Tag: horsepower

Basics of Compressors

Published on by johnball2014Leave a comment
Single Stage Portable Air Compressor

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

What is an Air Compressor?

Published on by johnball2014Leave a comment

Internals of an air compressor

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 states, air compressors are designed to compress air.  Unlike liquid, air is compressible which means that it can be “squished” into a smaller volume by pressure.  With this stored energy, it can do work for your pneumatic system.

There are two types of air compressors, positive displacement and dynamic.  The core component for 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 energy with an impeller.  (You can read more about types of air compressors HERE).

Compressed air is a clean utility that is used in many different 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 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 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 to operate 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 an expensive system to compress air to operate pneumatic systems.  So, efficiency in using compressed air is very important.  EXAIR has been manufacturing Intelligent Compressed Air Products since 1983.  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.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

 

Compressor internals image courtesy of h080, Creative Commons License.

Estimating the Total Cost of Compressed Air

Published on by johnball20142 Comments

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.

Compressed Air 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
Email: johnball@exair.com
Twitter: @EXAIR_jb