DD_BobK writes:

On Oct 23, 5:48Â*pm, blueman wrote:
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 Â*sells an 11 gallon portable air tank
(http://www.harborfreight.com/11-gall...ank-65595.html)
for 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!

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.

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

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.

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

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.

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

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.

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.

I'd plumb the tanks together with 1/2" pipe minimum and use Ed'
suggestion of a valve between the tanks.

That was already in my plan...

You can use the valve to select total compressed air volume and
control duty cycle.


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...