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
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
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
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
(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
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.