Humidification Humidifier Load Calculations and Calculator

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Humidification Humidifier Load Calculations and Calculator

When calculating energy requirements for a humidification system, the effect of dry air on any material supplying it with moisture should be considered. The release of liquid in a hygroscopic material to a vapor state is an evaporative process that requires energy. The source of energy is heat contained in the air. Heat lost from the air to evaporate moisture equals the heat necessary to produce an equal amount of moisture vapor with an efficient humidifier. If proper humidity levels are not maintained, moisture migration from hygroscopic materials can have destructive effects.

The true energy required for a humidification system must be calculated from the actual humidity level in the building, not from the theoretical level.

Preview Humidification Humidifier Load Calculator

The humidification load depends primarily on the rate of natural infiltration of the space to be humidified or the amount of outdoor air introduced by mechanical means. Other sources of moisture gain or loss should also be considered. The humidification load H can be calculated by the following equations:

Eq. 1
For ventilation systems having natural infiltration,
H = ρ V R ( W_i - W_o ) - S + L

Eq. 2
For mechanical ventilation systems having a fixed quantity of outdoor air,
H = ρ 60 Q_o ( W_i - W_o ) - S + L

Eq. 3
For mechanical systems having a variable quantity of outdoor air,
H = ρ 60 Q_t ( W_i - W_o ) ( ( t_i - t_m ) / ( t_i - t_o ) ) - S + L

where

H = humidification load, lb of water/h
V = volume of space to be humidified, ft³
R = infiltration rate, air changes per hour
Q_o = volumetric flow rate of outdoor air, cfm
Q_t = total volumetric flow rate of air (outside air plus return air), cfm
t_i = design indoor air temperature, °F
t_m = design mixed air temperature, °F
t_o = design outside air temperature, °F
W_i = humidity ratio at indoor design conditions, lb of water/lb of dry air
W_o = humidity ratio at outdoor design conditions, lb of water/lb of dry air
S = contribution of internal moisture sources, lb of water/h
L = other moisture losses, lb of water/h
ρ = density of air at sea level, 0.074 lb/ft³

References

ASHRAE Handbook HVAC System and Equipment - 2012