Design conditions

The processes described in the previous chapter are used to condition moist air in a real air — conditioning systems. The AC system is used to remove both sensible and latent heat from a space. Relationship between sensible and latent heat is defined as SHF (sensible heat factor).

Sensible loads: Q

Latent loads: O

Sensible and Latent loads: D


Dishwasher (100 dishes/h): sensible 167 W (570 Btu/h), and latent 65 W (220 Btu/h)

Person (male, moderate office work): sensible 70 W (250 Btu/h), and latent 30 W (105 Btu/h) Light bulb: sensible 100 W = 341 Btu/h

AC system provides airflow:

• at certain T to meet sensible loads

• at certain W to meet latent loads


Where 7"supply and fFsupply must give sensible and latent conditioning proportional to the loads.

Condition line represents line in the psychrometric chart through space conditions with the slope defined by SHF. This line contains all feasible supply air states.

Supply farther from space condition => Smaller mass flow required

Design condition is defined by:

• dry bulb temperature,

• humidity and

• pressure.

Design condition + SHF + 7^upply => Fix mass flow and supply air condition Example

Space design condition is 72°F and 50% RH. The total cooling load is 1,200,000 Btu/hr (lOOton), and the sensible cooling load is 720,000 Btu/hr (60 ton). Compare flow rates for (a) AT = 10°F and (b) AT= 20°F. AT = (Tspace — Tsupply)

ton=12,000 Bin h



Construct condition line by using the protractor.

Design conditions


Posted in Fundamentals of Heating. Ventilating, and Air-Conditioning