Delivered-To: primanis@xxxxxxxxxxxxxxx X-IronPort-MID: 1318085824 X-SBRS: 5.7 X-BrightmailFiltered: true X-Brightmail-Tracker: AAAAAA== X-Ironport-AV: i="3.93,300,1115010000"; d="scan'208"; a="1318085824:sNHT18390620" Subject: Cold Storage Date: Tue, 19 Jul 2005 12:15:48 -0400 X-MS-Has-Attach: X-MS-TNEF-Correlator: Thread-Topic: Cold Storage Thread-Index: AcWMfXPUn4wU7hfKRZenVXaF83jNQg== From: "D NISHIMURA" <dwnpph@xxxxxxx> To: "Olivia Primanis" <primanis@xxxxxxxxxxxxxxx>
PADG members: I thought that it was time to wade into the cold storage issue. Unfortunately, RIT has changed my server so the ALA lists don't recognize me anymore and my efforts to get the situation corrected have not been successful. However, I asked Olivia for help getting something to the list.
Many of the ideas we have about the interactions between our library objects and the environment are based on basics learned from inaccurate language. We are left with an erroneous picture of what is happening. One classic example with photographs is the idea that freezer storage will cause ice crystals to form in the gelatin that breaks up the gelatin and you'll be left with something resembling a frozen tomato (mush). Such erroneous images are perpetuated by the language that we use: "saturation," "the air can't hold any more water," and "moisture" when we mean the water contained in paper, books, and photographs or when we mean water vapor. (Yes, even scientists and engineers are guilty.)
First a little basic chemistry and physics. The water that we deal with in the cold storage of library materials exists in three phases (but not quite what you're expecting): vapor (gas), condensed, and possibly solid. I use the term condensed because it covers more than just liquid water (which IS a condensed phase.) However the water at equilibrium in books, magnetic tape, paper, and photographs, while it exists in a condensed phase, is certainly not a liquid. Condensed is a bit of a relative term in that ice is also condensed. Only the gas phase is not condensed. If you put a bunch of water molecules in vapor phase into any enclosed volume, the vapor phase will fill that volume. If I put a cup of liquid water into a 1 gallon (empty) milk jug, the liquid water certainly doesn't fill the gallon jug. In the liquid phase, the water molecules are loosely bonded together by intra-molecular forces, but not so strongly that the water molecules can't move around within the confines of the liquid. The water in library objects is also confined, but it is loosely attached along surfaces (perhaps of fibers) and this is why it is a condensed phase, but not liquid. There may be the odd water molecule stuck to another one here and there, but not enough to declare that the water is liquid. The water is adsorbed (spelled with a "d") to the book, paper, photograph meaning that it is bound along surfaces. ASHRAE points out in their Fundamentals book that practically everything adsorbs gases to some degree. Water in books, paper, and photographs is physisorbed meaning that it is loosely attached by weaker forces such as van der Waals forces or hydrogen bonding. Think of fridge magnets stuck to the refrigerator door. Chemisorbed molecules are bound by real chemical bonds that are as permanent and strong as any chemical bond can be. Think of the fridge magnets being bolted or welded to the door. If the water was aBsorbed (with a "b"), then it would be bound within the bulk of the material -- like water in a sponge is held in the holes throughout the sponge, not just along the surfaces of the holes, but completely filling the holes (until we squeeze them out.) Liquid water and our adsorbed water have a number of similar properties. If we add enough heat to a water molecule in either situation, it can break free of its bonds and evaporate into a vapor phase molecule. Similarly, a low enough energy water molecule can be stuck on to either other water molecules or to the surfaces of our object (condensation.) In addition, if we confined a puddle of liquid water into a container, the water vapor content would rise until equilibrium was reached between the liquid and vapor-phase water molecules. This is the point that people call "saturation" -- a poorly chosen term because it implies what many conservators wrongly state: that the air is holding the water. The air plays no part in the equilibrium. It just happens to be present. If I evacuated my container completely before adding the liquid water, I would find the same amount of water vapor present as when I do the experiment with air present. Air has as much to do with how much water vapor is present as water does to the number of fish that fit in some given volume. The water just happens to be present, but the real maximum number of fish that fit in some volume is strictly a function of the properties of the fish and have nothing to do with the water. If I did the same experiment, but put a book from your library into my evacuated container instead of a puddle of water, I would find an analogous situation: the water vapor content of the vessel would rise (water vapor pressure) until it reached some equilibrium point and then would stay constant. So there is an equilibrium between the condensed phase water in the book and the water vapor around it.
So what about temperature? Everyone apparently knows that "air holds more water at higher temperature." Using fundamental thermodynamic relations, one can derive the relationship between temperature and the equilibria in any multi-phase system. In general terms, the equation says that as temperature increases, the equilibrium between either solid (ice) and water vapor or between condensed-phase water and water vapor, shifts towards the vapor phase state and the degree depends on the amount of temperature change and the amount of energy required to break the bonds. This equation tells us that if we have an equilibrium state between water vapor and the condensed phase of water (often called the equilibrium moisture content, although moisture implies that the condensed phase is wet, which it's not) and the water vapor in a closed system, and we drop the temperature, the cold object will contain more water and there will be less water vapor. Wexler and Hyland used the basic Clapeyron equation (this is the equation derived from fundamental thermodynamic laws mentioned above) as a starting point to fit an equation with determined constants for water. Their equations (two are required: one for liquid/vapor equilibrium and the other for solid/vapor equilibrium) are used and cited in the ASHRAE Handbooks.
In the real world, our situation is more complicated because liquid water is all around us in lakes, rivers, and oceans, and therefore, the humidity in the environment is measured in relation to the equilibrium point between liquid water and vapor phase water. In addition, anyone who has watched the ice cubes in their ice cube trays in the freezer shrink and disappear knows that there is also an equilibrium between the solid water (ice) and water vapor. This is also why you can hang wet laundry on a clothesline in winter and it will dry. So RH at temperatures below 0 C are very real and are important to us even though the absolute water vapor content is very low even at 100% RH at those temperatures. (A 10 foot X 10 foot X 10 foot room at 70 F/100% RH contains 523 grams of water vapor (equal to slightly more than 2 cups of liquid water). At 0 F / 100% RH, that same room contains 30 grams of water vapor (equal to about 2 tablespoons of liquid water.))
The practical implications are that if an object is sealed in an enclosed environment and the object is dropped into the freezer, one would expect the RH to go up. However, as the object drops in temperature, it adsorbs water vapor at such a fast rate that a sensor in the air space around the object would show a small increase in RH initially and then a drop. One cubic foot of air (roughly 7.5 gallons) at 70F/50% RH contains roughly 0.26 grams of water. One hundred grams of paper would only have to increase in equilibrium water content by 0.26% to remove all of that water. It would never happen because equilibrium would be reached before that point since the water vapor pressure is decreasing as the water content of the paper increases. So even with very large air spaces in a package, the water vapor content is pretty insignificant. The result is that in most vapor-tight packages, the book, papers, or photographs are effectively being held at constant water content so the object behaves at cold temperature as if it was at equilibrium with a lower relative humidity. Sheet film starting at 68 F/50% RH and put into a 0 F freezer (in a sealed vapor-proof package) would come to equlibrium in the freezer as if it was was stored at 0 F and 40% RH. Paper is likely to be more responsive, but I don't have any numbers handy to use for illustration. Looking at this issue from the reverse side, you'll see that if an object is at equilibrium in cold storage with a moderately high relative humidity and it is removed to a warm room, it will likely be put into a vapor proof package to prevent condensation formation so the object will warm up moderately quickly, and will drop its water content fairly rapidly so it will end up behaving as if it at the warmer temperature and at a higher humidity than the freezer. This can be a particularly serious problem during the summer months. Suppose that you had a roll of motion picture film that was at equilibrium with at 0F freezer at 50% RH. If you take the can out and drop it into a car in the summer heat, it could get up to 100 F or more. At 104 F, the film would contain the same amount of water that it would at 104 F/70% RH and there is a risk of the film sticking together.
What is significant to Chris McAffee's question is that there is nothing magic about 0 F (It happened to be chosen by Daniel Fahrenheit as the zero point of his scale because some concentration of salt water happened to freeze at that temperature. In fact, as far as our books and papers go, even 32 F isn't special in any way since the water contained in our objects, while condensed, doesn't freeze into ice. All that 32 F does is change the strength of the bond that must be broken to release a water molecules (from the liquid or solid phase to gas phase) causing a very tiny hiccup in the equilibrium vapor pressure at 32 F. However, I point out that graphs of water content (in film) versus RH versus temperature don't have discontinuities appearing at 0 C at all. Curves of constant water content very smoothly vary in RH with changing temperature all the way from 60C (140 F) all the way down to 0F.
What may be problems that apply to any low temperature/low RH vault are A) dehumidification. At higher temperatures, the amount of dehumidification is limited by the cooling system. In buildings that use chilled water (more efficient for large spaces such as buildings), the chilled water can't be colder than its freezing point. One way around it is to use a mixture of ethylene glycol (anti-freeze) and water and those systems can be colder. However, if a heat exchanger containing the water-glycol mixture runs into a moderately high humidity or temperature, then it will ice up (and the ice will tend to insulate the heat exchanger from doing its job.) DX systems work like your refrigerator, freezer, or home air conditioner. Here a gas is compressed to form a liquid and is pumped to the heat exchanger where the liquid is allowed to evaporate back into a gas. The compressor gets hot and needs to get rid of that heat because work is done to compress the gas. (Your bicycle pump gets hot for the same reason.) An equal amount of heat energy is required in order for the liquid to evaporate into a gas. (In the same way, a steam burn is worse than a liquid water burn because there is extra heat generated by the steam condensing into liquid water on your skin. However, as water evaporates off our skin, we are cooled by the energy required to convert the liquid to gas.) In the DX systems (Direct Xpansion -- no one ever said that HVAC engineers could spell), if the heat exchanger gets too cold and there is too much water around, then they also ice up and more or less stop functioning. So to get either very low humidities or lower humidity at colder temperatures, desiccant bed dehumidifiers are required. These tend to use adsorbants such as silica gel in a rotating wheel. In one area, air to be dehumidified is blown through a column of desiccated silica gel. The silica gel behaves like our book, papers, or photographs so that if left, the silica gel will eventualy form an equilibrium between the adsorbed water and the water vapor content (in the air stream) so the silica gel needs to be constantly redesiccated. So the wheel rotates bringing the column of silica gel around to an oven area that heats the silica gel while an air stream carries away the water liberated from the silica gel. You'll remember that as temperature goes up, the equilibrium between the adsorbed water and water vapor heads towards greater water vapor and the water content of the silica gel goes down. Once the silica gel has been dried out, it continues the rotation until it's in position to dry the air again. (A large diameter wheel is used that turns slowly with many columns of silica gel so the wheel can turn continuously rather than stopping and starting. The areas where air is dried and the silica gel is dried are large enough that the silica gel can be well dried and well humidified.) Desiccant bed dehumidifiers are more expensive than coolant desiccation systems and have the added cost that the silica gel needs to be replaced periodically.
B) Kinetics laws tell us that the diffusion rate of gases through a leak in the vault is proportional to the gradient (the difference in water vapor pressure between both sides of the leak). In general, the colder or drier the vault is, the lower the absolute water vapor content on the inside and therefore, the higher the gradient is between the two sides and the faster the leakage rate is. With a little algebra applied to the diffusion equation, we find that the proportionate difference is constant and there is no dependence on absolute quantities of water inside and out. In my example for Chris I proposed an outside environment (based on Salt Lake City one morning) of 70 F/ 53% RH and two vaults with identical leaks. One vault is trying to hold -4 F/35% RH and the other is trying to hold 25F/ 35% RH. The leak is big enough that in some given time, t, each vault comes 1% of the way to equilibrium with the outside air. In that given time, t, the colder vault will be at 47.64% RH and will have leaked 2.66 milligrams more water (per pound of dry air leakage) than the warmer vault which will now be at 37.7% RH. While the absolute numbers look extremely small, the total amount of water vapour in the vault will also be quite small. (Ten cubic feet of air at -4F/35% RH contains only about 88 milligrams of water total.) Therefore small changes in the absolute amount of water vapor in the vault can result in significant changes in RH. Anyway, as the kinetics laws say, we had a faster leak with the colder temperature (and therefore a higher gradient) (assuming that all other things remain equal.) Water molecules are quite small and any hole bigger than about 0.4 nanometers should be large enough to let water vapor leak into the vault. That's a pretty small hole so you can see how difficult it is to build a vault wall with no water vapor leaks. So one thing that can help is to make sure that as many vault walls (including floor and ceiling) as possible are backed by the lowest humidity environments possible. This is why many vaults are built inside air conditioned buildings rather than left free standing. The building is going to be air conditioned anyway and the only cost that the vault adds to that air conditioning cost is initial cooling and drying of the air in the vault and dehumidification and cooling for heat and water vapor leakage. Usually these costs are lower than working with a free-standing vault. Notice that the temperature and water vapor gradient across the outside building walls and across the cold vault walls have been minimized.
One problem with fast leaks is that you get ice developing if your drier can get rid of the water as fast as it's coming in (although there will be a high concentration of water vapor at the leak point and a tendency to form frost, the dry air could allow the frost to evaporate as fast or faster than it's formed.) This was the problem that the Royal Historic New Orleans collection had with its cold vault many years ago. They had an ice ball that formed and grew on the wall and they couldn't get the leak sealed. Heat exchange laws say the same thing about the rate of heat exchange: the higher the gradient, the faster the heat flow. Fourier expressed it as heat exchange while Newton expressed it as temperature change. Down at the molecular level, they amount to the same thing. (Although, heat and temperature are not necessarily directly related unless we're talking about the same system.) Heat can be exchanged by conduction or radiation and if the object is surrounded by a fluid (chemical engineers consider gases to be fluids), then convection increases the effeciency of heat exchange by conduction and radiation. As a result, it is extremely difficult to produce a perfectly insulated wall. My materials science book points out that what we use as insulators are simply poor conductors, not absolutely non-conductors. So for a cold vault, it is best if it is surrounded by as cold of an environment as possible. This is one reason why very cold vaults tend to be accessible only through ante rooms with door interlocks that don't allow you to open the inside door unless the outside door is closed. The ante room provides a colder and drier environment than the outside when the inside door is opened, therefore reducing the rate of water leakage into the vault and reducing the amount of heat brought into the vault when access is required. In situations such as the planned nitrate storage vault at the National Archives of Canada, the vault was divided into smaller mini-vaults in order to minimize loss if a fire happened to break out. However, from an engineering point of view, as long as the walls were thick enough to contain the fire and fairly well insulated, energy losses through both water and heat leakages through the walls could be minimized by maximizing the number of common walls between vaults. If both sides of the wall are trying to hold the same environment, then the gradient is very close to zero and you've reduced the leakage rate to virtually zero.
So there's nothing specifically special about either 0 F or (for the objects), 32 F. The laws governing the temperature dependence of vault issues either arising from gradients or thermodynamics simply get progressively worse as temperature goes down. A large leak in the vault may result in ice at sub-zero Celsius temperatures, but is the ice any worse than forming a drip that runs down the wall and results in a puddle (for a cold, but above 0 C vault)? Single items will come to equilibrium much faster than masses of material so a single sheet of paper in a moderately vapor-proof enclosure will cycle in temperature and water content faster than a stack of sheets in a similar enclosure. Chemical deterioration won't be exaggerated by cycling, but it could be a concern because of mechanical changes, especially for laminated materials. Freezer burn is an interesting question, but I'm not so sure that it will be a big problem. Consider 100 grams of sheet film (measured at 70F/50% RH) in paper envelopes in a vapor proof enclosure that is moderately large. I ran a profile on a freezer in a UK institution so their numbers are handy. The freezer ran at -20.9 C (-5.6 F) normally. For 4.5 hours every 20 hours, the freezer rose to -8.8 C (16.16 F) during the defrost cycle. The film is normally being held at -20.9 C and 35% RH. The film will reach equilibrium effectively in less than an hour during which time it is losing about 0.25% of its dry mass in water. Given no constraints, the film would lose 0.24 grams of water when it warms up to -8.8 C. However, as the RH rises in the air space around the film, the film and the water vapor will come to equlibrium before frost forms (before 100% RH is exceeded). Assuming that the air space in the container is roughtly 500 mL or half a liter (about 30 cubic inches) at -20C/35% RH, the air space contains about 0.0002 grams of water vapor. As the film rises in temperature, it releases water as vapor, coming to equilibrium after losing approximately 0.00038 grams of water and coming to equilibrium at approximately 40% RH and -8.8C. The freezer drops back to -20.9 C and approximately 0.00038 grams of frost forms. The outside container gets cold first so the frost should form on the inside surface of the outside container first. The RH in the container is at 40%, but as the film cools back down, it adsorbs water vapor very rapidly. Given the opportunity, the film would pull the 0.00038 grams of water that it lost during the warm-up phase back into the film, but there is only 0.000068 grams (total) available as water vapor immediately. If the frost evaporates quickly as the air dries out, then the film should return back to its original condition and freezer burn isn't a problem. If the frost evaporates slowly, then the film will come to some equilibrium at about 33% RH and then will slowly adsorb water (by diffusion) as the frost evaporates. Diffusion will be quite slow potentially taking almost 200 days to reach equilibrium. Of course, in this time, the film will have gone through additional defrost cycles, further reducing the water content. (This process is unfortunately easier to show on graphs than to describe in words so take my word for it.) However, during the next cycle, the film is closer to its equilibrium point so it will lose less water and, to make things even more complicated, the equilibrium vapor pressure of whatever frosts remains should increase (following the same law that governs the film). At some point the rise in vapor pressure from the frost should stop the film from losing any more water vapor. However, if the frost is very slow to evaporate, then the film will continue to loss water at a slower and slower rate. While there are a number of possible reasons why freezer burn stop, it doesn't hurt to recommend that containers be selected to match the volume of their contents fairly well.
The difference between the situation with paper and photographs and food in the freezer is that you wouldn't want to eat most foods at their equilibrium water content. Lean beef and chicken, for example, are roughly 75% water by mass, whereas their equilibrium water content is probably closer to 10% or 15% water by mass so the meat at equlibrium in a moderately high humidity environment would have 80% to 87% loss of water. As a result, vegetables and meat frozen in big bags tend to be filled with frost because there is so much more water present than the equilibrium water content of the food. One pound of lean steak contains roughly 340 grams of water (while a pound of film at 0 F/60% RH contains only about 17 grams of water.) Fruits and vegetables are similarly high in water content typically. (Red Delicous apples are roughly 85% water and bananas are roughly 76% water, for example.)
In summary:
The problems associated with engineering a temperature and humidity controlled vault are almost always present no matter what conditions are chosen. However, the severity of the problems tend to increase as the vault temperature and RH decrease. In general, the construction and operating costs required to fight these problems increase significantly as the desired temperature and RH in the vault goes down.
-Doug