I'm not doing the math but how many gallons in 1 cubic foot, 2? At 8 something lbs per gallon its going to take more then 5 psi to push the water up and out the sprinkler heads.

If you blow through a straw gently into your glass of Coke, all you get is bubbles. If you take your little pancake compressor and blow into the same glass of Coke with a blowgun on the end, the Coke will fly everywhere.

It's just that simple. In order for force all of the water from the irrigation lines, you need enough air VOLUME at a decent pressure to move this liquid weight through the sprinkler heads. Also, keep in mind that an underground irrigation system has a trunk line with branch lines off of it. Air, water and electricity all take the path of least resistance.

You need enough air volume to make sure that every branch line is drained. The compressor must be large enough to literally overwhelm every branch line with air volume.

I use my 15 gal campbell hausfeld compressor to blow out my sprinkler. I am not sure what cfm it is capable of at 50 psi, but it is plenty adequate. One tankful per zone is all that is needed, and you don't have to expel every last drop!

A 3/4 inch line can pass 81 CFM.
Using any unit with good size tank and quickly releasing the air will give more flow than the compressor can make.

"A 3/4 inch line can pass 81 CFM"

Say what??
At what length? At what pressure? At what temperature?

You do not need 20 cubic feet per minute. Very few residential compressors put out that much volume anyway. Do use a compressor with at least 80 to 100 psi capability and a decent output like 4 to 5 cfm. The point being that you should not try to use a small 12 volt emergency tire compressor. That is not enough volume. Blow out one station at a time and repeat the process until you are satisfied that you have removed as much water as you can.

The whole reason behind using a compressor with lots of volume is to get as much water out as possible. Using a 4-5 cfm homeowner-type compressor is bad advice. It would *probably* be adequate if your lines were all nice and level underground with no dips anywhere. If your lines have a low spot anywhere though, whatever droplets that remain along the insides of the piping will eventually end up moving down the pipe walls and sitting wherever there's a low spot in the zone. Your lines don't need to be completely full of water to freeze and break.....all you need is a foot-long low section sitting full to freeze and pop.

When homeowners around here blow out their lines, they come in and rent a 70 cfm skid-mounted compressor that'll fit in a pickup bed, or they get a 100 cfm towable unit. They regulate the pressure down to whatever the lines and heads are rated to handle, and let the volume do the work.

madtripper,
The point I was trying to make was the difference in pressure. One shot from a blow gun at 125 psi would empty the glass instantly. But that's because the liquid in the glass is in a small, confined space.

The people in the irrigation industry mostly rent tow-behind compressors that put out 125 to 200 CFM when they winterize systems for their customers. They know from experience that high volumes of air blowing through the underground pipes will clear 99.9% of the water in a matter of seconds.

Small CFM compressors cannot sustain even 90 psi for very long. The pressure drops off quickly to the cut-in point also because these compressors have very small air receivers. And the larger the hose diameter you use, the faster this happens and the harder it is for the pump to play catch-up.

As air is blown into the water lines using one of these little compressors, water does indeed escape. However, as the water escapes, the level of the water in those pipes also drops and that creates an airspace above the water level which allows the air to take the easy route in finding an exit. The initial blast of air gets the much heavier water moving in the pipes. As the water moves along, it creates a space that must be taken up by the air. Water is not compressable. Air is.

You need to keep this now-moving-water moving at a rate that is sufficient to have it rise vertically and exit the sprinkler heads in totality. If the water pools in the elbow at the bottom of the sprinkler head, then it will freeze and break the elbow. Next spring, it's repair time.

Therefore, it is difficult to ensure that all the water has been removed from the system when using small CFM compressors. And since the whole object of the exercise is to remove the water to prevent freeze damage, then using a compressor with adequate volume (CFM) seems appropriate.

To me, this akin to saying the following. I need to winterize my cottage but I didn't buy enough RV antifreeze, so do you think I will be OK if I put only half as much anti-freeze in each toilet as written in the instructions?

You asked a question. Several people have told you how the Pros do it. It's your system, your dime. If you believe that you can blow it out with your compressor, then by all means, go for it. The proof is something that will be found next spring and every spring thereafter. Underground systems can be simple or quite complex. If you can simplify the system by isolating branches, then that might work. You have the luxury of time that a PRO does not. He needs to get in and get out quick in order to keep the cost down. To a degree, it's an apple and oranges thing. But either way, if water remains, so does the potential for damage.

You need enough CFM (speed) to create a turbulent boundary layer between the air and water. The high-speed turbulent airflow creates friction between the air and water. This friction then pushes and pulls the water out the pipe with the air. You probably get some Bernoulli effect as well from the change in pipe diameters and perhaps because the high flow out the main pipe could suck the water out of the smaller pipes along the way.

If you don't have enough airflow, you only get small waves and low friction because of the laminar boundary layer.

If you don't believe me, dive underwater with a snorkel in your mouth. When you come back up, gently blow into the snorkel to try to purge the water out. Not much happens except you blow a few bubbles and then choke on water. Now, go under water again and when you surface, let out a quick blast of air through the snorkel. All the water got blown out, didn't it? It's the same idea.

Another example is whitecap waves. When the wind blows fast enough, it pulls/pushes the water over the top of the wave. If you only have light winds, you get small waves because of the laminar airflow across the surface.

If you need more information, search google for Reynolds Number, turbulent or laminar fluid flow and/or boundary layer. Stick with liquids at first because when you get into compressible fluids (air), it can get tricky using shockwave tables and such.

I haven't really studied air flow in a sprinkler system, but I'll bet someone has and that's why they recommend 20 CFM or more. Who am I to doubt them?

The only time you need over 10 CFM is when you blowing commercial. Let's be real that's where the CFM comes in. Of course in between your blowing residentials at 120psi 185 CFM 1 minute per zone. Blasting their poly systems away. Trust me that's what happens.

So you drag up a 13 yr old thread to enlighten us of your brilliance ?

13-year-old thread... still relevant information. I have three thoughts to consider that I didn't see brought up specifically. I'm okay with getting into some hydraulic, hydrodynamic, and hydrostatic data, but will convey conversationally.

First; not all systems have simple nozzles on them. Some systems have wastegates, drains, large emitters, etc. In order to keep a constant 50-60 PSI when passing through PVB you would need 20-50 CFM when fully opening large drains and spigots. You may be able to build up a good 20-second blow if you could just isolate that line, but that would be time-consuming to be sure (see next thought).

Second; many compressors that are designed for high CFM are better engineered to consistently run for larger air movement applications. In this way, residents that take their time and have simple 1"- 3/8" lines with 4" heads equipped with misting nozzles should not have any issues using a 20gal 3hp ~5CFM to blow 4-5 zones in an hour or so. If you try that same compressor on zones with big brass emitters, impact, or spigots unless you plan on spending the whole day rotating zones with deliberate resting periods, you are likely to heavily stress out the compressor components, electrical circuits, and possibly overheat and still not get the water out of a belly. 20+ CFM systems are capable of workloads that far surpass simple single resident applications. There are some really well-engineered 15+ CFM systems too. Design vs use.

Third; some of the advice that is given in discussions and blogs are intended for folks who are looking to do this professionally. In that case, you want a 50+ CFM unit or you will be severely limited in systems you can service, how quickly you can service (neighbors run up all the time, can you do mine too?), and run a high risk of breakdown. If you depend on your machine for your income, you 1000% do not want to experience a head or motor burn out or have to order parts during peak season. Customers are fickle and will call someone else, or try it themselves in a split second especially if a cold snap is looming.

I've been blowing out my own sprinklers for 15+ years with a 20 gal air compressor at 45 -50 psi. I'm in Iowa where it gets plenty cold and have never had any issues with a line freezing. I can tell you that it takes FOREVER to do it this way as I drain the tank, recharge, then drain again for each zone. I may even do some zones 3 times just to be safe.

Sure, I'd love to have a 20 or even 50 cfm compressor but for do-it-yourselfer, what I have works. That said, I did pick up a 2nd 20gal compressor this winter and now have double the reserve. I'm hoping this will cut the number of times I clear each zone in half. Half of forever is better than FOREVER in my book.

anyone familiar with the term volume water only expands 30% in volume when it freezes if you get 50-60 % of the water out of the system you will be fine water doesn't freeze instantly as it does it follows the path of least resistance as long as you dont seal the system example closing the inlet you blow out your system through which would cause build up of air pressure if water is remaining when it freezes it expands creating air pressure that can build up damaging lines or not allowing frozen water to push liquid water into air voids the water may freeze but it will continue to push unfrozen water into the air voids as long as system is not closed !!!!!!!!! dont believe it take a two bottles of water fill them halfway put them both in the freezer one with a cap on it one without the thicker the bottles and better the cap the more you will see that volume expansion can create alot of pressure fast most of the time it is air pressure combined with water that causes pipe failure and other irritation system failures

Have a residential system, 6 zones, can blow it out with 4.5 cfm at 90 (metabol ”the tank” umit unit) holding at 50 psi . in winnipeg, lines never froze of broke