[Greenbuilding] Dehumidification by ventilation

nick pine nick at early.com
Fri Oct 29 08:18:46 CDT 2010


"thorthesailor" <sailorthor at ...> wrote:

> Psychrometric charts are pretty important to those cargo ships
transiting from one climate to another...

Charts are nice for understanding, but formulas let simple computers
control ventilation automatically... 

> ... the mates and captains aboard ship always had to know
how and when to ventilate the cargo holds to prevent condensation

To COOL a cargo or basement, ventilate whenever the outdoor air
temp Ta is less than the air near the cargo Tc. Condensation
can only occur if the dew point Td of the outdoor air is higher
than Tc. If Ta < Tc and Tc < Td, then Ta < Td, but Td is always
less than Ta by definition, so we only need to measure the Ta
and Td temperatures.  

To WARM cargo with outdoor air requires more control. To avoid
condensation, we can only ventilate if Td < Tc, which requires
measuring the RH fraction of the outdoor air and calculating its
dewpoint. In Rankine units, Tdr = Tar/(1-Tarln(RH)/9621), where
ln() is the natural log button on a $9 fx-260 Casio calculator
(After 1619, mates and captains might have known natural logs if 
not calculators http://en.wikipedia.org/wiki/Natural_logarithm.) 

If Tc = 40 F (Tcr = 500 R) and Ta = 60 F (Tar = 520 R) and
RH = 0.49 (ie 49%), Tdr = 520/(1-520ln(0.49)/9621) = 500.7 R,
so ventilation would cause condensation, but RH = 0.47 would
make Tdr = 499.6 R < 500 R = Tcr, so ventilation would be OK.

To DRY a basement or cargo, ventilate with outdoor air if its
absolute moisture content (aka humidity ratio) is less than
the moisture content of the air near the cargo. This requires
measuring the RH of the air near the cargo as well as the RH
of the outdoor air. Air at T (R) and RH fraction R has vapor
pressure P = Re^(17.863-9621/T) "Hg, so we can dry with
outdoor air if Ra < Rce^(9621(1/Tcr-1/Tar)).

For instance, if Tc = 80 F (Tcr = 540) and Ta = 70 F (Tar = 530)
and Rc = 0.6 (RH = 60%), we can dry by ventilation if 
Ra < 0.6e^(9621(1/530-1/540) = 0.84, ie 84%. If Tc = 40 F
and Ta = 100 F, we can dry if Ra < 0.6e^(9621(1/560-1/500))
= 0.08, ie 8%.

Air with vapor pressure P "Hg contains w = 0.62198/(29.921/P-1)
pounds of water per pound of dry air... 60 F (520 R) basement
air at 60% RH has Pc = 0.6e^(17.863-9621/520) = 0.317 "Hg and
humidity ratio wc = 0.62198/(29.921/Pc-1) = 0.00665. Outdoor
air at 70 F and 30% RH has Pa = 0.225 "Hg and wa = 0.0047.

With air weighing 0.075 lb/ft^3, a 15 watt 100 cfm fan can remove
100cfmx60m/hx0.075(wc-wa) = 0.88 pounds of water per hour (21 pints
per day) from the basement with an energy factor of 0.88/2.2/0.015
= 27 liters per kWh, 18 times more efficiently than a 1.5 EF
Energy Star dehumidifier http://www.energystar.gov/index.cfm?c=dehumid.pr_basics_dehumidifiers

Nick
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