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ChaChaMan (Industrial) (OP)
14 May 11 22:52
Hello,
Is it possible, with a heat pump, to do a Hot Gas Reheat, by diverting some of the Hot Gas from the Condensor to a reheat Coil. The goal is to respond to a call for dehumification without further cooling.

Please see attached diagram.

Equipment:
Heat Pump : (New-To Be supplied)3 Ton, 2 stage.
Controller : (Existing) Honeywell Vision Pro IAQ with dehumidifcation call capability.
Air Handler: (Existing) Custom built including plenum fan with VF Drive. The air flow can be varied.

Questions :
Can it be done ? or
Will this cause the heat pump to malfunction ?
Apart from the reheat coil, and 3-way valve, what other components would be required ?
Is there a manufacturer who manufactures such a Heat pump, ready for this of use.
What is the best way to execute such a design ?
DrRTU (Mechanical)
15 May 11 1:02
It can be done but why? You will need to control the supplemental condenser in the DA stream. I do not know of a small HP accu with the ability of HG to a  second condenser. Most larger application place condensers in series/bypass. Oil logging is an issue. Sporand has ORD/ORI valves you may want to review. You may want to look at a Carrier moisture humidi-mizer design on a CV rooftop to see the complex dx piping and controls. I have used an APR on small systems to improve dehumidification http://rawal.com A standard evap. coil will have poor latent performance. I would slow the air flow down, use electric reheat or a supplemental dehumidification unit before trying to utilize refrigerant re-heat on a small HP system.
walkes (Mechanical)
15 May 11 8:18
Water-to-air heat pumps have hot-gas reheat as an option for humidification control.  (Climatemaster, Florida Heat Pump).

One option though would be to use a heat pipe around the AHU coil.
http://www.heatpipe.com/ProductsServices/forengineers/engineer.htm

review their dehumidifier heat pipes.
ChaChaMan (Industrial) (OP)
15 May 11 15:34
DrRTU, Thx for the RAWAL suggestion...Therefor you are saying the RAWAL device + an electric coil c/w SCR is the more efficient solution to build and operate ? The idea of recouperating the heat was enticing, but if I understand you correctly, you are saying it's too complicated to properly execute ?

Walkes..Thx I already looked at heat pipes, they are very compelling, but I don't have the space in this case.

Thank you all for your insight !
DrRTU (Mechanical)
15 May 11 23:28
Did you perform a detailed load calculation? With HAP, you can specify the room RH and see the reheat required. Will the internal loads vary from design? Is this process related or just comfort cooling? Can you give us your design conditions and basis of design? Based upon a 3 ton system, the room should be very small. Many codes /standards prohibit non cycle rh for comfort cooling.
ChaChaMan (Industrial) (OP)
16 May 11 10:11
DrRTU this is a comfort cooling case in a private residence for elderly people.

What do you mean by 'Many codes /standards prohibit non cycle rh for comfort cooling. ' in this situation?

Back of the envelope calcs :
In dehumidification mode, run only first stage compressor. Therefor : 67% x 3 Tons = 24 000 btu(approx) of cooling.
At that capacity, run the blower at 800 cfm.
The reheat required would be that needed to maintain discharge air at 75 F. Therefor assuming a discharge air at the evaporator is 50F, The required reheat would be : 1.08 x 800cfm x (75F-50F)=21600 Btus...approx = to the cooling capacity.
(Difference could be explained by latent cooling perhaps)
I would therefor use a 6 KW electric coil c/w an SCR , with a temp sensor in the supply air stream to ensure the required reheat.

Is this better than a Hot gas reheat ? And is this good, or generally acceptable practice ?

Sincerest regards

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