Intelligent Compressed Air: Single Acting Reciprocating Air Compressors

Of all the types of air compressors on the market, you can’t beat the single acting reciprocating air compressor for simplicity:

Piston goes down: air is pulled in. Piston goes up: air is pushed out.

This simplicity is key to a couple of major advantages:

  • Price: they can cost 20-40% less than a similar rated (but more efficient) rotary screw model, up to about 5HP sizes.  This makes them great choices for home hobbyists and small industrial or commercial settings.
  • High pressure: It’s common to see reciprocating compressors that are capable of generating up to 3,000 psig.  Because the power is transmitted in the same direction as the fluid flow, they can handle the mechanical stresses necessary for this much better than other types of air compressors, which may need special modifications for that kind of performance.
  • Durability: out of necessity, their construction is very robust and rugged.  A good regimen of preventive maintenance will keep them running for a good, long time.  Speaking of which…
  • Maintenance (preventive): if you change your car’s oil and brake pads yourself, you have most of the know-how – and tools – to perform regular upkeep on a reciprocating air compressor.  There’s really not that much to them:

    The internals of a single acting reciprocating compressor.

Those advantages are buffered, though, by certain drawbacks:

  • Efficiency, part 1: The real work (compressing the air) only happens on the upstroke.  They’re less efficient than their dual acting counterparts, which compress on the downstroke too.
  • Efficiency, part 2: As size increases, efficiency decreases.  As stated above, smaller sizes usually cost appreciably less than more efficient (rotary screw, vane, centrifugal, etc.) types, but as you approach 25HP or higher, the cost difference just isn’t there, and the benefits of those other types start to weigh heavier in the decision.
  •  Maintenance (corrective):  Whereas they’re easy to maintain, if/when something does break, the parts (robust and rugged as they are) can get pretty pricey.
  • Noise: No way around it; these things are LOUD.  Most of the time, you’ll find them in a remote area of the facility, and/or in their own (usually sound-insulated) room.
  • High temperature:  When air is compressed, the temperature rises due to all the friction of those molecules getting shoved together…that’s going to happen with any air compressor.  All the metal moving parts in constant contact with each other, in a reciprocating model, add even more heat.
  • Oil in the air: If you’re moving a piston back & forth in a cylinder, you have to keep it lubed properly, which means you have oil adjacent to the air chamber.  Which means, no matter how well it’s built, you’re likely going to have oil IN the air chamber.

All that said, the benefits certainly do sell a good number of these compressors, quite often into situations where it just wouldn’t make sense to use any other type.  If you’re in the market for an air compressor,  you’ll want to find a local reputable air compressor dealer, and discuss your needs with them.  If those needs entail the use of engineered compressed air products, though, please feel free to give me a call to discuss.  We can make sure you’re going to ask your compressor folks the right questions.

Russ Bowman
Application Engineer
EXAIR Corporation
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How to Estimate Leaks and the Impact upon a Compressed Air System

In today’s age where compressed air is often referred to as the 4th utility in an industrial manufacturing facility, leaks throughout the system can add up to serious financial losses. It has been estimated that leaks can waste as much as 20-30 percent of an air compressor output.

waste

Not only are leaks a source of wasted energy, they can also contribute to other losses such as:

  • Causing a drop in system pressure, resulting in air tools to function less efficiently
  • Increasing the air compressor on/off cycles which shortens the life of it and other components in the system
  • Increased maintenance costs and more planned downtime for the maintenance to be performed
  • A need to install of additional compressors to make up for the inefficiencies caused by leaks

For compressors that have start/stop controls – the below formula can be used to estimate the leakage rate in the system-

Leakage Equation 1

To use the above formula, the compressor is started when there is no demand on the system –  all air operated equipment and devices are turned off.  As the air escapes the system through the leaks, the system pressure will drop and the compressor will turn on and cycle to bring the pressure back up to the operating level. Measurement of the average time (T) of compressor run duration, and time (t) of the system pressure to drop to the set-point can be plugged into the formula and a Leakage Percentage established.

Another method to estimate the leakage rate is shown below-

Leakage Equation 2

The above method requires knowledge of the total system volume, which includes downstream air receivers, air mains, and all piping.  To perform the check, bring the system pressure up the normal operating pressure (P1) and then measure the time (T) it takes for the system to drop to pressure (P2) which is generally around half the operating pressure.  The 1.25 is a correction factor to normal system pressure, since the leakage rate will be less as the system pressure is lowered.

A leakage rate greater than 10% typically shows that there are areas of improvement (leaks that can be identified and repaired)

Any leakage testing and estimating should be preformed regularly, at least each quarter, so as to minimize the effect of any new system leaks. The tests are only one part of a leak detection and repair program. The best way to detect leaks is the use of ultrasonic leak detector (shown below.)  To learn more about the EXAIR model 9061 Ultrasonic Leak Detector, check out this blog that was previously published.

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If you have questions about compressed air systems, or would like to talk about any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or any of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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Intelligent Compressed Air: Sliding-Vane Compressors

If you’re an active reader of the EXAIR blog, you’ve seen several posts over the last year about the various different types of air compressors. From the positive-displacement style of compressors (Rotary Scroll, Rotary Screw, Single and Double Acting Reciprocating Compressors,) as well as a review of a dynamic style (Centrifugal Compressors). In this blog, I’ll be discussing another of the positive-displacement variety: The Sliding-Vane Compressor.

Sliding Vane2
Air enters from the right, and as the compression chamber volume reduces due to counterclockwise rotation, the pressure increases until the air discharges to the left

In positive-displacement type compressors, a given quantity of air or gas is trapped in a compression chamber. The volume of this air is then mechanically reduced, causing an increase in pressure. A sliding-vane compressor will consist of a circular stator that is housed in a cylindrical rotor. The rotor then has radially positioned slots where the vanes reside. While the rotor turns on its axis, the vanes will slide out and contact the bore of the stator wall. This creates compression in these “cells”. An inlet port is positioned to allow the air flow into each cell, allowing the cells to reach their maximum volume before reaching the discharge port. After passing by the inlet port, the size of the cell is reduced as rotation continues and each vane is then pushed back into its original slot in the rotor.  Compression will continue until the cell reaches the discharge port. The most common form of sliding-vane compressor is the lubricant injected variety. In these compressors, a lubricant is injected into the compression chamber to act as a lubricant between the vanes and the stator wall, remove the heat of compression, as well as to provide a seal. Lubricant injected sliding-vane compressors are generally sold in the range of 10-200 HP, with capacities ranging from 40-800 acfm.

Advantages of a lubricant injected sliding-vane compressor include:

  • Compact size
  • Relatively low purchase cost
  • Vibration-free operation does not require special foundations
  • Routine maintenance includes lubricant and filter changes

Some of the disadvantages that come with this type of compressor:

  • Less efficient than the rotary screw type
  • Lubricant carryover into the delivered air will require proper maintenance of an oil-removal filtration system
  • Will require periodic lubricant changes

With the host of different options in compressor types available on the market, EXAIR recommends talking to a reputable air compressor dealer in your area to help determine the most suitable setup based on your requirements. Once your system is up and running, be sure to contact an EXAIR Application Engineer to make sure you’re using that compressed air efficiently and intelligently!

Tyler Daniel

Application Engineer

E-mail: TylerDaniel@exair.com

Twitter: @EXAIR_TD

Diagram:  used from Compressed Air Challenge Handbook

Estimating the Total Cost of Compressed Air

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

 

Compressor Control – A Way to Match Supply to Demand

Rarely does the compressed air demand match the supply of the compressor system. To keep the generation costs down and the system efficiency as high as possible Compressor Controls are utilized to maximize the system performance, taking into account system dynamics and storage. I will touch on several methods briefly, and leave the reader to delve deeper into any type of interest.

air compressor

  • Start/Stop – Most basic control –  to turn the compressor motor on and off, in response to a pressure signal (for reciprocating and rotary type compressors)
  • Load/Unload – Keeps the motor turning continuously, but unloads the compressor when a pressure level is achieved.  When the pressure drops to a set level, the compressor reloads (for reciprocating, rotary screw, and centrifugal type)
  • Modulating – Restricts the air coming into the compressor, as a way to reduce the compressor output to a specified minimum, at which point the compressor is unloaded (for lubricant-injected rotary screw and centrifugal)
  • Dual/Auto Dual – Dual Control has the ability to select between Start/Stop and Load /Unload control modes.  Automatic Dual Control adds the feature of an over-run timer, so that the motor is stopped after a certain period of time without a demand.
  • Variable Displacement (Slide Valve, Spiral Valve or Turn Valve) – Allows for gradual reduction of the compressor displacement while keeping the inlet pressure constant (for rotary screw)
  • Variable Displacement (Step Control Valves or Poppet Valves) – Similar effect as above, but instead of a gradual reduction, the change is step like (for lubricant injected rotary types)
  • Variable Speed – Use of a variable frequency AC drive or by switched reluctance DC drive to vary the speed of the motor turning the compressor. The speed at which the motor turns effects the output of the system.

In summary – the primary functions of the Compressor Controls are to match supply to demand, save energy, and protect the compressor (from overheating, over-pressure situations, and excessive amperage draw.) Other functions include safety (protecting the plant and personnel), and provide diagnostic information, related to maintenance and operation warnings.

If you would like to talk about compressed air or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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A Review of Centrifugal Air Compressors

Over the last few months, my EXAIR colleagues and I have blogged about several different types of air compressor types including single and double acting reciprocating, rotary screw, sliding vane and rotary-scroll air compressors. You can click on the links above to check those out. Today, we will examine centrifugal air compressors.

The types of compressors that we have looked at to date have been of the Positive Displacement type.  For this type, an amount of air is drawn in and trapped in the compression area, and the volume in which it is held is mechanically reduced, resulting is rise in pressure as it approaches the discharge point.

types of compressors

The centrifugal air compressors fall under the Dynamic type. A dynamic compressor operates through the principle that a continuous flow of air has its velocity raised in an impeller rotating at a relatively high speed (can exceed 50,000 rpm.) The air has an increase in its kinetic energy (due to the rise in velocity) and then the kinetic energy is transformed to pressure energy in a diffuser and/or a volute chamber. The volute is a curved funnel that increases in area as it approaches the discharge port. The volute converts the kinetic energy into pressure by reducing speed while increasing pressure. About one half of the energy is developed in the impeller and the other half in the diffuser and volute.

Centrifugal Compressor
Centrifugal Compressor Components

The most common centrifugal air compressor has two to four stages to generate pressures of 100 to 150 PSIG.  A water cooled inter-cooler and separator between each stage removes condensation and cools the air prior to entering the next stage.

Some advantages of the Centrifugal Air Compressor-

  • Comes completely packaged fort plant air up to 1500 hp
  • As size increases, relative initial costs decrease
  • Provides lubricant-free air
  • No special foundation required

A few disadvantages-

  • Higher initial investment costs
  • Has specialized maintenance requirements
  • Requires unloading for operation at reduced operational capacities

EXAIR recommends consulting with a reputable air compressor dealer in your area, to fully review all of the parameters associated with the selection and installation of a compressed air system.

If you would like to talk about air compressors or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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When to Use a Receiver Tank for a Compressed Air Application

Recently, I worked with a production engineer at a Tier 1 supplier for the auto industry.  An upcoming project was in the works to install a new line to produce headlight lenses.  As a part of the process, there was to be a “De-static / Blow-off” station, where a shuttle system would bring a pair of the parts to a station where they would be blown off and any static removed prior to being transferred to a painting fixture and sent off for painting.  For best results, the lenses were to be dust and lint free and have no static charge, ensuring a perfect paint result.

The customer installed a pair of 18″ Gen4 Super Ion Air Knives, to provide coverage of the widest 16″ lens assembly, that were staged in pairs.

112212
The Super Ion Air Knife Kit, and Everything that is Included.

The customer was limited in compressed air supply volume in the area of the plant where this process was to occur. 50 SCFM of 80 PSIG was the expected air availability at peak use times, which posed a problem –  the Super Ion Air Knives would need up to 105 SCFM if operated at 80 PSIG.  A further review of the design parameters for the process revealed that the system needed to blow air for only 4 seconds and would be off for 25 seconds to meet the target throughput.

This scenario lends itself perfectly to the use of a Receiver Tank.  Running all of the design numbers into the calculations, showed that the 60 Gallon Receiver Tank we offer, would allow for a 20 second run-time, and require 13.1 seconds to refill.  These figures were well within the requires times, and would allow for the system to work as needed, without having to do anything to the compressed air supply system.

receiver_tank
60 Gallon Receiver Tank

The moral of the story is – if you have a process that is intermittent, and the times for and between blow-off, drying, or cooling allows, a Receiver Tank can be used to allow you to get the most of your available compressed air system.

Note – Lee Evans wrote an easy to follow blog that details the principle and calculations of Receiver Tanks, and it is worth your time to read here.

If you would like to talk about any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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