Ice melting system

[m2e]

I am working on a project where there is a water filling station up north. The station is covered by a canopy, but the ambient temperature is about -40C plus some wind speed too. We have to design a heating system to melt the spilled water or rain water on the slab. Apparently we have three options: hydronic slab heating, electrical slab heating and overhead infrared heater. Gas is the cheapest so hydronic seem to have lower operating cost, but the installation cost is much higher; plus we don't know how responsive the system would be. Would we have to keep the system running 24 hours a day to keep the slab above 0C with the thermal mass? If so, we'll end up using more energy and more costly than infrared? How about electrical slab heating, how would that compare??

Which system would you use and why?

Thank you very much.

 

[IRstuff]

You don't specify how often, or what the criteria is. Does it clear the slab 24/7?

While electricity is expensive, hydronic implies a pump somewhere, doesn't it? In order to keep the heating system from dying, it would need to be flowing and/or above freezing whenever the temperature is below freezing.

I'd vote for standard electrical heaters. You need heaters to generate the infrared, so why waste the heat? An embedded heater could be configured to direct all its heat upward, with suitable insulation below the slab.

Presumably, you're doing a control system, so that would monitor the ambient temperature and only activate if below freezing, although, you might need to turn it on at night, even if the temperatures are well above freezing, because of infrared emission into the much colder sky.

TTFN

FAQ731-376

 

[MintJulep]

The energy required to maintain a slab at 40C above ambient with "some wind speed too" is a whole lot.

For bulk heating methods, the energy required is independent of method. Electric has the advantage of near 100% efficiency. Hydronic won't be anywhere near that after heat exchanger losses and pumping costs. Electric also has the advantage of being nearly maintenance free.

IR probably won't achieve the same amount of bulk heat into the slab. I would suspect that re-radiation to the canopy and sky combined with convection will whisk heat away faster than conduction within the slab will allow heating of the concrete. However, maybe that's ok for ice melting. I would be worried about thermal gradient and associated stresses.


By the way, how can you have rain with -40C temperatures?

I would focus on preventing spills and dealing with them as needed when the do happen. How about an adsorbent and a heating blanket?

 

[IRstuff]

Radiation into the sky will be on the order of 160 W/m^2, using 10°C surface temp and 230K sky temp. Obviously, the ground will probably frozen, or cold, so without insulation, there will be substantial conducted losses.

Convective loss for 10 W/m^2-K convection coefficient is about 500 W/m^2. That's a pretty windy condition; still air would be about 2.5 W/m^2-K

End result is that you'll need about 700 W/m^2 of heating capacity.

TTFN

FAQ731-376

 

[m2e]

Sorry guys, yeah, there's no rain, but snow drifted and carried by tires into the bays. For the operation time, it should only be operating during day time, but the client may want to open it 24/7. Yes, there'll be control. Moisture control on the ground.

From what we see, infrared will kind of have a double purpose for providing warmth to the users during the filling too. But yeah, there'll be radiation and convective heat loss thru the canopy and the ambient. That's why we're debating all these options...

 

[IRstuff]

IR can do that for the users, but what's the duty cycle on that? Do you really want to waste all that energy 24/7?

If user comfort is a strong desire, it might make more sense to have a slab heater AND a motion-sensing IR heater.

TTFN

FAQ731-376

 

[m2e]

From the comments, seems like electric wire slab heating are the best option, and then hydronic and infrared are similar. I agree that having pumps and having to run the hydronic to prevent freezing is bad. Also because of cheaper gas cost, infrared, although having less direct efficiency than electric slab heating, may still be a choice to consider.

Now, I think I still have some question about the response time comparing the two. For electric slab heating, you would have to warm up the whole slab and then the heat would slowly act upon the ice. However, for infrared, even though the heat will re-radiate toward the sky and canopy, at least the heat is immediate and it act direct to the ice. Suppose the cost of gas is half of electricity per energy unit, I can have a gas infrared heater twice the size of the electric slab heater. As long as the combined efficiency of infrared heater not less than half of electric, it would still be more advantages (both economical and practical). Is this right??

 

[IRstuff]

Yes, and no. I think the net efficiency for a radiant heater for this application is substantiall poorer than a direct heater. Morever, you could put the electric heater directly under the wear surface of the slab, insulate under the heater, and avoid having to warm up the entire slab. The net effect is that the electric heater would act more directly on the surface of the slab.

The radiant heater under conditions I posited above will lose at least 1.5 kW for every 1 kW emitted. Additionally, IR does not transmit well through fog, rain, or snow.

TTFN

FAQ731-376

 

[MintJulep]

Do you need to melt ice that has already formed, or only prevent water from freezing? If the former, don't forget the energy needed to melt the ice.

 

[m2e]

"Do you need to melt ice that has already formed.." This is something we're open to different options. The answer should be whichever require less energy. The original thought was that if we are using infrared, we'll fire it up half an hour or an hour prior to the operation hour of the station and melt away the ice (but shouldn't be much since it's covered), and then keep it on until the the station close. On the other hand, for slab heating (we assumed) it'll take several hours to warm up the slab, then it might be just easier to keep it on all the time to prevent the ice to form at all.

 

[IRstuff]

Again, the slab heating time depends on how that's done.

Let's say, for the sake of argument, that you have a rubber mat, a heater, insulation, then slab. Then, the heating requirement and time shouldn't be that different from the IR heating.

What's the required longevity of the setup?

TTFN

FAQ731-376

 

[GMcD]

Put an insulated building over this place with door air curtains, a heated slab sloped to a drain, and don't dump thousands of $$ worth of energy into the atmosphere. Do the Life Cycle Cost, yes, expensive up front cost for an insulated tin shed pre-engineeered building, but save a ton of $$ worth of on-going energy costs.....

 

[TBP]

You can easily run glycol in the hydronic tubing. You'd only need to operate it when conditions require. A consideration would be a boiler that could take the very low return temps, and associated condensing flue gases. I believe Viessmann makes a condensing boiler that can take at least minus 30 C return temps.

 

[ChrisConley]

Totally agree with GMcD. Not only will you dump pile of energy, you'll still have problems with ice and maintenance.

A cheap pre-eng, or garage package could be cheaper than a condensing boiler and snowmelt system.