Why
I tried this?
After buying the 1st
AC, it intrigued me that how the ac tonnage is calculated. I’d done that with
the various online calculators available in. Browsing thru several sites I
cudn’t gather the exact calculation and as is always the case I tried the hard
method viz., starting from the 1st principles. I don’t know whether
it’s correct but I think this will give a rough estimate, at least the min
requirement, while buying ur AC. In case u think otherwise, plz help me out
refining the model.
A
bit of a history
The ton that is referred in
AC manuals is actually a measure of power which is that power required by a short ton (i.e. 2000lb) of ice to be
converted to water at 00C in 24hr. Thus,
1 ton = mass of ice x latent
heat of ice / time = 3.5kW.
Calculations
The room where the AC is to
be installed must be measured for its length (L), breadth (B) and height (H).
Thus the total volume (V) of air, considering it to be an ideal rectangular
parallelopiped devoid of any furnitures (crazy right!) will be given by
V = LBH.
If u know the density (rair) of air, assume
a value that is denser nearer to room temp., which will give u a conservative
value (a good thing for a safer engg design, though the term safe is relative!),
u may calculate the mass of air needed to be cooled by
mair = rair x V.
For
my design I assumed it to be 1.2kg/m3.
In general for any temp diff
(DT) in Kelvin
(the temp diff will be same in 0C and K), the amount of heat (DHair) to be absorbed
will be
DHair = mairSairDT,
where,
Sair is the heat
capacity of air.
Now
generally u require AC to cool down to not more than a temp diff of 100C.
Again a good AC assures that
the cooling to any temp will be within 2min. Thus the power required for the
cooling in Dt will be.
Pair = DH/Dt.
Now the model may be refined
by adding other power devices (Pother) that u expect in the room. So
if T TR be the tonnage of the AC,
T = (Pair + Pother)/3.5kW.
Remember
the only hitches
U have to know certain
physical constants and the SI system of units. If that’s too much
then follow the shortcut given below.
Feedback
So plz tell me this helped!
Shortcut
For a 10' x 10' x 10' room u
will require 0.818 TR according to my logic. Taking this apply the unitary
method for any other volume.
Thanx
MSOffice