re: water vapor, rocks, air leakiness and relative humidity
8 feb 1997
andrew mckegney wrote:
>um, could you explain how this works , please?
>>liquid polyethylene glycol can absorb 11 times its weight in water.
mostly i've just read about this, and i think that may be 1x for glycol,
vs 11x for lithium chloride. here are some excerpts from pages 19.1-19.4
of the 1993 ashrae hof:
virtually all materials are dessicants; that is, they attract and hold
water vapor. wood, natural fibers, clays and many synthetic materials
attract and release moisture like commercial dessicants do, but they
lack the holding capacity of what are known as dessicant materials. for
example, woolen carpet fibers attract up to 23% of their dry weight in
water vapor, and nylon can take up almost 6% of its weight in water. in
contrast, a commerical dessicant takes up between 10 and 1100% of its
dry weight in water vapor, depending on its type and the moisture
available in the envrironment. furthermore, commmercial dessicants
continue to attract moisture even when the surrounding air is quite dry,
a characteristic that other materials do not share.
all dessicants behave in a similar way--they attract moisture until they
reach equiliburium with the surroundiung air. moisture is usually removed
from the dessicant by heating it to temperatures between 120 and 500 f
and exposing it to a scavenger airstream [eg a sunspace vented to outdoors
on a sunny summer day.] after the dessicant dries, it must be cooled so
that it can attract moisture once again. sorption always generates sensible
heat equal to the latent heat of the water vapor taken up by the dessicant,
plus an additional heat of sorption that varies between 5 and 25% of the
latent heat of the water vapor. [richard komp says materials that undergo
chemical change when absorbing water do not necessarily heat up while doing
so, which seems useful in summertime.] this heat is transferred to the
dessicant and to the surrounding air.
the hof says dessicants are used in many air-conditioning applications,
particularly when the cost of [solar :-)] energy to regenerate the dessicant
is low when compared with the cost of energy to dehumidify the air by chilling
it below its dew point. it also says they show promise in improving indoor air
quality in typical building hvac systems by adsorbing hydrocarbon vapors at
the same time they collect moisture from air.
the vapor pressure of a liquid absorption solution is directly proportional
to its temperature and inversely proportional to its concentration. figure
4 [showing surface vapor pressure of different concentrations of glycol
increasing with temperature, eg 98% glycol increasing from about 0.1" hg at
60 f to to 1" hg at 140 f with a dew point of 79 f (vs water at 6" hg at
140 f with a dew point of 140 f)] illustrates the [decreasing] effect of
increasing dessicant concentration on the water vapor pressure at its
surface. the figure shows the vapor pressure of various solutions of water
and triethylene glycol, a common commercial dessicant. as the glycol content
of the mixture increases, its vapor pressure decreases. this pressure
difference allows the glycol solution to absorb moisture from the air
whenever the vapor pressure of the air is greater than that of the solution.
...the behavior of a liquid dessicant can be controlled by adjusting its
concentration, its temperature, or both. dessicant temperature is controlled
by simple heaters and coolers. concentration is controlled by heating the
dessicant to drive moisture out into a waste air stream...
...liquid dessicants have an especially high water-holding capacity. each
molecule of lithium chloride, for example, can hold two water molecules,
even in the dry state. above two water molecules per molecule of licl, the
dessicant becomes a liquid and continues to absorb water. if the solution
is in equilibrium with air at a 90% rh air moisture condition, approximately
26 water molecules are attached to each molecule of licl. this represents
a water absorption of more than 1000% on a dry mass basis.
as a practical matter, however, the absorption process is limited by the
surface area of a dessicant exposed to the air being dehumidified and the
contact time allowed for the reaction. more surface area and more contact
time allows the dessicant to approach its theoretical capacity. commercial
dessicant systems reflect these realities either by spraying the dessicant
onto an extended surface much like in a cooling tower, or holding a solution
in a rotating extended surface with a large solution capacity...
which suggests a glycol pool in a sunspace with a glycol fountain in a house,
or a granular dessicant sunspace floor made from lava or expanded shale or
some sort of material containing lithium or calcium chloride or a silica gel
or some sort of zeolite...
figure 7 on page 19.5, "sorption isotherms of various dessicants at 72 f"
is a graph of mass of water vapor absorbed as a function of % rh at 72 f,
ranging from about 100% by weight for triethylene glycol (dow chemical) to
1000% for licl (cargocaire engineering division of munters usa, with a graph
by itself) at 90% rh. this dessicant chapter references "lithium chloride
technical data" bulletin 151, 1988, from foote mineral in exton, pa.
it also says
the useful life of dessicant materials depends largely on the quantity and
type of contamination in the airstreams they dry. in commercial equipment,
dessicants last between 10,000 and 100,000 hr and longer before they need
replacement... liquid absorbents are more susceptible to chemical reaction
with airstream contaminants other than water vapor than are solid[s]. for
example, certain sulfur compounds can react with lithium chloride to form
lithium sulfate, which is not a dessicant. if the concentration of such
compounds in the airstream were below 10 ppm and the dessicant were in use
24 h a day, the capacity reduction would be approximately 10% to 20% after
three years of operations. [at] 30 ppm, the capacity change would occur
after one year... in air-conditioning applications, dessicant equipment is
designed to minimize the need for dessicant replacement in much the same way
that vapor compression cooling systems are designed to avoid the need for
compressor replacement. unlike filters, dessicants are seldom intended to
be frequently replaced during normal service in an air-drying application.
i'd like to know more about this subject, for instance what are the rates
for water vapor absorption, and how much do these materials cost?