Component 206: Steam Spray Humidifier by HVACSIM+
General Description
The steam spray humidifier model calculates an outlet air temperature,
flow rate, and humidity ratio for a constant pressure process in which steam is
injected into an air flow stream to increase the humidity of the air. The
outlet air humidity ratio is limited either by the steam flow rate or by the
saturation humidity ratio (at atmospheric pressure) times a saturation
efficiency. The mass flow rate of the air increases slightly due to the
addition of moisture.
Nomenclature
Cpa - specific heat of dry air (kJ / kg-K)
Cpai - specific heat of moist inlet air (kJ / kg-K)
Cps - specific heat of steam or water vapor (kJ / kg-K)
Patm - atmospheric pressure (101.325 kPa)
Tai - temperature of inlet air (C)
Tao - temperature of outlet air (C)
Ts - temperature of inlet steam (C)
wai - mass flow rate of moist inlet air (kg/s)
wao - mass flow rate of moist outlet air (kg/s)
ws - mass flow rate of inlet steam (kg/s)
n - saturation efficiency ( - )
Hai - inlet air humidity ratio (kg water / kg dry air)
Hao - outlet air humidity ratio (kg water / kg dry air)
Haos - saturation humidity ratio (kg water / kg dry air)
Mathematical Description
The inlet air specific heat depends on the inlet air humidity ratio:
Cpai = (Cpa + Hai*Cps) / (1 + Hai)
where Cpa is the specific heat of dry air and Cps is the specific heat of steam
or water vapor. The air outlet temperature is a weighted average of the inlet
air and steam temperatures:
Tao = (ws*Cps*Ts + wai*Cpai*Tai) / (ws*Cps + wai*Cpai)
Assuming that all of the entering steam is absorbed by the air, the outlet
humidity ratio is
Hao = Hai + ws*(1 + Hai) / wai
The saturation humidity ratio at the outlet temperature is calculated next,
using the subprogram PSATS described in Chapter 3 to find the saturation
pressure at the outlet temperature. The constant in the following equation is
the ratio of water to air molecular weights.
Haos = (0.62198*PSATS(Tao)) / (Patm - PSATS(Tao))
where Patm is 101.325 kPa. The outlet humidity ratio, Hao, cannot exceed the
saturation efficiency times the outlet humidity ratio at saturation, n*Haos:
Hao = min(Hao, n*Haos)
Component 206 Configuration
Inputs Description
1 Ts - steam temperature (C)
2 Tai - inlet air temperature (C)
3 Hai - inlet air humidity ratio ( - )
4 wai - inlet air mass flow rate (kg/s)
5 ws - steam mass flow rate (kg/s)
Outputs Description
1 Tao - outlet air temperature (C)
2 wao - outlet air mass flow rate (kg/s)
3 Hao - outlet air humidity ratio ( - )
Parameters Description
1 n - saturation efficiency
Reference:
1. HVACSIM+ Building Systems and Equipment Simulation Program Reference
Manual (NBSIR 84-2996)
Daniel R. Clark
United States Department of Commerce
National Institute of Standards and Technology
Gaithersburg, Maryland 20899-0001