Heat exchanger design

[mortezaep]

Dear all

I'm trying to create a heat exchanger. In my first try, I wrote the below code that perfectly works:

from tespy.components import Sink, Source, HeatExchanger
from tespy.connections import Connection
from tespy.networks import Network
import shutil


nw = Network(p_unit='bar', T_unit='C', iterinfo=False)

so1 = Source('source1')
si1 = Sink('sink1')
so2 = Source('source2')
si2 = Sink('sink2')
des = HeatExchanger('heat_exchanger')

des.component()

so1_des = Connection(so1, 'out1', des, 'in1')
des_si1 = Connection(des, 'out1', si1, 'in1')
so2_des = Connection(so2, 'out1', des, 'in2')
des_si2 = Connection(des, 'out2', si2, 'in1')


nw.add_conns(so1_des, des_si1, so2_des, des_si2)

des.set_attr(pr1=1, pr2=1, ttd_u=5)
so1_des.set_attr(fluid={'water': 1}, T=120, p=10, m=50)
so2_des.set_attr(fluid={'water': 1}, T=10, m=20)
des_si2.set_attr(x=0)

nw.solve('design')
nw.print_results()

Nonetheless, I faced two problems in further development:

1. when I change the line (so2_des.set_attr(fluid={'water': 1}, T=10, m=20)) to (so2_des.set_attr(fluid={'water': 1}, T=10, p=1.692)) I get an error about (Singularity in jacobian matrix ...). The only difference is that instead of pressure, the mass rate is unknown.

2. By changing the working fluid to R410A, I get coolprop error. So the below code doesn't work.

from tespy.components import Sink, Source, HeatExchanger
from tespy.connections import Connection
from tespy.networks import Network
import shutil


nw = Network(p_unit='bar', T_unit='C', iterinfo=False)

so1 = Source('source1')
si1 = Sink('sink1')
so2 = Source('source2')
si2 = Sink('sink2')
des = HeatExchanger('heat_exchanger')

des.component()

so1_des = Connection(so1, 'out1', des, 'in1')
des_si1 = Connection(des, 'out1', si1, 'in1')
so2_des = Connection(so2, 'out1', des, 'in2')
des_si2 = Connection(des, 'out2', si2, 'in1')


nw.add_conns(so1_des, des_si1, so2_des, des_si2)

des.set_attr(pr1=1, pr2=1, ttd_u=5)
so1_des.set_attr(fluid={'water': 1}, T=120, p=10, m=50)
so2_des.set_attr(fluid={'R410A': 1}, T=10, m=20)
des_si2.set_attr(x=0)

nw.solve('design')
nw.print_results()

I do appreciate any help from your side.

[fwitte]

Hi @mortezaep,

thanks for reaching out!

1. On the hot side, specification of pressure instead of mass flow is not possible:

   • Temperature des_si2 is fixed due to ttd_u specification of the heat exchanger
   • Together with x, p is then also fixed
   • With the specification of pressure drop over that side of the heat exchanger (pr=1), the inlet pressure must be fixed as well
   • Therefore, you have two specifications of pressure for the pressure so2_des, which leads to the singularity.

2. In general it really helps if you can provide some good guess for the initial simulation. The issue with R410a appears within the first iteration, where some values for pressure and enthalpy might not be reasonable. Therefore, I usually suggest making two-step models:

from tespy.components import Sink, Source, HeatExchanger
from tespy.connections import Connection
from tespy.networks import Network
import shutil


nw = Network(p_unit='bar', T_unit='C')

so1 = Source('source1')
si1 = Sink('sink1')
so2 = Source('source2')
si2 = Sink('sink2')
des = HeatExchanger('heat_exchanger')

des.component()

so1_des = Connection(so1, 'out1', des, 'in1')
des_si1 = Connection(des, 'out1', si1, 'in1')
so2_des = Connection(so2, 'out1', des, 'in2')
des_si2 = Connection(des, 'out2', si2, 'in1')


nw.add_conns(so1_des, des_si1, so2_des, des_si2)

des.set_attr(pr1=1, pr2=1)
so1_des.set_attr(fluid={'water': 1}, T=120, p=10, m=50)
so2_des.set_attr(fluid={'R410A': 1}, T=10, m=20)
des_si2.set_attr(x=0, T=50)

nw.solve('design')

des_si2.set_attr(T=None)
# NOTE: You are setting a two-phase constraint (x=0) on the des_si2 connection
# therefore assume, your result will be in two-phase region. You must make sure
# your temperature is then below the critical temperature of the fluid, otherwise
# the simulation cannot converge successfully.
# The ttd_u specification has to be really high, since it is difference between so1_des.T - des_si2.T.
des.set_attr(ttd_u=50)

nw.solve("design")
nw.print_results()