Ray writes:

On Mon, 24 Oct 2011 19:40:31 -0400, blueman wrote:


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.

I am in the same situation. I also thought about the exact same tank.
Last year I paid the $20 for a commercial blow job.

Around here (big East Coast urban area), I was charged $75 to blow out
my 9 zone system, back SEVEN years ago. I imagine now it might be $100.
So, doing it myself has already paid for my trusty HF compressor 5 times
over...

His compressor put out 100 CFM. It took several minutes for the
remote heads to stop blowing mist.

The one time I went professional, he did 4-5 zones at a time (we have a
9 zone system), so even if had a 100 CFM system (which I'm not sure is
even possible to push through a 100 ft of 1" PVC feed pipe at a PSI of
50), we are talking only about 20-25 CFM per zone

air will blow out the near heads and is exhausted in a few seconds,
another 11 gallons may extend this to a fraction of a minute. The
consensus on the Internet is a min of 10 to 15 CFM is required. This
somewhat depends on the slope of the lawn. The steeper the slope the
more CFM required. In a previous house with a flatter lawn and auto
drains at the low spots I used my small compressor for years with no
problem.

Well, consensus doesn't equal experience. People told me the same thing
7 years ago, when I inquired. But my 8 gallon HF compressor which is
rated at 7.25 SCFM@40 PSI has done just fine.

I do each zone separately, blowing it out 5-6 times just to be sure,
though it starts blowing pure mist after about 2-3 times. Some of the
zones are on a mild hill. Each zone has 4-7 heads which together deliver
about 10gpm. The run lengths to the heads are about 100ft. When there is
still water in the tubing, I get effective pressure for about 45
seconds. When it's just misting, I get about 25-30 seconds of good
pressue (enough to lift heads & turn the rotors). By adding an 11 gallon
external tank which will bump my capacity from 8 to 19 gallons, I expect
to get over a minute per cycle of effective pressure.

As I mentioned in another post, I use either a home-built electronic
timer or a hacked computer remote control to automate the process. I go
slow to allow the compressor time to cool down between bursts so the
whole process takes several hours -- but it all runs automatically so it
takes almost none of my time.



Next I thought about the $129 HF 6 CFM in conjunction with my present
compressor. It is close, but I am not sure if my expected lifetime
would justify this expense.

So my solution this year was to blow out and use a shop vac to
evacuate the far heads. After doing that I decided to hire the pro
next year. Tomorrow it is going to be 14 so I hope I got enough water
out. A little water in the poly pipe won't cause a problem, sprinkler
heads are cheap, the expensive anti siphon is water free.

Lets us know how you make out.

Well I guess if I only paid $20 for a blowout, it would be worth it to
have someone else do it even though it's only a few minutes of work with
my setup now. But I don't know how anyone can come to your house for $20
nowadays given the cost of gas etc. I mean repair people often charge
$100 just to cross your threshhold...