# -*- coding: utf-8 -*-
# BioSTEAM: The Biorefinery Simulation and Techno-Economic Analysis Modules
# Copyright (C) 2020-2023, Yoel Cortes-Pena <yoelcortes@gmail.com>
#
# This module is under the UIUC open-source license. See
# github.com/BioSTEAMDevelopmentGroup/biosteam/blob/master/LICENSE.txt
# for license details.
"""
General functional algorithms for the design of heat exchangers.
"""
from numpy import log as ln
from numba import njit
__all__ = ('counter_current_heat_exchange',
'heuristic_overall_heat_transfer_coefficient',
'heuristic_pressure_drop',
'heuristic_tubeside_and_shellside_pressure_drops',
'order_streams',
'compute_Fahkeri_LMTD_correction_factor',
'compute_heat_transfer_area',
'compute_LMTD')
# %% Functional heat exchanger
def heat_exchange_to_condition(s_in, s_out, T=None, phase=None,
H_lim=None, f_equilibrium=None, heating=None):
"""
Set the outlet stream condition and return duty required to achieve
said condition.
"""
H_lim_given = H_lim is not None
H_in = s_in.H
if H_lim_given:
if heating:
if H_in > H_lim: return 0.
else:
if H_in < H_lim: return 0.
if phase:
s_out.T = T
s_out.phase = phase
if H_lim_given:
if heating:
if s_out.H > H_lim: s_out.H = H_lim
else:
if s_out.H < H_lim: s_out.H = H_lim
elif len(s_out.vle_chemicals) == 1 and heating is not None:
bp = s_in.bubble_point_at_P()
tol = 1e-3
dT = T - bp.T
s_out.T = T
if dT < -tol:
s_out.phase = 'l'
if H_lim_given:
if heating:
if s_out.H > H_lim: s_out.H = H_lim
else:
if s_out.H < H_lim: s_out.vle(H=H_lim , P=s_out.P)
elif dT > tol:
s_out.phase = 'g'
if H_lim_given:
if heating:
if s_out.H > H_lim: s_out.vle(H=H_lim , P=s_out.P)
else:
if s_out.H < H_lim: s_out.H = H_lim
else:
s_out.phase = 'g' if heating else 'l'
else:
s_out.vle(T=T, P=s_out.P)
if H_lim_given:
if heating:
if s_out.H > H_lim: s_out.vle(H=H_lim , P=s_out.P)
else:
if s_out.H < H_lim: s_out.vle(H=H_lim , P=s_out.P)
# Sanity check
if heating:
assert s_out.T > s_in.T - 0.01, "failed to solve temperature"
if s_out.T < s_in.T: s_out.T = s_in.T
else:
assert s_out.T <= s_in.T + 0.01, "failed to solve temperature"
if s_out.T > s_in.T: s_out.T = s_in.T
return s_out.H - H_in
[docs]
def counter_current_heat_exchange(s0_in, s1_in, s0_out, s1_out,
dT, T_lim0=None, T_lim1=None,
phase0=None, phase1=None,
H_lim0=None, H_lim1=None):
"""
Allow outlet streams to exchange heat until either the given temperature
limits or the pinch temperature and return the total heat transfer
[Q; in kJ/hr].
"""
# Counter current heat exchange setup:
# First find the hot inlet, cold inlet, hot outlet and cold outlet streams
# along with the maximum temperature approaches for the hotside and the
# cold side.
if s0_in.T > s1_in.T:
s_hot_in = s0_in
s_cold_in = s1_in
s_hot_out = s0_out
s_cold_out = s1_out
T_lim_coldside = T_lim0
T_lim_hotside = T_lim1
H_lim_coldside = H_lim0
H_lim_hotside = H_lim1
phase_coldside = phase0
phase_hotside = phase1
else:
s_cold_in = s0_in
s_hot_in = s1_in
s_cold_out = s0_out
s_hot_out = s1_out
T_lim_hotside = T_lim0
T_lim_coldside = T_lim1
H_lim_hotside = H_lim0
H_lim_coldside = H_lim1
phase_hotside = phase0
phase_coldside = phase1
if (s_hot_in.T - s_cold_in.T) <= dT: return 0. # No heat exchange
T_pinch_coldside = s_cold_in.T + dT
if T_lim_coldside:
if T_lim_coldside > s_hot_in.T:
return 0. # No heat exchange
else:
T_lim_coldside = max(T_pinch_coldside, T_lim_coldside)
else:
T_lim_coldside = T_pinch_coldside
T_pinch_hotside = s_hot_in.T - dT
if T_lim_hotside:
if T_lim_hotside < s_cold_in.T:
return 0. # No heat exchange
else:
T_lim_hotside = min(T_pinch_hotside, T_lim_hotside)
else:
T_lim_hotside = T_pinch_hotside
# Find which side reaches the pinch first by selecting the side that needs
# the least heat transfer to reach the pinch.
# Pinch on the cold side
Q_hot_stream = heat_exchange_to_condition(s_hot_in, s_hot_out,
T_lim_coldside, phase_coldside,
H_lim_coldside, heating=False)
# Pinch on the hot side
Q_cold_stream = heat_exchange_to_condition(s_cold_in, s_cold_out,
T_lim_hotside, phase_hotside,
H_lim_hotside, heating=True)
if Q_hot_stream == Q_cold_stream == 0.:
s0_out.copy_like(s0_in)
s1_in.copy_like(s1_out)
return 0.
if Q_hot_stream > 0 or Q_cold_stream < 0:
# Sanity check
if Q_hot_stream / s_hot_in.C < 0.1 or Q_cold_stream / s_cold_in.C > -0.1:
s0_out.copy_like(s0_in)
s1_in.copy_like(s1_out)
return 0.
raise RuntimeError('inlet stream not in vapor-liquid equilibrium')
if Q_cold_stream < -Q_hot_stream:
# Pinch on the hot side
Q = Q_cold_stream
if phase_coldside:
s_hot_out.H = s_hot_in.H - Q
else:
s_hot_out.vle(H=s_hot_in.H - Q, P=s_hot_out.P)
else:
# Pinch on the cold side
Q = Q_hot_stream
if phase_hotside:
s_cold_out.H = s_cold_in.H - Q
else:
s_cold_out.vle(H=s_cold_in.H - Q, P=s_cold_out.P)
return abs(Q)
# %% Heuristics
[docs]
def heuristic_overall_heat_transfer_coefficient(ci, hi, co, ho):
"""
Return a heuristic estimate of the overall heat transfer coefficient
[U; in kW/m^2/K]. Assume `U` is 1.0 kW/m^2/K if heat exchange is between
a condensing fluid and a vaporizing fluid and 1.0 kW/m^2/K otherwise.
Parameters
----------
ci : Stream
Cold inlet stream.
hi : Stream
Hot inlet stream.
co : Stream
Cold outlet stream.
ho : Stream
Hot outlet stream.
Returns
-------
U : float
overall heat transfer coefficient [kW/m^2/K].
"""
# TODO: Base U on Table 18.5, Warren D. Seider et. al. Product and Process Design Principles. (2016)
cip, hip, cop, hop = ci.phase, hi.phase, co.phase, ho.phase
phases = cip + hip + cop + hop
if 'g' in phases:
if ('g' in hip and 'l' in hop) and ('l' in cip and 'g' in cop):
return 1.0
else:
return 0.5
else:
return 0.5
[docs]
def heuristic_pressure_drop(inlet_phase, outlet_phase):
"""
Return a heuristic estimate of the pressure drop [dP; in psi]. If the fluid
changes phase, `dP` is 1.5 psi. If the fluid remains a liquid, `dP` is 5 psi.
If the fluid remains a gas, `dP` is 3 psi.
Parameters
----------
inlet_phase: str
outlet_phase : str
Returns
-------
dP : float
Pressure drop [psi].
"""
inlet_phase = inlet_phase.lower()
outlet_phase = outlet_phase.lower()
if ('l' in inlet_phase and 'g' in outlet_phase) or ('g' in inlet_phase and 'l' in outlet_phase):
# Latent fluid (boiling or condensing)
dP = 1.5
elif 'l' in inlet_phase:
# Sensible liquid
dP = 5.
elif 'g' in outlet_phase:
# Sensible vapor
dP = 3.
return dP
[docs]
def heuristic_tubeside_and_shellside_pressure_drops(ci, hi, co, ho,
tubeside_iscooling=True):
"""
Return an estimate of tubeside and shellside pressure drops.
Parameters
----------
ci : Stream
Cold inlet stream.
hi : Stream
Hot inlet stream.
co : Stream
Cold outlet stream.
ho : Stream
Hot outlet stream.
tubeside_iscooling : bool
True of tubeside fluid is cooling.
Returns
-------
dP_tube : float
Tubeside pressure drop (psi)
dP_shell : float
Shellside pressure drop (psi)
"""
dP_c = heuristic_pressure_drop(ci.phase, co.phase)
dP_h = heuristic_pressure_drop(hi.phase, ho.phase)
if tubeside_iscooling:
dP_tube = dP_h
dP_shell = dP_c
else:
dP_tube = dP_c
dP_shell = dP_h
return dP_tube, dP_shell
# %% General functions
[docs]
def order_streams(in_a, in_b, out_a, out_b):
"""
Return cold and hot inlet and outlet streams.
Parameters
----------
in_a : Stream
Inlet a.
in_b : Stream
Inlet b.
out_a : Stream
Outlet a.
out_b : Stream
Outlet b.
Returns
-------
ci : Stream
Cold inlet.
hi : Stream
Hot inlet.
co : Stream
Cold outlet.
ho : Stream
Hot outlet.
"""
if in_a.T < in_b.T:
return in_a, in_b, out_a, out_b
else:
return in_b, in_a, out_b, out_a
# %% Computational functions
@njit(cache=True)
def compute_fallback_Fahkeri_LMTD_correction_factor(P, N_shells):
"""Return LMTF correction factor using the fallback equation for
`compute_Fahkeri_LMTD_correction_factor` when logarithms cannot be computed."""
# A, J, and K are dummy variables
A = N_shells - N_shells*P
W = A/(A + P)
if 0.999 < W < 1.001:
Ft = 1
else:
J = W/(1. - W)
K = (J + 2**-0.5)/(J - 2**-0.5)
if K <= 1:
Ft = 1
else:
Ft = (2**0.5*J)/ln(K)
return Ft
[docs]
@njit(cache=True)
def compute_Fahkeri_LMTD_correction_factor(Tci, Thi, Tco, Tho, N_shells):
r"""
Return the log-mean temperature difference correction factor `Ft`
for a shell-and-tube heat exchanger with one or an even number of tube
passes, and a given number of shell passes, with the expression given in
[1]_ and also shown in [2]_.
.. math::
F_t=\frac{S\ln W}{\ln \frac{1+W-S+SW}{1+W+S-SW}}
S = \frac{\sqrt{R^2+1}}{R-1}
W = \left(\frac{1-PR}{1-P}\right)^{1/N}
R = \frac{T_{in}-T_{out}}{t_{out}-t_{in}}
P = \frac{t_{out}-t_{in}}{T_{in}-t_{in}}
If R = 1 and logarithms cannot be evaluated:
.. math::
W' = \frac{N-NP}{N-NP+P}
F_t = \frac{\sqrt{2}\frac{1-W'}{W'}}{\ln\frac{\frac{W'}{1-W'}+\frac{1}
{\sqrt{2}}}{\frac{W'}{1-W'}-\frac{1}{\sqrt{2}}}}
Parameters
----------
Tci : float
Inlet temperature of cold fluid, [K]
Thi : float
Inlet temperature of hot fluid, [K]
Tco : float
Outlet temperature of cold fluid, [K]
Tho : float
Outlet temperature of hot fluid, [K]
shells : int, optional
Number of shell-side passes, [-]
Returns
-------
Ft : float
Log-mean temperature difference correction factor, [-]
Notes
-----
This expression is symmetric - the same result is calculated if the cold
side values are swapped with the hot side values. It also does not
depend on the units of the temperature given.
Examples
--------
compute_Fahkeri_LMTD_correction_factor(Tci=15, Tco=85, Thi=130, Tho=110, N_shells=1)
0.9438358829645933
References
----------
.. [1] Fakheri, Ahmad. "A General Expression for the Determination of the
Log Mean Temperature Correction Factor for Shell and Tube Heat
Exchangers." Journal of Heat Transfer 125, no. 3 (May 20, 2003): 527-30.
doi:10.1115/1.1571078.
.. [2] Hall, Stephen. Rules of Thumb for Chemical Engineers, Fifth Edition.
Oxford; Waltham, MA: Butterworth-Heinemann, 2012.
"""
if (Tco - Tci) < 0.01:
R = 1
else:
R = (Thi - Tho)/(Tco - Tci)
P = (Tco - Tci)/(Thi - Tci)
if 0.999 < R < 1.001:
Ft = compute_fallback_Fahkeri_LMTD_correction_factor(P, N_shells)
else:
W = ((1. - P*R)/(1. - P))**(1./N_shells)
S = (R*R + 1.)**0.5/(R - 1.)
K = (1. + W - S + S*W)/(1. + W + S - S*W)
if K <= 0.001 or 0.999 < K < 1.001:
Ft = compute_fallback_Fahkeri_LMTD_correction_factor(P, N_shells)
else:
Ft = S*ln(W)/ln(K)
if Ft > 1.0:
Ft = 1.0
elif Ft < 0.5:
# Bad design, probably a heat exchanger network operating
# too close to the pinch. Fahkeri may not be valid, so give
# a conservative estimate of the correction factor.
Ft = 0.5
return Ft
[docs]
@njit(cache=True)
def compute_heat_transfer_area(LMTD, U, Q, ft):
"""
Return required heat transfer area by LMTD correction factor method.
Parameters
----------
LMTD : float
Log mean temperature difference
U : float
Heat transfer coefficient
Q : float
Duty
"""
return Q/(U*LMTD*ft)
[docs]
@njit(cache=True)
def compute_LMTD(Thi, Tho, Tci, Tco, counterflow=True):
r'''
Return the log-mean temperature difference of an ideal counterflow
or co-current heat exchanger.
.. math::
\Delta T_{LMTD}=\frac{\Delta T_1-\Delta T_2}{\ln(\Delta T_1/\Delta T_2)}
\text{For countercurrent: } \\
\Delta T_1=T_{h,i}-T_{c,o}\\
\Delta T_2=T_{h,o}-T_{c,i}
\text{Parallel Flow Only:} \\
{\Delta T_1=T_{h,i}-T_{c,i}}\\
{\Delta T_2=T_{h,o}-T_{c,o}}
Parameters
----------
Thi : float
Inlet temperature of hot fluid [K].
Tho : float
Outlet temperature of hot fluid [K].
Tci : float
Inlet temperature of cold fluid [K].
Tco : float
Outlet temperature of cold fluid [K].
counterflow : bool, optional
Whether the exchanger is counterflow or co-current.
Returns
-------
LMTD : float
Log-mean temperature difference [K]
Notes
-----
Any consistent set of units produces a consistent output.
Examples
--------
>>> compute_LMTD(100., 60., 30., 40.2)
43.200409294131525
>>> compute_LMTD(100., 60., 30., 40.2, counterflow=False)
39.75251118049003
'''
if counterflow:
dTF1 = Thi-Tco
dTF2 = Tho-Tci
else:
dTF1 = Thi-Tci
dTF2 = Tho-Tco
dTF21 = dTF2 - dTF1
if abs(dTF21) < 1e-8:
LMTD = dTF1
else:
LMTD = dTF21/ln(dTF2 / dTF1)
return LMTD