Heat pipe heat recovery

Artman

Russ, or any of the rest of you mechanical engineers out there, ever use something called an "Energy recovery heat pipe?"

I don't mean the old pumped heat exchanger type. The ones I am talking about are manufactured by Heat Pipe Technology, Inc.

They use passive exchange of R22 from a coil on the inlet side of a cooling coil piped to another coil on the outlet side. They only work during cooling season. The incoming hot outside or mixed air is precooled by giving up its heat to the R22 in the first coil, the gas moves to the second coil where it gives up its heat to the cold air stream, reheating it, and cooling the R22 enough to drop it back to the first coil again.

These seem so much more energy efficient than either electric reheat coils or running a boiler all summer to dehumidify.

Has anyone out there used one of these in the field?

russ_watters

It seems too good to be true - 99.3% efficiency. Typical heat recovery systems run about 60-75% efficient. I'll look at it more later though (and maybe ask my boss).

Artman

I know what you mean. It does sound too good to be true. I have specified them for a job where I have 100% outside air and can't return the exhaust for a conventional energy recovery system. I'm looking for some cheap reheat because I am using the outside air for comfort cooling as well as lab hood makeup air, so it needs to be dehumidified and not too cold. If they do work it seems like the perfect solution. (The system has reheat duct coils as well for some individual room control, so I'm not putting all of my eggs in one basket.)

Thanks Russ.

wimms

Not sure Russ where did you find 99.3% figure and whether its meant for entire system or heatpipe efficiency alone. Being totally passive device its efficiency can't be very low. I guess the 0.7% loss is due to mechanical movement inside heatpipe. Whether its very efficient for heat transfer is another issue. But it doesn't hurt.

Heatpipe as such does work, and is gaining interest in PC's, due to its totally silent operation. Although its not as efficient in heat removal as rude heatsink and fan, it definitely has its advantages.
Sure its not miraculous, so don't get caught in some marketing twists. It helps to move heat from hotspot to where its easier to get rid of it without forced energy usage.

Its surely quite far from application you're interested in, but in case you've not seen it, here are few applications of heatpipe for PCs in reviews:
http://www.hardcoreware.net/reviews/review-76-4.htm
http://www.ocprices.com/index.php?ac..._id=178&page=3
I find that interesting.

russ_watters

Its in the brocure for the product. 99.3% is thermodynamic efficiency. Those heatpipes (AFAIK) do not have refrigerant in them. They work by simply conducting heat from one place to another. Not the same thing.

wimms

perhaps a link? Can't find it. All I can see is claims that hp makes AC upto 30% more energy efficient than without.

no-no. Its exactly the same heatpipe thing, refrigerant and all. Its actually licenced from heatpipe inc. They don't have cooling coils of AC, perhaps you meant that.

russ_watters

http://www.innergytech.com/pdf/thermogain.pdf Looks like I got the wrong company. Page 3 is the efficiency number I saw. Looking deeper into the site, there is an actual efficiency test sheet here: http://www.innergytech.com/En/html/p...eat_pipe.html#

The numbers look a lot more reasonable there.

I'm still not so sure - I looked at the links you gave, but they didn't have any technical data that I could find. And with only one pipe, I don't see how it could have circulating fluid. And at $40 - that would seem too cheap for something with a refrigerant in it.

wimms

Seems you haven't met the ideas used in heatpipe and assume general meaning of the word, while its very specific term registered as trademark.
I couldn't find any better sources for now, perhaps you'd find better details in their patent, but here is short description of the principle.
http://www.heatpipe.com/heatpipes.htm

Also, as more text from http://www.systemcooling.com/modules...ticle&sid=1430 which is also mere $29 device
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Heat pipe Technology

The CpuMate cooler uses two heat pipes to transport heat from the heatsink base up to the large surface area provided by the copper and aluminum fins. “So why use heat pipes?” you might ask. A heat pipe is a highly efficient conductor of heat. A properly constructed heat pipe has a very low thermal resistance, which is roughly independent of its length (unlike ordinary metal rods whose thermal resistance increases with length). Heat pipes are commonly used to transport heat from one location to another.

Heat pipes work on the principle of evaporation and condensation. A working fluid (frequently distilled water) evaporates inside one end of the heat pipe (the hot-end) absorbing heat in the process. A partial vacuum inside the heat pipe allows the water to evaporate at low temperatures. Once formed, the water vapor diffuses from an area of high vapor pressure (where it is being generated) to the other end of the tube where the vapor pressure is lower.

The vaporized fluid then condenses back to liquid (cold-end) and the heat is dissipated into the air from the metal fins. The working fluid returns to the hot end via capillary action thru an internal wicking structure (sintered metal coating, fine wire mesh, or grooves) so the heat pipe does not have to rely on gravity to recycle the working fluid.

The key to a heat pipe’s high efficiency is the latent heat of vaporization. One gram of water absorbs 540 calories of heat when it changes state from a liquid to a gas (without any increase in temperature). It then gives up this same amount of heat when it condenses back into a liquid. By contrast, adding 540 calories of heat to 100 grams of copper (small heatsink) would raise its temperature 173ºC!
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So, I understand that there are many different ways to implement it, but principle is about the same. Usually it relies on gravity to cause the liquid circulation, thus the requirement of placement at angles. It isn't any ordinary metal rod, that wouldn't simply work. And it isn't any classical refrigerant based heat pump either. But as its so small and so simple, I assume its also quite cheap.
As very general idea, it seems to be heat conductor that allows to transfer heat from high concentration areas where its difficult to get rid of to better suited place where large surface area allows more efficient heat exchange.

As you are dealing with cooling systems, your opinion on this thing is very interesting. I for eg didn't have any doubts that it works as claimed.

Artman

Wimms is correct this is the same technology, but is using distilled water in a partial vacuum as the refrigerant. The link offers a hint to the reason for the high efficency. It says it is because it takes advantage of the latent heat of vaporization. This makes sense.

tyroman

The heat pipe system described was put in use in the early 1970's during construction on the Alyeska pipeline which carries crude oil from the North Slope down to Valdez terminal in Alaska.

The heavy crude had to be heated to lower viscosity for pumping. To prevent the heated pipe from heating the pipe supports and melting the permafrost which would undermine them in above-ground segments of the line, heat pipes filled with amonia were mounted in each leg of a support. The finned (cold) end of each heat pipe can be seen in attached photo of the pipeline.

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mdk42

Not a bad technology, but I'm not sure about the 99+% efficiency. Heat pipe recovery systems do have one drawback--they only recover sensible heat. Active and Passive Dessicant systems, heat wheels, can recover both sensible and latent heat and can work in both the summer and winter.

russ_watters

The efficiencey clearly can only be for sensible heat. For HVAC applications, its nowhere near 99% total. And even a dessicant wheel can only give you like 80% or so.

Artman

The type of heat pipes I am talking about are wrapped around a cooling coil, they drop the temperature of the incoming air making the cooling coil more efficient at removing moisture, then the outgoing air is reheated by the heat pipe changing the phase back of the refrigerant.

These have no effect on heating. They require a temperature difference across the cooling coil to work. This form of energy recovery is useful in 100% outside air situations where you can't recovery energy from the exhaust air stream. Either because it is corrosive or contaminated, broken into many small amounts, or just too difficult to get the two air streams cose to one another.