Blowing out water pipes
I am planning to buy a compressor to blow out water pipes for winter. The pipe is 3/4 inch and approximately 50 feet long.
What size compressor would I need?
Yes, it's getting to be the time of the year in the Northern Hemisphere where we have to be concerned about water freezing in pipes that are exposed to the frigid wintertime conditions.
What you need to blow out a fairly small water pipe as you've outlined above is a small compressor.
Let me ask you, are you just going to use the compressor for this? If so, then perhaps you might rent an air pig or small air tank, fill it up with compressed air at the local gas station, and use that to blow the lines instead of buying an air compressor.
On the other hand, once you've got an air compressor, you'll find lots of uses for it.
In order for the water line to be voided of water, you need to fill it with compressed air fairly quickly. The compressed air, flowing into the water pipe, will drive the water before it and out the other end.
If the water pipe is metal, you can use compressed air at industrial pressures...perhaps 120 PSI or so. If it's not, make sure that the pressure you use doesn't exceed the burst pressure of the pipe, or you might be creating more problem than you wish.
If my math is correct (and yes, you do want to check it) a 3/4" pipe x 50' will hold about .2 cubic feet of air. If you introduce a few cubic feet of compressed air at, say, 30 PSI, you'll have plenty to drive the air out of the pipe, as long as the outlet is below the air inlet.
If there's convoluted piping, add higher pressure (maybe 50 PSI if the pipe can stand it) and get the flow of compressed air going into the pipe as fast as you can to build the volume of air inside the pipe quickly, to drive the water out before it.
It will help if the outlet from the pipe is as large a possible to allow the water to get out more easily. Open the drain tap fully.
When all that's coming out of the outlet at the end of the process is a little mist or just air, you're done.
So, a small compressor that has a tank that holds 2-3 cubic feet of air, one with a regulator to reduce the outflow pressure if need, and an air hose with a high flow blow gun is what you'll need.
Hope this helps,