Leon lcb11211@swbelldotnet writes:

On 10/24/2011 2:25 AM, DD_BobK wrote:
On Oct 23, 5:48 pm, 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 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.

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

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.

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...