
re: re. alternative heat rejection to cooling towers
26 nov 2001
mike hardy wrote:
>hello,
hello,
>i am working on a job at the moment with around 300kw of cooling...
about a million btu/h.
>as this is a theatre (mainly constant cooling) with quite a few other
>areas, with a heating requirement, some heat recovery is possible.
thermal mass comes to mind. is the theater occupied at 2 am? you might
need to dump 12hx300kwx3412btu/kwh = 12.3 million btu/day.
>the heat rejection water from the gas fired (absorption) chillers is around
>37.5ºc and needs to be cooled to around 32ºc...
so the average temp is about 95 f. you might replace the chillers with
12.3m/10f/4.4btu/f = 279,164 2liter soda bottles and night ventilation :)
>has anybody seen a modified sort of fountain used for heat rejection?
the 1964 john deere hq (arch eero saarinen, mech burns & mcdonnell
engineering co.) in moline, il (6412 (f) cdd) uses a lake and fountains
as a spray pond to reject heat from its 1600 ton (5.6 mw) cooling plant.
>any experiences or thoughts would be most gratefully received.
a shaded pond or waterwall would consume less energy than a fountain...
nrel says the average humidity ratio w = 0.0133 pounds of water per pound
of dry air in phila in july. the average windspeed is 8 mph and the daily
min and max temps are 67.2 and 86.1 f. this corresponds to a dew point
tdp = 9621/(14.46+ln(1+0.62198/w))460 = 64.9 f. if the night air temp
to = 72 f (halfway between the 24hour average and daily min) and the
outdoor vapor pressure pa = 29.921/(1+0.62198/w) = 0.626 "hg, the wet bulb
twb (r) = 9621/(22.47ln(460+to+100patwb)), ie 67 f, after some iteration.
phil niles says a pond loses 1.63x10^9((t+460)^4a(to+460)^4) btu/hft^2
by radiation, where a = 0.002056tdp+0.7378 = 0.87 in this case, so a 95 f
pond in 72 f air radiates 41 btu/hft^2, and loses qc = (0.74+0.3v)(tto)
= 72 btu/hft^2 by convection, and qe = b(ttwb)qc by evaporation, where
b = 3.01(0.74+0.3v)((t+twb)/651) = 14.1, so qe = 14.1(9567)72 = 323
btu/hft^2. the total is 436 btu/hft^2, so a 48'x48' 95 f roofpond might
lose 12hx48x48x436 = 12 million btu over an average 12 hour july night...
nick

