Ceiling Radiant cooling/Heating

[DDaneshi]

I'm planning to use waste heat and some redundant cooling from a refrigeration system (Water Glycol-40%) for an office building. I'll go with radiant ceiling to get dual function, but I'm not sure about the temperatures of cooling and heating liquid.
- Should heating temperature for ceiling panels be higher compared to floor heating.
- Does heating liquid temp depend on room height
- What temp should I use for cooling.
- Does WG mix vs water have a big impact on heating/cooling performance?
Thanks,

 

[BronYrAur]

Too many unknowns here to offer any practical advice but I will throw out a few things.

Yes, ceiling radiant panels will generally be hotter than underfloor systems. Cooling in a radiant panel can only be sensible. You will have no way to remove condensate, so water temp will need to be higher than a chilled water coil. This type of system does not bring in any ventilation air, so you have that whole issue to address. Adding glycol to the fluid reduces heat transfer, reduced heat capacity, and increases viscosity. This is particularly a problem with cooling. Not so much with heating but it can't be ignored.

 

[DDaneshi]

Thanks Bron,
Let me give you some numbers, the primary heat/cold source is refrigeration engine room that we're designing:
- I can get a high grade heat as water of 120 F from superheated refrigerant HEX. (around 600 MBH)
- Condensing temp on my refrigeration system is 95 F, which shall be WG mix. I have plenty of this low grade heat.
- For cooling, I can have an HEX and get whatever temperature required for cooling ranging from 20 F to 50 F.

Another question, what should be liquid velocity in pipes for heating and cooling systems? And what's temperature difference on supply and return lines on each manifold?

 

[BronYrAur]

These questions are all job specific. There is no one answer. I try not to exceed the 7 or 8 feet per second range on fluid velocity, but you need to account for friction loss. Delta-T is another wide open question. Many chilled water systems have a 10 deg delta, but some as high as 20. Hot water routinely have 20 degree delta's, but many systems have larger temp drops.

I don't know where you are located, but 120 deg water would not be hot enough in my neck of the woods.

 

[DDaneshi]

I'm in Toronto, Canada.
So, you mean the numbers are very close to conventional heating/cooling systems.
What if I increase the velocity and minimize temp difference, I'm hoping to maintain relatively constant surface temperatures on panels and work with lower temperatures.
Many panel manufacturers claim that 95 F is a good temperature for heating especially for ceiling radiant systems.

 

[BronYrAur]

Again too many variables to give any real answer. Although that temperature may work for the core office area it may not be enough for the perimeter. And you still have the outside air to address that will need to be heated. It's great that you have a heat Source readily available but you might need to supplement it with something. Latent cooling is a big challenge as well as ventilation Cooling. I'm not trying to dismiss your idea but there's a lot of engineering required to make it work

 

[GMcD]

What BronYrAur said- from what you are asking, it seems like you are trying to rationalize the use of the heat and cool source, and then try to make the radiant system work based on your sources of heating and cooling. I've been designing and operating radiant systems (slabs, panels, floors and ceilings) for over 17 years and the first thing you do is check the room loads and cooling and heating requirements. In an office building, the interior zones are generally cooling only all year around, and then the perimeter zones are either heating or cooling depending on the season. Radiant cooling from the ceiling is most effective (and heating too), but radiant cooling has a hard limitation on effective comfort cooling capacity of about 25 btuh/SF of panel area or 75 watts/sq. Meter, because the surface temperature of the radiant cooling panels (whether a slab surface or suspended radiant panel) must stay above the ambient dewpoint (usually around 62F to 64F minimum). You say you are in Toronto - you better have a really good air system with dehumidification to keep the indoor dewpoint below 59F or so (check your psych chart - 74F indoor dry bulb at 60% RH is about where your maximum aiming point will be). With the limitation on the radiant panels - you my need large ceiling panel surface areas to get enough cooling at the perimeter zones if it's typical double glazed curtainwall windows and no exterior solar shading. You WILL need supplemental air side cooling for that. Or, reverse-engineer by beating up the architect to get the window to wall ratio down, get exterior solar shading, and use triple glazed windows to get the cooling loads down to within the effective "output" of the radiant cooling panels.

Heating from radiant panels is dead easy and what you need to do is size the perimeter radiant panels to get your maximum effective area for cooling (the worst case) and then back-calculate what your entering heating water temperature should be for that panel area, based on the manufacturers data.

That's a very quick summary of what to think about for a starting point - you need to do a heck of a lot more design research to understand how to design and operate a radiant cooling system.

 

[SAK9]

If the radiant cooling system can not cope with 100% of the sensible load in the space, the air system will need to step in and then the temperature control gets tricky. How do you control room temperature then? Assuming you assign temperature control to only one system, which of the two systems will do it and why? Ventilation rate will be decided by a lot of a factors such as latent load, extent of sensible cooling required, and supply temperature of ventilation air.