It is important to know the cost of compressed air at your
facility. Most people think that compressed air is free, but it is most
certainly not. Because of the expense, compressed air is considered to
be a fourth utility in manufacturing plants. In this blog, I will
show you how to calculate the cost to make compressed air. Then you can
use this information to determine the need for Intelligent Compressed Air®
products.
There are two types of air compressors, positive displacement
and dynamic. The core construction for both is an electric motor that
spins a shaft. Positive displacement types use the energy from the
motor and the shaft to change the volume in an area, like a piston in a
reciprocating compressor or like rotors in a rotary compressor. The
dynamic types use the energy from the motor and the shaft to create a
velocity energy with an impeller. (You can read more about air
compressors HERE).
For electric motors, the power is described either in kilowatts (KW) or
horsepower (hp). As a unit of conversion, there are 0.746 KW in 1
hp. The electric companies charge at a rate of kilowatt-hour
(KWh). So, we can determine the energy cost to spin the electric motors.
If your air compressor has a unit of horsepower, or hp, you can use Equation
1:
Equation 1:
hp * 0.746 * hours * rate / (motor efficiency)
where:
hp - horsepower of motor
0.746 - conversion to KW
hours - running time
rate - cost for electricity, KWh
motor efficiency - average for an electric motor is 95%.
If the air compressor motor is rated in kilowatts, or KW, then
the above equation can become a little simpler, as seen in Equation 2:
Equation 2:
KW * hours * rate / (motor efficiency)
where:
KW - Kilowatts of motor
hours - running time
rate - cost for electricity, KWh
motor efficiency - average for an electric motor is 95%.
As an example, a manufacturing plant operates 250 day a year
with 8-hour shifts. The cycle time for the air compressor is
roughly 50% on and off. To calculate the hours of running time, we have
250 days at 8 hours/day with a 50% duty cycle, or 250 * 8 * 0.50 = 1,000
hours of running per year. The air compressor that they have is a 100
hp rotary screw. The electrical rate for this facility is at $0.08/KWh.
With these factors, the annual cost can be calculated by Equation 1:
100hp * 0.746 KW/hp * 1,000hr * $0.08/KWh / 0.95 = $6,282
per year.
In both equations, you can substitute your information to see
what you actually pay to make compressed air each year at your facility.
The type of air compressor can help in the amount of
compressed air that can be produced by the electric motor. Generally,
the production rate can be expressed in different ways, but I like to use
cubic feet per minute per horsepower, or CFM/hp.
The positive displacement types have different values
depending on how efficient the design. For a single-acting piston type
air compressor, the amount of air is between 3.1 to 3.3 CFM/hp. So, if
you have a 10 hp single-acting piston, you can produce between 31 to 33 CFM
of compressed air. For a 10 hp double-acting piston type, it can
produce roughly 4.7 to 5.0 CFM/hp. As you can see, the double-acting
air compressor can produce more compressed air at the same horsepower.
The rotary screws are roughly 3.4 to 4.1 CFM/hp. While
the dynamic type of air compressor is roughly 3.7 – 4.7 CFM/hr. If you
know the type of air compressor that you have, you can calculate the amount
of compressed air that you can produce per horsepower. As an average,
EXAIR uses 4 CFM/hp of air compressor when speaking with customers who would
like to know the general output of their compressor.
With this information, we can estimate the total cost to make
compressed air as shown in Equation 3:
Equation 3:
C = 1000 * Rate * 0.746 / (PR * 60)
where:
C – Cost of compressed air ($ per 1000 cubic feet)
1000 – Scalar
Rate – cost of electricity (KWh)
0.746 – conversion hp to KW
PR – Production Rate (CFM/hp)
60 – conversion from minutes to hour
So, if we look at the average of 4 CFM/hp and an average
electrical rate of $0.08/KWh, we can use Equation 3 to determine the average
cost to make 1000 cubic feet of air.
C = 1000 * $0.08/KWh * 0.746 / (4 CFM/hp * 60) = $0.25/1000ft3.
Once you have established a cost for compressed air, then you
can determine which areas to start saving money. One of the worst
culprits for inefficient air use is open pipe blow-offs. This
would include cheap air guns, drilled holes in pipes, and tubes. These
are very inefficient for compressed air and can cost you a lot of
money. I will share a comparison to a 1/8” NPT pipe to an EXAIR Mini Super Air Nozzle.
(Reference below). As you can see, by just adding the EXAIR nozzle to
the end of the pipe, the company was able to save $1,872 per year. That
is some real savings.
 
Making compressed air is expensive, so why would you not use
it as efficiently as you can. With the equations above, you can calculate how
much you are paying. You can use this information to make informed
decisions and to find the “low hanging fruit” for cost savings. As in
the example above, targeting the blow-off systems in a facility is a fast and
easy way to save money. If you need any help to try and find a way to
be more efficient with your compressed air system, please contact an Application Engineer
at EXAIR. We will be happy to assist you.
John Ball
Application Engineer |
Comentários
Enviar um comentário