Complicated selection process can make these common equipment items mysterious to specifiers.
by Collin Boettcher
September 20, 2019

These heat exchangers can be as large as the machine and mounted adjacent to the compressor. Multistage models are often sited on a mezzanine elevated 15 or 20 feet above grade to accommodate the motor, which can be a large diameter synchronous design running at 400 or 500 rpm. These units are most commonly found compressing process gasses in refineries and large chemical plants.

Screw machines: Screw machines are used in both the air and gas market. This market uses 100 to 125 psig industrial plant air machines, provided in a noise enclosure and usually between 100 and 250 hp. Screw machines can be dry screw with Teflon-coated rotors or lubricated. Electronic and aerospace facilities insist on very pure plant air to avoid contamination in manufacturing. They lean toward dry screw technology. There are lots of older reciprocating plant air machines still pounding away in the basements of many manufacturing plants worldwide.

Large centrifugal: These compressors in the 30,000- to 100,000-hp range are routinely built and can deal with pressure ratios from single digit to over 100, depending on the process variables.

Specialty gas diaphragm: These compressors are used for very high-pressure ratio exotic gasses in aerospace and cylinder filling. They will be 50 to 100 hp and achieve ratios of 500 or more. They are best with very low flow rates at very high pressures.

Fans: On the other end of the gas-handling spectrum are fans, which are low-ratio machines. Fans are usually rated in inches of water column pressure and come in a wide variety of designs depending on the application. Fans produce large volumetric flow rates at low pressures.


Compressors are a wide variety of sizes, strengths and designs and there is no correlation from one application to the next. Unlike a pump where size visualization becomes more or less intuitive as you gain experience, a compressor offers no clue when you initially tackle the above sizing procedures.

Take note of the wide range of hps. Also note the effect inlet pressure has on hp with machines with similar pressure ratios. Remember that doubling the inlet pressure doubles the hp at the same volumetric flow. A compressor expert can help with the right selection.

Blower History

As with many inventions, the blower was discovered by accident. In the mid-1800s, the Roots Brothers were working on a better design for the water wheel, which ran machinery in their woolen mill. However, the water caused the counter rotating wooden paddles, or impellers, to swell and the device stuck. One of the brothers forced rotation and a burst of air blew his hat off. They immediately recognized the new machine—a rotary blower.

The blower was patented in 1860. By 1870, the major use of blowers was in foundry cupola furnaces in the steel industry and the mining industry for shaft ventilation. Other early uses included aeration, pneumatic conveying and supercharging.

Today, blowers are commonly used in oil and gas, chemical processing and wastewater treatment.