Thermodynamics: it's not rocket science

ASHRAE Journal, Dec, 2007 by Joseph W. Lstiburek

Some things are so obvious we miss them. The second law of thermodynamics is like that. Of course, the crazy way we speak about the second law does not help. "In an isolated system, a process can occur only if it increases the total entropy of the system." Huh? It makes you want to hate Rudolf Clausius. Couldn't he just say that heat goes from warm to cold? *

[ILLUSTRATION OMITTED]

Most of us understand heat goes from warm to cold. It's the other simple applications of the second law that we miss:

* Moisture goes from warm to cold;

* Moisture goes from more to less; ([dagger])

* Air goes from a higher pressure to a lower pressure; and

* Gravity acts down.

The two moisture rules: "warm to cold" and "more to less" are simply a "thermal gradient" and a "concentration gradient." If you combine the thermal gradient and the concentration gradient, you get the "thermodynamic potential" and according to the second law, water always follows the thermodynamic potential. The psychrometric chart is a graphical representation of "warm to cold" and "more to less" (thank you Willis Carrier).

This "warm to cold" and "more to less" thing should be easy. However, seemingly smart folks get it wrong all the time. Consider the space shuttle. The problem with it is that foam keeps coming off of the external tank of the orbiter (Photo 1). Think of it this way. We have an aluminum pressure vessel filled with a cryogenic liquid on a beach in Florida in July (picture yourself holding an aluminum soft drink can at the beach).

Most of us know intuitively that the can will sweat. And most of us know we need to insulate the can. But how many of us know that the insulation needs to control the vapor drive? And what is the magnitude of the vapor drive anyway? Figure 1 shows the boundary conditions. The surface of the foam goes to 150[degrees]F (66[degrees]C) on a sunny day (the foam is the color brown after all). In the summer it rains in Florida every day at 1 p.m. So let us add a film of liquid-phase water to the surface of the foam and wait for the sun to come out. Now, try to find where the saturation line (i.e., 100% RH) crosses the 150[degrees]F (66[degrees]C) line on a psychrometric chart. Nope. Not there. Can't find it. Off the chart. It's so far off the chart that we are in uncharted territory. Then, try the steam tables. Holy vapor pressure, Batman, it's 13 kPa (98 mm of mercury). Now, find the vapor pressure at -420[degrees]F (-251[degrees]C). This one is easy--you don't need no stinkin' chart for that--it's zero. We are talking some kind of serious inward vapor drive: 13 kPa to 0 kPa (98 mm of mercury to 0 mm of mercury).

Now ask the question that certain rocket scientists apparently have not asked: how resistant is the foam insulation on the external tank to vapor flow? The foam has a permeance of 3 perms per inch. The foam is 2 in. (50 mm) thick. Yup, that would make the resistance to flow 1.5 perms. I am no rocket scientist ([double dagger]) but that sounds kind of high. Seems to me that water vapor will flow right through the foam under that magnitude of vapor drive.

I wonder what happens when the vapor hits the aluminum lining of the tank? It's not just foam coming off the tank, but hunks of ice. Basically, we have a vapor barrier on the wrong side of the assembly. It seems to me that we need a vapor barrier on the outside of the assembly. Search online for photos of the early shuttle launches and ask yourself what looks different? Notice the "white" stuff on the external tank of these early launches. It looks like a low perm aluminum skin to me. Apparently, the vapor control got value engineered out of the assembly after the first few launches to save weight. Violate the second law at your peril.

[FIGURE 1 OMITTED]

Let's move on to the U.S. Department of Homeland Security's Federal Emergency Management Agency (FEMA), New Orleans and Hurricane Katrina. It is hard to believe this if you have not seen it with your own eyes--thousands of trailers (Photo 2). New Orleans is located in the ASHRAE hot-humid climate region. Most of us intuitively know this means it is hot and humid outside most of the time in New Orleans. Here is the tricky part. We air condition the inside in New Orleans. Yes, it is true. We make the inside colder and drier than the outside in New Orleans. So, work with me on this--the outside is hot and humid, and the inside is cold and dry. It's that pesky second law again. What direction is the vapor drive? We can actually use a psychrometric chart for this example and not have to go to the steam tables. It isn't as hard as NASA's problem.

[ILLUSTRATION OMITTED]

With a dominant inward vapor drive (there is not much of a winter in New Orleans), what is the last item you want to see on the inside of an assembly in New Orleans? Yes, that is right, a vapor barrier. You would never want a vapor barrier on the inside of an air-conditioned assembly in New Orleans. However, all FEMA trailers have an interior vapor barrier (Photo 3). The FEMA trailers are lined with gypsum board wrapped with vinyl facings, which are turning the trailers into moldy messes. And, it is not like FEMA wasn't warned. FEMA, the U.S. Department of Housing and Urban Development (HUD) and the mobile home/ manufactured home industry all blindly continue to believe that this is not a problem. But hey, it is poor people in these trailers, so who will notice?