Wrap around Heat Pipe 1

[walz]

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.

 

[EnergyProfessional]

you mean something like the dehumidifier heat pipe

http://www.heatpipe.com/

? Or which manufacturer are you looking at? The manufacturer should be able to tell you the operating range when you still have evaporation/condensing of the fluid. In CPU coolers it seems to work over a wide range. I guess this re-heat method is more needed when temperatures are lower, then i typically have humidity problems due to small cooling load. If it gets too cold (below heat pipe operating range) i guess it just doesn't do much anymore.

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).

 

[walz]

I asked the manufacturer, but they never came back.

I am using the their free software available online. SPC

http://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

http://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

 

[AbbyNormal]

I am curious why you would even need the reheat in your application, is this for a process, it surely is not for air conditioning

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[EnergyProfessional]

the ERV we have installed actually is a Semco unit. bu ti had worked with Greenheck rep before and since he is the rep for the heat pipes too, i hopefully get some good insights.

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.

 

[AbbyNormal]

To me you would use the heat pipe to enhance dehumidification. Walz's 115DB and 80 WB, is not a big humidity chllenge, it is not a very steep process line, it is mainly sensible cooling

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

 

[walz]

Abby ....

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

 

[AbbyNormal]

Manufacturers like Heat Pipe Technology have selection software that will let you see what happens when you change the outdoor air condition, but when there is less heat availble to remove when pre-cooling there will be less heat available to reheat.

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

 

[walz]

On the software .. that i have mentioned which iam using to select my heat pipe. Doesnt give me the performance at low at side temperature of an already selected heat pipe at higher temperature....

 

[Compositepro]

The boiling point of the refrigerant in a heat pipe is not really very important. It merely defines the broad operating temperature range. The pressure in the heat pipe will be the vapor pressure of the refrigerant at the cold end temperature. It can be above 10 atmospheres or below one atmosphere and it will still work. As for using it for humidity control, that is a completely separate issue that depends on many factors other than than the refrigerant in the heat pipe.

 

[walz]

Compositepro

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.

 

[Compositepro]

Normal "boiling point" is the temperature at which the vapor pressure of a fluid is equal to one atmosphere. This is completely irrelevant to a heat pipe. A heat pipe is evacuated of all air and all that is inside is liquid and vapor refrigerant. The liquid is always at its boiling point, because the only pressure in the heat pipe is the vapor pressure of the refrigerant. If the temperature of the heat pipe goes down then the pressure inside the pipe goes down. The basic principle of the heat pipe is that if the liquid end of the heat pipe (usually the lower end) is higher than the vapor end the liquid will vaporize (boil) and the vapor will condense at the other end. This occurs over a very wide range of temperatures, and has nothing to do with the "normal boiling point" of the refrigerant.

 

[walz]

Having a heta pipe selected. its giving pre-cool as well as reheat. Vapourized liquid is causing pre-cooling and at other end the same vapour condense back to provide reheat. There must be a temperature point dividing the two. Heat pipe can only work if both happens either me call it pre-cool / reheat or you call it vapourize / condense out.

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"

 

[AbbyNormal]

Think of it this way

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

 

[walz]

Abby

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)

 

[Compositepro]

A heat pipe is very much like a termosiphon, but more efficient, because the density difference between vapor and liquid is greater, and because the heat transfer coefficients of boiling and condensing are much greater than between liquid and solid.

 

[AbbyNormal]

walz

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

 

[toughguy1]

As Abbynormal explained, the heat pipe will work as long as there is a temperature differential. The most simple way to explain the benefit of using heat pipe is that the heat pipe moves the sensible heat ratio. It doesn't give you any free energy, but it does convert sensible cooling into latent cooling, so in effect, you get better dehumidification. The total capacity should stay the same.