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Correct way of calculating cooling load??

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SoD

Mechanical
Nov 21, 2018
8
In calculating cooling load of outside air,

Data:
Inside -
DBT (in ℃) = 25
Wi (in Kg/ Kg of air) = 0.010983205​

Outside
DBT (in ℃) = 40
Wo (in Kg/ Kg of air) = 0.026247157​

Air flow rate = 3 m3/s

Equation:
SH Load = mass rate * Cp * dT
LH load = mass rate * Cp * dW

Method 1: (It is mostly used in HAVC applications)
Considering standard air properties,
cp = specific heat of air = 1.006 kJ/kg oC
ρ = density of air = 1.202 kg/m3

OR

Method 2:
Considering separate conditions and values such as,

Outside condition Inside condition
Temp (deg cel) 40 25
Humidity ratio (in Kg/ Kg of air) 0.026247157 0.010983205
Cp (Kj/Kg K) 1.007 1.006
density (Kg/m3) 1.127 1.184



Both methods give huge difference in answer.
Please help in choosing correct way.



 
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If I'm interpreting the above correctly then your method 1 outlines sensible cooling which accounts only for a difference in temperature.
a cooling coil will typically remove moisture which so the formula you should use should be based on the change in enthalpy which will encompass changes in temperature and moisture.
I suggest you mark your points up on a psychrometric chart and go from there.
 
Use psychometric charts as these have enthalpy values, the difference of which will allow you to estimate heat removal.
 
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