
re: what is the best method for calculation how much a/c you'll need?
11 jul 2001
john wrote:
>>what is the best method for calculation how much a/c you'll need?
ohm's law for heatflow, with a few extra bits and pieces.
>>i also just wondered what would be considered a reasonable level of
>>humidity to live in? i think where i am it is far to humid.
ashrae says about 60% max, depending on air and radiant temps and air
speed and activity level (eg 1 met) and clothing resistance (eg 1 clo.)
anyone know a definition of "comfortable" (min ppd) that relates these
factors with a single equation? a maylasian might say 60% is very dry.
bigbear wrote:
>>you need to figure how much heat is coming into the conditioned space via
>>walls, ceilings/roof, floor, windows etc. also how much heat is being
>>generated by people, lights, appliances etc.
>that's easy! shitloads of heat in summer...
is a shitload an inverse rat's ass? :) let's ask george ghio.
he's an expert on upsidedown rat's asses.
say it's 90 f with 80% rh on a summer day, and 70 f at 60% indoors.
this corresponds to vapor pressures of 0.8exp(17.8639621/(460+90))
= 1.16 "hg outdoors and 0.45 "hg indoors, with humidity ratios wo
= 0.62198/(29.921/1.161) = 0.0251 and wi = 0.0095, so wowi = 0.0156
pounds of water per pound of dry air. since it takes about 1000 btu
to condense a pound of water vapor, every cfm of outdoor air adds
about 60m/hx0.075lb/ft^3x1000btu/lbx0.0156 = 70 btu/h to the cooling
load for moisture removal plus about 20 btu/hf to lower the temp.
>my double glazing has a few cracks in the outside window, which i
>haven't got around to replacing yet.
say you do that and reduce the air infiltration to 15 cfm (a min
ventilation standardyou would have to caulk inside very carefully
to achieve this, and open the windows or turn on a ventilation fan
once in a while), so you need 90x15 = 1,350 btu/h for ventilation.
>...i don't think my curtains help... so i plan to replace them with
>large thick curtains which are white on one side and black on the other.
>i'll use the white side towards the sun in summer and the other way
>in winter.
good idea. let's say that eliminates summer solar heat gain.
(otherwise, we might add a peak 200 btu/hft^2 of window.)
btw, have you filled your apartment with thermal mass, say
65 cases of coca cola (tm)?
>i also live directly above a kitchen, so that can contribute to the
>warming and cooling quite a lot...
say you convince the cooks to open the windows downstairs...
>i have a lot of electronic stuff which gives off heat, and i estimate
>that will increase by quite a lot in the next year as well. i plan to
>add a few more appliances like fridge, kettle, bigger tv etc.
let's suppose you are using 300 watts on average, with another
100w for your body. that adds 400wx3.41btu/hw = 1364 btu/h.
>how would i go about calculating all this?
>i have the size of my room as well.
say you live in a 16' cube with us r16 walls, and one of the walls
is an r4 window with a 16'x16'/r4 = 64 btu/hf thermal conductance.
the rest add 5x16x16/r16 = 80 to that, making the total 144 btu/hf,
so the room gains (90f70f)144btu/hf = 2880 btu/h by conduction
plus 1350 plus 1364 = 5594 btu/h or 1.64 kw of cooling.
with a cop of 3, you might use a 600 w 6000 btu/h air conditioner.
nick

