.. _heatexchangers: Heat Exchangers ================ Introduction ------------- The module ``heatexchangers`` contains methods to perform heat transfer calculations related to heat exchangers. The module has methods for the following: 1. Calculation of F factors 2. Calculation of effectiveness and NTU 3. Calculation of overall heat coefficient from NTU .. container:: custom **How to use** It is recommended that the module be imported as ``from pychemengg import heatexchangers as hx`` The following examples demonstrate how the module `heatexchangers`` can be used to solve heat transfer problems. Examples -------- Example 1: Counter flow ``LMTD`` ................................. ``Example 1.`` Hot fluid (Cp=2.09 kJ/kg-K) flows through a counter flow heat exchanger at a rate of 0.63 kg/s. It enter at 193 C and leaves at 65 C. Cold fluid (Cp=1.67 kJ/kgK) exits at 149 C at a rate of 1 kg/s. What area is required if the overall heat transfer coefficient based on the inside area is 0.7 kW/m2K. ``Ans: Area = 8.5 m2`` .. code-block:: python # EXAMPLE 1 from pychemengg.heattransfer import heatexchangers as hx from pychemengg.heattransfer import heatcommonmethods as hcm # Use change in internal energy to compute heat transfered heattransfer_hot = hcm.calc_internalenergychange(mass=0.63, specificheat=2.09e3, deltaT=193-65) # heattransfer_hot = heattransfer_cold, use this to find inlet # temperature of cold fluid T_cold_in = 149 - heattransfer_hot/1.67e3 # compute LMTD deltaT1 = 65-T_cold_in deltaT2 = 193-149 LMTD = hcm.calc_LMTD(deltaT1=deltaT1, deltaT2=deltaT2) correctionfactor = 1 # Because counterflow # compute area using convection equation area = heattransfer_hot/0.7e3/LMTD print(f"Area required = {area: 0.1f} m2") # PRINTED OUTPUT Area required = 8.5 m2 Example 2: Double-pipe ``Effectiveness-NTU`` ............................................ ``Example 2.`` A fluid (Cp = 4.18 kJ/kgK) enters a parallel flow, double-pipe heat exchanger at 40 C at 0.75 kg/s. It is heated by a second fluid (Cp = 1.581 kJ/kgK) flowing at a rate of 1.5 kg/s with inlet temperature 115 C. If the area is 13 m2 and overall heat transfer coefficient is 0.205 kW/m2K, find the total heat rate. ``Ans: heat rate = 87.17 kW`` .. code-block:: python # EXAMPLE 2 from pychemengg.heattransfer import heatexchangers as hx # Use effectiveness-NTU method. Ccold = 0.75*4.18e3 Chot = 1.5*1.581e3 # Print both to see which is Cmin print(f" Chot = {Chot} and Ccold = {Ccold}") # This produces: Chot = 2371.5 and Ccold = 3135.0 # Based on this, assign Cmin and Cmax. Cmin = Chot Cmax = Ccold # Calculate NTU = UA/Cmin NTU = .205e3 * 13/Cmin # Next given this NTU find effectiveness. # First create instance of EffNTU class doublepipe = hx.EffNTU(Cmin=Cmin, Cmax=Cmax, NTU=NTU, effectiveness="?") # The keyword - 'effectiveness' is assigned the string "?". # This alerts the function that effectiveness is to be computed. # Then call the appropriate exchanger type to compute effectiveness. effectiveness = doublepipe.doublepipe_parallelflow() heatratemax = Cmin*(115-40) heatrate_actual = effectiveness * heatratemax print(f"Heat rate = {heatrate_actual: 0.3e} W") # PRINTED OUTPUT Heat rate = 8.719e+04 W Example 3: Comparison of cross flow and shell and tube ``F correction factors`` ................................................................................ ``Example 3.`` An exchanger is desired for operation with T\ :sub:`hot,in`\ = 400 C, T\ :sub:`hot,out`\ = 130 C, and T\ :sub:`cold,in`\ = 25 C. The following is known about the system: **Hot fluid**: mass flow rate = 2 kg/s, specific heat = 2000 J/kg k **Cold fluid**: mass flow rate = 6.857 kg/s, specific heat = 1050 J/kgK **Overall heat transfer coefficient** = 150 W/m2K Compare i) shell and tube heat exchanger design with ii) cross flow exchanger with both fluids unmixed. Find the suitable configurations. ``Ans: 2 Shell - 4 tube: F = 0.93, area = 48.64 m2; Cross flow: F = 0.85, area = 53.8 m2`` .. code-block:: python # EXAMPLE 3 from pychemengg.heattransfer import heatexchangers as hx from pychemengg.heattransfer import heatcommonmethods as hcm # Use energy balance to find T_cold_out mass_hot = 2 specificheat_hot = 2000 T_hot_in = 400 T_hot_out = 130 deltaT_hot = T_hot_in - T_hot_out hot_internalenergychange = hcm.calc_internalenergychange(mass=mass_hot, specificheat=specificheat_hot, deltaT=deltaT_hot) mass_cold = 6.857 specificheat_cold = 1050 T_cold_in = 25 T_cold_out = T_cold_in + hot_internalenergychange/mass_cold/specificheat_cold # case: shell-tube # Calculate LMTD. deltaT_1 = T_hot_in - T_cold_out deltaT_2 = T_hot_out - T_cold_in LMTD = hcm.calc_LMTD(deltaT1=deltaT_1, deltaT2=deltaT_2) print(f"LMTD = {LMTD: 0.1f} C") # Calculate F correction factor. exchanger = hx.FCorrectionFactor() oneshelltube_F_factor = exchanger.oneshell2ntubepasses(T_tubein=T_cold_in, T_tubeout=T_cold_out, T_shellin=T_hot_in, T_shellout=T_hot_out) print(f"1-Shell 2-Tube F Factor = {oneshelltube_F_factor: 0.2f}") # The F factor is 0.58, which is quite low. # Therefore the 2 shell-4 pass configuration can be examined. twoshelltube_F_factor = exchanger.twoshell4ntubepasses(T_tubein=T_cold_in, T_tubeout=T_cold_out, T_shellin=T_hot_in, T_shellout=T_hot_out) print(f"2-Shell 4-Tube F Factor = {twoshelltube_F_factor: 0.2f}") # The F factor for 2 shell and 4 tube is 0.93. # Calculate area for this using convection heat rate equation and internal energy change # hot_internalenergychange = U * A * LMTD * F overallheattransfercoefficient = 150 area_shelltube = hot_internalenergychange/overallheattransfercoefficient/LMTD/twoshelltube_F_factor print(f"Area needed for 2 shell-4 tube heat exchanger = {area_shelltube:0.2f} m2") # case: cross flow both fluids unmixed # Calculate F factor. crossflow_F_factor = exchanger.singlepass_crossflow_bothunmixed(T_tubein=T_cold_in, T_tubeout=T_cold_out, T_shellin=T_hot_in, T_shellout=T_hot_out) print(f"Cross flow F factor= {crossflow_F_factor: 0.2f}") # Calculate area as was done for shell-tube case area_crossflow = hot_internalenergychange/overallheattransfercoefficient/LMTD/crossflow_F_factor print(f"Area needed for cross flow heat exchanger = {area_crossflow:0.2f} m2") # PRINTED OUTPUT LMTD = 157.5 C 1-Shell 2-Tube F Factor = 0.58 2-Shell 4-Tube F Factor = 0.93 Area needed for 2 shell-4 tube heat exchanger = 49.32 m2 Cross flow F factor= 0.85 Area needed for cross flow heat exchanger = 53.70 m2