Compressor Air Tank

What difference does it make what size compressor air tank you have on your air compressor?

Is a large compressor air tank better than a small one?

What can you do if your compressor air tank is too small?

All good questions. We hope we have some good answers, as follows.

What the compressor air tank does

While there are air compressors that run on a demand system, that is, the compressor motor runs all the time but the pump only compresses air when the air system calls for it and have no tank to speak of, the air compressors that most of us are familiar with in our basement or garage workshops are those that have a compressed air tank.

Air compressor tanks

The compressor air tank on the DIY or small workshop type air compressor can be pancake shaped, it can be round, hot dog shaped, and some air compressors even come with two small, parallel air tanks instead of just one compressor air tank.

Compressor motors don’t like to cycle on and off

Without the air tank on your air compressor the compressor motor and the pump would have to start every time you went to use compressed air, and when you let go of the air wrench trigger, you would no longer be using air, and the compressor motor and pump would stop. Start using air again, and the motor and pump would have to start.

The thing is, the electric motor on your air compressor – and almost all similar electric motors for that matter – are not designed to fast cycle. Too much starting and stopping leads to excessive motor wear, capacitor wear and will result in more frequent compressor motor and motor component breakdowns and repair costs.

The compressor air tank provides a reservoir of air that increases in pressure as the compressor pump runs, until the pressure switch cut out pressure is reached. At that time the compressor stops on high pressure cut out. Even with the compressor stopped, which allows it time to cool down, you can still do work with your compressed air, using the reservoir of air in the compressor air tank.

When your use of compressed air drops the tank pressure to the pressure switch cut in level, the air compressor will start, and then the compressor will run until the pressure in the compressor tank again reaches the cut out pressure setting. The cut in and cut out pressure settings for different air compressors vary. Your air compressor cut in might be in the 50 PSI – 90 PSI range and the cut out may be in the 90 – 150 PSI range, or even higher on some more industrial type air compressors.

Being  a reservoir for pre-compressed air and reducing the frequency of compressor motor cycling on and off is one use for the compressor air tank.

Compressor air tank helps steady compressed air flow

If you were air brushing for example  (a pressure and flow sensitive use of compressed air – for sure) and your air compressor kept cycling on an off, there would be increases and decreases in the pressure and the flow to your paint gun. It would be difficult to a first class job painting the artwork or any paint job if the flow of compressed air, and the pressure of the compressed air, to your gun continuously varied as the motor cycled on an off.

Naving a compressor air tank full of air, and setting the air regulator at a pressure level below the normal cut in pressure of the compressor, means that the air flow and air pressure to your paint gun would not vary while you were air brushing. The ON/OFF of the compressor and the resulting flow and pressure change in the tank would occur at a pressure setting above the regulated pressure of the compressed air to the paint gun, allowing the paint gun to have a steady, uninterruped flow for best performance while painting.

Are bigger compressor air tanks better?

By and large, we subscribe to the theory that a bigger air tank is better than a smaller one.

The larger the compressor air tank, the more pre-compressed air that you have available  to use before the air compressor has to start again to to rebuild the tank pressure. The longer the period is between compressor motor starts the better.

Electric motors that run longer and start less frequently are more efficient and have less wear, a benefit of having a larger compressor tank. There is a diminishing return for this, however. Even though some compressors profess to have a 100% duty cycle – which is supposed to mean that you can run your air compressor for extended periods with no damage, it seems to us as though the 100% duty cycle as it relates to the home workshop compressor has become more marketing spiel than fact.

If your air compressor is not built to run for extended periods of time, such as those that might be necessary to fill a 60 gallon air tank versus a 5 gallon air tank, then having a bigger tank may lead to maintenance issues. You need to be sure that your air compressor can run for longer than the typical 10 minutes on – 10 minutes off duty cycle if you are planning on using a bigger air tank.

The only reliable source for that information is the air compressor manufacturer. Sometimes it is hard to find out who that is, but that is grist for another mill!

Can I add a tank?

If your air compressor comes with a too small tank and it is forever cycling on and off to keep you in compressed air supply, then adding a compressor air tank to increase the store of pre-compressed air is a good solution.

In fact, in industrial plant air compressor installations, it is a recommended practice to have a second tank to allow more time in the tank for the compressed air to cool and naturally de-water. That compressed air de-watering process is a subject for another page and will be covered elsewhere on this site.

As air flows out your discharge coupler on the compressor, into the air line, and down to your air tool, if there were another tank in that line somewhere, that would only add to your capacity.

Plumb and extra compressor air tank

The extra air tank can be as big as you need or want – again, as long as the duty cycle of the air compressor will suit running long enough to fill another tank.

Of course, adding a tank means that you will have to wait that much longer for the air compressor to reach cut out pressure after it starts.

Make sure that the extra compressor air tank has a drain, and use it regularly.

Comments

  1. I picked up compressor tank that I retasked as a boiler for steam. It was just the bare tank and I’ve added fittings – initially I fitted a pressure release valve for safety and connected the smaller outlet to copper piping (steam outlet) That didn’t work as my pressure valve tripped with no steam coming out the smaller outlet. So I reasoned the smaller outlet is a one way valve inlet so I made it the water inlet and T’ed into where I had connected the pressure release valve thinking I should be able to get steam from that line. But now I’ve got no steam and no pressure release (the test release on top the valve gives nothing) – so I’m guessing there’s some sort of internal one way valve system in these tanks? Can anyone help? I either need a tank internal schematic or some guidance.
    I’m using a NR compressor, AC100 Air-Craft

  2. Hello, I have a Sealey SAC5020E 50ltr compressor which has a plug at each end of the tank, one of the plugs has begun to leak air so I ordered a new one (hoping it would come with a seal) it didn’t arrive with any seal and Sealey say the plugs don’t have seals. The new plug is also leaking so again I phoned Sealey. They suggested PTFE tape, which didn’t work, or thread lock, which is currently drying and I’m not holding my breath. Is there anything else you could suggest to put on the treads to seal the plug in the tank?

    • No, normally plugs like those in a compressor tank do not come with any sort of seals. We tend to use paste thread sealant for things like this. A liberal amount applied, the plug threaded in, and then a 24 hour wait should resolve the leak issue. That is, assuming, that the thread pitch and size of the new plug is comparable to the old.

  3. My compressor has a 5L tank built-in to top up to a high level of 5 BAR and have down time until 3 BAR. The demand for the tank is not connected to a tool but has a constant bleed off of 3L per minute to keep a particular line of mine dry (corrosion free). I’ve calculated that the duty cycle is 37%. I understand that having a larger tank, will mean a longer down time for the compressor but a longer build up time too. Am I correct in saying that in my circumstance, installing a further 25L tank will not reduce the duty cycle but only increase the build up and down time of the same long term ratios.

    • If the air demand remains constant, adding air tanks will only prolong the fill. On the other side, the compressor will not run as frequently. If, in filling the multiple tanks, your compressor must run longer than it’s designed duty cycle, this could cause compressor damage.

  4. why is there an air line coming out of the side of a dv air pump body, on the lower side by the oil reservoir and then feeding back into the main air discharge line?

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