Rotary screw air compressor buying guide

In this rotary screw air compressor buying guide, it's all about the workhorse of many industries: the rotary screw compressor. Rotary screw compressor can supply compressed air 24 hours a day, 365 days a week, if necessary. In this buying guide, I will reach you everything you need to know when buying a rotary screw air compressor. If you are still not sure whether a rotary screw compressor is the right choice for you, be sure to start at the air compressor buying guide main page. Are you looking for a reciprocating compressor? Visit my reciprocating compressor buying guide instead.

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Buy a rotary screw air compressor

Do you want to be sure you buy the right air compressor, the air compressor that is right for you and your situation? Don’t worry, I am here to help you.

In this buying guide I will teach you:

• Who needs a rotary screw compressor, and who better stay away from screw air compressors.
• What the right size air compressor is for you.
• How to save money on your air compressor purchase.
• What options you need and don’t need
• The most common mistakes when buying a rotary screw compressor.

Who needs a rotary screw air compressors anyway?

Rotary screw air compressors are perfect for:

• Large and small production facilities (factories)
• Bigger garages and workshops
• People that need lot's of compressed air

But... I have seen a lot of business that bought a rotary screw compressor, where a reciprocating compressor would have been better (they listened to the salesman who sold them the biggest machine they could). If any of the following applies to you, a rotary screw compressor is not for you:

• You only need compressed air for hand tools
• You don't need air continuously
• You need a compressor for home/diy type of work
• You need a compressor for light construction work (to power nailers, etc).

In these cases, a reciprocating air compressor would be a better choice. ## How to choose the right capacity and pressure for your new rotary screw air compressor.

It is very important to buy a compressor that can supply you with enough amounts of compressed air, at the right pressure. Unforatunately, most people don't know exactly what they need, and end up buying a too big or a too small air compressor. Let me help you.

If you buy a rotary screw compressor, it is usually for one of the following reasons:

1. This is your first air compressor.
2. It's a replacement for an old rotary screw compressor
3. You now have a reciprocating compressor, but need more capacity (cfm).
4. You already have a rotary screw compressor, but need more capacity.

For each of the above cases, here is how to easily determine the right pressure and capacity for you:

1. This is your first air compressor

Check the manuals or spec sheets of your pneumatic equipment. It should tell you the pressure at which the equipment operates and the amount of air it needs. The part that needs the highest pressure dictates the required pressure. Add 1.5 bar to to minumum required pressure to account for pressure drop and load-unload differential of your air compressor. Now, add up the air consumption of all your equipment. This is the capacity of your air compressor.

It's a good idea to buy a bigger air compressor, but not too big. Rotary screw compressors don't like standing still. Keep in mind that some equipment state the 'average consumption', while other equipment state the 'maximum (or peak) consumption').

2. It's a replacement for an old rotary screw compressor

Check the current pressure and capacity of your old compressor. If your old compressor was adequate, buy a new compressor of the same pressure and capacity. Take a moment to decide whether is a good idea to buy a (slightly) bigger compressor, to account for future expansion of your business.

3. You now have a reciprocating compressor, but need more capacity (cfm).

Check the pressure at which your current compressor is set. Don't look at the maximum pressure of your current reciprocating compressor, as this is often much higher than the pressure needed to operate your machinery. If you want to replace the compressor, check the capacity of your current compressor. Now you know you need a bigger compressor than this. How much bigger? It depends on the tools/equipment you added that made your old compressor too small.

Also keep into account future expansion of your business when determining the right capacity. If you want to add the screw compressor as an additional compressor, keep in mind that rotary screw compressors don't like standing still for a long time. It makes them old and rusty. Always make the rotary screw compressor your primary compressor and the reciprocating compressor your secondary/backup compressor. Rotary screw compressors are more energy efficient anyway.

4. You already have a rotary screw compressor, but need more capacity.

Most of the time, I see people buy the same type or capacity of compressor as the current one. Which is a good idea. It allows you to easily use the two compressors alongside each other, and use one as the backup for the other. Another idea is to buy a bigger compressor and use the old one only as an emergency backup.

You might think about getting a variable speed drive (VSD) compressor to take care of the compressed air need when your first compressor can't keep up. ## Don't be fooled: buy only the options and extras you really need!

Rotary screw compressors come in a large variety of flavors, with many optional extras and options. But what do you really need? Do you need an integrated air dryer? Do you need extra filters? Do you need electronic condensate traps?

Integrated air dryers and filters.

Most rotary screw air compressor manufacturers offer an all-in-one solution. This means you get an air compressor with integrated compressed air dryer and sometimes also compressed air filters. For example, at Ingersoll Rand they call it the "Total Air System" ("TAS" added to the model name), at Alas Copco they call it "Full Feature" ("FF" added to the model name).

For most systems, a compressed air dryer is highly recommended. Same goes for compressed air filters. Compressed air is wet and dirty by nature. Oily too, with oil lubricated compressors. Clean compressed air saves your equipment from premature failure. Keep in mind that all compressed air dryers and filters create and addition pressure drop in the system. This means addition energy costs (to overcome the pressure drop). This extra energy cost can run into the hundreds or thousands of dollars over the lifetime of the compressor.

You will get efficient and thoughtful service from kapa.

Additional reading:
What is the Advantage and Disadvantage of Strapping Machine Head
Laser welding of battery components
How your oil home heating system works. - Petro

Something to keep in mind.

Electronic condensate traps (instead of fully mechanical ones)

Do you need it? Not really. Is it better? Yes. A dirty, clogged and non-functioning (mechanical) condensate trap (the one with the floater), is maybe the number 1 maintenance issue with air compressors.

A non-functioning condensate trap will result in lots of water in your compressed air system. However, if you manually drain or check the drain on your air receiver every day / week, you will quickly discover when your air compressors condensate drain stops working. But, if you're like most people and don't want to check on your compressor every day, an electronic condensate drain is a good idea.

An electronic condensate drain can always be installed afterwards if you don't want to fork out the cash right now.

Variable speed drive (VSD)

A variable speed drive compressor matches the output capacity to the actual demand. In simple terms, it simple let's the compressor run faster or slower, depending on the current pressure. A VSD compressor is often advertised as the one and only way to save energy. This is true, but not always.

While it is true that a VSD compressor can save you huge amounts of energy, it is not always the case. A VSD compressor is NOT a good choice when:

• It runs at 90-100% speed all the time. In fact, a VSD that runs at full speed consumes more energy than a standard fixed-speed compressor
• The compressors only runs at 10-20% all the time. In fact, the compressor is way too big, but I have seen this so many times! It would be more energy efficient to get a smaller fiexed speed compressor. Furthermore, compressors that run only at low speeds will experience a lot (a lot!) of maintenance issues.

A VSD compressor IS a good choice when it runs at 50 - 80% load. This is where the money-saving is. PLus it allows the compressor to heat up, which will prevent lot's of maintenance issues.

Belt drive or direct drive

Personally I would choose direct drive. Belt drive units are usually more compact, but are more noisy and in my experience require more maintenance (on the pulleys and belts), especially in dusty environments.

Aftercoolers

On most stationary screw compressors, an aftercooler in standard. I would always choose to have an aftercooler anyway. Without the aftercooler, the compressed air leaving your compressor would be around 80 - 100 degrees, which is too hot to fed into most air dryers, air piping or air tools.

Besides that, the aftercooler removes huge amounts of water from the compressed air. If you don't have an aftercooler, all this water stays in the compressed air (as gas) and condensates when it cools (in piping system, tools, equipment.) ## Energy: the single biggest expense

Most people buying an air compressor don't realize that the purchase of the compressor is just the beginning. Over the lifetime of the compressor, the purchase cost is only 10%. Another 15% is maintenance and spare-parts and w whooping 75% is energy cost.

If you have a choice between two otherwise comparable air compressors. Always choose the most energy efficient one. In can save you thousands of dollars per year. Unfortunately, most of the time this goes unnoticed. Even if the more energy efficient compressor is more expensive, I would still prefer it over a less energy-efficient one. I

n 10 years time, the payback will be huge, you can buy a brand new compressor just from the savings!

Must know: the most popular rotary screw air compressor series

Everywhere you look online, you will see the same makes and models of rotary screw air compressors pop up. The main competitors are Quincy, Atlas Copco, Ingersoll Rand, Chicago Pneumatic. Sometimes you will see the odd cheap rotary screw manufacturer listed, but I would stay away from those and choose one of the established brands.

The difference between the series are usually the size range (HP / Capacity), or the maximum pressure. Some manufacturers have series for 'heavy duty' use, or 'high efficiency' use. The real difference between the series is sometimes hard to see. I'm sure some is only for marketing purposes (wouldn't you like to buy a 'carpenter compressor' when you are a carpenter?)

Inside a series, the difference between the individual models is mostly only the pressure and the capacity. Here's an overview of air compressor manufacturers and their rotary screw series available. I also created an extensive overview of screw air compressors with much more information.

Quincy

• QGS Series
• QGD Series
• QSB/T Series
• QSF Series
• QGV Series
• QSI Series

Chicago Pneumatic

• CPN Serries
• CPM Series
• CPA Series
• CPB Series
• CPC Series
• CPD Series
• CPE Series
• CPF Series
• CPG Series
• CPVS Series

Ingersoll Rand

• IRN5K Series
• UP5 Series
• Up6 Series
• R Series

Atlas Copco

• GX Series
• GA Series
• GR Series
• Z Series (oil-free)
• AQ Series (water injected)

Sometimes it seems like manufacturers or dealers like to confuse you with all their different models, options and extras.

For a list of manufacturers and available series and ranges, check the rotary screw air compressor brands and series overview.

Clive603 said:

Just recalled where the manual for my retired Atlas KE Vee twin compressor was hiding. These units came in 3 sizes and 15 versions. A quick look shows the single stage KE series are rated at 100 psi and the otherwise identical two stage KT versions are rated at 200 psi. Interestingly the output is given as piston displacement at specified run speed not directly as air delivery. I guess this avoids many potential specification mis-read problems as its then down to the user to figure out exactly how much air he (or she) gets at what temperature. My 3 hp KE2 was rated at 16.2 cfm piston displacement running at 1000 rpm. The lower pressure KE23 version running at 1200 rpm delivers 45 psi from 19.5 cfm piston displacement.

The KT2 version has two piston displacement ratings 7.32 and 9.75 cfm covering 2 HP 900 rpm and 3 hp 1200 rpm drive. The manual also lists a higher pressure 18 suffix version rated at 255 psi and 8.13 cfm piston displacement running at 1000 rpm.

Its interesting to see that specified running speed varies from 720 to 1200 rpm for various versions despite similar design. My mate Andy also got a KE2 from the same scrappy and ran his at 130 psi but the last 10 psi were very slow. I haver seen single stage units installed to deliver 150 psi. Hot air!

Clive

Click to expand...

Heating the air in compression can actually help. If you start with room temperature air and compress it, the heating makes it expand so it has a larger volume and lowered viscosity, making it push out the exhaust valve faster. If the tank is large enough that the air in it is reasonably cool, that air temperature defines the back pressure, not the air going through the exhaust port. If the heat conduction between the intake and exhaust ports is high enough to significantly heat the incoming air, then the compression stroke starts with a smaller charge and resultant loss in output. Cool in, hot out improves the throughput.

The manufacturer only listing displacement (swept volume) is just dodging the issue. A good spec sheet gives the actual output at various pressures. These compressors never deliver the full swept volume. Running at slow speeds and against low back pressure, they may come close, but all drop off when they have to work hard. Configured as a two stage where the low pressure cylinder was only working against about 15 PSI in the manifold between the cylinders, at 62 CFM swept volume and 100 PSI it actually delivered 56 CFM. That is about the best you could expect. I sized the orifice in the bead gun to match and sent the air through the cooler and to it, only feeding the 200 gal. tank when not blasting. With the large storage, other people in the shop never noticed.

Another factor usually neglected is the force needed to open an atmospheric intake valve. Running my Gardner Denver ACR as a vacuum pump, it would rapidly go up to 25 in hg gauge and stop. The approximately 4-5 in difference between that and atmospheric pressure was the differential needed to actuate the valve. That difference is also subtracted from the fill pressure when it is used as a compressor, although that is mitigated a bit because once the valve snaps open, it will tend to stay.

The viscosity of the air can have a surprising effect. I once repaired some huge heaters for an aluminum annealing oven big enough to handle truck loads. The operator commented that even though he had the blower motors rewound for the maximum performance possible, he had to start with the blowers off and heat the air until it got thin enough to avoid overloading them.

All these factors get in the equation, which isn't simple.

Bill

Heating the air in compression can actually help. If you start with room temperature air and compress it, the heating makes it expand so it has a larger volume and lowered viscosity, making it push out the exhaust valve faster. If the tank is large enough that the air in it is reasonably cool, that air temperature defines the back pressure, not the air going through the exhaust port. If the heat conduction between the intake and exhaust ports is high enough to significantly heat the incoming air, then the compression stroke starts with a smaller charge and resultant loss in output. Cool in, hot out improves the throughput.The manufacturer only listing displacement (swept volume) is just dodging the issue. A good spec sheet gives the actual output at various pressures. These compressors never deliver the full swept volume. Running at slow speeds and against low back pressure, they may come close, but all drop off when they have to work hard. Configured as a two stage where the low pressure cylinder was only working against about 15 PSI in the manifold between the cylinders, at 62 CFM swept volume and 100 PSI it actually delivered 56 CFM. That is about the best you could expect. I sized the orifice in the bead gun to match and sent the air through the cooler and to it, only feeding the 200 gal. tank when not blasting. With the large storage, other people in the shop never noticed.Another factor usually neglected is the force needed to open an atmospheric intake valve. Running my Gardner Denver ACR as a vacuum pump, it would rapidly go up to 25 in hg gauge and stop. The approximately 4-5 in difference between that and atmospheric pressure was the differential needed to actuate the valve. That difference is also subtracted from the fill pressure when it is used as a compressor, although that is mitigated a bit because once the valve snaps open, it will tend to stay.The viscosity of the air can have a surprising effect. I once repaired some huge heaters for an aluminum annealing oven big enough to handle truck loads. The operator commented that even though he had the blower motors rewound for the maximum performance possible, he had to start with the blowers off and heat the air until it got thin enough to avoid overloading them.All these factors get in the equation, which isn't simple.Bill

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