficient control mode. This is one of the most common efficiency problems. Compressors must be controlled to maintain constant system pressure. If uncontrolled compressors put too much air into the system, the pressure goes too high; if they put in less than you use, the pressure goes too low. The onboard compressor controller in each unit is tasked to ensure that just enough compressed air is added to your system to maintain a more-or-less constant pressure between set limits.
When matching the compressor capacity to the load, normally at least one compressor is operating at partial load, that is, not putting out its full capacity. And this is where using an efficient compressor control mode is important. Common control modes in order of most efficient to least efficient are:
• Variable speed — An inverter drive or special motor changes the speed of the compressor. The power in this mode decreases in almost direct proportion to the flow.
• Variable displacement — Bypassing through controlled ports reduces the effective length of the compression element. Useful for the top 50% of flow, power turns down about 7% for each 10% reduction in flow.
• Load/unload — The compressor alternately produces full or zero flow between two pressure set points. The average power decreases about 5%–6% per 10% drop in flow but only if the system has large storage receiver capacity.
• Modulation — The inlet of the compressor is choked off to control the output flow. The power turns down about 3% for every 10% reduction. Thus, if a compressor with this type of control is putting out 40% of its capacity, it’s consuming about 85% of full-load power.
What mode are your compressors operating in? Ask your service provider or someone in your plant who knows compressors. If your compressors are in modulation, you likely have some work to do to improve your efficiency. However, there usually are lots of potential opportunities to get compressors operating better with other modes of operation. Often in a system of multiple compressors, one or more may need replacement due to age. For a new unit, consider a more efficient mode.
Lack of storage. Too often a system doesn’t include enough storage receiver capacity. The presence of one or more large storage receivers near the compressor room helps the compressors maintain stable system pressure and run more efficiently. Receivers store a significant quantity of air that can prevent the system pressure from falling or rising too fast. This simplifies compressor control and could very well allow one or more compressors to turn off when not required instead of running unloaded, wasting power.
If a compressor is running in load/unload mode, the presence of large storage will make the compressor more efficient by reducing the frequency of load cycles. Figure 1 shows the effect of adding storage to lubricated screw compressors. As more storage is available, the power consumption at part loads decreases.