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- In the movie “The Mosquito Coast”, a guy (played by Harrison Ford) constructs a tin house in the jungle that is a wood-fired freezer. I do know that technically it is possible to build such a device, but how well it works I don’t really know. In the movie, the one he built seems to make barrels of ice in a matter of twenty or thirty seconds. Later in the movie, they are taken hostage by some armed thugs, who they convince to sleep in the freezer house, which the bad guys do, because they don’t really know what it is for. Harry later lights the fire (to get it to freeze the bad guys inside) but the bad guys shoot some holes in it, and it explodes, burning up the whole camp. -So whatever he used for a working fluid seemed to be flammable, if the story is to be believed.
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- We were sitting around at work, and someone had commented that they had just watched this movie again, and asked if the freezer could actually work that fast. We work in a store that has a -20 F freezer, and we know that it takes a room-temperature cup of water about 25 minutes to begin icing up and 35 or so minutes to freeze, exactly depending on the container it’s in. How can any freezer make a barrelful of ice in thirty seconds? Even if you had to put a barrel if water into it to begin with, I cannot see anything cooling off that fast. Was this device a Hollywood fictional construct, or could it really work? - MC
Just a guess… but from my, admittedly, miniscule knowledge of physics I don’t see any way that a barrel full of water would go from room temperature to freezing in 30 seconds… my guess is:Hollywood fiction.
Assuming a 42-gallon oil barrel full of water, which at 60 F weighs about 350 lbm (v[sub]f[/sub] = 0.01604 ft[sup]3[/sup] per lbm), we have to see how much heat ro remove until the thing reaches ice.
The heat removed to go from 60 F (admittedly cool for the jungle, but maybe in the shade) to 32 F is:
350 lbm * (28.08 Btu/lbm - (-0.01 Btu/lbm)) = 9832.52 Btu
Then, to change this 32 F water into ice, we need to remove the latent heat:
350 lbm * 143.4 Btu/lbm latent heat of fusion =
Summing these, we get 50,190 Btu.
Thus, we would need to remove 60,023 Btu per barrel of 60 F water to just freeze it. Doing this in 30 seconds requires removing 120,045 Btu/min, or 7.2 MBtu/hr for a continuous operation.
Now…how efficient is this refrigerator? I’m assuming that the refrigerator was some sort of heat-driven device, which can be used when one has large amounts of cheap or waste heat to superheat the refrigerant in an adsorption cycle. These cycles have very low performance, which is why they are mainly practical if one has lots of things to burn (like in a jungle, I guess). These cycle has COP’s (coefficients of performance) of less than 1, so let’s assume 0.5 actually.
That means that for every Btu which is removed by the system from the water, we need 2 Btus of heat to come in to the system. I think. So let’s say then that we would need 240,045 Btu/min of heat energy in some form. Now, assuming that we are using wood waste from the jungle. We could say sugarcane or baggasse, but I don’t recall them growing that in the movie. Anyhow, 45% moisture wood waste has a heating value (gross) of about 4490 Btu/lbm. Now assuming that a wood combustor in these conditions burns and produces usable heat with about 50% efficiency, we have the following energy requirement:
240,045 Btu/min / (4490 Btu/lbm * 0.5) = 106.9 lbm per minute of wood waste.
So if this system could even exist, it seems you would have to fully combust 107 lbm per minute of wood. And this is where I think my speculation will have to end. Is it possible? If they want to make ice continuously, they will need about 3.25 tons/hour of wood waste. And I make no spculation as to the process, and how you get the mixing required to chill the water that fast, etc. But it seems like, in theory, they might be able to get the energy (for a while) from the jungle…
Time to disappear again…
Great summation, Anthracite! Memories of hogh-school physics are coming back to me.
Except we did it all in metric, of course.
It’s been a while since I’ve seen the movie, but I am certain that big tanks of gas were involved in the process. I believe that they were ammonia. In the movie, he explains how the process works. I think it’s an exo (endo? I can never remember this) thermic chemical reaction that sucks up heat. The movie is based on a novel also. Should someone own that novel, perhaps they could look it up?
Wow…my grammar was especially poor this morning when I wrote that. I do apologize.
I didn’t see the movie or read the book, but if it is a refrigeration cycle fired by wood involving ammonia, then it is probably an ammonia-water absorption system. A large one of these systems would have lots of liquid ammonia around at a temperature of about 0 F. People shooting guns around tanks of liquid anhydrous ammonia (the home version of ammonia is 5% ammonia, 95% water, wheras anhydrous means water-free) is incredibly dangerous because the vapors are explosive and the liquid will suck up incredible amounts of heat while flashing to gas. A gallon of 0F liquid ammonia flashing to 95F gas sucks up 3257 Btu’s of heat, so based on Anthracite’s numbers, and an assumed heat transfer efficiency of 75%, about 20 gallons could freeze the drum solid. The max sustainable refrigeration rate would be slower, but the liquid could be generated over a period of days or weeks, and then flashed in a very short period.
The difficulty is transfering the heat to the drum. A normal refrigeration system would have evaporator coils and a fan. Getting enough air cold enough to flash freeze a drum of water would be a real bitch, especially when absorption cycles have a fairly high lower temp limit compared to standard refrigeration cycles. If it was liquid nitrogen, or even liquid CO2 it would be easy.