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