Contents
EvaporativeCondenser2_CL
This procedure determines the outlet refrigerant and air states of an evaporative condenser of specified surface area. The condensing refrigerant (R$) can be two-phase or superheated. Heat transfer coefficients for the air and refrigerant in the superheated, saturated and subcooled regimes are provided to the routine, along with the ratio of surface area on the air side to that on the refrigerant side. The heat transfer coefficients can be determined using the functions in the Heat Transfer Library, as illustrated in the example. The heat transfer enhancement resulting from the wetted surface is internally calculated using the method described by Mitchell and Braun (2013). . See Condenser1_CL and Condenser3_CL for similar routines in which condensation occurs by heat transfer to dry air. See EvaporativeCondenser1_CL for a model that determines the required surface area of an evaporative condenser, given the outlet subcooled state.
This library file can be used in the design stage of the evaporative condenser by inputting the design inlet conditions to determine the required area.
Reference: Mitchell and Braun, Principles of Heating, Ventilation, and Air Conditioning in Buildings, Wiley, 2013, section SM 4
Inputs:
R$: name of the refrigerant
m_dot_R: refrigerant flow rate (kg/s, lbm/hr)
h_R_in: specific enthalpy of entering refrigerant (J/kg, kJ/kg, Btu/lbm)
P_R: refrigerant pressure (Pa, kPa, bar, MPa, psia, atm)
V_dot: volumetric flow rate of air (m^3/s, cfm)
T_air_in: air inlet temperature (C, K, F, R)
rh_in: inlet air relative humidity (-)
P_air: air pressure (Pa, kPa, bar, MPa, psia, atm)
h_air: effective outside (air) heat transfer coefficient per unit refrigerant tube area (W/m^2-K, Btu/hr-ft^2-R)
A_air\A_R: ratio of effective heat transfer area on the air side to the heat transfer area for the refrigerant*
h_R_SH: heat transfer coefficient for refrigerant in the superheat regime (W/m^2-K, Btu/hr-ft^2-R)
h_R_sat: average heat transfer coefficient for refrigerant in the saturated regime (W/m^2-K, Btu/hr-ft^2-R)
h_SC: average heat transfer coefficient for refrigerant in the saturated regime (W/m^2-K, Btu/hr-ft^2-R)
A_R: required heat transfer area for the refrigerant (m^2, ft^2)
Outputs:
Q_dot: overall heat transfer rate (W, kW, Btu/hr)
h_R_out: refrigerant outlet specific enthalpy (J/kg, kJ/kg, Btu/lbm)
T_air_out: outlet temperature of the air (C, K, F, R)
w_out: humidity ratio of exiting air (-)
f_sh fraction of the surface area in the superheat regime
f_sc fraction of the surface area in the subcooled regime
*A_air\A_R is the ratio of the product of the overall fin efficiency and total (finned and unfinned) area on the air side to the total surface area inside the refrigerant flow passages. For unfinned surfaces, A_air\A_R is the ratio of the outer surface to inner surface areas.
Example:
$UnitSystem Eng F psia
V_dot_air=10000 [cfm] "volumetric air flowrate"
T_air_in=80 [F] "air inlet temperature"
P_air=14.7 [psia] "air pressure"
rh_in=0.4 "air relative humidity"
R$='R134a' "condensing refrigerant"
m_dot_R=800 [lbm/hr] "refrigerant flow rate"
T_R_sat=70 [F] "refrigerant saturation temperature"
P_R=P_sat(R$,T=T_R_sat) "refrigerant saturation pressure"
Superheat=10 [DELTAF] "degrees of superheat at inlet"
T_R_in=T_R_sat+Superheat "inlet temperature"
h_R_in=enthalpy(R$,T=T_R_in,x=1) "inlet specific enthalpy"
h_air=21.6 [Btu/hr-ft^2-R] "heat transfer coefficient on air side -see below"
A_air\A_R=12.1 "ratio of area on air side to that on refrigerant side"
h_R_sh=36 [Btu/hr-ft^2-F] "heat transfer coefficient for refrigerant in superheat region"
h_R_sat=444 [Btu/hr-ft^2-R] "heat transfer coefficient for refrigerant in saturation region"
h_R_sc=31.7 [Btu/hr-ft^2-F] "heat transfer coefficient for refrigerant in subcooled region"
A_R=65 [ft^2] "surface area on refrigerant side"
call EvaporativeCondenser2_CL (R$, m_dot_R, h_R_in, P_R, V_dot_air, T_air_in, rh_in, P_air, h_air, A_air\A_R, h_R_sh, h_R_sat, h_R_sc, A_R : Q_dot, h_R_out, T_a_out, w_out, f_sh, f_sc)
T_R_out=temperature(R$,h=h_R_out,P=P_R) "outlet refrigerant temperature"
"!Calculation of the heat transfer coefficients using the Heat Transfer Library functions"
$VarInfo A_air\A_R Guess=10 Lower=1 Upper=100
N_circuits=24 "number of parallel circuits"
A_fr=2*4 [ft^2] "frontal area"
W=4 [ft] "length of tubes"
N_tubes=A_R/(pi*W*D_in) "required number of tubes"
Call CHX_geom_finned_tube('fc_tubes_s80-38T': D_o, fin_pitch, D_h, fin_thk, sigma, alpha, A_fin\A) "air side geometry"
"Determine ratio of area on air side to the refrigerant heat transfer area"
m_dot_R_min=m_dot_R*convert(lbm/hr,lbm/min) "refrigerant flow rate in lbm/min"
th = 0.9 [mm]*convert(mm,ft) "tube wall thickness"
D_in=D_o-2*th "inner tube diameter"
A_unfinned=pi*D_o*N_tubes*W*(1-fin_thk*fin_pitch) "unfinned area on air side"
A_tot=A_unfinned+A_finned "total finned and unfinned area on air side"
A_finned/A_tot=A_fin\A "ratio of finned to total area is provided by Compact HX library"
eta_o=0.97 "overall fin efficiency - See Heat Transfer, Nellis and Klein, section 8.5"
A_air\A_R1=eta_o*A_tot/(N_tubes*W*pi*D_in) "ratio of heat transfer area on air side to that on refrigerant side"
"Determine heat transfer coefficients for air and refrigerant "
m_dot_air=V_dot_air/volume(air,T=T_air_in,P=P_air)*convert(lbm/min,lbm/hr) "air mass flow rate"
Call CHX_h_finned_tube('fc_tubes_s80-38T', m_dot_air, A_fr, 'Air', T_air_in, P_air: h_air1) "compact hx correlation"
call PipeFlow('R134a',T_R_in, P_R, m_dot_R/N_circuits, D_in, N_tubes*W*f_sh, 0: h_T_sh, h_H_sh, DELTAP_sh, Nusselt_T_sh, f_R_sh, Re_R_sh) "superheat section"
T_w=65 [F] "estimated wall temperature"
Call Cond_HorizontalTube_avg(R$, m_dot_R_min/N_Circuits, T_R_sat, T_w, D_in, 1, 0 : h_cond) "saturated section"
call PipeFlow('R134a',T_R_sat-1 [F], P_R, m_dot_R/N_circuits, D_in, N_tubes*W*f_sc, 0: h_T_sc, h_H_sc, DELTAP_sc, Nusselt_T_sc, f_R_sc, Re_R_sc) "subcooled section"
{Solution:
f_sc=0.2413 [-]
f_sh=0.04778 [-]
Q_dot=65801 [Btu/hr]
T_a_out=65.45 [F]
w_out=0.01342 [-]
}