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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 upside-down 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(17.863-9621/(460+90))
= 1.16 "hg outdoors and 0.45 "hg indoors, with humidity ratios wo 
= 0.62198/(29.921/1.16-1) = 0.0251 and wi = 0.0095, so wo-wi = 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/h-f 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 standard--you 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/h-ft^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/h-w = 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/h-f thermal conductance.
the rest add 5x16x16/r16 = 80 to that, making the total 144 btu/h-f,
so the room gains (90f-70f)144btu/h-f = 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.


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