js2j@mhuxt.UUCP (sonntag) (06/25/85)
> >>Stillsuits wouldn't work. As I read it, the idea behind a stillsuit is > >>that it allows cooling by evaporation while trapping the moisture. > >>Thermodynamics does not allow this. Any cooling action gained by the > >>evaporation inside the suit would be offset by the condensation process. > >> > >>The net result would be a steam box that would be hot on a cold day with > >>any insolation at all. > > > >Sorry to disabuse you, but all Thermodynamics says is that you can't get > >something for nothing. If you recall, the description of the stillsuit > >includes some form of "pump" which operates as the wearer walks. This > >provides the necessary "work" to circulate fluid. A refrigerator works well > >just by circulating fluid. I haven't done any calculations, but on the > >surface a stillsuit should be feasible. Mike > >-- > > I too, felt that stillsuits would not work, and having read _Dune_ a long > time ago I don't remember what powered them. But, if the above description > is accurate, then the stillsuits definately can't work. The "engines" for > the "pump" are obviously human muscles which will generate heat. > And because, as we all know "... 2) You can't break even ...", the wearer > will produce more heat than they can remove; resulting in "friedman" > (boiled might be more accurate). Can some thermodynamics whiz in net.physics clear this up? The stillsuit uses work provided by muscles to move heat from the interior of the suit (which would have to be a little less than 37 celcius) to the exterior of the suit, which could be around 45-65 celcius, I guess. The question is, could this work, in principal? It seems as though it depends on the efficiency of human muscles and the efficiency of the heat pump and the efficiency of the external radiator. Obviously, if human muscles were 100% efficient (ie, they change chemical to kinetic energy with no loss), then this could work, regardless of the objections of the last person quoted above. -- Jeff Sonntag ihnp4!mhuxt!js2j "Well I've been burned before, and I know the score, so you won't hear me complain. Are you willing to risk it all, or is your love in vain?"-Dylan
friedman@h-sc1.UUCP (dawn friedman) (06/30/85)
> Can some thermodynamics whiz in net.physics clear this up? The stillsuit > uses work provided by muscles to move heat from the interior of the suit (which > would have to be a little less than 37 celcius) to the exterior of the suit, > which could be around 45-65 celcius, I guess. The question is, could this > work, in principal? It seems as though it depends on the efficiency of > human muscles and the efficiency of the heat pump and the efficiency of the > external radiator. Obviously, if human muscles were 100% efficient (ie, > they change chemical to kinetic energy with no loss), then this could > work, regardless of the objections of the last person quoted above. Gah. I had hopes of staying out of this one, but can no longer retain my objectivity. What I want to do is separate the entropy question from the energy question, if this is possible. Is anyone still saying that you simply can't move heat from a colder object to a hotter object? This is the entropy part of it. You CAN, but it's not spontaneous: you have to put work into it. So it isn't going to work the way that was suggested earlier, that is, by evaporation balanced by condensation. (Unbalanced evaporation will cool you nicely and leave you a raisin; but if you evaporate the water within the stillsuit AND recondense it WITHIN the stillsuit, the stillsuit-bounded system remains at the same temperature; that was the point being made earlier.) So what was the cooling mechanism proposed in place of evaporation? After all, wasn't saving water the main point of the stillsuit? I'm quite willing to believe in a human-powered refrigerator suit; but I want to hear a proposal for a cooling mechanism. The two tablespoons of water or whatever minute quantity Kynes mentioned is not going to cool anything larger than a sandrat. dsf (Dina)
hes@ecsvax.UUCP (Henry Schaffer) (06/30/85)
I thought that the Dune stillsuit was a water *reclaimation* device, not a cooling device. --henry
throopw@rtp47.UUCP (Wayne Throop) (07/01/85)
> > Can some thermodynamics whiz in net.physics clear this up? > > After all, wasn't saving water the main point of the stillsuit? > I'm quite willing to believe in a human-powered refrigerator suit; > but I want to hear a proposal for a cooling mechanism. The two > tablespoons of water or whatever minute quantity Kynes mentioned is > not going to cool anything larger than a sandrat. > > dsf (Dina) Here is a model to work with, using convection cooling to cool the stillsuit, and evaporative cooling to cool the inhabitant. Consider these layers, working from the wearer out: 1 wearer 2 layer that lets water out, but insulates heat very well, and also prevents the water from getting back in. 3 water & humid air reservoir 4 layer that prevents water from escaping, but is a thermal conductor. 5 the outside environment The "human muscle power" here is used only to pump various substances around in the water & humid air reservoir for convenience (this is more like what Dune says they are used for). Layer 1 is cooled by evaporation. The heat from the evaporate is deposited in layer 3, which is in turn cooled by convection. Layer 3 is the hotest, layer 5 the next hotest, and layer 1 the least hotest :-). The key to making it work is the "magic" properties of layer 2. It allows water to pass one-way, and is a terrific thermal insulator. It may be that in order to have the properties I state, some energy would have to be expended... I'm not sure on this point. Someone has already mentioned the present-day desert-dweller's basic uniform, which covers essentially the entire body. My understanding of how it works is that it is layers 1-thru-3 from above. That is, the interior is cooled by evaporation, the evaporate is allowed to escape through porous clothing, and the clothing insulates against the now higher exterior temperatures. Without the added properties of layer 2 and layer 4, the "stillsuit prototype" used by current desert-dwellers must allow the water to escape. So: are the "magic" properties of layer 2 theoretically possible? If they are impossible in a simple sense, can it be done "with mirrors", that is, by clever (but minimal) expenditure of energy? If layer 2 could be made to work, it seems to me that stillsuits would work just fine. -- Wayne Throop at Data General, RTP, NC <the-known-world>!mcnc!rti-sel!rtp47!throopw
mangoe@umcp-cs.UUCP (Charley Wingate) (07/04/85)
In article <82@rtp47.UUCP> throopw@rtp47.UUCP (Wayne Throop) writes: >Here is a model to work with, using convection cooling to cool the >stillsuit, and evaporative cooling to cool the inhabitant. Consider >these layers, working from the wearer out: > > 1 wearer > 2 layer that lets water out, but insulates heat very well, > and also prevents the water from getting back in. > 3 water & humid air reservoir > 4 layer that prevents water from escaping, but is a > thermal conductor. > 5 the outside environment > >The "human muscle power" here is used only to pump various substances >around in the water & humid air reservoir for convenience (this is more >like what Dune says they are used for). Layer 1 is cooled by >evaporation. The heat from the evaporate is deposited in layer 3, which >is in turn cooled by convection. Layer 3 is the hotest, layer 5 the >next hotest, and layer 1 the least hotest :-). > >The key to making it work is the "magic" properties of layer 2. It >allows water to pass one-way, and is a terrific thermal insulator. It >may be that in order to have the properties I state, some energy would >have to be expended... I'm not sure on this. Here's the problem: a packet of dry air picks up water from (1), and also heat (since (1) is supposedly cooler than (3)). We let it sit there until it reaches some sort of equilibrium (assuming it can only pick up a particular quantity of water). Now we take it to layer (3), where it has to get rid of the water. To do this, it has to get rid of some heat, which it must dump in layer (5). The important question is: how much heat? The answer: not as much as it started out with. Therefore the vapor pressure in (3) has to grow, or (4) has to be refrigerated. If we take the first possibility, eventually this pressure must rise high enough to prevent the flow of water from (1) to (3). In addition, there is a net flow of heat INWARD; when the water vapor cannot flow, heat is still being produced in (1), and thus there is no cooling. You can't cool a device in a hotter environment without disposing of the heat in some manner other than radiation or convection, or without some sort of refrigeration. The problem with the stillsuits is that they explicitly forbid the former, and that the energy supplied for refrigeration is insufficient. Charley Wingate
gdmr@cstvax.UUCP (George D M Ross) (07/05/85)
Sweating is a means of cooling down. If you sweat into a stillsuit then the suit has to get rid of the heat somehow, otherwise you will become rather uncomfortable.... -- George D M Ross, Dept. of Computer Science, Univ. of Edinburgh Phone: +44 31-667 1081 x2730 UUCP: <UK>!ukc!cstvax!gdmr JANET: gdmr@UK.AC.ed.cstvax
hollombe@ttidcc.UUCP (The Polymath) (07/09/85)
In article <82@rtp47.UUCP> throopw@rtp47.UUCP (Wayne Throop) writes: >So: are the "magic" properties of layer 2 theoretically possible? If >they are impossible in a simple sense, can it be done "with mirrors", >that is, by clever (but minimal) expenditure of energy? If layer 2 >could be made to work, it seems to me that stillsuits would work just >fine. This probably doesn't fulfill all the requirements, but it's a present-day start. Damart Corporation's Thermawear products are made of a cloth with some of the required properties. It's an excellent heat insulator, and body heat drives moisture through it and away from the skin. The only missing property is the one-way permeability to water. Heat will drive water through it in either direction, as I found out the hard way by standing next to a radiant heater after coming in from a rain storm. (The embarrassing result is left as an exercise ... etc. (-: ). If one adds cooling fins to the stillsuit and a stiff desert breeze it may be possible to get rid of the heat. Another possibility is a mechanism for storing the heat energy until night or a cooler environment arrives (human body energy output is about 600 btu/hour, I think). Heating up dehydrated fecal matter before dumping it would be a partial help, though probably not enough heat could be got rid of solely in that manner. --- -_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_- The Polymath (aka: Jerry Hollombe) Citicorp TTI Common Sense is what tells you that a ten 3100 Ocean Park Blvd. pound weight falls ten times as fast as a Santa Monica, CA 90405 one pound weight. (213) 450-9111, ext. 2483 {philabs,randvax,trwrb,vortex}!ttidca!ttidcc!hollombe