Inlet temperature, outlet pressure and storage

Will reducing the inlet air temperature to a compressor increase its efficiency? In a world of sound bites, people become obsessed with simple, easy answers. The oft-quoted rule about inlet air temperature goes something like this: Each 5°F reduction in inlet air temperature nets a 1% gain in efficiency. How many times have you heard statements like this? Others include:

• Install the air intake on the north side in the shade.
• Pre-cooling the inlet air always increases efficiency.

The unvarnished truth is this: The temperature rule holds true for some compressors, but not all compressors.

Before you ask why, you need to know what we mean by an increase in efficiency. Efficiency doesn’t mean operating longer between overhauls or running smoothly. Efficiency means delivering more compressed air output per unit of input electrical power. Throughout the industry, this metric often is denominated as bhp per 100 cfm of compressed air delivered. People like me, who want things simple and easy, use cfm per input kW — the amount of air delivered per input kW of power. We call it specific power.

Weight versus volume

Another concept that affects the math is the difference between the actual volume of inlet air (acf) and the standard volume of inlet air (scf). The most valid efficiency measure is scfm delivered at full-load input power. The actual cubic feet per minute, also called free air delivered (FAD), is based on air at the inlet conditions. There are no corrections made for temperature, pressure or relative humidity. The standard cubic feet per minute metric, on the other hand, is a function of those variables. Remember:

• A fixed volume of cold air weighs more the same volume of warm air.
• A fixed volume of air at a higher ambient pressure weighs more than the same volume at lower ambient pressure.
• Water vapor in the inlet air is compressed, discharged and removed by the compressed air drying system, and represents a reduction in the weight of air compressed and delivered to the systems.

In short, inlet temperature, pressure and relative humidity affect both the density and weight of the air ultimately delivered to the users in the plant.

Measuring in scfm requires that the inlet air conditions be corrected to 60°F, 14.5 psia and 0% relative humidity, the standard conditions established by the Compressed Air and Gas Institute (CAGI) and Pneurop, its European counterpart. Most, if not all, manufacturers of air compressors and dryers use these standard conditions. Unfortunately, production machinery manufacturers and flowmeter manufacturers don’t always use this standard. 

A big difference

You can use the following equation to connect the inlet and outlet conditions for a compressor:

scfm = acfm x ((p1-pw) x (460°F + 60°F))/(14.5 x (460°F + t1))

where:
p1 = inlet ambient pressure (psia)
pw = water vapor partial pressure at inlet (psia)
t1 = inlet air temperature (°F)

You can find the water vapor partial pressure in Table 1, which is sufficiently accurate for informed decisions about selecting the proper equipment. Multiply the saturated values given there by the percent relative humidity at the inlet conditions. Whatever water vapor the filters, dryers and other cleanup equipment removes represents a relative reduction in inlet air pressure to the compressor.