Air is one of the basic needs of human being without which we cannot survive and air is total free. But most people think that compressed air is free. No way, it is not free at all. Because of the expense, compressed air is considered to be a fourth utility in manufacturing plants.
In this blog I am going to show the calculation of the cost to make compressed air and hope this information will be sufficient to understand the uniqueness and energy saving features of EXAIR intelligent compressed Air products.
For electric motors, the power is described either in kilowatts (KW) or horsepower (hp). 1 hp=0.746 KW
The electric companies charge at a rate of kilowatthour (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:
Energy Cost= 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 90%.
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 90%.
Example: a manufacturing plant operates 250 day a year with 8hour 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 Rs 9.4 /KWh. With these factors, the annual cost can be calculated by Equation 1:
100hp × 0.746 KW/hp × 1,000hr × Rs 9.4 /KWh / 0.90 = Rs 779,156 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 type compressors have different values depending on how efficient the design. For a singleacting piston type air compressor, the amount of air is between 3.1 to 3.3 CFM/hp. So, if you have a 10 hp singleacting piston, you can produce between 31 to 33 CFM of compressed air. For a 10 hp doubleacting piston type, it can produce roughly 4.7 to 5.0 CFM/hp. As you can see, the doubleacting 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.
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 (Rs 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 Rs 9.40 /KWh, we can use Equation 3 to determine the average cost to make 1000 cubic feet of air.
C = 1000 * Rs 9.40 /KWh * 0.746 / (4 CFM/hp * 60) = Rs 29 /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 blowoffs. 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 per year. That is some real savings
The table above shows the air consumption for typical homemade blowoff s.
The pages that follow give the air consumption and other data on EXAIR's
Air Nozzles and Jets.
Consider the following example where a Model 1102 Mini Super Air Nozzle
replaces an 1/8" open pipe. The compressed air savings is easy to calculate
and proves to be dramatic. Payout for Air Nozzles and Jets, including filter and
installation cost is measured in weeks  not years, as is the case for other cost
reduction equipment. Based on a 40 hour work week, 52 weeks a year.
Example:
1. Existing blowoff is 1/8" open pipe at 80 PSIG (5.5 BAR) supply.
Air consumption, from the table above, is 70 SCFM (1,981 SLPM).
2. Use a 1/8 FNPT Model 1102 Mini Super Air Nozzle also at 80 PSIG
(5.5 BAR) supply. Air consumption, from the image below,
is 10 SCFM (283 SLPM).
3. Compressed air saved = 70  10 = 60 SCFM (1,981  283 =1,698 SLPM)
4. For this example, the blowoff is continuous. If the duty cycle was 20%,
then air saved would be 60 x .2 = 12 SCFM (1,698 x .2 = 340 SLPM).
5. Most large plants know their cost per 1,000 standard cubic feet of
compressed air (10,000 standard liters). If you don't know your actual cost
per 1,000 SCF, Rs 29 is a reasonable average to use.
(Cost per 10,000 standard liters is approximately Rs 10 )
6. Money saved per hour = SCFM saved x 60 minutes x
cost/1,000 SCF
(SLPM saved x 60 min x cost/10,000 SL)
= 60 x 60 x Rs 29 /1,000
(= 1,698 x 60 x Rs 10 /10,000)
=Rs 104/hour that is =Rs 4160/week and Rs 216,320/year savings for one nozzle.
Padmalochan Nayak
 Vivek Engineers #22, 1st Floor, 1st Cross, Bilekahalli Indl. Area, Adj. IIMB Compound, Bannerghatta Road, Bangalore  560 076 Ph : 080  2648 1309, 4170 1145.



 