Thermal engineering: Sizing a plate-type heat exchanger?

[Just a Sparky]

Brainstorming some ideas for constructing a poor man's compressed air dryer and aftercooler. One of the ideas which has stuck so far is buying a used plate-type heat exchanger on the cheap. Garden hose to supply naturally chilled water, compressed air in on one side, water trap on the other and voila: No more hot, wet air spitting out of my sand blaster after things warm up and less water getting into my air tank.

Only difficulty for me is determining how big of an exchanger I need for this job. Used parts are readily available from 12x5" 50-plate Kleenex-box sized stacks weighing 10 pounds to 24x7.5" 120-plate behemoths weighing over 100 pounds for reasonable prices. I've got a 10 horse single stage pump that moves about 35 ACFM. Figuring roughly 70% efficiency, that comes out to about 2.2 kW of waste heat being put into the discharge air stream which will have to be removed rapidly by the heat exchanger.

I don't have much experience with refrigeration systems. Could someone here offer some input on whether, say, a typical 12x5", 100-plate unit would offer adequate performance for this task? Or whether it would be too big/small? Looking at approximately 50-55*F water flowing at around 1/2 to 1/4 gallon per minute for a 10-20*F rise to room temperature if my math is correct. Guessing the pump discharges somewhere between 250~300*F. Just not sure how to work out my "approach" figure for various different sized stacks.

With a 1" hose feeding the stack, air will be jetting into it at around 80 miles per hour so I want to make sure whatever I hypothetically buy is going to have sufficient surface area for the task and not end up being pathetically undersized... nor an excessive waste of cash.

 

[technocrat]

Plate heat exchangers are are usually used in liquid-liquid situations. It is unlikely that you will obtain sufficient thermal coupling in a liquid-air situation to effectively exchange heat at high air flow rates. Plate-fin type exchangers may be a better choice.

 

[dian]

if you go to the setrab site they have a btu rating of thei oil coolers. that might give you an idea.

i have an internal document from them on how to calculate the performance. lets see if i can upload it.

 

[Just a Sparky]

I understand the reasoning for the smaller sized plate exchangers likely being inadequate to this task, but the larger size I described (23.5" x 7.5" x 13") offers some serious surface area and volume to slow down the air flow and divide it up. Figuring the useful surface of each plate at roughly 22.5" x 6.5" comes out to about 1 square foot per plate. That means a 120 plate unit would offer about 120 square feet of thermal interface. Said units are stamped with a volumetric capacity of 3.9 gallons for each loop. That comes out to about 0.52 cubic feet. So with a 35 ACFM pump, I would get 67.3 air changes per minute, or one air change every 0.9 seconds.

A 0.9 second dwell time over a 120 square foot thermal interface ought to be sufficient, no? That's roughly a 2~3 foot per second air velocity over the plates, +/-. Considering the traditional tube-and-fin type aftercoolers get away with much less active surface, push the air much faster through it and still manage a 25*F approach temperature with pumps twice-and-a-half the size of mine: https://www.grainger.com/product/4UJG5
That particular unit is advertised as pushing roughly 8.3 feet per second of air velocity, but judging visually by the size of the tubes, that figure seems extremely low for 100 CFM. A 1" NPT port as pictured will be pushing closer to 117 feet per second at only 35 CFM.

(Not ruling out that option either, but the appeal of cheap sub-ambient cooling & drying via air-water exchange is real. Sub-ambient air entering my air tank means zero condensate forming inside of it. Maybe a little on the outside, but that will evaporate on it's own.)

 

[Joe Gwinn]

For this kind of thing, the classic approach is two metal tubes thermally bonded to one another along their length, with coolant and coolee fluids flowing in opposite directions (counter-current heat exchanger). The thermal bonding is usually soft soldering. For reasonable size, both kinds of fluid must be liquid, not gas. If the intent is condensation, the tubes must be near-vertical, with a condensate tank at the bottom. A vertical spiral would also work. The condensate tank may have a float controlling a little drain valve to prevent overflows.

 

[Silent_Man]

Why not use something designed for the application?

This is what I used when re-pluming my compressor not too long ago. I mounted some fans to it since i didnt want to mount it on the belt guard.

Doesnt break the bank, and again, designed for the application.
https://www.akgcoolers.com/compressor_coolers/Belt_Guard_Aftercoolers.html

 

[Just a Sparky]

Larger size makes one challenge worse, not better.

Not an "expert" but all the plate-type exchangers I've ever dealt with were low-pressure goods. Flat surface-area thing?

How big can you go before your total pressure per square unit vs total square units subject to it is a problem for deflection and failure of a flat plate?

Whilst the tube and fin went to far higher PSIG, not only for refrig, but steam, oil & hydraulics. Round tubes are predictable goods, even at arbitrary length. Very predictable.

Not a lot of square air or hydraulic lines in general use, are there?

Go figure that under pressure they want to become round?

Looking at brazed-plate units rated for refrigeration service. I.e. condensing R-410A at 400 PSI. The one I'm looking at is good for 435 PSI and 437 *F. Plenty.



For scale, those are 2-1/2" NPT fittings.

Why not use something designed for the application?

This is what I used when re-pluming my compressor not too long ago. I mounted some fans to it since i didnt want to mount it on the belt guard.

Doesnt break the bank, and again, designed for the application.
https://www.akgcoolers.com/compressor_coolers/Belt_Guard_Aftercoolers.html

Because I'd like to achieve true air drying, not merely aftercooling. That requires sub-ambient temperatures prior to entering a condensate separator. An air to air cooler only gets close - some moisture still condenses inside of the tank with those.

 

[BT Fabrication]

I would personally get a refridgeration air dryer and be done with it. cooling with a plate chiller just builds up moisture that just gets fired into the tank anyways or restricts your air flow when it gets full.

 

[Just a Sparky]

I would personally get a refridgeration air dryer and be done with it. cooling with a plate chiller just builds up moisture that just gets fired into the tank anyways or restricts your air flow when it gets full.

Moisture trap. Exact same M.O. as a reefer unit.

Also, $300 vs $3,000 - plus the cost of an aftercooler since most reefer units aren't built for hot air.

 

[Just a Sparky]

I am pleased to report that I went forward with this project and it is a resounding success. Here are my findings:

• I used an Alfa Laval CB27-100H plate-type heat exchanger. (100 plates, appx. 12" x 4" profile.)
• Compressor is a Quincy 244; Single stage, rated 10 horsepower. 4" x 3.5" stroke duplex running at 900 RPM.
• Test was performed at 80 PSI
• Heat exchanger is configured in a counter-flow arrangement.
• A moisture trap is placed immediately after the heat exchanger and positioned slightly below it to encourage drainage.

• Ambient temperature: 76*F
• Compressor discharge temperature: 300*F
• Heat exchanger air inlet temperature: 190*F
• Heat exchanger air outlet temperature: 69*F
• Heat exchanger water inlet temperature: 63*F
• Heat exchanger water outlet temperature: 68*F

The top of the heat exchanger was too hot to touch. The top two inches of the exterior were dry. The remainder of the heat exchanger from that point downwards was chilly to the touch and was condensing ambient moisture much like a cold beverage. The air coming out of my test orifice was bone dry - none of the usual spits, spats or ice shards I am used to. Upon inspection after draining the air system, the moisture trap had indeed collected some nice chilly water which the heat exchanger had condensed.

Note that this test was conducted without any sort of aftercooler in the system. Hot air was being fed directly into the heat exchanger, which is something many of the more affordable refrigerated air dryers are not rated for.

Total project expenses were only marginally above that of a commercially produced aftercooler - about half went into the heat exchanger and the other half into fittings. The end result performs substantially better than an air-air aftercooler by achieving sub-ambient discharge temps and is at least half an order of magnitude both cheaper and more compact than a comparable high-temp rated refrigerated dryer. Granted one of those will drop the dew point much further, but hey... that's an extra grand (or four) worth of investment that my sand blaster isn't going to notice or appreciate. If I can get the same work done and save a few thousand dollars in the process... that's a success in my books.

 

[Joe Gwinn]

I am pleased to report that I went forward with this project and it is a resounding success. Here are my findings:

• I used an Alfa Laval CB27-100H plate-type heat exchanger. (100 plates, appx. 12" x 4" profile.)
• Compressor is a Quincy 244; Single stage, rated 10 horsepower. 4" x 3.5" stroke duplex running at 900 RPM.
• Test was performed at 80 PSI
• Heat exchanger is configured in a counter-flow arrangement.
• A moisture trap is placed immediately after the heat exchanger and positioned slightly below it to encourage drainage.

I'm not visualizing the physical arrangement. A rough sketch would be helpful.

• Ambient temperature: 76*F
• Compressor discharge temperature: 300*F
• Heat exchanger air inlet temperature: 190*F
• Heat exchanger air outlet temperature: 69*F
• Heat exchanger water inlet temperature: 63*F
• Heat exchanger water outlet temperature: 68*F

The air and water inlet and outlet temperatures suggest that this is not counter-current. The air outlet temperature should be close to the water inlet temperature. With a 6 F difference, not that it really matters - it's still cold enough.

The top of the heat exchanger was too hot to touch. The top two inches of the exterior were dry. The remainder of the heat exchanger from that point downwards was chilly to the touch and was condensing ambient moisture much like a cold beverage. The air coming out of my test orifice was bone dry - none of the usual spits, spats or ice shards I am used to. Upon inspection after draining the air system, the moisture trap had indeed collected some nice chilly water which the heat exchanger had condensed.

Note that this test was conducted without any sort of aftercooler in the system. Hot air was being fed directly into the heat exchanger, which is something many of the more affordable refrigerated air dryers are not rated for.

Total project expenses were only marginally above that of a commercially produced aftercooler - about half went into the heat exchanger and the other half into fittings. The end result performs substantially better than an air-air aftercooler by achieving sub-ambient discharge temps and is at least half an order of magnitude both cheaper and more compact than a comparable high-temp rated refrigerated dryer. Granted one of those will drop the dew point much further, but hey... that's an extra grand (or four) worth of investment that my sand blaster isn't going to notice or appreciate. If I can get the same work done and save a few thousand dollars in the process... that's a success in my books.

It certainly is a win. And this setup is essentially bullet-proof, so maintenance should be rare.

 

[Bobw]

Just for my own curiosity..

You are cooling this thing with water running from the tap? Correct?

Garden hose to supply naturally chilled water,

What's the water bill looking like this month?

 

[Just a Sparky]

Just for my own curiosity..

You are cooling this thing with water running from the tap? Correct?

What's the water bill looking like this month?

At a local rate of $2.71 per thousand gallons and a flow rate of 1/2 gallon per minute, that comes out to $0.08 per hour of blasting. The same cost in electricity to run a 750W refrigerated air dryer at the local rate of $0.12/kWh. The air compressor itself on the other hand is closer to $1.12 per hour. The cost of water is a drop in the bucket (pun) compared to the 10kW sucked down by a 10 horse induction motor after losses.

I'm not visualizing the physical arrangement. A rough sketch would be helpful.

The air and water inlet and outlet temperatures suggest that this is not counter-current. The air outlet temperature should be close to the water inlet temperature. With a 6 F difference, not that it really matters - it's still cold enough.

It certainly is a win. And this setup is essentially bullet-proof, so maintenance should be rare.

I can snap a picture some time. Pretty sure that was set up for counter-flow unless I had a bonehead moment and reversed the hoses. I taped each end of one to ensure I wouldn't do that. Entirely possible too that I'm just seeing a 5*F approach temperature since the air velocity is pretty high inside of this heat exchanger. 35 ACFM through the 1" nipples brazed into it comes out to about 80 miles per hour. There isn't much area in between each of the plates in this unit, so I'm guessing the total cross-section isn't too terribly far off from that.

Regardless it still does a pretty good job. I spent a good two hours blasting last night and discovered the moisture trap I installed has a self-draining feature; It must have pissed out a good six to eight ounces or so all over my floor. That was fun to clean up but at least I know it's definitely working.

 

[Joe Gwinn]

An afterthought: Sometimes domestic water comes with some sand et al mixed in. I ran into this some years ago when I had to replace a new water pressure regulator that died young. Mystified, I took the old one apart, and found it full of fine sand. Perhaps the town was flushing the water pipes then.

Sand could plug up the fancy heat exchanger, so it could be worthwhile to add a fine-mesh water filter before the heat exchanger. If I recall, the exchanger core is rated for something like minimum 1 mm particle clearance diameter, so the filter mesh should pass no larger than say 0.5 mm diameter particles.

 

[Just a Sparky]

I'm not visualizing the physical arrangement. A rough sketch would be helpful.

Here you go.

 

[Bill D]

Link to his heat exchanger.
Bill D

CB27 / CBH27 - Alfa Laval - PDF Catalogs | Technical Documentation | Brochure

pdf.directindustry.com

 

[Joe Gwinn]

Here you go.

From the three photos, I gather the following:

The hot compressed air comes in at the top, in the big long red hose, and cold compressed air leaves at the bottom, in the short big red hose to the water separator, and thence to the sandblasting booth. This is as expected.

The cold water enters and leaves in the thin yellow hoses with green sleeves, but I cannot tell which way the water flows. For counter-current, one would assume that the water should enter at the bottom and leave at the top. But I never did find an enginnering document that stated this explicitly. It may be thought to be obvious.

 

[Just a Sparky]

The cold water feed is from the bottom of the heat exchanger. Feeding from the top would prevent it from priming and result in a pneumatic lock. It simply wouldn't work that way. Some of the plates would get splashes of water, while the rest towards the back would stay dry.

Now when I'm done blasting, I do reverse the hoses for blow-down. I fitted a hose bib to the bottom of my receiver tank for the dual purpose of draining moisture from the tank and blowing down garden hoses in one smooth motion. Makes them easy to store and keeps my garage floor dry. They stay permanently connected to the heat exchanger, snaked behind all my machines and tool boxes along that wall. I simply coil the excess underneath one of the latter once I'm done blowing them down. Simple and easy.