Contents
Condenser2_CL
Condenser2_CL determines the outlet state of an air-cooled condenser given the inlet states of the air and refrigerant and the heat transfer surface area. The refrigerant is expected to enter the condenser in a superheated state. Thermal energy is rejected to an air stream. It is necessary to provide the total heat exchanger area, the heat transfer coefficient on the air side of the heat exchanger and for the superheated, saturated, and subcooled sections on the refrigerant side. The heat transfer coefficients can be determined using the functions in the Heat Transfer Library, as illustrated in the example. See Condenser4_CL, which provides the same capability except that it internally calculates the heat transfer coefficients.
This library file is intended to be used to determine the outlet states for a condenser of fixed design. Use Condenser1_CL to determine the required heat exchanger area at design conditions.
Reference: Heat Transfer, Nellis and Klein, 2009, Cambridge University Press, section 8.5
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)
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: heat transfer area on the refrigerant side (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)
f_sh: fraction of the condenser area that is used for desuperheating
f_sc: fraction of condenser area that is used for subcooling
*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 SI C kPa kJ mass
$TabStops 0.2 4.5 in
R$='R134a' "refrigerant"
m_dot_R=0.0028 [kg/s] "refrigerant mass flow rate"
P_R=1 [MPa]*convert(MPa,kPa) "refrigerant pressure"
h_R_in=enthalpy(R$,T=60 [C], P=P_R) "specific enthalpy of entering refrigerant"
V_dot_air=0.06 [m^3/s] "volumetric flow rate of air"
T_air_in=20 [C] "air inlet temperature"
P_air=1 [atm]*convert(atm,kPa) "air pressure"
h_air=44.3 [W/m^2-K] "convection coefficient for air. See h_air1 calculation below"
A_air\A_R=A_air\A_R1 "ratio of air on air side to refrigerant side. See A_air\A_R1 calculation"
h_R_sh=h_H_sh "convection coefficient for superheated R134a. See h_H_sh calculation"
h_R_sat=h_cond "convection coefficient for saturated R134a. See h_cond calculation"
h_R_SC=h_H_sc "heat transfer coefficient in subcooled section. See h_H_sc calculation"
A_R=0.11 [m^2]
Call Condenser2_CL(R$, m_dot_R, h_R_in, P_R, V_dot_air, T_air_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_air_out, f_sh, f_sc)
"!Calculation of the heat transfer coefficients using the Heat Transfer Library functions"
N_circuits=1 "number of parallel circuits"
A_fr=0.26*0.2 [m^2] "frontal area"
W=0.2 [m] "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"
th = 0.9 [mm]*convert(mm,m) "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 "
T_R_in=temperature(R$,h=h_R_in,P=P_R)
T_R_sat=T_sat(R$,P=P_R)
m_dot_air=V_dot_air/volume(air,T=T_air_in,P=P_air) "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=T_air_in "estimated wall temperature"
Call Cond_HorizontalTube_avg(R$, m_dot_R, T_R_sat, T_w, D_in, 1, 0 : h_cond) "saturated section"
call PipeFlow('R134a',T_R_sat-1 [C], 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=0.08871
f_sh=0.1709
h_R_out=101.5 [kJ/kg]
Q_dot=0.5372 [kW]
T_air_out=27.4 [C]
}
Index