Adding external air tank to existing air compressor to give equivalent bigger tank

[blueman]

I have a low-end Harbor Freight 8 gallon air compressor that has many times over earned its $100 cost.

However, there are times when I would like to have a larger air supply tank such as when I am blowing out my irrigation system. Of course the steady-state is limited by the compressor motor to about 4-5 CFM@90 psi.

But I was wondering whether I could extend at least the initial volume by adding an additional tank.

Harbor Freight for example sells an 11 gallon portable air tank

about $38. It is rated up to 125 psi (same as my compressor) and comes with a gauge and a tire-type fitting.

I was thinking that I could re-plumb to NPT and attach it to the drain hole on my compressor tank (with a T-fitting and ball valve to still allow drainage).

This would then give me effectively 19 gallons of initial air supply.

Of course, I would need to be careful about duty cycle since filling up

19 gallons vs. 8 gallons would be like doing 2 1/2 continuous fills of my original tank.

But assuming that I am careful about duty cycle is this a reasonable way to temporarily extend the initial air supply for occassional uses where I need to get the advantages of a larger tank?

(Note that it seems that several of the Harbor Freight compressors use the same HP motor with similar CFM ratings for a range of tank sizes -- probably because the HP is ultimately limited by the 110V 15A supply circuit)

Thanks!

[P.H. T.]

Don't know if that is possible. I don't know who makes that Harbor Freight that you have, but if they have a U.S. number, I would call them and talk to them about doing that. Or talk to someone local that knows air compressor's and could guide you on that.

Paul T.

[Swingman]

Had an add-on tank for a long time but recently gave it away when I moved to a smaller shop. I never found it to be much of a practical advantage, all things considered.

Hardly noticeable, AAMOF.

YMMV ...

[Artemus]

Your logic and reasoning are just fine. Go ahead and add an external tank, it shouldn't harm your existing unit. Art

[Ed Pawlowski]

There are numerous ways to pipe the tank in line. Your reasoning is correct, you get the initial burst, but that is all.

I'd pipe it in, but I'd put a valve in the line to shut it off. Two reasons for that. There is no good reason to fill that tank if you are not going to need the backup of air. Saves both time and energy at startup for normal work.

Second, you can fill it, close the valve, and have it ready for use. You can also open the valve and use that air if you only need a little shot of it and not have to start the compressor.

[Leon]

If you are talking about blowing out a sprinkler system I doubt the capacity you are gaining will help. Long ago I had a 80 gallon tank with a small pump, I could empty it in about 15 seconds when blowing a lot of air. My nephew used to be in the sprinkler business and rented a commercial compressor for blowing out the systems.

[Puckdropper]

blueman wrote in news: [e-mail]:

*snip*

Where it might make a difference is in air hungry intermittent duty tools like air wrenches. The extra reserve of air would allow the tool to run longer (perhaps enough to finish the job) before the compressor kicks on.

If you do this, make sure to open the compressor regulator to charge the tank fully and then put another regulator before the tool connection. If all you charge the tank with is 30 psi, you won't see much difference at all. Charge to 100 or 120 psi and you'll notice the difference.

Puckdropper

[DD_BobK]

Yes, your idea will be useful in application you describe (blow out sprinkler system)

I did some "back of the envelop" calcs.

Assuming a sprinkler zone has about 100ft of 3/4" PVC pipe & zone's total flow is about 15 gpm.

Charge your compressed air tanks to maximum pressure but I'd recommend using a pressure regulator in the supply hose to the sprinkler system.....set at no higher than 50 psi.

PVC piping is not meant to be used with compressed air.. ...... there is a danger of brittle failure. Safety glasses & cleared area are highly recommended.

Your original compressor setup will blow out a single zone for about

25 secs ....kinda short.

Add the 11 gallon tank & bump your total storage to 19 gallons, you can blow out a single zone for about 70 secs. Two blown outs per zone should do the trick.

I'd plumb the tanks together with 1/2" pipe minimum and use Ed' suggestion of a valve between the tanks. You can use the valve to select total compressed air volume and control duty cycle.

cheers Bob

[Josepi]

Usually morning muddleheadedness. Leon? LOL

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[Leon]

Did you calculate for the fact that pushing a liquid "up" requires a lot of continuous air flow? And concerning your blow time, how much of that is way below 50 psi which would be much less effective? From some that did this for a living he would run the compressed air at a continuous pressure for 10 -15 minutes. And calculations for 100' might be correct but I had a small yard and probably had 250' of underground line on one run, not to mention all the heads that are leaking air.

[Josepi]

Do you have to do aerobics when you hold up a glass of water?

It takes pressure, not flow.

[blueman]

All good points -- I actually am planning on doing that for all the reasons you mention...

[blueman]

Well, I have been doing it successfully for the past 7 years on a 9 zone sprinkler system (with 1" feed pipe) and about 10 gpm per zone. We live in the Boston area with deep cold spells and I have not had any problems.

It takes about 3-4 full tank boluses to blow out each zone (i.e. to the point where i just get 'steam' rather than water coming out). Just to be sure, I run it about 6-7 times per zone.

On each tank full, I get about 30-40 seconds of good initial blow before the pressure drops down too low to lift the heads or move the rotors.

My hope was that with 2.5x the volume of the current configuration, that I would get a better blowout and/or require fewer repeats per zone. In particular, I was hoping to get the initial blow to last longer, say maybe 1 minute or so. Of course, one could say if it ain't broke don't fix it, but I figure it can't hurt.

[blueman]

Well, my reasoning for attaching in via the drain hole is that then I would be connecting before the regulator and at full pressure (which maxes out on my compressor at about 110 psi). That way both tanks would be before the regulator...

[blueman]

Actually it's about 10gpm per zone (for each of 9 zones).

That is exactly what I have been doing the past 7 years.

I get about 30-40 seconds. So your calculations are on target.

Currently, I get a good blowout (only steam) after about 3-4 tankfulls @8 gallons. I do about 6-7 runs just to be sure.

I was hoping with 19 gallons to get 60+ seconds per run so that the runs are both longer and better (less 'wasted' air just opening up the heads and filling the system as a fraction of the run time). My thinking was I could do it then in 3-4 runs with a good margin of safety.

That was already in my plan...

I actually first built an electronic circuit using a 555 timer to control the valve to get me an adjustable on/off pulse width. Currently, I let it discharge for about a minute and then close for about 4 minutes. Since the tank fills in about 1.5 minutes, I have an approximate compressor duty cycle of 33%.

Last year, I hacked my controller to be able to run it via a plugcomputer, so now I can use simple bash shell scripts to open and close the valves at any interval and in any order I want. I can even control it over my phone using sms messages or remotely via any laptop over wifi.

Finally, I position a portable fan near the compressor motor and housing to give it some extra cooling...

[blueman]

All I can say is that it works for me... and after a few tank fulls it is just blowing steam (not even mist) -- just humidified air.

Even if there are some residual drops somewhere in the line, there is now enough air and expansion space in the system to prevent any ruptures when the tiny residual water (if any) freezes...

[nailshooter41]

On Oct 23, 11:26=A0pm, Puckdropper wrote: !

Exactly my experience. When I spray finish with an hvlp conversion gun, the pressure moves too far up and down to get a completely consistent flow to the gun. The extra storage of the tank give me more compressed air available at once.

I turn up the compressor side to 100 psi, and pressure the tank and the second tank. On the downside (line to gun connection from the pony tank) I have a line conditioner/water removal regulator combo that I set at 50psi. Gun is fine tuned as needed wit another gauge at the handle to about 30 - 40 depending on the material and conditions.

Having that much volume of 100 psi air metered out at 50 psi (some fan of Bernoulli can figure out the exact benefit) and ultimately a little less than that made a world of difference in spraying. I can spray much longer without line pressure drop, and if i can see it by the quality of spray deteriorating as it comes out of the gun I simply bleed off the remaining air needed to kick the machine to on to recharge the two tanks. Wait a couple of minutes and I am back in business with a lot of newly compressed, consistently pressured air.

For a couple of cab doors and a drawer or two, the compressor never starts up, and I am finished.

To the OP, as far as cleaning out pipes is concerned, I think you would have about the same effect. A lot higher volume of compressed air initially, but to benefit you would have to let the tank charge up each time to charge both tanks after you depleted them. I am thinking that if your system is working now, don't screw with it. And heck, at $38, you are almost half way to a while new compressor from HF!

Robert

[DD_BobK]

My calcs's & sprinkler zone design were merely SWAGS......... .

I've done I fair number of sprinkler systems (new & rework)....never seen 250' on a single zone with 3/4" pipe. I had a hard time seeing his system from my keyboard so I guessed.

Read my post...all of my blow out is with the tanks at 50 psi or higher.

The task at hand was to evaluate his idea of adding 11 gallons of reservoir volume not determine optimum blow out compressor size & blow out.

As I said my calc's & assumptions were SWAGS...... and based on blueman's followup posts, they appear to be "in the ballpark". The addtiotnal informatin he provided also seem to confirm that the added 11 gallons & the way he intends to plumb up the addition will result is system that does the job for me.

Mission accomplished?

cheers Bob

[Twayne]

Simply put, you'll get a longer run time and a longer tank-fill time. Your plan sounds fine as is.

HTH,

Twayne`

[blueman]

Now that a new Harbor Freight has opened up about 9 miles, I went to the store and bought the tank for $37, an 8 ft 3/8" 200psi hose on sale for $4 and a couple of fittings for a few bucks. And I used a 20% coupon on the entire purchase...

So for less than $40 total, I have it all working beautifully.

The setup now looks like:

Portable tank-------> Tee ---> 3/8" hose --> Female quick connect | | Gauge Ball valve & pressure release (included)

(Note: I added the ball valve to allow for easy emptying rather than having to exert constant pressure on the built in release

Compressor ----> Drain ---> Ball valve ---> Male quick connect

When I want to fill the external tank, I quick connect from the tank to the drain, open the ball valve on the compressor drain and close the ball valve on the tank.

Then if I want to use the combined volume, I just attach my hoses and tools to the normal regulator side of my compressor and get a combined

19 gallon volume

If I want to use the portable tank remotely, I just disconnect the quick connect and then connect hoses and tools to the female quick connect on the portable tank (I may add an inline regulator later).

If I want to drain both tanks, then I disconnect the quick connect and open the ball valves on both the compressor tank and the portable tank.

Thanks for all the helpful comments that gave me the confidence to go ahead with my plan.

Now I just have to wait a few weeks until it's time to blow out my irrigation system.

Also, as a few others have mentioned, it will be good for high volume, relatively limited time operations like HVLP spraying.