Half a croissant, on a plate, with a sign in front of it saying '50c'
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Dry Steam Dehumidifer

Regenerate liquid desiccant using dry steam
  [vote for,

When coming up with this idea (and when thinking up reasons for it to be bunned), I made several assumptions:

First, packed column heat exchangers are generally more efficient than indirect contact heat exchangers.

Second, R718 (water) is a more energy efficient refrigerant than what's used in most dehumidifiers.

Third, that the idea's only indirect contact heat exchanger is across such a low pressure differential that it could easily be cheaper than either the evaporator or condenser used in a refrigerative dehumidifier.

Fourth, the cost of two packed column heat exchangers together is less than that of either the evaporator or condenser used in a refrigerative dehumidifier.


Start with an aqueous solution of calcium chloride (CaCl), and spray it into the top of a packed column heat exchanger, and blow air from the room into the bottom of the packed column. The CaCl solution starts out concentrated, and becomes more dilute; the air starts cool and humid, and becomes warm and arid.

The dilute CaCl solution, after passing through the outer packed column, moves through a preheater (from which the liquid gains heat), then through a small turbine (or other hydraulic motor), to the top of a second packed column heat exchanger; this second column is inside of a pressure vessel.

The pressure in this pressure vessel is /equal/ to the vapor pressure of the liquid entering it, such that any added heat will cause evaporation; note that the pressure is *not* so low that flash evaporation occurs.

Upon reaching the bottom of the second column, the CaCl solution will be warmer and more concentrated than it entered; it will be removed by a pump, passed back through the preheater (this time losing heat), and cycled back to the top of the first column.

(The preheater is optional; it improves efficiency at the cost of increased capital cost).

The gas used to concentrate the dilute CaCl will be dry (aka superheated) steam. The steam of course moves from the bottom to the top of the packed column heat exchanger. As the steam loses heat to the liquid, it gains mass and volume, due to the liquid evaporating.

Some of the steam leaving the top of the inner column will be moved, via a blower, through a counter-current heat exchanger, and back to the bottom of the column.

The rest of the steam leaving the top of the inner column will be compressed, then pass through a very small desuperheater (like the type used for domestic hot water, not the steam engineering type), then the counter-current heat exchanger, in which the steam condenses. The condensate enters a gas/liquid separator, from which it is removed via the condensate pump, which sends it through the desuperheater, then either down the drain or into a removable collection tank.

(The desuperheater is optional; it improves efficiency at the expense of a slightly higher capital cost).

There is a float sensor in the gas/liquid separator; when gas is present (when the liquid level is low enough), a valve is temporarily switched, causing the steam compressor to stop moving steam from the packed column to heat exchanger, and instead move the gas from the gas/liquid separator to the atmosphere. Also, if the liquid level is particularly low, the condensate pump is temporarily shut off.

This should only occur when air enters the pressure vessel; a lot of air will normally enter when the machine is serviced (e.g., when the packing in the inner column is replaced); a very small amount of air will continually enter due to becoming dissolved in the liquid as it flows through the outer packed column. This occasional use of the steam compressor as a vacuum pump should consume a negligible amount of energy over the life of the machine.

The turbine that allows dilute CaCl into the pressure vessel should be able to generate about the same amount of power as the concentrate and condensate pumps consume together.

Since the packing material in the outer column acts as a dust filter, it needs to be replaceable; furthermore, there's always a chance that some luser will contaminate the system (duhr, what happens if I pour in some coffee?), the machine still needs a way for both inner and outer packed columns to be cleaned, and the packing in both replaced. I'm not really sure what the best packing material should be -- /probably/ a random packing, which the user can pour in, for convenience, but the specifics are beyond my expertise.

The steam compressor, the heat exchanger, the gas/liquid separator, and the condensate and concentrate pumps, should all be inside of the pressure vessel, to minimize pressure differentials and minimize material costs.

Since no part of the system is at a temperature below that of the surrounding room, there's no chance of ice forming in the machine when air temperature is at or above 0C.

The steam compressor raises the steam's pressure less than would be necessary if this were a refrigerative dehumidifier; this means that there is a relatively small pressure differential across the surfaces of the indirect contact heat exchanger; this in turn means the heat exchanger can be cheaper and more efficient than it otherwise would be. And did I mention that there's only one indirect contact heat exchanger?

The (slightly corrosive) calcium chloride solution is only passed through direct contact heat exchangers (the packed columns), not through an indirect contact heat exchanger. This means that the sole indirect contact heat exchanger in the system does not need to be as corrosion resistant as it otherwise would be. The packed columns can of course be plastic lined, and I'm pretty sure the packing material isn't going to be corroded by CaCl.

Possible variations on this idea:

Lithium bromide could be used instead of calcium chloride -- this would probably increase the efficiency, but it would also increase the setup and maintenance cost, since (in winter) I can get a 10 pound jug of CaCl ice melt for about $10, whereas I've no clue where to get LiBr.

If a plastic, copper or stainless steel heat exchanger is used, it should be very easy to make the machine into an atmospheric water generator, producing potable water.

goldbb, Aug 10 2011

Drawing of idea https://docs.google...raIjoHOE/edit?hl=iw
[pashute, Aug 15 2011]


       Just so we're clear, this is intended for large-scale industrial applications, right? It seems a tad complex for the thing I use to dry out my basement...
Alterother, Aug 10 2011

       Alterother, it's entirely possible to put a lot of complexity in a small casing, so it could be used to dry out your basement.   

       On the other hand, if the device is made portable, someone is sure to tip it over, or store or transport it on it's side, resulting in the absorbent liquid spilling out. CaCl isn't particularly toxic, nor excessively caustic... but once spilt, the liquid doesn't dry out, eventually resulting in water damage to one's floor, unless mopped or vacuumed up (possibly requiring a steam cleaner, if spilled on a carpet).   

       Thus, it's most practical use is as a stationary device, dehumidifying a whole home, warehouse, office building, and so on.   

       Conveniently, if the system is used for dehumidifying a building with separate zones, we can use a separate packed column direct contact heat exchanger for each zone, plumbed to a central distiller. The aqueous CaCl can be pumped at close to atmospheric pressure, so the pipes involved don't have to withstand high pressure.
goldbb, Aug 10 2011

       I normally like your ideas, they have a strait engineering style that I wish I could manage. This one seems a bit more complex than necessary   

       Why not a bowl of Calcium Chloride crystals, two electrodes to electrolyse any water that is collected. The Hydrogen can be released into the room, air need to contain 4% Hydrogen by volume before there is a problem.
j paul, Aug 11 2011

       Wouldn't the other electrode evolve chlorine?
mouseposture, Aug 12 2011

       That would work wonderfully, as long as one doesn't mind that the electrolysis will also generate chlorine gas.   

       I'm sure you could sell such a dehumidifier to 8th_of_7. :)
goldbb, Aug 12 2011

       I'd suggest MgCl2 rather than CaCl2 as it is a smaller molecule than calcium chloride and more hygroscopic to boot. Inexpensive compared to lithium bromide.
AusCan531, Aug 14 2011

       AusCan, thanks :) Considering that I've actually seen Magnesium Chloride available to buy for use as ice melt, so it's definitely a readily available substance, and not unreasonably priced.   

       That said, how much practical difference will it make in terms of humidity absorption? Remember, whatever water we absorb from the air, we must desorb into the superheated steam.
goldbb, Aug 14 2011

       OK trying to draw this. (BTW BB Why don't you? Its easy with google docs)   

       Here goes, see link.   

       I lost you at the desuper heater and counter current. You'll have to draw this yourself, or wait for me to find more time.   

       In any case, its clear that this should be done with a presentation showing layers being added for clarity.
pashute, Aug 14 2011

       [goldbb] 1 kg of magnesium chloride will absorb more water than 1 kg of calcium chloride and is slightly less corrosive than CaCl2. It will also absorb and desorb water quite nicely at the temperatures you propose.
AusCan531, Aug 15 2011


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