DIY full size air compressor - am I mad?
Discussion
Firstly, apologies if I posted this in the wrong section, I did look for more suitable areas but couldn't find any!
So, I've had this idea running around my head for a while and now I'm finally serious about doing it. So I thought I'd come on here and see what sort of reception the idea gets!
I've wanted to have an air-compressor for quite a while now, unfortunately I cannot afford a decent new one, I could buy a second hand one, but they seem to really vary in quality and price but most importantly where's the fun in that?!
So, three main parts to a compressor...
Motor - I'm going to go electric as don't fancy dealing with more flammable substances than I need to and I have a 3 phase 32amp supply available, be rude not to use it.
Tank - I'll find an old gas tank or something, might do two sizes so I can choose whether I want the big one for large jobs or just fill a small one for quick jobs like inflating a tyre, or both for a days work. I was thinking ball valves to shut off/open a tank.
Air pump - This is where it gets interesting, I was considering using a small 4 pot petrol engine, remove the cam shaft(s)/rockers etc then fit a series of one way check valves into the spark plug holes. Bit of plumbing and I reckon that's an easy way to a very high cfm & psi air pump. It wouldn't need to run at too high rpm due to the displacement, would last forever (provided it doesn't sit too long) as it's designed for so much harder work. I'd probably fill it with water but would it really get that hot? I guess there is the problem of oil entering the air supply but i guess that only goes to lubricate the tools. I realise this may not be the best setup for painting, but I doubt I'll do much of that, and if I do, I'll go buy or hire a proper compressor. Could always add oil and water catches in the lines I guess?
My calculations for 8cfm at 90psi require about 3.14hp of work done, or 4hp for 10cfm at 90psi, or 4hp again for 8cfm at 100psi. So considering losses, let's say the system is only 70% efficient, a 5hp (or maybe just above) should be able to run the system. Has my thermodynamics gone wrong here, have I overestimated the pump's efficiency etc? My main worry is all the extra energy required to rotate the entire engine assembly, even with as much as I can removed, I fear there could be quite a lot of losses, my question is, is it excessive.
Put a pulley on the motor, work out the ratios, put a pulley on the motor/drive it by the crank pulley. Remove everything not required on the engine, plumb a load of check valves, blow off valve, pressure switch, gauge etc. Could this work?
Also, ideas on where to source a cheap second hand high power motor would be much appreciated!
P.S. the answer is yes, I am mad, but it would probably make a good "how not to" video at least...
So, I've had this idea running around my head for a while and now I'm finally serious about doing it. So I thought I'd come on here and see what sort of reception the idea gets!
I've wanted to have an air-compressor for quite a while now, unfortunately I cannot afford a decent new one, I could buy a second hand one, but they seem to really vary in quality and price but most importantly where's the fun in that?!
So, three main parts to a compressor...
Motor - I'm going to go electric as don't fancy dealing with more flammable substances than I need to and I have a 3 phase 32amp supply available, be rude not to use it.
Tank - I'll find an old gas tank or something, might do two sizes so I can choose whether I want the big one for large jobs or just fill a small one for quick jobs like inflating a tyre, or both for a days work. I was thinking ball valves to shut off/open a tank.
Air pump - This is where it gets interesting, I was considering using a small 4 pot petrol engine, remove the cam shaft(s)/rockers etc then fit a series of one way check valves into the spark plug holes. Bit of plumbing and I reckon that's an easy way to a very high cfm & psi air pump. It wouldn't need to run at too high rpm due to the displacement, would last forever (provided it doesn't sit too long) as it's designed for so much harder work. I'd probably fill it with water but would it really get that hot? I guess there is the problem of oil entering the air supply but i guess that only goes to lubricate the tools. I realise this may not be the best setup for painting, but I doubt I'll do much of that, and if I do, I'll go buy or hire a proper compressor. Could always add oil and water catches in the lines I guess?
My calculations for 8cfm at 90psi require about 3.14hp of work done, or 4hp for 10cfm at 90psi, or 4hp again for 8cfm at 100psi. So considering losses, let's say the system is only 70% efficient, a 5hp (or maybe just above) should be able to run the system. Has my thermodynamics gone wrong here, have I overestimated the pump's efficiency etc? My main worry is all the extra energy required to rotate the entire engine assembly, even with as much as I can removed, I fear there could be quite a lot of losses, my question is, is it excessive.
Put a pulley on the motor, work out the ratios, put a pulley on the motor/drive it by the crank pulley. Remove everything not required on the engine, plumb a load of check valves, blow off valve, pressure switch, gauge etc. Could this work?
Also, ideas on where to source a cheap second hand high power motor would be much appreciated!
P.S. the answer is yes, I am mad, but it would probably make a good "how not to" video at least...
There are quite a few under £100, or a little over but they aren't very high capacity, I'd rather buy something that isn't going to kick in every 3 minutes and not going to have a very long lifetime. Maybe I'm being a little overkill about this?
I guess my main problem is that the actual air compressor pump units are really quite expensive to buy second hand. I wonder if I could tie two pairs of pump and motors together into one large tank. It's easier and cheaper to get the small motors, but again, the difficulty is finding a suitable air pump cheaply.
Not a great fan of Chinese cheese either if possible!
I guess my main problem is that the actual air compressor pump units are really quite expensive to buy second hand. I wonder if I could tie two pairs of pump and motors together into one large tank. It's easier and cheaper to get the small motors, but again, the difficulty is finding a suitable air pump cheaply.
Not a great fan of Chinese cheese either if possible!
Air tools mainly, so impacts, ratchets, drills, nibblers, cut offs, possibly grinders, sanders and hammers. That's all I can think of the top of my head.
Then the usual tyre inflation and blow off stuff.
So my thoughts were about 8cfm at 90psi preferably with at least a 150l tank would be a good size to aim for compressor wise. Then I had ideas on how to make it a project...
I can get hold of old small engines quite cheaply and easily, hence my thought of using that as an air pump rather than the proper ones which seem quite expensive (£150+ for a mediocre one).
Doubt I'll be blasting or spraying with it. At least not in the near future. I'm just not a fan of buying twice, would rather buy or build a solid, heavy duty second hand unit than a cheap or undersized small one. I also, like a good project (not that I don't already have far too many of them...)
Then the usual tyre inflation and blow off stuff.
So my thoughts were about 8cfm at 90psi preferably with at least a 150l tank would be a good size to aim for compressor wise. Then I had ideas on how to make it a project...
I can get hold of old small engines quite cheaply and easily, hence my thought of using that as an air pump rather than the proper ones which seem quite expensive (£150+ for a mediocre one).
Doubt I'll be blasting or spraying with it. At least not in the near future. I'm just not a fan of buying twice, would rather buy or build a solid, heavy duty second hand unit than a cheap or undersized small one. I also, like a good project (not that I don't already have far too many of them...)
Thanks for all the advice, very much appreciate it, a lot of food for thought! Oh and thanks for moving the thread, sorry for not getting it in the correct place first time!
My reason for using 3 phase was only motors running better off it, nothing to do with the high power draw. I know there are debates whether motors run more efficiently etc, but for the same power output, they tend to be cheaper for what I can see.
Will do that with the air receiver, I was thinking at least 150litres, preferably 200+
85% ineefficient or 85% efficient? 85% inefficient means only 15% of the input power goes into compressing air, is that what you meant to say? Not that I disbelieve you, it just seems exceptionally low.
That's true, the air will be saturated, what is the best way to dry it, drains and filters?
as for 10CFM free air delivery at 100psi requiring only ~2kw of power, trying to figure where the extra work done comes from. Sorry, trying to remember all my thermodynamics here!
I'd love to get a nice oil injected piston compressor but find they're very expensive, so trying to look at alternatives like engines and aircon units. And seeing if they could be adapted to the task for a considerably reduced cost?
Cheers for all the help and advice though, really do appreciate it!
My reason for using 3 phase was only motors running better off it, nothing to do with the high power draw. I know there are debates whether motors run more efficiently etc, but for the same power output, they tend to be cheaper for what I can see.
Will do that with the air receiver, I was thinking at least 150litres, preferably 200+
85% ineefficient or 85% efficient? 85% inefficient means only 15% of the input power goes into compressing air, is that what you meant to say? Not that I disbelieve you, it just seems exceptionally low.
That's true, the air will be saturated, what is the best way to dry it, drains and filters?
as for 10CFM free air delivery at 100psi requiring only ~2kw of power, trying to figure where the extra work done comes from. Sorry, trying to remember all my thermodynamics here!
I'd love to get a nice oil injected piston compressor but find they're very expensive, so trying to look at alternatives like engines and aircon units. And seeing if they could be adapted to the task for a considerably reduced cost?
Cheers for all the help and advice though, really do appreciate it!
I was originally thinking running a T piece above spark plug holes, have a check valve on both sides, one letting air in, the other letting air out. If I'm honest though, it probably would limit airflow a little and using the original valves would be ideal. But in a 4 stroke motor, the valves only open every other rotation which isn't ideal, so some major valve train modification would be required. The motor would be spinning at a much lower rpm though so hopefully would allow enough time for all the air to get in the restricted opening.
Gassing Station | Home Mechanics | Top of Page | What's New | My Stuff