throopw@rtp47.UUCP (Wayne Throop) (07/10/85)
I still haven't seen a real "thermodynamics whiz" take on this issue in a way that satisfies my curriosity about the situation. Charley Wingate comes closest. However, I have some additional questions. > 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: 1-way vapor barrier & insulation 3: water & vapor > > 4: vapor barrier 5: outside ] > > [... to condense water in layer 3] 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). Why wouldn't the increase in partial pressure cause the water to condense in (3)? I don't have the skills to to the quantitative calculations to model this situation, but it seems that (4) *is* "refrigerated" in a sense (by convection with the outside air). I'm not sure I see the crucial point here, which is why can't the vapor that passed through (2) lose *all* the heat it picked up in (1) (assuming that (2) can support a high enough "backpressure")? > ... > 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. I can see that this must be so. That is, in the absence of refrigeration, the temperature of an object must be higher than that of the environment in order to lose heat to the environment. But surely temperature of the object needn't be uniform. That is, there *is* a process pumping heat from (1) to (3) other than convection or radiation. Why can't this process raise the temperature of (3) to the point where it can be "cooled" in turn by convection to (5)? (I imagine that the "total heat" in (1) & (3) must add up to an "average temperature" that is higher than the "average temperature" of (5).) > The problem with the stillsuits is that they explicitly > forbid the former, and that the energy supplied for refrigeration is > insufficient. I suspect that energy might have to be invested in supporting the pumping of heat from (1) to (3), which Charley suggests, but I don't know just *how much* energy, and I'm not yet convinced that it must be expended at all (I just *suspect* that it must be so). Can someone show just how much energy would have to be invested in the mechanism in (2)? Is it zero? Is it more than could be supported by human muscle power? Could it be expressed as a function of the temperature of (5), given that the temperature of (1) must be ~90 degrees F? (After all, stillsuits would clearly work in very cold temperatures, so could an upper bound on the ambient temperature of Arakis be derived, given the 600-btu/hr guestimate of human heat production, the thermal conductivity of air, etc etc?) > mangoe@umcp-cs.UUCP (Charley Wingate) -- Wayne Throop at Data General, RTP, NC <the-known-world>!mcnc!rti-sel!rtp47!throopw