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re: hvac ???????
21 feb 1998
marshall e. wrote in sci.engr.mech:
>can you please help with the following.....
i can try, or you might consult your local hvac criminal...
>i want to place a coil in an area where the normal temperature is
>105-120 degrees f. i want to pump water at 20 gallons per minute
>at 70 degrees f through the coil. i am thinking that there will
>be approximatly 25 square feet of coil surface. there will be
>a fan blowing the 105-120 degree air through the coil. i am truly
>guessing that the fan will move around 1500 cfm. the coil would be
>similar to ones found on residential outside units for condensing...
perhaps you need a high-temperature fan, eg a fine dutch 16"
2700 cfm multifan from grainger, stock # 4c861, $380, 311 f,
or a few of their 10" 560 cfm 4c688 $60 149 f cooling fans.
or you might use a junkyard $35 1984 dodge omni automobile radiator with
its attached 12 volt fan. did it lose heat from burning a gallon of gas
per hour when idling, say 100k btu/hour, with 200 f water and 100 f air,
and a thermal conductance of about 100k/(200-100) = 1000 btu/h-f?
>1. what is the drop in air temperature?
>2. what is the increase in water temperature?
what the air loses, the water gains, so these are related by flow rates
and heat capacities, but more information is needed to predict heatflow.
for instance, the air velocity past the coil surface determines its thermal
resistance, and warmer air that first encounters colder water (an efficient
"counterflow heat exhanger") works better than some other flow patterns.
you might assume the water doesn't change temperature at all as it flows
through this heat exchanger, for starters, since it has a much higher heat
capacity flow rate, about 20x8x60 = 9600 btu/h-f vs about 1500 for the air.
>3. what are the formulas for calculating answers to questions
> like these?
"ohm's law for heatflow," aka newton's law of cooling works pretty well.
q = (thot-tcold)/r. if the air is 110 f and the water is 70 f, and both
flows are large, and the coil has an air film resistance of 0.008 f-h/btu,
the heatflow is about (110-70)/0.008 = 5000 btu/h.
>4. what is a great book to help a non trained engineer like myself
> to understand more about hvac?
first, learn about solar house heating. read physics books. ignore hvac
books and hvac people and hvac practices, or watch what they do and do
otherwise. think of the sun as your friend, not your enemy.
heat exchangers are described on page 3-4 of the 1993 ashrae handbook of
fundamentals (the 1997 edition of this great and sometimes inscrutable
bible just came out, in inch-pound and si units, i guess.) in their terms,
if air enters the box at say thi = 110 f and water enters the coil at
tci = 70 f, with 1500 cfm of air flowing past the coil surface at say,
500 linear feet per minute (about 6 mph) and the coil has a roughish
outside surface, the airfilm surface conductance would be about 2+6/2
= 5 btu/h-f-ft^2 (0.008 "ohm's for 25 ft^2), probably smaller than water
film and coil wall conductances. the minimum heat capacity flow rate is
ch = cmin = 1500 btu/h-f for air, approximately, assuming no condensation,
and cc = cmax = 9600 btu/h-f, so capacity rate ratio z = cmin/cmax = 0.156.
with area a = 25 ft^2, the "number of transfer units" ntu = au/cmin
= 25ft^2x5/1500 = 0.083, and exp(-ntu(1-z)) = 0.932 (where "exp" is the
"e-to-the-x" or inverse natural log button on a $20 casio calculator)
so "effectiveness" e = (1-exp(-ntu(1-z))/(1-zexp(-ntu(1-z)) = 0.079
= (thi-tho)/(thi-tci) = (110-tho)/(110-70) and the outgoing air temp
tho = 110 - 0.079(110-70) = 107 f, with heatflow q = 1500(110-107)
= 4500 btu/h, and the outgoing water temp tco = 70 + q/cc = 70.5 f.
the all-copper 2'x 2' ($200?) shw 2347 duct heat exchanger made by magicaire
(aka united electric at (817) 767-8333) transfers 45k btu/hour between 125 f
water and 68 f air at 1400 cfm, a thermal conductance of about 800 btu/h-f
(r = 0.00127 "ohms") with a 0.1" h20 air pressure drop...
there's an hvac newsgroup too, btw, mostly peopled by freon pumpers who
talk a lot about broken axles and flat rate fees, with occasional humor.
if you sign up, remember to bash paul mulligan.
nick
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