re: a superinsulated cat house
17 dec 2000
>>how can i measure and predict and control the indoor humidity?
btw, this is economically important in greenhouses, where plants produce
lots of water vapor. the usual way to get rid of it is to ventilate with
outdoor air in the winter, but that wastes lots of energy...
the simplest method for relative humidity control in cool or cold
weather is to bring in outside air, heat it, and allow it to absorb
moisture before exhausting it to the outside... a greenhouse filled
with mature pot plants may lose up to about 0.15 lb. of water vapor
per square foot of greenhouse floor area per hour during the day
[about a pound per square foot per day]; loss at night will be less.
if evaporated moisture is not removed, rh will increase until the
air is saturated or until condensation begins on a cold surface...
if outside air at 20 f and 80% rh is brought into the greenhouse
and heated to 60 f, it will absorb 0.0005 lb. of water vapor per
cubic foot of air if the final rh is 70%. it would take 300 ft^3 of
air/hr. to remove the 0.15 lb. of water vapor produced per square foot
of greenhouse floor area. it would require about 300 btu/hr. [about
10 gallons of oil per hour for a 30x100' greenhouse!] to warm the air
to 60 f...
from greenhouse engineering, nraes-33, 3rd revision, august, 1994,
northeast regional agricultural engineering service,
152 riley-robb hall/coop extension/ithaca ny 14853-5701
this also happens in human houses: the andersen windowfolk estimate that
the activities of an average family of 4 (breathing, washing, cooking...)
evaporate 2 gallons per day of water inside a house, about 4 pounds per
person per day or 167 btu/h of latent heat, less than the (70f-30f)15cfm
= 600 btu/h needed for heating ashrae's minimum standard 15 cfm per person
of winter ventilation air, so we can potentially use vs waste all of that
latent heat, using incoming outdoor air as a cold sink.
>i assume sorption rate is just about zero in your case? (is the
>shipping container waterproof?) so that's rate of production minus
>rate of exchange...
the polystyrene board is waterproof, but the towel on the floor isn't.
it might absorb condensed water from the beadboard walls when the cat
is in situ and evaporate in his absence. condensation from inside the
cat door would tend to drip off the bottom edge and end up outdoors.
this might freeze the door closed in very cold weather.
>has some relevant models.
>>can i condense the water vapor in a passive chimney air-air heat
>>exchanger to save that latent heat, with only moderate indoor
with no heat exchanger, 1 cfm of outdoor air would make the indoor rh 53%
at 70 f, but that raises the box conductance to about 2.56 btu/f. with
no condensation, the latent heat is wasted, and only 68-22.5 = 45.5 btu/h
is available to warm the box. this makes the winter temp rise 45.5/2.56
= 17.8 f for an indoor temp of 47.8 on a 30 f day. too cool.
>how would you attach a double-wall chimney, or were you thinking
>some other strategy?
we might model this as 2 heat exchangers in series, with no condensation
in the first stage, and condensation in the second...
cooling 1 cfm of indoor air at 70 f and 50% rh to its 50 f dew point
requires a heat exchanger effectiveness e = (70-50)/(70-30) = 0.5.
ntu = e/(1-e) = 1 = au/cmin = a(1.5btu/h-f-ft^2/2)/1 for a counterflow
heat exchanger with slowly-moving air films on both sides of a good
conductor and heat capacity rate ratio z = cmin/cmax = 1, ie we need
at least a = 1.3 ft^2 of heat exchange area. sensible cooling vs cooling
with condensation is a more difficult job for the heat exchanger, since
condensation raises the airfilm conductance by something like 1000x.
recovering half the ventilation air heat lowers the box conductance to
2.56-e = 2.06 btu/h-f and increases the box temp rise to 45.5/2.06 = 22 f,
making the box 52 f on a 30 f day, still too cool.
suppose the second stage (condensing) heat exchanger lowers the outgoing
air temp to 34 f, so it escapes with a saturated water vapor pressure
pw = exp(17.863-9621/(460+34)) = 0.199 "hg and w = 0.62198/(29.921/pw-1)
= 0.00417. at 1 cfm, about 60 ft^3/13.5 = 4.44 lb/h of air flows out of
the box with about 0.00417x4.44 = 0.0185 pounds of water vapor containing
about 944x0.0185 = 17.5 btu/f of latent heat, but the 30 f air that flows
into the box with average w = 0.0032 has 944x0.0032x4.44 = 13.4 btu/h of
latent heat, so the cat only needs to supply the difference, 4.1 btu/h,
and 22.5-4.1 = 18.4 btu/h more heat is available for incoming air, ie
a total of 45.5+18.4 = 63.9 btu/h.
meanwhile, a lower outgoing temp means the incoming air needs less heat:
if e = (68-34)/(68-30) = 0.89 for the combined heat exchanger, the box
conductance becomes 2.56-0.89 = 1.67, and the box becomes 30+63.9/1.67
= 68.4 f inside on a 30 f day.
the second exchanger cools 1 cfm of 50 f air to 34 f with effectiveness
e = (50-34)/(50-30) = 0.8 = 1-exp(-ntu) with z = 0 (ie condensation),
so ntu = -ln(1-0.8) = 1.61 = a(1.5)/1, and a = 1.61/1.5 = 1.07 ft^2.
considering natural stack-effect airflow, the descending outdoor air
enters the chimney at 30 f and enters the box at 30+0.69(68-30) = 56 f,
so it has a mean temp of 43 f, while the ascending indoor air enters
the chimney at 68 f and leaves at 35, so it has a mean temp of 52. the
temperature difference between the two air columns is 9 f. making 1 cfm
flow with a 9 f temp difference and a 1 foot height difference requires
that 1 cfm = 16.6avsqrt(1'x9f), ie vent area av = 0.0201 ft^2 (2.9 in^2)
at the top and bottom.
the outdoor air entry of the "chimney" might be the 1/8" air gap between
the inside box wall and a 2' wide x 1' tall layer of plastic film inside
the box (24"x1/8"= 3in^2), so outdoor air falls down from a 24"x1/8" slit
in the top of the box and exits via a 1/8" slot between the bottom edge
of the film and the floor of the box. indoor air might rise up the other
side of the film through a 1/8" air gap between the film and a vertical
1" layer of insulation board, and exit the top of the box through another
slot. a projection above the box and between the slots might help keep
the airstreams from mixing.
how can we scale this up for a greenhouse, using 3 layers of film (a 3-
vs 2-layer cover) and 2 small blowers? bring in fresh air from the south
to the north side through the outer film gap while exhausting indoor air
from the north to south side through the inner film gap...
and why do greenhouses need outdoor air? why not dehumidify using the
inner film air gap as a closed system? plants need co2, but that might
come from compost. one local prizewinning orchid grower's house is
completely sealed up from the outdoors, with an air conditioner inside
for cooling and some radiators for heating and an alcohol lamp for co2
and some fans to wiggle the plants to increase co2 uptake...