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h_2phase=10000 "Refrigerant side heat transfer coefficienct, [W/m^2 K]" h_ev_air=90*(m_dot_air/m_dot_air_max)^0.8 "Air side heat transfer coefficient, [W/m^2 K]" alpha=7.8 "Fin to refrigerant area ratio" T_dp_in = dewpoint(airh2o, P=P_atm, T=Teai, R=RH) T_dp_out=dewpoint(airh2o, P=P_atm, T=Teao, R=RH_o) "First calculate air outlet temperature and humidity for various mass flow rates at given SHR" cfmperton = m_dot_air*volume(air,T=Teai,P=P_atm)*convert(m^3/s,ft^3/min)/((q_s+q_lat)*convert(kW,ton)) q_s + q_lat = Q_supplied SHR= 1/(1+q_lat/q_s) q_s = m_dot_air*specheat(air, T=Teai)*(Teai - Teao) q_lat = m_dot_air*(omega_i - omega_o)*h_fg h_fg = (enthalpy(water, x=1,T=Teao) - enthalpy(water,x=0,T=Teao)) c_pm = 1.02 omega_i = humrat(airH2O,T=Teai, R=RH, P=P_atm) omega_o = humrat(airH2O,T=Teao, R=RH_o, P=P_atm) "Results showed that outlet air conditions are essentially independent of air flow rate" "Now calculate relation between outlet air temperature and humidity, consistent with heat/mass transfer relation" SHR = 1/(1+(h_fg*LMwD*((h_2phase+alpha*h_ev_air)/1000))/(c_pm*Le^(2/3)*(h_2phase/1000)*LMTD_a)) LMTD_a =(L_t - S_t)/ln(L_t/S_t) L_t = Teai - T_surf_ev S_t = Teao - T_surf_ev LMwD =(L_w- S_w)/ln(L_w/S_w) L_w = omega_i - omega_s S_w = omega_o - omega_s omega_s = humrat(airH2O,T=T_surf_ev, R=1, P=P_atm) DELTAT_approach = DELTAT_air + DELTAT_ref T_surf_ev = Tero + DELTAT_ref Teao = T_surf_ev + DELTAT_air h_in=Enthalpy(airH2O, P=P_atm, T=Teai, W=omega_i) h_out=Enthalpy(airH2O, P=P_atm, T=Teao, W=omega_o) DELTAT_ref = Q_supplied/(h_2phase/1000*A_ev_logmean/alpha) A_ev_logmean=Q_supplied/(h_ev_air/1000*((h_2phase/1000*LMTD_a)/(h_2phase/1000+alpha*h_e v_air/1000)+(LMwD*h_fg)/(c_pm*Le^(2/3)))) "Evaporator air-side area" E.4 Results In Figure E.2 the required evaporating temperature is plotted as a function of SHR for two inlet conditions, based on the following assumptions (typical for R744): href: hair,max: α: Le: 10,000 W/m2 K 90 W/m2 K 7.8 1 75PDF Image | Comparison of R744 and R410A
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