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Regaining Control of Controls
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Regaining Control of Controls

Author: Dan Wise, Webmaster

The search for cost savings has increased the awareness of the high electrical cost of operating a compressed air system.

There are specific rules (not rules of thumb) that govern the performance of a compressed air system. You, or someone you trust, must know the rules to prevent problems and to get the most from your air system.

Compressor control systems are a common cause of wasted energy. However, it takes more than an understanding of controls to correct the problem and recover the energy savings.

There is an interdependent relationship between the various components of a compressed air system. An good example of this is the impact that the air delivery piping and the air storage system can have on compressor controls.

An Industrial Plant will either add compressors to handle growth or replace older units with newer compressors. The air system must be able to accommodate different types and brands of control systems and compressors.

Excessive pressure loss between the compressor discharge and the system can preclude capacity control efficiency by eliminating the required effective storage. The effective storage requirement varies by the type of compressors and controls.

The following illustrates how a few changes can dramatically lower your energy bill for compressors.

Air System Profile

An auto parts manufacturing company operates a plant with 2 compressors. One is a 400 HP, double acting, reciprocating unit rated at 2006 CFM. The other is a 350 HP oil cooled, 2 stage rotary screw unit rated at 1685 CFM.

The reciprocating compressor uses a 5 step regulator for control. The rotary screw compressor uses modulation/on line, off line with Automatic Control Selector. Controls were set to hold a minimum 95 psig system pressure.

The average demand for compressed air is between 3000 CFM and 3200 CFM. The compressors operate 6240 hours a year. They have a blended power cost of $.045 kWh and the motors have a .93 efficiency.


The compressors were set up under the classic method of operating the rotary screw as the base load machine. The reciprocating compressor operates as the trim unit to handle the fluctuating air demands that are over the capacity of the rotary screw.

The reciprocating unit loads in at 95 psig and begins to add extra air, raising the system pressure. At 105 psig, the reciprocating compressor is supposed to partially unload and the rotary screw is supposed to continue on as the base load unit.

The problem starts as soon as the reciprocating units loads into the system. It takes over as the base load unit. This put the 350 HP rotary screw into trim with modulation.

Air Audit Findings

An Air Audit was conducted to determine why the reciprocating compressor was forcing the rotary screw compressor into the role of trim unit.

The investigation revealed that the actual sensed pressure in the pipe, although registering as 99 psig at the compressor panel gauges, was bouncing back and forth between 101 psig and 103 psig. This was caused by the reciprocating unit's 6 inch discharge line feeding into the rotary screw's 4 inch discharge line with a crossing pipe tee.

This area of high turbulence created back pressure to both compressors. However, the compressors reacted differently because the control systems sense system pressure in different locations.

The rotary senses system pressure at the exit point from the unit, in its discharge line before the crossing pipe tee connection that is causing the back pressure. The reciprocating compressor senses system pressure at the air receiver which is downstream from the crossing pipe tee connection.

This is why the back pressure in the discharge lines affects the rotary more than the reciprocating unit.

The more the rotary tried to load in, the higher the back pressure goes, which pushes it to unload. When the system pressure would once in a while get high enough to unload the reciprocating unit, it would only do so momentarily and reload because the turbulence continues to hold the rotary back from full load.

Therefore, when the sensed system pressure reaches 101 psig, the rotary modulation control immediately starts to back down and make less air. This continues on until the sensed system pressure reaches 105 psig and the rotary is at 50 percent load (842 CFM) and the reciprocating unit is still at full load (2006 CFM).

The total flow of the 2 compressors is 2848 CFM. The systems continued to run at this condition which is the rotary at 50 percent capacity and 85 percent power, while the reciprocating unit is 100 percent capacity and 100 percent power.

The actual brake horse power (BHP) was calculated at 373 BHP for the reciprocating unit and 280.5 BHP for the rotary. This is a total of 653.5 BHP.

Annual Power Cost

The numbers from Air Audit provided the data needed to calculate the annual electric power cost for these 2 compressors.

The first step is to multiply the horsepower of the compressor times .746 times the hours of operation times your power rate (HP x .746 x hours x power rate). Then, divide that number by the motor efficiency.

What were they spending on power? The formula would be 653.5 HP x .746 x 6240 hours x $.045 which would then be divided by .93 motor efficiency.

They were spending $147,197 in annual energy costs to operate the 2 compressors.


The strategy for making improvements was based on two compressed air system principles.

  1. Always use 30 or 45 degree angle entry connections when introducing air into a flowing stream of air. This eliminates the energy waste caused by the turbulence and back pressure of a pipe tee connection.
  2. Always install an air receiver between rotary screw compressors and reciprocating compressors when running them in parallel. Direct connections to the discharge air line of a reciprocating compressor produces pulsations which may be harmful to the pressure gauges, check valves, controls and the air oil separator element in the rotary screw compressor.

The solution was to add an air receiver and connect each compressor to its own air receiver tank. Then, install air piping from the two receivers to the main header for air distribution to the plant. Angle entry connections were used at every point where air was introduced into a flowing stream of air.


The changes made it possible to operate the rotary screw compressor as the lead machine with the reciprocating unit as the trim unit.

The result was to run the rotary at 100 percent capacity and 100 percent power, while the reciprocating unit is 43 percent capacity and 47 percent power. This lowered the combined horse power used to 505 BHP.

What was the impact of the changes? The annual energy cost was lowered because less horse power was required to operate the same 2 compressors.

The improvements lowered their annual energy costs to $113,748. This is shown in the formula of 505 HP x .746 x 6240 hours x $.045 which would then be divided by .93 motor efficiency.

The $33,449 in annual energy savings were more than enough to cover the one time expense of an Air Audit, the piping modification and installation of an extra air receiver.


Air Audits are as unique as the different facilities that operate compressed air systems. However, there are opportunities to save money that are common to most situations.

If you think your system has room for improvement and you would like more details, send an email to Give us a short description and a phone number and we will have someone call to help with your questions.

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Revised: Thursday, 08-Oct-2015 11:51:56 AEDT
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