HVAC System / Compressed Air Systems / Power Quality

Inlet temperature, outlet pressure and storage

Part 1: Don’t let misapplied rules of thumb become your compressed air rules of dumb.

By Hank van Ormer

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. 

Figure 1. Correcting for temperature, ambient pressure and moisture content can have a large effect on the measured air volume.

Figure 1. Correcting for temperature, ambient pressure and moisture content can have a large effect on the measured air volume.

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.

Air  temp, °F

Water vapor PP (psia)

Air temp, °F

Water vapor PP (psia)

Air temp, °F

Water vapor PP (psia)

32 

0.08854 

60 

0.2563 

86 

0.6152

  34    

0.09603

62

0.2751 

88

0.6556

36  

0.10401 

64 

0.2951 

90

0.6982

38  

0.11256 

66 

0.3164

92 

0.7432

  40    

0.1217 

68 

0.339  

94 

0.7906  

 42

0.1315 

70 

0.3631  

95 

0.8153 

44

0.14199  

72 

0.3886  

96 

0.8407 

  46

0.15323 

74 

0.4156 

98 

0.8935 

48  

0.16525  

76  

0.4443  

100 

0.9492  

50   

0.17811  

78 

0.4747  

102 

1.0078  

52   

0.19182  

80 

0.5069  

104 

1.0695  

   54      

0.20642 

82 

0.541 

106 

1.1345 

56   

0.222  

84  

0.5771 

108 

1.2029  

  58     

0.2386 

85 

0.5961 

110 

1.2748  

Table 1. Water vapor partial pressure at 100% RH as a function of temperature.


For example, assume an ambient temperature of 90°F and 50% relative humidity. If the air was fully saturated, the water vapor partial pressure, according to Table 1, would be 0.6982 psia. But, with 50% RH, the partial pressure is 0.6982 x 0.50 = 0.3491 psia. Deduct this value from the recorded inlet pressure (psia) to compensate for the water vapor to be lost. This effectively reduces the weight of the compressed air.

Always measure the compressor’s inlet pressure with a vacuum gauge. A reading below 14.5 psia increases the volume of the air required to meet the target flow rate. But, weight and volume also are functions of inlet filtration, inlet piping and weather. Hence, compressed air is dynamic, so you should be aware of the effect the inlet filter size and condition, and the piping size and length have on performance and specific power. Any inlet condition variable that makes the air denser increases the scfm delivered per unit of acfm (inlet air at inlet conditions).

That background is preparation for answering the original question: Will reducing the inlet air temperature to a compressor increases its efficiency? It seems kind of simple now, doesn’t it? The hotter the air, the lower is its density. The colder the air, the more a unit volume will weigh, thereby producing greater scfm. And, in a positive-displacement compressor, the actual swept volume of inlet air has more effect on power draw that does the scfm or weight of air. As the temperature falls, delivered air scfm increases at a much greater rate than the power. The net result leads to a relatively accurate guideline: Every 5° of cooling leads to a 1% improvement in specific power or efficiency.

Reducing inlet temperature or otherwise increasing the air density is equivalent to increasing the mass flow when the volume displacement (icfm/acfm) remains constant. The input power remains the same or about the same. This rule is fairly accurate for reciprocating and oil-free rotary compressors (vane, screw, lobe, scroll, etc.)

A new ballgame

What about lubricant-injected cooled units (rotary screw and vane)? Generally, compressed air representatives are going to give you one of two answers: “Sure, the 1% per 5°F rule applies,” or “No, it doesn’t apply.” The facts are these:

  • The rule certainly doesn’t apply in its entirety.
  • Compressor manufacturers have invested many hours of laboratory testing unsuccessfully trying to establish an accurate guideline.

Positive-displacement, lubricant-cooled, rotary screw or vane compressors inject lubricant (at 150°F to 160°F) into the compressed air, either at the inlet or immediately after seal-off. The oil acts as a coolant and absorbs some of the heat of compression. Regardless of design, some of this hot coolant/lubricant finds its way back to the inlet, where it can have a significant effect on the temperature of the air entering the rotary compression element(s). This localized heating of the inlet air can greatly reduce the beneficial effect of a colder inlet air temperature.

Over the years, we chased all over trying to find a universally appropriate way to quantify this effect. Finally, we gave up, accepting the test result for what it was: a normal change in inlet air temperature had no consistent measurable effect on efficiency. The methods for injecting lubricant/coolant into rotary screw compressors have evolved from the old-style standard injection ports located after the seal-off point to methods such as:

  • Female rotor injection on the inlet side to enhance sealing
  • Rotor inlet oil injection

Both approaches probably warm the inlet air, so the net result is that you can’t count on colder inlet air to increase performance (more scfm/kW) from a lubricant-cooled rotary screw or vane compressor.

The centrifugal story

Because they’re mass-flow units, the required input power to centrifugal compressors is a function of the weight/density or mass flow of the air. When the inlet air is colder (Figure 2), these compressors deliver more air using a commensurate increase in driving power. This doesn’t improve efficiency, but if you require the extra air and have the power available to produce it, it might not matter. If you don’t have the power available or haven’t reset the current or power limiters, then the centrifugal compressor will just blow off sooner.

Figure 2. Air temperature has a large effect on centrifugal compressor power consumption, but not necessarily on its efficiency.

Figure 2. Air temperature has a large effect on centrifugal compressor power consumption, but not necessarily on its efficiency.

Temperature and power

This rule of thumb can become a rule of dumb. When someone takes things out of context and says, “My inlet air is 100°F now. When it goes to 30°F this winter, I’ll increase my air flow by 14% (1% per 5°F) so now my 1,000-cfm unit will deliver 1,140 cfm and I can add another bench.” Real life doesn’t work that way.

Most of the guidelines in use were developed for 100-psig-class compressors, which operate between 80 psig and 135 psig. Keep this thought in mind when applying them.

Hank van Ormer owns Air Power USA Inc., in Pickerington, Ohio. Contact him at hank@airpowerusainc.com and (740) 862-4112.

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