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same air flow rate, psychrometrically the sensible heat ratio is the ratio between the latent and sensible change in enthalpy. The enthalpy ratio fixes the slope of a cooling line on a psychrometric chart (Pita, 1989). As shown in Figure E.1, the inlet condition and slope of the relative change in enthalpy then uniquely defines the evaporator surface temperature as the intercept of this line with the saturation curve on a psychrometric plot. In the equations above, the relative latent and sensible changes in enthalpy are defined in terms of the heat transfer coefficients and log- mean differences. As a result, the evaporating temperature for a given SHR is dependent only on the inlet conditions, and the air mass flow rate can be adjusted for capacity. RH=100% Uniquely defined surface Decreasing Inlet Cond. temperature/surface humidity ratio SHR Dry Bulb Temperature Figure E.1 Graphic psychrometric determination of evaporator surface temperature E.3 EES program The following EES program calculates the required evaporating temperature from given inlet air conditions and SHR based on the equations above. Additionally, the air-refrigerant temperature difference at the exit from the evaporator can be specified, which determines the air exit enthalpy; from that, the required air flow rate and heat exchanger area for a given capacity can be calculated. The program is arranged to simulate the effect of a variable speed fan by adjusting the air side heat transfer coefficient as Re0.8 . EES listing: "!Main psychrometric program" "Sensible heat ratio and air flow requirements" P_atm = 101 RH = 0.5 Teai = 27 DELTAT_air =1 SHR=0.75 m_dot_air_max=0.1 Q_supplied = 1 Le=1 "Relative humidity" "assume 1 degree approach on air side" “sensible heat ratio” "Total heat transfer in kW" 74 Humidity RatioPDF Image | Comparison of R744 and R410A
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