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Old 03-22-2007, 10:26 AM   #1
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Pex as heat exchanger?

I was at Home Depot yesterday trying to get the last things I need for my veggie set up. I was gonna create a coiled copper heat exchanger and was flinching at the cost of copper. I saw this stuff called Pex which is a flexiable plastic compound made to substitute for copper. It's rated to 200 degrees and seems to bend well.

Unless there are any drawbacks I'm not aware of, it seems to be a good canidate for an in tank heat exchanger to move coolant. It costs $12 for 50ft of 1/2in oppossed to $55 for copper.

Any history or opinions?

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Old 03-22-2007, 12:02 PM   #2
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Pex is cool stuff. Find out how much they charge to rent the crimping tool for the day. My only concern with it would be whether or not it was resistant to oil, acid, and other nasties in the veggie oil. Of course parts of my system are made of rubber fuel line that isn't resistant to that junk so......
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Old 03-23-2007, 11:04 AM   #3
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I was on biodiesel.infopop forum, (lots of info) and some guys who are liscenced pex installers were saying that heating the pex a bit, not to the point where it's clear, but enough will cause it to expand and become pliable, after you put it over the barb it will contract and hold tight as hell. One guy said he had about 150 of those types of connections running it as floor heating in his house. No leak, but then again he's not getting it up to 160+.

I haven't worked with this stuff before but if it works it would be a great alternative to copper.
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Old 03-23-2007, 12:13 PM   #4
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When it comes to water circulating in a closed loop through my fuel, I'd prefer to use the code approved attachment method, but that's just me. Did I mention that I have a hard time with heating things up? I usually burn them, myself, or both.
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Old 03-28-2007, 06:01 PM   #5
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you say it's good to 200? you'll be running coolant through this? seems like you might be pushing the temp rating a bit.

copper is cheap compared to what could happen if you start injecting antifreeze into your motor.
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Old 03-28-2007, 07:15 PM   #6
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Well I figuered I'll set up the pex line, at least to test it for future reference. With the busses temp gauge reading 180-190 the pex got hot enough so that I can only hold it for about 4-5 seconds until it was to hot to hold. It seemed very strong and I tried to bend at the conections and It felt like it would kink if I really tried but not just from any average force. The connections are outside of the tank anyway, so the only way to leak coolant into the tank is if it burst in the pex line itself.

I need to get one on those Infrared Thermometers but the closest autozone is like 1 hour 15 mins away. They got one for $40, the only one I found in my area is $130 from napa.
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Old 03-28-2007, 07:51 PM   #7
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Old 03-28-2007, 08:13 PM   #8
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They sell those IR thermos here for $15 on sale sometimes. I'd look on eBay if I were you. Sure, shipping can be a killer, but if you can get one for $40 shipped without going to get it....it's that time-value of money thing.
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Old 03-29-2007, 07:35 PM   #9
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I would be concerned about what might happen if you start to over heat for some reason, you may push that temperature envelope. In general, electrical conductivity is a good indication of a materials thermal conductivity.
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Old 03-29-2007, 10:55 PM   #10
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Quote:
Originally Posted by asnowsquall
I In general, electrical conductivity is a good indication of a materials thermal conductivity.
That is a VERY good point.
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Old 04-01-2007, 07:44 AM   #11
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it seems to me that pex tubing, being plastic won't be very good at exchanging heat. This is good if you want to transport hot liquids over a distance ie: outdoor boiler, but is not so good if you want to use it as a heat exchanger.

why not use a heater core (small alluminum radiator) i have purchased several of them from the scrap yard for $10 each. I haven't done the math, but would estimate that they transfer heat 10x better than a 10 foot length of 1/2" copper tubing. Copper tubing must tranfer heat at least 10X better than pex tubing i would think.
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Old 04-03-2007, 03:38 PM   #12
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PEX is great for waterlines and infloor or radiant heating systems. But for heatexchangers copper is defenetly better. However I second lapeer20m with his idea of using an old small radiator, you will have way more transfer area thanks to the fins.

If you work with PEX pipes and you accidently kink one, just use a LP torch to heat it up and it will take up its regular shape again. No need of cutting out kinked pieces an joining them.

I'm not familier with Veggie oil setups so I dont know why you need this heat exchanger and where,but an other idea might be to have an external "FlatPlate Heatexchanger" http://www.flatplate.com/
I have one of those in my radiant heat system in my home, they dont take up much space and work great.
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Old 04-03-2007, 05:11 PM   #13
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The reason for the heat exchanger is to heat up the oil so that it flows easier. At 160 degrees F it is still about 3 times more viscous than diesel fuel from what I've read, but is thinned out enough that you shouldn't have problems. Thicker stuff can hurt parts and won't inject well leading to injector damage and coking.

My bus uses a flatplate heat exchanger and they RAWK for getting the oil up to temp for injection. My issue is with getting the oil to the heat exchanger so I'm working on heating my lines and tank pickup enough to get adequate flow to that heat exchanger. Pound for pound and volume to volume you can't beat a crossflow flat plate heat exchanger for efficiency.
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Old 10-08-2007, 02:27 PM   #14
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Re: Pex as heat exchanger?

I am wanting to make the conversion to WVO and am at the beginning of the entire bus conversion process. For heat in the living area I plan on a 6 gal water heater, circ pump and expansion tank with pex tubing and baseboard radiators. Now I also thought about doing a similar setup and run the lines to my WVO tank/s. The heaters are propane and the circ pump is 12VDC so I could start heating the fuel the night before and should have a tank that is nice and warm.

Instead of water, RV antifreeze (for winterizing RV tanks) since it will allow for higher temps if needed. I was planning on setting the temp at 140 to start and adjust from there. The heat exchanger will most likely be somesort of radiator inside the tank.

I figure that I can also run the pex next to the fuel line and insulate both of them to keep the fuel warm as it goes to the engine.

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Old 10-08-2007, 08:10 PM   #15
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Re: Pex as heat exchanger?

Well I have used the pex for the past 10K miles and it seems to have put up well with no signs of wear or breaking down. We'll see.
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Old 10-11-2007, 08:58 AM   #16
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Re: Pex as heat exchanger?

for my veggie heat exchanger, I was looking at using the one I took out of my rear floor heater in the bus. It's kind of big though and 14 years old. I'd hate for it to spring a leak in the tank. I thought about getting a new heater core from Auto Zone, but again, they sell those things because they spring leaks. I think the SAFEST thing to do would be to use coiled up copper line. I would NEVER EVER use Pex. A temp rating of 200 degrees? I'd be scared the whole time I was driving my bus. An engine at normal operating temperature runs about 195 degrees. Start climbing a hill or sit in traffic on a hot day and you've just exceeded your rating. Imagine that you spring a leak in your tank. Not only are you losing coolant, but you're pumping it into your engine. Now, you're on the side of the road with no coolant and jacked up injectors because you wanted to save a few bucks. In my opinion, this isn't an area where you want to save some money. Just buy the copper tubing.
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Old 10-11-2007, 11:38 AM   #17
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Re: Pex as heat exchanger?

The only saving grace I can think of when it comes to that 200 degree rating is that that is in ambient, uncirculating air. Basically what I'm saying is that hte oil is going to act as a much better means of conducting the heat away from the tubing.

For example...when cooling a computer fan blown air is the standard. For those running serious overclocks or otherwise needing to get rid of heat there is water cooling which runs FAR cooler, but still needs a radiator somewhere in the system. But what about those people that build computers in aquariums full of mineral oil? They have no heat sink other than the uncirculating oil and can run plenty cool (and quiet).

How about boiling water in a plastic bottle over a fire? Ever done that? I have. The plastic distorts, but it does not burn through as long as you do it properly and the #2 plastic used in a 20 ounce pop (soda or coke for you people in other regions) bottle is not nearly as tough or as thick as PEX.

I agree that it might be pushing the limits, but maybe that's what bus conversion is all about.
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Old 10-11-2007, 04:50 PM   #18
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Re: Pex as heat exchanger?

That's one limit I don't plan to test! If it had say a 450 degree rating, then I'd try it out. I'm sure that there is a margin of error built into that 200 degree rating, but when you're right at the materials limit, it's going to wear out pretty fast. I'd rather pay the extra $50 for some copper tubing than pay for a new engine.
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Old 12-03-2007, 09:59 PM   #19
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Re: Pex as heat exchanger?

Well 12,000 miles later the pex hasn't flinched. I agree that testing this material may be stupid, for my sake I hope not. I have fiddled with the pex when it is at it's hottest and it seems very rigid still. Not trying to advocate the use of pex just saying that mine is working......so far.
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Old 12-24-2007, 03:44 PM   #20
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Re: Pex as heat exchanger?

Might I suggest a length of stainless steel gas appliance connector hose inside your tank. Under $20 around here and a 5' length would I think give you plenty of heat in your veggie tank, installation not too difficult, they have regular 1/2" pipe fittings. Personally I worry about putting anything 'inside' the tank likewise HIH lines, one leak and you're pretty much gerfunked. I'm using a hotfox and a hose on hose bundle no chance of coolant in the veggie and with the hotfox you don't have to wait for the entire contents of the tank to heat up. Just my 2 cents.

Quote:
Originally Posted by dlor1
I was at Home Depot yesterday trying to get the last things I need for my veggie set up. I was gonna create a coiled copper heat exchanger and was flinching at the cost of copper. I saw this stuff called Pex which is a flexiable plastic compound made to substitute for copper. It's rated to 200 degrees and seems to bend well.

Unless there are any drawbacks I'm not aware of, it seems to be a good canidate for an in tank heat exchanger to move coolant. It costs $12 for 50ft of 1/2in oppossed to $55 for copper.

Any history or opinions?
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