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by Scott
(Ontario, Canada)

I have a compressor with a 20-gallon tank that has developed a pinhole leak and needs to be replaced.

I have a replacement tank available that has a more than adequate rating (removed from a Natural Gas Vehicle conversion so rated greater than 1000 psi).

This tank however has only one port instead of the two ports on my existing tank. One port is used for compressed air in from the ‘head’ and the second is for compressed air out to the regulator, filter, etc. (Actually there are three if you include the drain port).

My question is if I use this tank by teeing it into a line connected directly between the compressor head and the regulator, will it operate OK or will there be losses that will decrease the overall CFM rating? The tank would be installed ‘inverted’ so the port would be facing down and a ‘drop pipe’ installed directly below the port with an automatic drain.

All new units I see at Home Depot and the auto supply stores all have separate connections to the tank for ‘inlet’ and ‘outlet’ air so I assume there must be a good reason.

Any recommendations?

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Hello Scott:

Let’s look at what the ports do.

One port, as you point out, is for the drain on the bottom of the tank, and this port is very important. You need to drain the tank regularly to help eliminate rust and to help stop compressor-generated water from flowing down your airline. Since the drain port is on the bottom of the tank, and outlets are on the top, free water generated would tend to stay in the bottom of the tank until it was drained.

Port two is the inlet port from the compressor head into the tank. This line is checked, meaning that the air only flows one way, from the compressor itself and into the tank. The air cannot flow back up to the compressor head, and once the compressor unloads, no air pressure can build up on the piston(s) head from the tank to prevent start-up.

Port three is the discharge port, leading to (preferably) a filter, then a regulator so that you can modulate the downstream air pressure to your needs.

Now, let’s combine the ports.

When the compressor pressure switch calls the compressor to start, air would flow down the line from the compressor, past the check valve, and into the tank. At the same time, compressed air would flow into the airline leading to your FR & L, and to your application.

If your air-consuming application was running, the compressor would never generate enough air pressure to reach kick out, might not even reach MOP for the application, and it would run until it overheated and kicked out on thermal overload.

Having said that, if your airline has a checked coupler on the far end (air can’t escape unless a connector is inserted into it), and there was no air-consuming appliance connected and running, then the airline too would be part of the air reservoir/tank.

This would lead to a fair amount of water vapor and water in that airline since as it condensed, water would have nowhere to go but down the airline to your tools.

The reservoir/air tank of the compressor allows some dwell time for the compressed air to cool and drop out water, and now this warm compressed air would cool in the airline where water vapor would condense.

So, from a strict plumbing standpoint what you present seems reasonable, if impractical, as long as you are OK with lots of water running into your airlines.

Make sure you’ve got a check valve in the line between the compressor and the Tee.

Good luck with this experiment. If you go ahead with it, how about an update after you get it working? And maybe send a picture?


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