Component 204: Evaporative Humidifier by HVACSIM+
General Description
This component, which is based on a model developed by Chi [1],
represents an air stream passing over a water pan or through a porous pad,
increasing the humidity and decreasing the temperature of the air. The model
assumes that the process is adiabatic, which means that the water temperature
is assumed to be equal to the entering air wet bulb temperature. In addition,
the process is assumed to occur at constant pressure. Any pressure drop
through the humidifier may be modeled by increasing the flow resistance of an
adjacent component.
The evaporative humidifier model requires the inlet air temperature,
humidity ratio, and mass flow rate as inputs. It computes the exit humidity
ratio, temperature, and mass flow rate. The wet bulb temperature and total
enthalpy of the air remain constant while the moisture content rises and the
exit dry bulb temperature decreases. The mass flow rate increases slightly due
to the addition of moisture to the air.
Nomenclature
Cp - specific heat at constant pressure
H - enthalpy
hA - water to air heat transfer coefficient times area
T - temperature
w - mass flow rate
Omega - humidity ratio
Subscripts
a - air
fg - latent heat of vaporization
i - inlet
o - outlet
ref - reference point at which H=0 (-20 C)
s - saturation
v - water vapor
wb - wet bulb
Mathematical Description
The total enthalpy at the inlet is determined by the following
equation:
Hai = {Omegaai*[Cpv*(Tai - Tref) + Delta(Hfg)] +
Cpa*(Tai - Tref)} / (1 + Omegaai)
Next, the inlet wet bulb temperature is calculated, using a correlation from
reference [2]:
Twb = 30.9185 - 39.682*C + 20.5841*C^2 - 1.758*C^3
where
C = loge(Hai)
The outlet saturation humidity ratio is calculated next, using the subprogram
PSATS described in Chapter 3 to find the saturation pressure at the inlet wet
bulb temperature. The constant in the following equation is the water to air
molecular weights ratio.
Omegaaos = 0.62198*PSATS(Twb) / (Patm - PSATS(Twb))
where Patm is 101.325 kPa. The outlet air humidity ratio, temperature, and
mass flow rate can now be determined.
Omegaao = Omegaai + (Omegaaos - Omegaai)*(1 - e^(-R))
where
R = hA / (Cpa*wai)
The outlet air temperature and mass flow rate are given by
Tao = Tai - [(Omegaao - Omegaai) / Cpa]*Delta(Hfg)
wao = [(1 + Omegaao) / (1 + Omegaai)]*wai
Component 204 Configuration
Inputs Description
1 Tai - inlet air temperature (C)
2 Omegaai - inlet air humidity ratio ( - )
3 wai - inlet moist air mass flow rate (kg/s)
Outputs Description
1 Omegaao - outlet air humidity ratio ( - )
2 Tao - outlet air temperature (C)
3 wao - outlet moist air mass flow rate (kg/s)
Parameters Description
1 hA - water to air heat transfer
coefficient times area (kW/C)
Reference:
1. Chi, J. "Building Environmental Control System Analysis (BECSA)
Computer Program, Volume 1: Component Mathematical Models." HCP Syste
ms, Inc., final report to the National Bureau of Standards under
Contract No. NB82SBCA1577, Sept. 1982.
2. Tobias, J.R. "Simplified transfer function for temperature response of
fluids flowing through coils, pipes or ducts." ASHRAE Transactions,
Vol. 79, pp. 19-22, 1973.
3. 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