re: winter humidification wastes energy
29 oct 2004
>> in cold climates in the winter, a large factor you have missed is
>> radiative heat transfer to the cold exterior walls. since radiative
>> heat transfer depends on the fourth power temperature delta, cold
>> walls can make you feel cold even when the air is warm and humid.
>how cold? to investigate this, you might make ta = 70 f and
>tr = 70-r0.66(70-30)/(r20+r0.66) = 68.7 in that basic program,
>for an r20 wall on a 30 f day.
making 1 exterior wall of a 20x20' room 68.7 f makes the mean radiant
room temp about 69.82, which requires raising the room air temp from
70 to 70.15 f to keep the same comfort level. no big deal. perhaps you
came up with a similar answer, or maybe you had no idea of the answer.
the latter seems more likely, given your posting.
don ocean wrote:
>...we do use forced air heat to the outside walls and returns
>to the inside walls.
that wastes energy...
>...warm air rises. harry thomason used a central supply with the heat
>source in the basement and a long slotted grill near the ceiling to
>make a thermal chimney and floor returns near outside walls to make
>good natural airflow.
which can save 400 watts of blower power and significant heat energy.
adding a 4'x8' r2 window to the exterior wall above with a supply vent
underneath which creates 4 btu/h-f-ft^2 of moving airfilm conductance
makes the indoor window surface temp 65.77 f and the interior surface
of the exterior wall 68.94 and mean radiant room temp about 69.71, which
requires raising the room air temp to 70.24 to keep the same comfort level.
this room requires about (65.77-30)4'x8'/r2 = 572 btu/h for the window
+ (68.94-30)128ft^2/r20 = 249 btu/h for the wall, a total of 821 btu/h.
>this keeps the outside walls slightly cooler. we might make up for that
>by slightly raising the room temp, and end up using less energy.
with a lower-velocity lower-temp return vs supply under the window and
a 1.5 btu/h-f-ft^2 slow-moving airfilm conductance near the window, its
indoor surface becomes 60.30 f, the interior surface of the exterior
wall becomes 69.10, and the mean radiant room temp becomes about 69.52,
which requires raising the room air temp to 70.40 to keep the same comfort.
this room requires about (60.30-30)4'x8'/r2 = 485 btu/h for the window
+ (69.10-30)128ft^2/r20 = 250 btu/h for the wall, a total of 735 btu/h,
about 10% less than the previous room, with a supply register under
the window. this room is equally comfortable, unless we sit close to
the unshaded window.
10 clo = 1.2276'clothing insulation (clo)
20 met=1.1'metabolic rate (met)
30 wme=0'external work (met)
40 taf=70.23841'air temp (f)
50 ta=(taf-32)/1.8'air temp (c)
60 twallf=30+(taf-30)/(20+2/3)*20'inside wall temp (f)
70 gw=4'indoor window air film conductance (btu/h-f-ft^2)
80 twindf=30+(taf-30)/(2+1/gw)*2'inside window temp (f)
100 tr=(trf-32)/1.8'mean radiant temp (c)
110 vel=.1'air velocity
120 rh=50'relative humidity (%)
130 pa=0'water vapor pressure
140 def fnps(t)=exp(16.6536-4030.183/(ta+235))'sat vapor pressure, kpa
150 if pa=0 then pa=rh*10*fnps(ta)'water vapor pressure, pa
160 icl=.155*clo'clothing resistance (m^2k/w)
170 m=met*58.15'metabolic rate (w/m^2)
180 w=wme*58.15'external work in (w/m^2)
190 mw=m-w'internal heat production
200 if icl<.078 then fcl=1+1.29*icl else fcl=1.05+.645*icl'clothing factor
210 hcf=12.1*sqr(vel)'forced convection conductance
220 taa=ta+273'air temp (k)
230 tra=tr+273'mean radiant temp (k)
240 tcla=taa+(35.5-ta)/(3.5*(6.45*icl+.1))'est clothing temp
250 p1=icl*fcl:p2=p1*3.96:p3=p1*100:p4=p1*taa'intermediate values
290 n=0'number of iterations
300 eps=.00015'stop iteration when met
310 xf=(xf+xn)/2'natural convection conductance
330 if hcf>hcn then hc=hcf else hc=hcn
360 if n>150 goto 490
370 if abs(xn-xf)>eps goto 310
380 tcl=100*xn-273'clothing surface temp (c)
390 hl1=.00305*(5733-6.99*mw-pa)'heat loss diff through skin
400 if mw>58.15 then hl2=.42*(mw-58.15) else hl2=0'heat loss by sweating
410 hl3=.000017*m*(5867-pa)'latent respiration heat loss
420 hl4=.0014*m*(34-ta)'dry respiration heat loss
430 hl5=3.96*fcl*(xn^4-(tra/100)^4)'heat loss by radiation
440 hl6=fcl*hc*(tcl-ta)'heat loss by convection
450 ts=.303*exp(-.036*m)+.028'thermal sensation transfer coefficient
460 pmv=ts*(mw-hl1-hl2-hl3-hl4-hl5-hl6)'predicted mean vote
470 ppd=100-95*exp(-.03353*pmv^4-.2179*pmv^2)'predicted % dissatisfied
480 goto 500
500 print taf,twallf,twindf,trf,pmv
70.23841 68.9404 65.76747 69.71472 1.408771e-04
innova airtech instruments has an excellent comfort web site...