Wrap around Heat Pipe
Wrap around Heat Pipe
(OP)
I am using the pretreated fresh air handling unit to dehumidify outside air for cooling.
I've designed the cooling coil with heat pipe wrap around.
My design figures for coil and heat pipe selection are as follows.
Air enters the heat pipe pre-cool section at 115/80 (DB/WB)deg F. After Pre-Cool (sensible free cooling only), air is at 95/75 (DB/WB).Then after cooling coil air comes out at 55/54 (DB/WB). and finally after the reheat of heat pipe (sensible heating), the air becomes 75/61.6 (DB/WB).
Sensible (free reheat)
My question is that, heat pipes removes sensible heat from the raw fresh air because of a refrigerant gets vopourized inside the pre-cool section. The amount of refrigerant vapourized (and hence the sensible cooling removed) depends upon the temperature of fresh air. Msx = 115 deg F
This fresh air temperature wont remains the same throughout the day. Now what will happen when the temperature of air comes down. will this heat pipe still able to pre-cool.
while selecting the heat pipe using a selection software or whatever, we never know what's the boiling temperature of refrigerant inside the heat pipe. which cause pre-cool and reheat.
I've designed the cooling coil with heat pipe wrap around.
My design figures for coil and heat pipe selection are as follows.
Air enters the heat pipe pre-cool section at 115/80 (DB/WB)deg F. After Pre-Cool (sensible free cooling only), air is at 95/75 (DB/WB).Then after cooling coil air comes out at 55/54 (DB/WB). and finally after the reheat of heat pipe (sensible heating), the air becomes 75/61.6 (DB/WB).
Sensible (free reheat)
My question is that, heat pipes removes sensible heat from the raw fresh air because of a refrigerant gets vopourized inside the pre-cool section. The amount of refrigerant vapourized (and hence the sensible cooling removed) depends upon the temperature of fresh air. Msx = 115 deg F
This fresh air temperature wont remains the same throughout the day. Now what will happen when the temperature of air comes down. will this heat pipe still able to pre-cool.
while selecting the heat pipe using a selection software or whatever, we never know what's the boiling temperature of refrigerant inside the heat pipe. which cause pre-cool and reheat.





RE: Wrap around Heat Pipe
I'm having a problem that we have a geothermal system with heat pumps over the building. The ERV does not have a cooling coil and we have high humidity, so that people set thermostats to 68F. I now want to add a cooling coil to the ERV and was thinking to add such heat pipe as well. I see their rep is the same one as for Greenheck I'll ask them. One of my concerns is the actual heat transfer rate the heat pipe can do. and the pressure drop, if that makes it worth it. since I have an existing fan that might be a problem. they also seem to have a ready-cooling coil with heat pipe (combination coil).
RE: Wrap around Heat Pipe
I am using the their free software available online. SPC
www.spcoils.co.uk
and by the way, i am also using the same ERV you mentioned with cooling coil + heat pipe. there i have this same concern. We do have a local representer of Greenheck here. But they supply fans only. Yes their ERV is is without cooling coil.
This combination of Ehnthalpy wheel + cooling coil + heat pipe, i am looking since long time one single manufacturer to provide complete package.
I attended one seminar of VTS Air Handlers and Recovery units.... where i learned that they are using this combination with online selection tool. Got excited
www.vtsgroup.com and then goto climaCAD online..
upon chosing the cooling coil with combi heat pipe combinaiton, the software was not giving results...
Unfortunately problem stays there.,....
The reason was that these VTS people were also getting HEat pipe from other supplier. and they never returned with an answer.
Somebody once told me that heat pipe manufacturere's dont declare their refigerant and its boiling point. But its a damn good thing for coil and energy consumption
RE: Wrap around Heat Pipe
The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install
RE: Wrap around Heat Pipe
The reheat probably is not that useful since air being a bit colder for fresh air doesn't hurt. But that reheat, is what enables pre-cooling of air before it enters the cooling coil. with that the air gets colder int eh cooling coil and more moisture is removed. I think the heat pipe is a way to change the sensitive/latent heat removal property. Total energy removed stays the same.
It is nothing different than sub-cooling with DX, and reheating with hot water. But it is much more efficient. but for the air stream it is the same.
RE: Wrap around Heat Pipe
I think a simple HRV pre-treating Walz's outside air would be a lot more effective
The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install
RE: Wrap around Heat Pipe
I am using two kinds of Pre-treating ..
One which you told ...using HRV (Enthalpy Wheel) where i have to have considerable extract to recover the energy
But sometimes i have no extract but still need the pretreated outside air.... for that i am using cooling coil with heat pipe..
HerrKaLeun... Total energy isn't the same in both cases.
Please find attached the chart for the process. It is air-conditioning purpose.
The purpose is to bring the OA at 115 to Off-coil (dehumidification between OA and room set point which is at about 50% RH and 73-75 F.
Now cooling coil load without heat pipe would have been between OA and Off-Coil.... which is reduced (due to free sensible cooling) to On-coil to Off Coil.
and i dont bother about reheat since this pretreated fresh air 75 DB and 50% RH is the return condition of my room recirculating AHU. Since both air streams are at the same condition, AHU has no ventilation load to bear. it is just taking care of the room loads.
Now we have deviated the topic actually. the original thread was about the case when outside temp is low. will heat pipe be able to do sensible cooling....say e.g.,
at 100 outside air temp, what will be the amount of free sensible cooling, will my Fresh air cooling coil load be increased. I am confused about how to size this cooling coil
Regards
RE: Wrap around Heat Pipe
I guess you want to try and deliver 'neutral ventilation air' directly to the space and want some free reheat on top of what something like a draw through fan gives you.
But if you are worried about how much reheat you will get when it is only 100F outside, I think you are still running cooling so I would probably just use an HRV to transfer sensible heat out of that fresh air and just let the cooling coil itself pull out the small latent laod that air will add. I would tend to mix the fresh air into the return air myself, but then again sitting here far away on the internet, I do not see your big picture.
The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install
RE: Wrap around Heat Pipe
RE: Wrap around Heat Pipe
RE: Wrap around Heat Pipe
I agree that heat pipe manufacturer's play with the vapour pressure to vary the boiling point inside. and yes there has to be a range...
Now that range is also critical. Go back to my process points. 115 is the max temperature, where refrigerants is being boiled and making precooling. When air comes out of cooling coil at 55, it is again condensing the refrigerant vapour back ... thats what gives me the reheat. It means that this refrigerant has a boiling point somewhere higher than 55.
Now lets assume that in summer i always have an off coil temperature of 55 deg F
living in a perfect world...let us suppose that this refrigerant gets boiled at say 90F.
so this means that any temperature at or higher than 90, is gonna give me the pre-cool and my chilled water cooling coil load will also be smaller. since any way i have to have off-coil of 55.
but if this boiling point is 100 and my outside temp is 98 ... now i will not get the pre-ccoling and my chilled water cooling coil will be undersized.
all i need is to know the refigerant boiling point so that i can analyze + economize my chilled water cooling coil for all cases.
RE: Wrap around Heat Pipe
RE: Wrap around Heat Pipe
since the pressurse of that liquid refrigerant cannot be changed and the only pressure is the vapour pressure. there is a boiling point though i agree its not the normal boiling point. Boiling point is the temperature where vapour pressure becomes equal to atmospheric pressure. IF this atmospheric pressure increases / decreases so do the boiling point respectively.
Since the refriegerant inside is getting boiled, and it is condensing back, there must be a point called "whatever boiling point"
RE: Wrap around Heat Pipe
Get a 30 poung jug of refrigerant and connect a pressure gauge to it.
Let it sit in a conditioned sapce for a couple hours and observe the pressure and look up its saturation temperature that corresponds to that pressure.
Then place the jug outside. Assuming it is up to your triple digit ambient conditions, keep an eye on the pressure in that jug, its going to increase. The saturation temperature or 'boiling point' will also increase.
Your heat pipe is going to respond in a similar manner and as long as your entering air condition is warmer than the leaving coil condition you will get pre cooling and reheat
Heat pipes can also be used for heat recovery. In winter they can warm up oustide air be transfering heat from warmer exhaust air. In summer they can transfer heat from warm fresh air to the cooler exhaust air. What you have to do to change from summer operation to winter operation is to change the tilt of the heat pipes.
I suggest you get on heat pipe technology's website, and pick out a heat pipe similar to the one you selected for 115F air using their selection software. Then, once you have a comparable heat pipe selected, similar rows deep etc, alter the entering air condition to 90F or whatever temperature you are concerned about and see what happens.
The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install
RE: Wrap around Heat Pipe
what i get from you is that .... as far as heat pipe's pre-cool side and reheat side are at different temperatures, pre-cool and reheat is going to happen, whatsoever these temperatures are. Cuz its a kind of differential that it needs across both the ends. If that's true than what compositepro keeps on telling is also valid.
But then it turns out to act like a thermosiphon, which has no boiling point issue. it is just working beacuse it has two differnet temperatures at both of its ends..
What you suggested me to select a heat pipe at desired ambient and then see the response at lower temp. I have been trying to do this since long, but the software which i am using doesnt have that option in it.
But one thing i have noticed is that when the temperature on the heat pipe entering side gets down the total delta T across pre-cool as well as reheat (pre-cool delta T = reheat delta T) also dercreases much.
How can we explain this behaviour. This obviously means that at relatively lower temperature heat pipe is producing less pre-cooling and less reheat ...( total energy = 0)
RE: Wrap around Heat Pipe
RE: Wrap around Heat Pipe
I am saying perhaps check this company
http://www.heatpipe.com/
they have selection software, I have an older CD with it but perhaps if you click on the engineering manual link, and then register with them, you can maybe get the software.
Then pick out a similar performing heat pipe using their selection tool, then alter parameters and see what happens
The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install
RE: Wrap around Heat Pipe