Don't err with Air Compressors

Understand what's involved in getting a suitable supply of compressed air

By by David M. McCulloh, Mac Consulting Services

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Figure 2. Oil-injected rotary screw compressors require ample capacity to avoid cycling with this type of control.

Oil-injected rotary screw compressors can use various means of capacity control. The most common but least energy efficient is by inlet valve throttling. At about 40% of capacity, the power requirement is more than 80% of full load power. Below 40%, the compressor is unloaded, with no flow to the system, but will then require about 25% of full load power. Again, ample storage capacity is required. Load/unload (full capacity/zero capacity) control can cause excessive cycling unless ample storage capacity is provided (Figure 2).

Some models are available with variable displacement, which provides capacity control by changing the effective length of the rotors. This offers some improvement over inlet valve throttling but also is limited in the capacity control range.


Figure 3. VSD oil-injected rotary screw compressor provides more efficient capacity control than possible with constant speed unit.

VSD rotary screw compressors provide much more efficient capacity control, although they sacrifice a bit of efficiency at full capacity. Compare the percent power at various capacities (Figure 3), with the comparable data for inlet throttling (Figure 2).

For the oil-free rotary screw compressor, advantages include:

  • complete compact package
  • relatively low first cost
  • no need for a special foundation.

Disadvantages include:

  • less efficiency than water-cooled reciprocating type
  • higher long-term maintenance costs.

Its operating cost at full capacity and 100-psig discharge pressure runs 18–22 kW/100 cfm two-stage. For the oil-injected unit, advantages include:

  • complete compact package
  • relatively low first cost
  • no need for special foundation
  • routine maintenance (oil, filter, separator changes).

Disadvantages include: 

  • less efficiency than water-cooled reciprocating type
  • potential problem of oil carryover.

Operating cost at full capacity and 100-psig discharge pressure runs 18–19 kW/100 cfm single stage and 16–17 kW/100 cfm two-stage. 

  • Reciprocating compressors. These units come in single-acting and double-acting configurations. The single-acting type uses only one side of the piston to compress the air, whereas the doubleacting uses both sides. Single-acting compressors generally are small in size, air-cooled, and limited in capacity and time of operation but can be conveniently located near the point of use. Double-acting units usually are water-cooled and can have two or more stages of compression, which increases efficiency. They also can have multiple steps of capacity reduction, providing 100/75/50/25/0% capacity within a specified pressure control band. Both lubricated and non-lubricated piston/cylinder versions are available.

The advantages of double-acting compressors include:

  • efficient compression
  • efficient multi-step capacity control
  • relatively routine maintenance.

Disadvantages include

  • relatively high first cost
  • need for special foundations due to vibrations
  • oil carryover on lubricated versions.

The operating cost at full capacity and 100-psig discharge pressure for a water-cooled double-acting unit runs 15–16 kW/100 cfm. In contrast, an air-cooled single- acting compressor runs 22–24 kW/100 cfm.

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