What is an air compressor? This may seem like a simple question, but it is the heartbeat for most industries. So, let’s dive into the requirements, myths, and types of air compressors that are commonly used. Like the name implies, air compressors are designed to compress air. Unlike liquids, air is a compressible gas, which means that it can be “squished” into a smaller volume by pressure. With this stored energy, it can do work for pneumatic systems.
There are two main types of air compressors, positive displacement and dynamic. The core component of most air compressors is an electric motor that spins a shaft. Positive displacement uses the energy from the motor and the shaft to change volume in an area, like a piston in a reciprocating air compressor or like rotors in a rotary air compressor. The dynamic types use the energy from the motor and the shaft to create a velocity with an impeller like centrifugal air compressors. This velocity converts to a rise in pressure.
How do they work? Most air compressors are driven by an electric or gas motor. The motor spins a shaft to push a piston, turn a rotor, or spin a vane. At the beginning of the air compressor, we have the intake where a low pressure is generated from the displacement to bring in the surrounding ambient air. Once trapped, Boyle’s law states that when the volume decreases, the pressure increases. For the dynamic type, the velocity and design will increase the air pressure. The higher pressure will then move to a tank to be stored for pneumatic energy. The amount of power required is dependent on the amount of air that needs to be compressed.
Compressed air is a clean utility that is used in many ways, and it is much safer than electrical or hydraulic systems. But most people think that compressed air is free, and it is most certainly not. Because of the expense, compressed air is considered to be a fourth utility in manufacturing plants. For an electrical motor to reduce a volume of air by compressing it, it takes roughly 1 horsepower (746 watts) of power to compress 4 cubic feet (113L) of air every minute to 125 PSI (8.5 bar). With almost every manufacturing plant in the world utilizing air compressors much larger than 1 horsepower, the amount of energy needed to compress a large volume of air is extraordinary.
Let’s determine the energy cost to operate an air compressor to make compressed air by Equation 1:
Equation 1:
Cost = hp * 0.746 * hours * rate / (motor efficiency)
where:
Cost – US$
hp – horsepower of motor
0.746 – conversion KW/hp
hours – running time
rate – cost for electricity, US$/KWh
motor efficiency – average for an electric motor is 95%.
As an example, a manufacturing plant operates a 100 HP air compressor in their facility. The cycle time for the air compressor is roughly 60%. To calculate the hours of running time per year, I used 250 days/year at 16 hours/day for two shifts. So operating hours equal 250 * 16 * 0.60 = 2,400 hours per year. The electrical rate at this facility is $0.10/KWh. With these factors, the annual cost for operating the air compressor can be calculated by Equation 1:
Cost = 100hp * 0.746 KW/hp * 2,400hr * $0.10/KWh / 0.95 = $18,846 per year in just electrical costs.
So, what is an air compressor? The answer is a pneumatic device that converts power (using an electric motor, diesel or gasoline engine, etc.) into potential energy stored as pressurized air. Efficiency in using compressed air is very important. EXAIR has been manufacturing Intelligent Compressed Air Products since 1983. We are able to save you money by reducing the amount of compressed air you use. If you need alternative ways to save money when you are using your air compressor, an Application Engineer at EXAIR will be happy to help you. We even have a Cost Savings Calculator to find the annual savings and payback period; and you will be amazed at how much money can be saved.
John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb
Photo: Technical Illustration of a portable single-stage air compressor by Brain S. Elliot. Creative Commons CC BY-SA 4.0