Utilizing Condenser Water to thaw walk bridge 1

[hvacman]

A client has requested the feasability of diverting a 4" condenser water return main through a suspended crawlspace below an exterior pedestrian bridged crosswalk with the intent to force 85-90 deg. water through a manifold of fintubes (to provide radiant heat to slow paver freezing above)and back into the main building return. The area below the bridge is approx. 20ft x 30ft x 4FT clear. It currently is not a conditioned space. We could punch through the existing ceiling into this space however I am concerned whether an industrial fintube to fit this application even exists, and how much supplemental heat transfer can actually be made available? Location is Delaware (cold winters). There are re-insulation issues/freeze protection issues to also consider. Any input or suggestions would be appriciated.

 

[Rodale]

Try looking at it as a snow melt situation. What is the construction of the deck? How many BTU's are required? Is the condenser ever off line during peak weather conditions?

 

[MintJulep]

Delaware isn't that cold, and isn't below freezing for very many hours annually. Many of the hours that are below freezing will be late night and early morning.

How does the condenser duty match up against the hourly weather data?

Pump losses for pumping fluid through all this extra piping when it is above freezing would seem significant.

This would suggest looking at an isolated secondary loop for this.

My guess is that you will find that insufficient energy is available when needed, and that a dedicated direct fired snow melt system will be more viable.

 

[chicopee]

Lot to consider: backpressure, flow interruption, insulation,thickness of concrete, size of manifolds, material selection, corrosive effect of salt thru joints to name a few.

 

[ChasBean1]

I agree with Mint. When you need heat most, less will be supplied by a condenser water system unless you have some extraordinary cooling loads during the winter. Extra tubing and head loss could prove a cost burden versus a cost benefit.

 

[lilliput1]

Check energy codes. I think they don't allow snow melting systems. It may be better to put in a roof covered walkway. It would have the added benefit of protecting pedestrians from the rain. There would still be some snow accumulation from drifts.

 

[chicopee]

I dont think that I agree with you lilliput1. Hartford,Ct has district heating and I believe that some of the walkways around Constitution Plaza are heated.

 

[sterl]

You can certainly melt snow and warm the walk with 85-90 deg water but moving heat through air to the underside of an existing slab will need some pretty sweet design, considering the temperatures of the warm water.

Lots of data on the web about snow melt heat loads; I suspect in Delaware something like 110 BTU/ Sq Ft at design though I haven't researched it....But the grids that are normally employed have tubing at 6-12" centers, and that's cast in place...That spacing would correspond with using hot fluid temperatures of 130 to 160 deg.

As an energy justification in that climate, I would guess that the short duty proportion and the low temperature make it a bad investment...But that doesn't make it a bad project.

 

[tombmech]

MintJulep and ChasBean are correct. Even if it's available during the conditions you most need it, condenser water is most likely providing water-side economizer operation as chilled water. At the very least, if you're operating with condenser water at such temperatures, you're more concerned with freezing the condenser water itself - much less using it to thaw ice somewhere else.

Sufficient temperature differentials for ice melting leverage is questionable, regardless. Pull-down ratios and response become very important. Electric heaters are probably the best option unless a central steam plant is nearby, and that would still encompass many issues. A heat exchange process with a pumped glycol - freeze-proof - mixture is prudent.

Also, please excuse the further comment, but energy codes will never trump life safety issues.