Component 43: Desuperheater

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Component 43: Desuperheater

Specifications

Line connections
1	Primary side inlet (cold stream, inside tubes)
2	Primary side outlet (cold stream, inside tubes)
3	Secondary side inlet (hot stream, outside tubes)
4	Secondary side outlet (hot stream, outside tubes)

General User Input Values Characteristic Lines Physics Used Displays Example

General

Component 43 models a separate desuperheater for the feed water pre-heating. It is used when the superheated steam is not to be passed directly to a preheater, in which the steam is condensed, but instead is to be desuperheater first. Normally, the desuperheater steam is then led in another feed water preheater further upstream.

The degree of desuperheating does not be complete. In the design case, the specification value DT44SN enables the user to set the degrees fo superheat present at the steam outlet i.e. the temperature difference between the steam coming out and the saturation temperature corresponding to the pressure. For DT44SN = 0, the steam leaving the desuperheater component is not superheated any more (i.e. saturated steam).

The desuperheater can be operated selectively in counter current or concurrent. Upper or lower terminal temperature difference see Heat Exchanger General Notes

The characteristics can be corrected or replaced by an adaption polynomial or a Kernel expression.

There are two identification modes for this component: T2-user (-5) input and T4-user (-4) input. Based on these defaults, k*A is calculated in all load cases. If these methods are used in the design case, then the super heating DT44SN is not an input, but instead the calculated value for DT44S will be copied into DT44SN. In an off-design case, KAN and the characteristic lines are not used.

The degrees of superheat achieved at the outlet is in those cases shown as result value DT44S.

Binary mixtures for the desuperheater:

This component can be used for desuperheating binary mixtures as well.

Here the specification value DT44SN refers to the temperature difference to the dew point temperature of the mixture.

Pressure drop limitations:
As the pressure drop rises quadratically with the mass flow, pressure drops that are significantly too high can quickly arise in the event of a transgression of the nominal mass flow. These will then cause phase transitions and convergence problems (see Heat Exchanger General Notes) .

Design in the Case of Concurrent Flow

In the heat exchanger (Components 43) it is possible to carry out a design via the upper and lower terminal temperature difference also in the case of concurrent flow (FFLOW=1).

If both inlet temperatures are specified, the upper terminal temperature difference can only be determined iteratively. Usually this is no problem. If convergence problems occur in more
complex models, another design mode will have to be used.

Identification mode

In analogy to other components, a flag FIDENT for activating the identification mode has been implemented for the desuperheater too. It has the settings

0: no identification
2: identification of KA by specifying the outlet temperature T2
4: identification of KA by specifying the outlet temperature T4

To prevent the behaviour of existing models from changing, the flag FSPEC can still be used. In this case, the settings for FIDENT are ignored.

Note about the result values:

Performance factor RPFHX

The quotient from the current value for k*A (result value KA) and the k*A expected in the respective load point due to the component physics and characteristic lines respectively (result value KACL) serves to assess the condition of a heat exchanger.

The quotient KA / KACL is displayed as a result value RPFHX.

For more information on general notes applicable to most common heat exchangers, see Heat Exchanger General Notes

Similar components:

For more information on how this heat exchangers compares to other heat exchangers, see Heat Exchanger General Components

User Input Values

FMODE	Flag for calculation mode design/off-design Like in Parent Profile (Sub Profile option only) Expression =0: GLOBAL =1: local off-design (i.e. always off-design mode, even when a design calculation has been done globally) =2: special local off-design (special case for compatibility with earlier Ebsilon-versions, should not be used in new models, because the results of real off-design calculations are not consistent) = -1: local design
DP12N	Cold side pressure loss line 1 to 2 (nominal)
DP34N	Hot side pressure loss line 3 to 4 (nominal)
FIDENT	Specifications Like in Parent Profile (Sub Profile option only) Expression =0: No Identification =2: identification of KA by specifying the outlet temperature T2 (also in off-design) =4: identification of KA by specifying the outlet temperature T4 (also in off-design)
DT44SN	Super Heating temperature difference (nominal)
TOL	Tolerance in the energy balance
FFLOW	Flag for the direction of flow, see Heat Exchanger General Notes Like in Parent Profile (Sub Profile option only) Expression =0: counter current flow =1: concurrent flow
FADAPT	Flag for adaptation polynomial ADAPT/ adaptation function EADAPT Like in Parent Profile (Sub Profile option only) Expression =0: not used and not evaluated =1: Correction for kA [KA = KAN char line factor * polynomial] =2: Calculation of kA [KA = KAN polynomial] =1000: Not used but ADAPT evaluated as RADAPT (Reduction of the computing time) = -1: Correction for kA [KA = KAN char line factor * adaptation function ] = -2: Calculation of kA [KA = KAN adaptation function ] = -1000: Not used but EADAPT evaluated as RADAPT (Reduction of the computing time)
EADAPT	Adaptation function for KA (input)
FFU	Switch OF / ON Like in Parent Profile (Sub Profile option only) Expression =0: Heat exchanger off (no heat transfer, but calculated pressure loss =1: Heat exchanger on (active)
FSPEC (deprecated)	Combined switch (deprecated) Like in Parent Profile (Sub Profile option only) Expression -999: unused (FIDENT used instead) deprecated: =11: T2 and T4 are calculated (in the design mode from DT44SN, in off-design from KAN and the characteristic lines) =-5: T2 is specified (also in off-design), KA is calculated. DT44SN or KAN and the characteristic lines are not used. =-4: T4 is specified (also in off-design), KA is calculated. DT44SN or KAN and the characteristic lines are not used.
KAN	Heat transfer coefficient * area (nominal) - Design Heat Transfer Capability
M1N	Cold side mass flow (nominal)
M3N	Hot side mass flow (nominal)

The parameters marked in blue are reference quantities for the off-design mode. The actual off-design values refer to these quantities in the equations used.

Generally, all inputs that are visible are required. But, often default values are provided.

For more information on colour of the input fields and their descriptions see Edit Component\Specification values

For more information on design vs. off-design and nominal values see General\Accept Nominal values

Characteristic Lines

1^st Correction factor dependent on primary mass flow CKAM1 FK1 = f (M1/M1N)

2^nd Correction factor dependent on secondary mass flow CKAM3 FK2 = f (M3/M3N)

(K*A)/(K*A)N = FK1 * FK2

Characteristic line 1: Correction factor dependent on primary mass flow CKAM1: (k*A)1/(k*A)N = f (M1/M1N)

     X-axis          1         M1/M1N                       1^st point
                        2          M1/M1N                     2^nd point
                        .
                      N         M1/M1N                     last point

     Y-axis       1          (k*A)1/(k*A)N              1^st point
                        2          (k*A)1/(k*A)N             2^nd point
                        .
                        N         (k*A)1/(k*A)N             last point

Characteristic line 2: Correction factor dependent on secondary mass flow CKAM3: (k*A)2/(k*A)N = f (M3/M3N)

   X-axis          1          M3/M3N                      1^st point
                        2          M3/M3N                     2^nd point
                        .
                        N         M3/M3N                     last point

     Y-axis          1          (k*A)2/(k*A)N               1^st point
                        2          (k*A)2/(k*A)N           2^nd point
                        .
                        N         (k*A)2/(k*A)N            last point

Physics Used

Equations

Design case

(Simulation flag:

GLOBAL = design

and

FMODE = GLOBAL)

P4 = P3 - DP34N

T4 = T4S + DT44SN

H4 = f (P4,T4)

M4 = M3

Q4 = M4 * H4

DQ = Q3 -Q4

P2 = P1 - DP12N

Q2 = Q1 + DQ

M2 = M1

H2 = Q2 / M2

T2 = f (P2,H2)

for Fflow = cross-counter flow {

DTL = T4 - T1

DTU = T3 - T2

}

for Fflow= counter current flow {

DTL = T4 T2

DTU = T3 T1

}

LMTD = (DTU - DTL)/(ln(DTU) - ln(DTL))
(k*A) = DQ/LMTD

(k*A)*LMTD = M2*H2 - M1*H1

(k*A)*LMTD = M3*H3 - M4*H4

Off-design case

(Simulation flag:

GLOBAL = off-design

FMODE = local off-design)

F1 = (M1/M1N) ** 2

at GLOBAL = design: F1=1.0

P2 = P1 - DP12N * F1

M2 = M1

FK1 = f (M1/M1N) characteristic line connection 1 for GLOBAL = design: FK1=1.0

FK2 = f (M3/M3N) characteristic line connection 2 for GLOBAL = design: FK2=1.0

(k*A) = (k*F)N * FK1 * FK2

F3 = (M3/M3N) ** 2

at GLOBAL = design: F3=1.0

P4 = P3 - DP34N * F3

M4 = M3

maximum/minimum values for the iteration

H2max = f (P2,T3)

Q12max = M1 * (H2max - H1)

H4min = f (P4,T1)

Q34max = Q3 - M4 * H4min

for FFLOW= (cross-counterflow/backward flow) {

Qmax = min(Q12max,Q34max)

}

for FFLOW=(counter current) {

Estimation for the start of the iteration

QA = min(Q12max,Q34max)

QM = QA*QA/(Q12max+Q34max)

Iteration mark 1

H2 = h1 + QM / M2

T2 = f (P2,H2)

T4 = T2

H4 = f (P4,T4)

QP = Q3 -M4 * H4

DQ = QM - QP

regula falsi method

(QM - QMalt)

grad = --------------------

(DQ - DQalt)

QM = QM - DQ * grad

End of the regula falsi method

| DQ |

DQ = |--------------------|

| (QM+QP)*0.5|

if DQ < TOL, then end iteration 1
else continue Iteration 1

Qmax = QM

}

Q12 = 0.5* Qmax

Iteration mark 2

H4 = (Q3 - Q12)/M4

T4 = f (P4,H4)

H2 = H1 + Q12/M2

T2 = f (P2,T2)

for FFLOW = counter current flow {

DTL = T4 - T1

DTU = T3 - T2

}

for FFLOW = concurrent flow {

DTL = T4 -T2

DTU = T3 -T1

}

LMTD = (DTU - DTL)/(ln(DTU) - ln(DTL))

QQ = (k*A) * LMTD

DQQ = Q12 - QQ

regula falsi method

(Q12 - Q12alt)

grad = -------------------------

(DQQ - DQQalt)

Q12 = Q12 - DQQ * grad

End of the regula falsi method

| DQQ |

DQ = |--------------------|

|(Q12+QQ)*0.5|

if DQ < TOL then end iteration 2
, else continue iteration 2

(k*A)*LMTD = M2*H2 - M1*H1

(k*A)*LMTD = M3*H3 - M4*H4

Component Displays

Display Option 1

Example

Click here >> Component 43 Demo << to load an example.

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