Shop Air Compressor Aftercoolers

[mikedrom]

I’m working on a new air compressor build for my garage, and I want to add an after cooler between the pump and tank of my air compressor setup. Dozens of creative folks have uploaded pictures of their intricate coolers—pipes spanning entire walls, coils submerged in Home Depot buckets, etc.—but for my less-hardcore use, I’m thinking something much simpler could work: a furnace water-to-air heat exchanger with an electric fan strapped to it.

The temp, pressure, and CFM ratings for these all appear to meet what any shop compressor can muster up, but I’m assuming there HAS to be a reason I haven’t seen people using these. Dedicated aftercoolers for air compressors are also at least 10x the cost of these, so I’m guessing I’m missing something.

Someone want to talk me out of this idea before I accidentally blow myself up?

 

[DaveInDenver]

@mikedrom, I have no first hand experience with doing what you're thinking but a couple of things jump to mind.

Watch safe working pressure, pressure drop and flow rate. I'd also verify that materials are compatible. If those all work then I don't see why it would be a problem.

Can't imagine anything in compressed air is different than you'd see in boiler water or steam. You install compressed air lines using threaded iron pipe, soldered copper or PEX plumbing so not sure why a heat exchanger would be any different.

The question I would have is let's say you use a fairly large one, say 36" x 36", and it's got 15 passes and two layers, made with 1/4" pipe. That might be 90 feet of 1/4" pipe and I'd think that would have some impact depending on the circuit. They'd put manifolds on each side so the path isn't through the whole length but rather 30 parallel 36" tubes if they're not worried about a phase change. But one designed for condensing might not use manifolds and is one long tube.

Just thinking aloud.

 

[ScaldedDog]

That's designed to cool pressurized water, right? Pressure is pressure, but there is way, way less energy stored by water under pressure than by air under the same pressure. In a failure water will leak. Air may tear the thing apart.

Mark

 

[mikedrom]

That's designed to cool pressurized water, right? Pressure is pressure, but there is way, way less energy stored by water under pressure than by air under the same pressure. In a failure water will leak. Air may tear the thing apart.

Mark

You have a good point. These exchangers are “rated” (however you interpret that) to a claimed 375psi, which would be almost 3x the pressure produced by the compressor. If it does blow, though, it would definitely go boom.

 

[DaveInDenver]

That's designed to cool pressurized water, right?

Depends. Some are just exchanging heat from steam or hot water to air with no phase change. Some are condensing steam, too.

Pressure is pressure, but there is way, way less energy stored by water under pressure than by air under the same pressure. In a failure water will leak. Air may tear the thing apart.

The forces on the pipe aren't significantly different in practical application. Pressure exerted by a liquid may be greater than by gas due to increased density but it takes more work to pressurize gas than liquid.

Pressure exerted by a liquid is different to gas?

Hello everyone, I have made number of topics recently I think they have all stem from my poor understanding in this matter. Ok in a gas, molecules can take up a volume and exert a pressure. Inside a fixed container when you increase the temperature pressure increases because more gas molecules...

www.physicsforums.com

Case in point, typical vessel and pipeline testing is hydrostatic, where you fill the system with dyed liquid water and pressurize. This is how your welding gas cylinders are certified. Water is used because it's incompressible and usually not reactive to much so it's the lowest risk way to do it.

 

[ScaldedDog]

What made me think of it is guys using PVC pipe for air lines, since it's rated for the pressure. It's a great idea until it isn't and a failure occurs, and PVC shards are sent flying all over the place. Burst PVC water pipes don't do that. A burst radiator wouldn't be as bad, but I wouldn't want to be in the vicinity if it happened.

Mark

 

[ScaldedDog]

Now that I'm looking at his product photo on my PC and not phone, it appears it's just copper tubes with fins. If that's the case, it's probably not any different than copper tubing when it bursts.

Mark

 

[DaveInDenver]

What made me think of it is guys using PVC pipe for air lines, since it's rated for the pressure. It's a great idea until it isn't and a failure occurs, and PVC shards are sent flying all over the place. Burst PVC water pipes don't do that. A burst radiator wouldn't be as bad, but I wouldn't want to be in the vicinity if it happened.

Mark

Yeah, PVC is a bit unique. It's not a pressure problem, in sizes you'd normally see in compressed air it's just fine. It's actually better than copper on paper. In 1/2" SCH 40 PVC is good to 1900 psi operating while plain drawn Type M copper is only 825 psi working. Although copper burst is very high in reality, too.

PVC Pipes - Pressure Ratings vs. Size

Maximum operating and required burst pressure of PVC - Polyvinyl Chloride - pipe fittings.

www.engineeringtoolbox.com

Copper Tubes Type K, L and M - Working Pressures vs. Size

ASTM B88 seamless copper water tubes - working pressures.

www.engineeringtoolbox.com

The problem you run into is preventing damage, a nick or cut changes everything. Temperature does as well. Those PVC ratings are 23°C/73°F. At 60°C/140°F you have to derate to 20% of capacity. That's what gets you mostly. The air you compress is heated and can be quite significantly so with some compressors.

Also aging affects PVC and the fittings do not share the same capacity necessarily.

Anything can explode and shrapnel from a copper or iron pipe isn't better. It's just that PVC systems are less robust. Underground PVC is allowed for transporting compressed gases. It's just above ground that it is not.

 

[mikedrom]

@mikedrom, I have no first hand experience with doing what you're thinking but a couple of things jump to mind.

Watch safe working pressure, pressure drop and flow rate. I'd also verify that materials are compatible. If those all work then I don't see why it would be a problem.

Can't imagine anything in compressed air is different than you'd see in boiler water or steam. You install compressed air lines using threaded iron pipe, soldered copper or PEX plumbing so not sure why a heat exchanger would be any different.

The question I would have is let's say you use a fairly large one, say 36" x 36", and it's got 15 passes and two layers, made with 1/4" pipe. That might be 90 feet of 1/4" pipe and I'd think that would have some impact depending on the circuit. They'd put manifolds on each side so the path isn't through the whole length but rather 30 parallel 36" tubes if they're not worried about a phase change. But one designed for condensing might not use manifolds and is one long tube.

Just thinking aloud.

Yeah, I’m thinking efficiency may drop with the exchanger. Most of the folks online use one continuous run of copper pipe that’s either coiled or spread over a huge distance. This exchanger is designed to radiate heat, not necessarily cool/condense the contents inside. While those two are essentially the same processes, I think that design to optimize one or the other plays a factor.

 

[AimCOTaco]

Yeah, I’m thinking efficiency may drop with the exchanger. Most of the folks online use one continuous run of copper pipe that’s either coiled or spread over a huge distance. This exchanger is designed to radiate heat, not necessarily cool/condense the contents inside. While those two are essentially the same processes, I think that design to optimize one or the other plays a factor.

This is the way I think for home, just a long pipe run and well placed drains, no fans to fail.
Forced air over a heat exchanger in outdoor or shop conditions requires lots of maintenance in my experience.
I also know guys who have seen the PVC pipes explode when the boss used them for compressed air, definitely don't do that.

 

[IoN6]

I run a dryer between my compressor and machines in the shop. The cooling element appears to be all aluminum and of similar construction as the one pictured. Does not seem to be anything more than a mini-"fridge", though I am sure there is more to it than that.

Refrigeration Dryers

Refrigeration drying is the most cost-effective technology for the drying of compressed air. First choice for applications with pressure dew points of + 3°C

www.beko-technologies.com

 

[mikedrom]

I think I’ll try it out. If it doesn’t work, I’m only out a hundred bucks. I’m thinking of installing two pressure gauges—one before the exchanger and one after—to make sure the exchanger isn't a bottleneck in the system. I’ll measure the temp change before and after the exchanger as well to gauge how efficient it is. I may end up building a submerged coil to cool instead of this doesn’t work great (or honestly may go with that first if I can find room for it in my extremely tight shop…)

 

[60wag]

What kind of equipment will use the compressed air? What CFM is the compressor cable of producing? Will the compressor be running constantly?

 

[mikedrom]

What kind of equipment will use the compressed air? What CFM is the compressor cable of producing? Will the compressor be running constantly?

Air tools, paint guns, plasma cutter. Compressor pump is 15CFM @ 90psi. It’ll see weekend use mostly.

 

[60wag]

I doubt the temperature of the air will affect your needs much, water and oil in the air might. I'd run the air into the receiver tank to let it cool a bit and have most of the water and oil drop out in the tank, then add a decent coalescing filter (big) and a particulate filter on the exit from the tank. Maybe an additional trap at the point of use just to see if anything is making it through the main filters.