Air Tank for Compressor

Submitted: Sunday, Jun 23, 2013 at 02:04
ThreadID: 102896 Views:3228 Replies:4 FollowUps:7
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I want to mount a air tank under the tray above the tail shaft on a GU Patrol Ute. The compressor is located in a under tray tool locker which makes the compressor head lower than the air tank.I have read the correct installation is to have the compressor higher than the air tank so no moisture can drain down to the compressor.

I had thought that if the hose from the compressor didn't go to the bottom of the air tank it would not matter ? I am unsure whether I can still mount the air tank higher using a loop in the hose so it comes a bit lower than the head and enters from below and up into the head or whether I need to add a water trap or simply locate the compressor above the air tank.

Anyone with some knowledge or experience with this that could help I would appreciate it.................
Cheers Peter
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Reply By: Allan B (Member, SunCoast) - Sunday, Jun 23, 2013 at 09:19

Sunday, Jun 23, 2013 at 09:19
G'day Peter,
Perhaps some understanding of the physics involved may help you sort this.

Atmospheric air always contains some water in vapour form. As the air is compressed its volume is reduced but the water content remains until the air becomes saturated and further compression causes some of the water vapour to condense into liquid form and 'drop out' of the compressed air. This action is compounded by the temperature of the compressed air. As it is compressed its temperature rises and it can support more water in vapour form but as the air cools the vapour condenses into liquid water.

Typically, in an air compressor installation, the air leaving the compressor is hot and supports the water in vapour form but as it cools, either in the piping or in the receiver, some water will condense and become liquid in the pipe or receiver.

It is usually easiest to drain this collected water from the bottom of the receiver so the system is arranged so that the compressor is above the receiver (look at all the portable sets) with a drain in the bottom of the receiver. Piping from the compressor should fall continuously from the compressor to the receiver without dips to avoid water collection in the dip.

If the compressor is below the receiver and some cooling takes place in the piping then water will drain back to the compressor. Air will bubble through this collected water until the compressor stops whereupon it may drain into the compressor which is most undesirable. If the compressor must be mounted below the receiver then a water trap needs to be incorporated at a low point following the compressor with either an automatic drain or with sufficient volume capacity to enable a manual drain.

It is also possible to mount the compressor below the receiver if the discharge piping or hose is thermally insulated so that minimal condensation will occur in the hose and connect it to the top of the receiver tank. Cooling and condensation will then take place in the receiver and can be regularly and manually drained from the bottom of the tank.

Hope this helps.

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Follow Up By: Member - kwk56pt - Sunday, Jun 23, 2013 at 14:12

Sunday, Jun 23, 2013 at 14:12
Thanks Allan for the detailed it makes sense that the water condenses in the pipe as it cools. The compressor is mounted up high in the under tray toolbox so I will look at running a hose that has a low point and fit a water trap so that water will run back to it from the compressor. I would prefer to leave the compressor where it is as its pretty big 1.5 hp unit and its present location is away from dirt and water and out of the way. It is a sealed unit so I could look at relocating it to the chassis perhaps.
I have noted the suggestion of insulating the pipe as its easy and cheap to do and this would help minimise the water that makes it to the water trap. The tank is a after thought rather than part of the original fit out. My ute has a Telstra style body which I have already fitted out.
I could take the cheap and easy option yet and do nothing but you have helped a lot in my understanding of the issues.
Thanks Peter
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Follow Up By: Allan B (Member, SunCoast) - Sunday, Jun 23, 2013 at 15:53

Sunday, Jun 23, 2013 at 15:53
Peter, Having given the lesson on compressed air/condensate behaviour, maybe now I could suggest a simple effective arrangement for you.

As already expressed, the water in vapour form in the delivery pipe will remain in vapour form until cooled somewhat. So if you are using a flexible hose as your delivery pipe rather than say copper tubing, then condensation will not take place until the vapour-laden air reaches the receiver and cools sufficiently. The condensate will collect in the bottom of the receiver and can be conveniently drained from the bottom by means of a manually operated drain dock. This will not need to be frequent given the relatively small amount of compressed air you will be using.
Small automatic drains are not easy to come by at a low price and also can be unreliable. They operate by means of a float in a chamber rising to open a very small valve. The available energy is low and sticking can occur.

If you go this way position both the air inlet and outlet at the top of the tank (they can share the same connection) and the drain at the bottom of the tank. The manual drain valve does not have to be right at the tank, it can be further down the drain pipe. Ensure that the hose from the compressor rises continually to the tank. Experience will teach you how often you need to drain.
Any small amount of condensation which may form in the delivery hose when the compressor is idle will quickly be evaporated by the hot delivery air when the compressor next starts and carried to the receiver.

Of course, having the compressor above the tank is ideal but the arrangement described above will work quite well provided that you ensure that the air cools but little before reaching the receiver.

How can I be sure about this? Well, I've had a lot to do with providing clean dry air to industrial instruments and controls.

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Follow Up By: Member - kwk56pt - Sunday, Jun 23, 2013 at 16:59

Sunday, Jun 23, 2013 at 16:59
Allan ,
thanks for the suggestions. I had thought you must be a instrument fitter or something. Yes I was going to use flexible hose to the tank from the compressor with a tee and a petcock at a low point that I would make between the tank and compressor.

I would probably leave this open after I have finished with the compressor so the air tank would discharge and the line could drain. The inlet was going to be at the top of the tank, but I had not though much about the outlet but will take your advice and put it at the top even if the inlet and outlet need to share the same port via a Tee. I might need a manifold as I would prefer a quick disconnect at the front and another at the rear I assume the outlet is at the top rather than the side lower port to ensure the air is dryer for air tools ? When you suggested insulating the hose going to the tank I had thought initiately of that Bunnings pipe insulation but suspect the hose will be too hot for it, probably no need with a petcock added at the low point ?

The plan for the drain on the air tank was to put it at the bottom and run a hose down to somewhere accessible and put a petcock on the end of it.

Ok think I have understood your suggestions and now have a plumbing plan that will not lead to the premature failure of my not cheap compressor. Your time taken to assist me is greatly appreciated.

Thanks Peter
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Follow Up By: Allan B (Member, SunCoast) - Sunday, Jun 23, 2013 at 17:24

Sunday, Jun 23, 2013 at 17:24
Probably not a good idea to further insulate a flexible hose from the compressor outlet. It could cause a failure due to over heat, and anyway is not needed.... the hose would probably be enough insulation in itself.

The air inlet and outlet connections do not need to be at the very top of the tank. Halfway up on the side would be OK. So long as they are well above the possible condensate level. But the condensate drain does need to be at the tank bottom for proper operation.

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Reply By: olcoolone - Sunday, Jun 23, 2013 at 09:36

Sunday, Jun 23, 2013 at 09:36
Peter....In a theory point of view you're correct BUT in real life especially with a small air compressor on a vehicle the chances are unlikely.

The way to avoid and how most are done is to run the air inlet and outlet on top of the tank with a drain point on the bottom.

If you want to stop condensation entering the tank you could run a water trap.
AnswerID: 513625

Reply By: desray (WA - Sunday, Jun 23, 2013 at 18:47

Sunday, Jun 23, 2013 at 18:47
Peter why do you want /need a air tank, What will you use it for.
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Follow Up By: Member - kwk56pt - Sunday, Jun 23, 2013 at 21:38

Sunday, Jun 23, 2013 at 21:38
I thought it would provide a source of compressed air to operate a air duster for jobs like cleaning filters, the radiator and under the bonnet, good for re seating tyres, maybe a drill to make light work in hard ground getting tent pegs in by drilling a pilot hole. I thought I might buy a second hand rattle gun and use that as well. It should also make a small improvement in tyre filling as the tank is recharging while I am checking tyre pressure and shifting to the next tyre.
I don't have a compressor at home but as I have a good one on the vehicle why not use that. To be honest my reasons are not that good, its a fair bit of trouble and expense. Have you had a experience using a air tank or formed a opinion on them ?

Cheers Peter
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Follow Up By: desray (WA - Sunday, Jun 23, 2013 at 22:18

Sunday, Jun 23, 2013 at 22:18
As you say its " a fair bit of trouble and expense" The tank would be ok for the duster/blowing out filters. Shouldn't really, need it to reseat tyres ,, lots of 12volt drills around,, the rattle gun uses a LOT of air ,you would need a big tank, also 12volt rattle guns available . You would take longer blowing the tyres up, the first tyre is quick as you have stored air but the second,third and forth take longer because the compressor now has to fill your tyres and the air tank to 40psi or whatever is going in your tyres. Yes I have a air tank on my van at work but find it very limited in what you can use it for.
FollowupID: 792432

Follow Up By: Member - kwk56pt - Sunday, Jun 23, 2013 at 22:44

Sunday, Jun 23, 2013 at 22:44
Thanks for your thoughts something to consider, maybe I am making a lot of work for not much gain.
FollowupID: 792434

Reply By: Echucan Bob - Tuesday, Jun 25, 2013 at 00:19

Tuesday, Jun 25, 2013 at 00:19
Peter, one convenient reservoir for compressed air that you may have forgotten, and you already have on board, is your spare tyre(s). Inflated to 60 psi they hold quite a bit of ready air for blowing, tyre inflation etc Best of all they can be inflated at a servo. A single tyre at 60 psi will inflate a completely flat tyre to 30 psi in seconds.

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