Fan Coil Units & Reheat Cycle

[JOrozco]

I plan to have fan coil units operating in a "Reheat Cycle) for the purpose of controlling humidity. Basically, I plan to cool down and de-humidify the air mass as it goes thru the cooling coil of the fan coil unit and then reheat the air mass as it goes thru the heating coil of the unit. I have both chilled and hot water available throughout the year. I'll be happy for any suggestions regarding this particular application for fan coil units.

Thanks,

Mike Orozco

 

[bigfoote]

I have a related question. Can you use fan coil units with chilled water? If you did would you remove any Humidity? What would the chilled water temp have to be to have any effect? At a glance I would think that you need to have a higher flow through a fan coil unit used for cooling to get results. I've heard of people using a large radiator out of a Cat. and a buried tank of water for this. I am wondering how large a tank you would have to bury so the return water would not warm the tank water? I was thinking of setting up something like this in my shop I have a hydronic heat system with a fan coil unit on a seperate stat. I thought I would probably need a larger fan & coil unit to get any effect. this turned out to be quite longer than I intended. If this is an inappropriate post please let me know. Thank you

 

[ChasBean1]

Mike, I have no problem with your plan - this is common for dehumification and temperature control. The key to do it on this small scale is to cool to a controlled temperature (at the outlet of the cooling coil but prior to the inlet of the reheat coil) as needed for dehumidification, which is typically a 55°F target air temperature. Then you reheat as needed to satisfy the space thermostat.

 

[quark]

Yes, this is a standard system and hot water control is better compared to steam heating. But The minimum humidity you can maintain is 50% at 22+/-2 deg.C. Can't give you any equation or reference but it is my experience. Below that limit temperature shots up abruptly.

Regards,

 

[lilliput1]

Control the cooling coil chilled water valve with a room humidistat to open when Room RH is 50% (adjustable)& above & to close if room RH is 40% (adjustable) below. Valve should modulate between 40 & 50 RH.

Control the heating coil hot water control valve with the room thermostat to close when temperature is 75deg. F (adjustable) and to open when the temperature is 73°F and below (adjustable). Valve should modulate between 75 & 73 degrees. If larger temperature swing can be tolerated, consider closing valve at 75 & opening at 70.

 

[JOrozco]

Thanks you all for the helpful suggestions, they are appreciated.

Mike

 

[ChasBean1]

lilliput, your post got me thinking... say the room starts with no reheat and no CHW, and is at 45% RH. With a constant moisture level in the air, and when temperature starts to drop, the room RH goes up. The CHW valve opens to further reduce temperature but could also further increase relative humidity (with a constant moisture content, temperature drops, RH rises). When the room gets too cold, the reheat comes on. I would be concerned that both would run full-bore perpetually during dehumidification.

Initial condition repeated: Room starts with no reheat and no CHW, and is at 45% RH. With a constant moisture level in the air, and when temperature starts to RISE, what's to stop the rise? When temperature rises, RH goes down so then your cooling source would be disabled.

The controls would be interlocked. Changing one would affect the other, and vice-versa. This could turn into a one-foot-on-the-brake and one-foot-on-the-gas control that could lead to disastrous energy use during dehumidification and no cooling when needed.

If the CHW (or DX) always maintains a 55°F outlet temperature prior to the reheat coil and the HW controls as necessary to satisfy the room thermostat, I think that control would prevent these problems and perform better.

 

[lilliput1]

Definitely constant volume reheat gives you better control but this is energy extensive. I have used a McQuay DX constant volume aircooled rooftop unit with electric reheat to serve an auditorium with large amount of OA for ventilation. I specified the controls as in my post above. The sales rep gave me a hard time. I told him it is a matter of DDC programing that they should provide because this type HVAC requirement is typical. They should configure their units to meet the need. This situation reminded me of when DX VAV AHU first came in use and the problem was that at low loads the DX coil froze. Eventually the industry developed control to sequentially reset the discharge air temperature after the fan speed had turn down to the minimum CFM, about 250 FPM coil face velocity, to solve the freezing problem. So the industry should work on the application needed for spaced with high outdoor air requirements. He made calls to the factory and found out that McQuay do have a dehumidification Control Package Option that works in the same manner as that specified for the job.