Energy to melt snow

[bikermark]

We are working on a melting system for melting snow (250 tons/hr) and trying to calculate the energy required. We are depositing it in heated water. One system on the market claims an energy requirement which is less than what you would expect from a pure heat of fusion for ice calculation. How can the energy be less than what one would calculate from heat of fusion?

 

[prex]

Well, it can't.
The misunderstanding could be in the fact that a liquid fraction is considered to be present in the snow (but this wouldn't be conservative), or perhaps in the units of measurement (density of snow is quite lower than that of water, and if the plant capacity is referred to a volume of snow...)
Can you provide the figures you are dealing with?

prex

http://www.xcalcs.com

Online tools for structural design

 

[bikermark]

Thank you prex.
Here are the numbers. We have 250tons/hour of snow being dumped into 2250 gallons of water that is 60 degrees F. Most often this would be done as snow is falling so perhaps air temperature of 15 to 32 degrees F. The density of the snow being blown into the tank will range from 10 to 30 lbs/ft3. Is this enough information? We are trying to determine how many million BTU/hr is required. The 250tons then becomes about 1000gpm of water that will be continuously discharged to maintain the 2250 gallons of heated water that snow is being blown in to. THX!

 

[pablo02]

Not to deviate off the request, but did anyone see the article about using a combustion turbine or jet engine exhaust to melt snow? It was a city in the eastern USA, don't remember all the details -- trailer mounted unit?... They melted the snow about as fast as they could scoop it and dunp it....

 

[TD2K]

I don't have my reference books here but the calculation is simple. It's the sensible heat of the snow being heated from its inital temperature to 32F plus the latent heat of fusion plus the sensible heat of the water from 32F to 60F.

you don't need the density as you have the mass flow, 250 tonnes per hour. Just look up the other data and do a check. If the vendor is trying to say they can do it for less energy, they are not being truthful.

 

[bikermark]

Thank you TD2K:
I think you are correct. The next thing I guess is determining the fraction of water in various snow conditions? Someone told me then that since there is a fraction of water in the snow then some is just sensible heat of water from initial temp to 60F?

 

[IRstuff]

Nope, whatever is still ice still needs a jolt of heat of fusion, plus, liquid water content varies considerably.

Assuming 250 ton/hr, I get something like 113 million BTU/rh

TTFN

 

[TD2K]

If the snow is below 32F, I would assume it's all frozen.

I suppose with things like road salt leaving slush in the mixture, that would introduce another degree of complication but I don't have any data to quantify the effect.

If I was designing it, I doubt I would want to take credit for it unless I was willing to accept a reduced capacity if the water wasn't there at some time OR you have good data that the water fraction is ALWAYS there.

 

[25362]

Possible explanations for using less heat than the estimated rate to bring the snow up to 60 deg F are:

1. Temperature measuring instruments may not read the true average water temperatures, because the water segregates due to density differences.

2. It takes (residence) time to melt the floating ice (a typical unsteady state heat tranfer with phase change).
For a 1 cm deep snow layer I have estimated the melting time at about 50 seconds. It would take longer if the snow layer is thicker -4 times longer for a 2 cm layer, since the melting time is proportional to the square of the thickness-.

3. When continuously removing a volume of water it may be that the snow's residence time is short, and the temperature of the effluent water containing the molten snow layer, depending on the point of removal, is, in fact, lower than 60 deg F.

Just a thought.

 

[bikermark]

Thank you all.
I'm not sure how TD2K got 113MMBTU/lb? Using 144BTU/lb for heat of fusion and 500,000lbs of snow, isn't the worst case 72MM? I've still got an "expert" saying much less than that... Interesting from 25362 on residence time...

 

[IRstuff]

Sorry, forgot to multiply by 5/9..

72MBTU/hr just for converting ice to water.

Then you have to heat it to 60F from 15F, that should be 22.5 MBTU/hr

Total of 94.5 MBTU/hr

TTFN

 

[25362]

A layer of water below the floating snow, would be a serious resistance to heat flow. The thermal conductivity of water at 32-40^oF is 0.57 W/(m*K) -less even than that of a common building brick- while that of ice is 2.33 W/(m*K), an if snow contains occluded air its actual value may be lower, and become similar to that of water.

When I estimated the time for melting I used the higher value; while using the lower value it would result in a period four times longer to melt. As a consequence, it is indeed possible that not all the snow melts because of lack of (residence) time. This could be one reason for the lower-than-normal heat duties for the heater.

 

[JKEngineer]

If you take the incoming snow at 15F, as was done above, and make 60F water out of it, the heat loads are:

warm ice to melting: 0.44BTU/lb degF * (32-15)= 7.48 BTU/lb
melt ice: 144 BTU/lb
warm water from melting: 1 BTU/lb degF * (60-32)= 28 BTU/lb
----------
Total: 180 BTU/lb

at 250 tons/h * 2000 lb/ton = 500,000lb, this becomes
180 * 500,000 = 90 MMBTU/h

Note that the specific heat of ice is considerably lower than that of water. I did not look it up, but memory says 0.44, which is what I used above.

Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com

 

[samv]

Just out of curiousity - why do we want to melt so much snow? I'm guessing large city, no room for snow... is this common practice where you are to melt it??

from snow loving samv, Canada

 

[bikermark]

Yes-
The application is for cities with storage problems.

 

[chicopee]

Seems to me that it would be a lot cheaper to let mother nature do it thing. After all cities like Albany,NY does not plow their streets. I am sure several cities in Canada don't bother plowing either. Why waste all that fuel with the price the way it is.

 

[samv]

We do need to plow in my part of Canada - because of the huge blanket of snow we get every year, but we just stack it in less used areas (ditch, corner of parking lot, empty lot etc)and let it melt in spring.. (Ever try driving your Honda Civic thru a meter of snow!!) However, yes, some Canadian cities with milder winters will get away with little or no plowing. It does seem like an expensive practice to want to melt it artificially - anyone tried selling it to a nearby ski resort or something??

 

[IRstuff]

Ugh, have you ever seen how much crud winds up in de-iced and plowed snow?

I wouldn't risk my skis or my face on that stuff.

TTFN

 

[samv]

If the quantity of snow is large enough (and I'm guessing it is if you have such storage problems), there would be a large quantity of clean snow that would be salvageable I think. ok so maybe you would have to invest in harvesting the top layer after a snow storm and find a couple ski resorts willing to buy it regularly - but this huge snow melting machine is also an investment as are the snow making machines of ski resorts...

just throwing out ideas - the concept of having to melt snow just sounds so energy innefficient and foreign to me...

(how about buying empty fields?)

I'll keep quiet now unless somebody asks me a direct question as I've brought this thread off the original question and city engineers would know more about the dirtiness of plowed snow than me!

 

[IRstuff]

Actually, there are a number of additional issues with re-cycling in this fashion, not the least of which is the adhesiveness of the snow on the ski slope and the difficulty in getting a nearly uniform deposition, randomized ice/snow/water distribution, etc.

Most of resorts that need snow would probably wind up melting it anyway, so that they can pump it through their snow making machines.

TTFN