Saturday, 4 March 2017

Decoding the Duty Cycle Rating of Piston Compressors

Selecting the correct air compressor for your application can be a complicated process. Before making a purchase, operators need to specify the quality and amount of air their application requires. Each compressor technology (rotary, piston, centrifugal, etc.) has an optimal flow output, so users should choose compressor technology based on the compressed airflow requirement of their application.
Some compressors, like rotary screw and centrifugal, are designed to run continuously at full speed while maintaining peak airflow (defined in cubic feet per minute, or CFM). The motors and cooling systems of these compressors are engineered to run 100 percent of the time without overheating. However, this isn’t true for all compressors.
Piston compressors do not have the cooling capabilities to run continuously for extended periods of time. Therefore, each piston compressor must have a specified flow output and pressure, as well as the percentage of time it can run without damaging the compressor. This percentage time running and stopped is referred to as the duty cycle of a compressor.

What is duty cycle?
Duty cycle, a term typically used as a rating for electric motors, is the time a compressor runs compared to the total cycle time (total time running and stopped). For example, a compressor with a total cycle time (Tc) of 10 minutes and a running time (Tr) of six minutes has a duty cycle of 60 percent.
Duty cycle is not an industry definition
While the compressed air industry has many standards that manufacturers adhere to, there is not an official definition for the duty cycle of a compressor. This lack of definition can create confusion, and operators may end up purchasing expensive compressors that do not meet their expectations or needs.
Some piston compressors claim to have a 100 percent duty cycle, but that is impossible. So what does is mean when you see a piston compressor with that rating?
Decoding the 100 percent duty cycle rating 
Typically, piston compressors can generate over 150 psi of compressed air and have a large storage tank per horsepower compared to a rotary screw compressor of the same horsepower. (Remember, rotary screw compressors can run continuously, so they don’t need a large air receiver.) The piston compressor will fill the tank with high-pressure air that the user will deplete over time. Eventually, the compressor will turn back on to refill the tank.
A piston compressor that specifies a 100 percent duty cycle does not mean it can run continuously, but that it can provide air at a specific pressure and flow 100 percent of the time with the help of a storage tank. Let’s take a look at an example.
A compressor advertises a 100 percent duty cycle rating of 25 cfm at 100 psi. In reality, this compressor can create 42 cfm and has a tank size of 130 gallons.
Piston compressors typically use a pressure switch to control system pressure. This pressure switch controls when the compressor starts and stops depending on the system pressure. The pressure switch will have two pressure settings called a pressure band that uses a lower pressure setting as the cut-in or turn on pressure and a higher pressure setting as the cut-out or shut off pressure. A typical pressure band for a piston compressor is 30psi. For our example, this will allow the unit to turn on at a system pressure of 115 psi and turn off at a 145 psi.
As advertised, 25 cfm can be continuously provided from the storage tank for the application. When the system pressure dips to 115 psi, the compressor turns on and pumps the system up to 145 psi for 125 seconds before turning off.
After 83 seconds of continuous use at 100 psi and a flow of 25 cfm, the system pressure will have dropped back down to 115 psi, and the cycle will start over.
Tr (total running time) = 83 seconds
Tc (total cycle time) = 208 (83+125) seconds
Tr/Tc = Duty cycle              83/208 = .60 = 60%
From the above calculations, we can see that the piston compressor is actually running for 60 percent of the time. 60 percent is the typical running duty cycle for a piston compressor, ensuring a long and reliable life for the machine.
Don’t let duty cycle ratings disrupt your system
It is critical to know that the 100 percent duty cycle on a piston compressor does not mean the machine can run continuously. Doing so can damage the compressor, resulting in premature wear and higher maintenance costs.
If you have an application that requires your piston compressor to run for more than six minutes at a time (60 percent of the ideal cycle time of 10 minutes), consult an expert. You might need a larger compressor or different compression technology to meet the demands of your system.

Splash and Pressure Lubrication in Piston Compressors

Piston compressors have been around for centuries. They can be either oil-injected or oil-free, depending on the application and end use. In oil-injected models, the oil typically serves three crucial purposes: cooling, sealing and lubricating. But not all oil-injected piston compressors lubricate components the same way. There are two common methods of lubricating the pump in piston compressors: splash and pressure lubrication.

Splash lubrication
In splash lubrication, oil is applied to the cylinders and pistons by rotating dippers on the connecting-rod bearing caps. Each time they rotate, the dippers pass through an oil-filled trough. After running through the oil trough, the dippers splash oil onto the cylinders and pistons to lubricate them.
While splash lubrication is effective for smaller engines and pumps, it’s not a precise process. Parts of the pump may be insufficiently oiled or oiled too much. The amount of oil in the trough is vital for proper operation. If there is not enough oil, wear between critical components may occur, and too much oil will cause excessive lubrication, which can lead to hydraulic lock.
The type of oil used and its viscosity is also important in a splash lube system. The oil must be thick enough to provide sufficient lubrication and cling to the dippers, but not so viscous that it heats up as it is churned about in the oil trough. Oil purity is also critical; oil should be filtered regularly and replenished when necessary.
Pressure lubrication
Pressure lubrication is the second type of method used to lubricate piston compressors. It is a more technically advanced and usually more costly method, but it results in longer life for a compressor.
Pressure lubrication is a process where an oil pump precisely distributes oil to key areas of the pump. Typically, the oil is pumped through an oil filter and into the pump where it is then recycled and reused; using a replaceable oil filter can further improve the life of the oil. The oil is transported to the key area by use of an oil pump. Therefore, the viscosity of the oil is not as critical as with a splash tube system.
Either method has been used extensively in many various pump and engine applications, and both are suitable for piston compressor applications. When purchasing a new piston compressor, decide what’s important for you. If upfront cost is important, a splash lubricated compressor may be the way to go. But if you are willing to invest more in a pressure lubricated piston compressor, you’ll be rewarded with added longevity and reliability.
What kind of lubrication does your piston compressor use? Let us know in the comments below. You may also enjoy the following articles:

Wednesday, 4 January 2017

Using Compressed Air? Think again!

Using Compressed Air? Think again!

We know this might sound strange from a guy who's all about compressed air...
... but did you ever wonder WHY and IF you really need to use compressed air?
It might have been a very bad decision to make use of compressed air for your tools, machines, actuators, etc.
Sure, some applications DEMAND compressed air. It's simply the only real option for it.
And we know, compressed air and has many upsides:
  • compressed air tools are very powerful
  • they are safe to use
  • can operate in very dirty environments
  • are cheaper compared to their electrical counterparts
  • can last a lifetime.
I love all the benefits that using compressed air for your application brings.. but there is one very big downside..
Compressed air is EXPENSIVE.
Very expensive.In fact, it's 7x - 8x more expensive compared to electricity.
We talked about the cost of compressed air. And here's an example that shows exactly how and why using compressed air is 7x more expensive than electricity.


As you can see, we put in a lot of (electrical) energy to compress the ambient air.
When compressing air, we waste a lot of energy to... heat.
We can't 'destroy' energy. We can only transform energy.
Energy in = Energy out.
Energy in = energy out. Always.
So nothing is lost. But we can have 'useful' energy and 'useless' energy.
In the case of the air compressor, the generated heat is mostly useless and we just try our best to get rid of it (which might even be a problem by itself, especially in hot climates).
If we're lucky, we can use heat-recovery to 'capture' some of the heat and do something useful with it, like heating water for the shower or heating up a cold factory space.
Of all the energy we put into the compressor, most of it is wasted before we can use it at the point of use (pneumatic tools, machines, etc). In fact, for every 100HP input, only 7 - 14 HP will be left over when we use the compressed air in our application.
2 HP compressed air motor VS 2 HP electric motor
We have comparable motors. A compressed air one, and an electric one. They have the same shaft output power, and about the same rotational speed.
2HP compressed air motor2HP electric motor

In reality, the compressed air motor is half as big and only 1/3 of the weight of the electric motor!
Energy needed
How much energy do we need to power these motors? Let's check the specifications and do some calculations!
How much electrical energy do we need to power the 2 HP electric motor? Quite simple, the output power divided by the efficiency.
How much electrical energy do we need to power our compressed air motor? From the motor specs can get the compressed air requirements.
Compressed air motor:Electrical motor:
80 psi (5.5 bar)
68 cfm (32 l/s or 115 m3/hr)
2 HP shaft power
86.5% efficiency (taken from the specs)
Electrical input power: 2 / 0.865) = 2.3 HP (= 1.7 kW) 

Alright, that doesn't really tell us anything (yet!).
Now let's find an air compressor that can give us the required compressed air.
For example, we could use the following air compressor: Chicago Pneumatic QRS 20.
Specs for this comrpessor:
100 psi (6.9 bar)
85 CFM (40l/s or 144 m3/hr)
(I just picked one, it could be any other brand or model with the same specs).
From the datasheet we learn that this compressor consumes 21.3 HP (or 15.9 kW) at full power.
But this is a 100psi unit and we don't need 100psi, we just need 80.
And we don't need 85 CFM, we just need 68 for our motor.
Adjusted for these values (I will save the calculations for another post), we still need 13.5 HP (which is 10.1 kW) electrical input to generate the required compressed air of 68 CFM at 80 psi
Or, we could have used the following Rule of thumb:
Air compressors deliver 4 to 5 CFM (at 100 psi) per HP input power
In metric units:
Air compressor deliver 2.5 to 3.2 l/s (at 6.9 bar) per kW input power
Using this rule of thumb, we come to about the same values, so this rule of thumb should be about right.
Yearly costs compared
Let's assume we use the motor for a full 1-shift work week for the whole year, which is 2080 hours.
And lets assume electricity costs us $0.05 per kWh.
Compressed air motor:Electrical motor:
10.1 kW x 2080 hrs x $0.05 = $10501.7 kW x 2080 hrs x $0.05 kWh = $104 

The compressed air motor costs of a whooping $1050 per year to operate, compare to only $104 for the electric motor, which is about 10 times more expensive!
Consider the energy cost of compressed air
Next time you buy a power tool or design a machine.. compare the operational costs of a compressed air part (motor, piston, actuator, gripper, etc) tot that of it's electrical equivalent.
I know that using compressed air is in many cases so much easier (to design, to install, to use, to maintain).
But considering the potential energy savings (year after year), sometimes it's worth thinking about an alternative!
For example, the above compressed air motor costs around $1.000, the electric motor around $420.
And that compressor sets you back about $10.000
10 years of use
After 10 years, the electric options would have cost you 420 (motor purchase) + (10 * 104 electricity) = $1460
The compressed air options would have cost you $10.000 (compressor) + $1000 (motor) + (10 * 1050 electricity) = $21.500
Compressed air motor:Electrical motor:
Total cost after one year:

Total cost after one year:


Even if we would need to spend a thousand or 2 on electrical cables, cabinet and motor starter for the electric motor option, it would still be way cheaper!
.. even if the electric motor burns out every other year!
I know the comparison is not complete or 100% precise, but it's food for thought!
Many times, using compressed air is just simply to only real option, because of adverse working conditions, needed reliability, availability of spare-parts, etc.

Friday, 30 December 2016

A VSD Compressor Doesn’t Save You Money

VSD (Variable Speed / Frequency Drive) Compressors

The energy savings were huge… of course. The pay-back time would be a few months, or days, or less.. for sure!

Stop here!

Yes, a VSD compressor can save huge amounts of energy (and thus money), but only if a propoer homework is one, make some calculations and use your common sense.

[For those in the dark: a VSD or variable-speed compressor uses a frequency drive to match the compressor speed (and thus capacity) to the amount of compressed air needed. This as opposed to 'load-unload' machines that runs at a fixed speed but 'loads' and 'unloads' between a minimum and maximum pressure).

And IF a VSD compressor is a good idea in your situation, buy the right size of VSD compressor.

Here's when a VSD compressor will save you money: when it runs at less than 80% speed for most of the time and when it runs continuously (daily).

In these cases, the cost-savings can be huge. Tens of thousands of dollars per year in energy costs can be saved.

But: A VSD compressor that runs at 100%, fully loaded, uses 5 to 10% more energy than a fixed-speed compressor! This is because of the 'overhead' of the electronic frequency drive.

The optimum frequency range for a VSD is 50 – 80% speed for maximum savings.

Don't just believe the salesman with his fancy brochures and his pitch about all the wonderful things a VSD compressor does to your budget.

I hate to say this, but does he know what he's talking about? As an engineer I have had many talks with salespeople and their pitches and brochures and claims… and 90% is either hyped up, not applicate to your situation, or simply not true.

The sad truth is that in my years as a compressor mechanic, I have seen many, many VSD compressors that were just not needed, were only costing money, or were simply slowly dying.

Here are a few examples:

1) A VSD compressor that was way too big. It ran for only 5 to 10 minutes every hour, at maximum 30% speed.

This machine was slowly dying from water and rust because it never really heated up. A smaller fixed-speed machine, matched to the actual needed output would be way more logical. (this was a 90 kW machine!)

2) A VSD compressor that ran between 30 and 50% all the time, every day.

Sure, it was saving money over a load-unload machine of the same size. But a VSD compressor that was half the size and ran at 80% would be more cost-effective (both in investment and running costs).

3) A VSD compressor that was needed for one specific machine and was only used intermittently

This compressor was like one unit with the machine it supplied air to, and was way too big. The air demand was basically stable, and was only needed for 10 or 20 minutes at a time, a few times per day.

A basic compressor of correct size with a large air receiver would be far more cost-effective in this case.

4) A VSD compressor that ran at 100% each and every day, all day.

That customer invested extra money for a variable speed compressor, and ended up paying 5 to 10% MORE in energy costs each and every year! Yeah, great!

Again, a VSD compressor CAN be a great choice, and can save you a lot of money. And this is true in many cases.

But, beware of the salesman with a hyped-up story about energy savings, who doesn't know when NOT to try to sell the a VSD compressor.

Or you end up paying MORE in energy costs, MORE in maintenance costs and MORE in up-front capital investment.

And the compressor mechanics can try and clean up the mess afterwards ;)

Wednesday, 28 December 2016

3 Ways to Save Money On Compressed Air

3 Ways to Save Money On Compressed Air

What if you knew that your compressed air system was completely optimized… running happily day after day.

No more waste of money and electricity…

Because let's face it: wasting thousands of dollars per year on electricity, without you knowing it, sucks.
Want to optimize your compressed air system?

It's impossible to share the complete step-by-step system in an email, but I can get you started on the right path.

Compressed air optimization #1: Air leaks

As discusses in our previous blog: fix those leaks!

Compressed air leaks are the number one energy wasters!

The biggest, most rewarding, leaks will be easy to find. For the smaller leaks, you might have to do some extra work, but it will pay off!

But more importantly, once you fixed most of the leaks, put a system in place to continually check for and repair new compressed air leaks.

You can also use some stealth tactics like dividing your compressed air network in zones and shutting of those zones that are not in use (during the night, during the weekend, during low season).

Or create an award system where employees get a price for finding and fixing leaks. It will pay off!

I have to stop here or we could be here all day talking about compressed air leaks!

Compressed air optimization #2: Drains

Condensate drains… they remove condensate water from out compressed air, which is a good thing.

But, there are two problems with condensate drains:

  1. They often leak, especially the older mechanical types. These leaks should be treated as any other leak: they should be fixed as soon as possible!
  2. They use a lot of compressed air when discharging condensate (water).
Nowadays, there are many types of condensate drains available. Simple, cheap ones and more expensive ones.

Always go for a 'zero loss' drain.

A 'zero loss' drain is a drain that only discharges water, not air.

You see, many simple drains work by a timer. The drain just opens for 10 seconds every 5 minutes. Every 5 minutes, even if there is no water to discharge.

Smarter drains only discharge when they sense that there is water to discharge. But the discharge time is a fixed setting, for example 10 seconds.

These drains still waste compressed air, because all the water might be already discharged after 2 seconds… wasting 8 seconds of compressed air!

Zero loss drains only discharge the water. When all the water is discharged, they close immediately.

Realize that compressed air is very expensive. Any waste of compressed air should be avoided. Those simple drains could cost you hundreds to thousands dollars per year in wasted compressed air!

Compressed air optimization #3: eliminate pressure loss

Pressure loss or pressure drops. What is it?

It's like a ghost in your compressed air system sometimes.

One moment it's there, the next it's gone.

Pressure drops are created when the compressed air flows through a restriction. It's the difference in pressure before and after the restriction.

The restriction could be anything: a filter, a bend in the pipe, a valve, a pipe with a smaller diameter… in fact, each and every part in your compressed air system introduces a pressure drop.

But notice that I said '… when compressed air flows through..".

Pressure drops are only created when the compressed air flows through your system.

If you shut down the air consumers and you don't use compressed air, there is no pressure drop.

The higher the flow, the larger the pressure drop.

That's why it's best to measure pressure drop when all your machinery and tools are running.

The total pressure drop in your system is the difference between the pressure at the consumer (machine, actuator, etc) and the pressure in your compressor room.

For example: the pressure at your machine is 6.5 bar. At the same time, the pressure gauge in your compressor room reads 7.3 bar.

Is one of the gauges broken? No! You have a pressure drop of 7.3 – 6.5 = 0.8 bar in your system.

Why is pressure drop bad?

Always remember that compressing air takes energy (electricity). Compressing air to a higher pressure costs you more energy.

If your equipment needs a minimum of 6.5 bar in the above example. You need to set your compressor at 7.3 bar, to account for the pressure drop.

Say you could reduce the pressure drop from 0.8 to 0.1 bar. Now you can lower your compressors pressure to 6.5 + 0.1 = 6.6 bar. A reduction of 0.7 bar

That 0.7 can save you thousands to tens of thousands per year (depending on the size and number of air compressors you have).

Friday, 23 December 2016

Rules of Thumb

Power and installation:

  • For every horsepower, a compressor delivers 4-5 cfm (m3/hr), at 100 psi (7 bar) pressure .
  • A 50 HP air compressor can produce up to 12 gallons / 45 liters of water per day (with only 8 running hours daily).
  • Air receivers should be sized about 4 gallon capacity for each CFM of compressor capacity. That's 9 liters per m3/h in metric units.

For example, a small 20 cfm compressor needs an 80 gallon air receiver.

Costs and energy savings:

  • On average, compressed air costs about 30 cents per 1000 cfm (1700 m3/hr). This depends on the size of the compressor and includes electricity, purchase price of compressor and maintenance costs.
  • A 50 HP air compressor will cost you about 25.000 dollars in electricity cost per year (at 6.000 running hours per year).
  • Every 2 psi (0.15 bar) of pressure drop in your system will cost you 1% extra in energy cost (this adds up quickly!!!)
Pressure - Rule of Thumb

  • Lowering Compressor Inlet Air Temperature 10° F (5.5 °C) will result in a 2% energy savings.
  • A two stage reciprocating air compressor is about 15% more efficient compared to a single stage unit.


  • The typical discharge temperature of a rotary screw compressor (before aftercooler) is: 175°F or 80°C
  • The typical discharge temperature of a single stage reciprocating compressor is: 350°F or 180°C
  • The typical discharge temperature of a two stage reciprocating compressor is: 250°F or 120°C
  • In rotary screw compressors, every 18 °F (or 10 °C) above 200 °F (or 95 °C) reduces the compressor oil life by 50%!

The one rule of thumbs that amazes people the most is the amount of water that an air compressor produces!

Tuesday, 20 December 2016

Multi Compressors Controller - SmartAir Master Compressed air management system

SmartAir Master
Compressed air management system

Superior control solution with great energy saving potential

Compressed air systems typically comprise of multiple compressors delivering air to a common distribution system. The combined capacity of those machines is generally greater than the maximum site demand.

With CompAir's advanced demand responsive sequencer SmartAir Master, the efficiency of compressor stations with up to twelve compressors including downstream equipment can be maximised. Apart from the energy savings, the compressed air management system also contributes to decreased downtime, optimum performance, service and monitoring and ultimately leads to increased plant productivity.

A profitable investment

Up to 35% energy savings can be achieved by implementing a central SmartAir Master multi-compressor controller.
  • Harmonises the workload of up to 12 fixed or regulated speed compressors
  • Eliminates energy waste by tightening the network pressure to the narrowest pressure band
  • Equalises the running hours for economic servicing and increased uptime

Versatile control functions

The SmartAir Master sequencer calculates the system demand and selects the most suitable compressor combination to meet exactly the plant requirements, which in turn offers significant energy savings.
Unlike conventional control systems, further equipment such as dryers, filters and condensate drains can be included, ensuring the complete compressed air system works at optimum performance.