Look beyond the sticker price when buying an air compressor

It’s the total cost of ownership that should guide you.

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By Bob Anderson

PlantServices.com

When shopping for air compressors, it pays to look beyond the sticker price before you make your decision. The startling fact is that the initial price represents a mere 15% of compressor costs over a 5 to 7 year period. Savvy purchasing professionals know how to look under the hood, so to speak, when they make their choice.

Energy

The first place to kick the tires is in the area of energy efficiency. Buying an energy efficient compressor saves you tens of thousands of dollars in the long run. Compressors are not by their nature very energy efficient. In fact, it can take 7 to 8 horsepower of electric power to produce 1 horsepower of compressed air power. Energy costs for operating a compressor can exceed the initial cost of the compressor in the first year alone. So, if all you are comparing when choosing a compressor is the sticker price, you could end up paying a premium in the end. Look for a compressor that has the best energy saving features.

The most efficient compressor on the market today is the double acting reciprocating compressor. However, it has been replaced in many plants by oil-flooded rotary screw compressors. Once the standard for plant air, the double acting reciprocating compressor is now used mostly for higher pressure or specialty applications. The demise of these compressors is due partly to high initial costs, complex installations, and the need for experienced maintenance crews.

This leaves the majority of plant air being provided by rotary screw compressors. The best rotary machines have energy saving features built into their design.

The most efficient rotary compressor is one that is fully loaded, providing the maximum flow, measured in cfm, per brake horsepower. When an application has long periods of little or no demand for compressed air you should have a compressor that can shut itself off. Or, at the very least, you can blow the oil reservoir down and operate at reduced horsepower, usually between 18 and 22% of full load horsepower.

Efficient operations

One way to run a compressor efficiently can be to operate the compressor in a load-no load mode. When in this mode the compressor is either running full loaded, or, when system demand is satisfied, running completely unloaded and blown down.

Of course, it's not wise having a compressor that is always unloaded. To avoid paying a premium for something that sits idle, ensure that your compressor is sized properly to match your needs, both present and in the near future. Most major reputable compressor companies have distributors that do a good job of sizing compressors.

Although load-no load is touted as a high efficiency feature, there is a catch. Unless there is a sufficient amount of air receiver capacity, including the volume of air in the piping system, this feature can be energy inefficient.

There needs to be a minimum of 10 gallons of receiver capacity for every cubic foot per minute the compressor delivers. The larger the capacity of stored air, the longer the compressor can remain blown down and operating at a reduced horsepower.

Whenever a compressor's system blows down, compressed air vents to atmosphere. In effect, you're throwing money away by discharging air you paid money to compress. The key is ensuring the compressor stays blown down long enough for you to benefit from the lower horsepower consumption. If your system has long periods of unloaded operation, look for a compressor that also has a feature that turns the motor off after a preprogrammed time interval. This drops the horsepower to zero.

If your system has varying load cycles requiring different volumes at different times of the day and you do not have the required storage to run load-no load, you need to have a system that runs efficiently with partial loads. With partial loads the compressor produces compressed air at less than full capacity. The trick is to reduce compressor capacity while maintaining the minimum pressure required. Various methods of part load compressor controls achieve this end.

One way of doing this is with inlet valve modulation. This works by controlling the position of the inlet valve that opens and closes in response to system demand. Although extremely responsive, it is the least efficient way to run a compressor partially loaded.

For example, running a compressor at 70% capacity still requires 91 to 93% of full load horsepower. The reason for this inefficiency is that, when at less than full capacity, a partially closed throttle restricts the opening and creates a vacuum at the inlet. This lowers the inlet pressure. At sea level, a partial vacuum of 4.7 pounds per square inch lowers the inlet pressure to 10 pounds per square in absolute from 14.7 pounds per square inch atmospheric.

More work is now required to raise the inlet pressure to the final discharge pressure. Compressing this air to 100 psig, 114.7 psia, requires a higher compression ratio, the ratio between the discharge and inlet pressures. The system had 114.47 divided by 14.7 = 7.8 compression ratio, and now at part load it is 114.7 divided by 10 = 11.5.

Overall, less power is required to compress the smaller volume of air, but with the above inefficiencies, more energy must be expended per cubic foot of air.

Alternate approaches

A better way to part load a compressor is by controlling the rotor length. A turn-valve or poppet-style valve unit opens windows up as demand decreases. The inlet valve remains fully open so the compression ratio is not affected.

Instead, the length of the rotor is shortened. At 70% capacity, a turn valve unit can achieve 78% brake horsepower, a 13 to 15% energy savings over inlet valve modulation.