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
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
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
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
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
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
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
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
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.
- 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.
- 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
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
If you think your system has room for improvement and you
would like more details, send an email to
firstname.lastname@example.org. 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