Component 88: Steam Generator - Flue Gas Zone

Specifications

 

General       User Input Values       Characteristic Lines       Physics Used       Displays       Example

 

General

Together with components 89 (steam generator, main heating surface) or 91 (steam generator, auxiliary heating surface) respectively, component 88 allows the geometry based calculation of the heat exchange in the heating surfaces of a steam generator.

Component 88 represents a flue gas zone, which houses a main heating surface (component 89) and up to two auxiliary surfaces. PIN 3 of the flue gas zone (component 88) should be
connected with PIN 3 of a component 89 (main heating surface). PIN 4 or PIN 5 of component 88 can be connected with PIN 3 of component 91. In addition, the radiation flow from the flue gas volume in the zone to the upstream and / or downstream heating surface component 89 can be taken into account. For this purpose, PIN 7 can be connected with PIN 5 of the main heating
surface (component 89) in the upstream flue gas zone and correspondingly PIN 6 with PIN 4 of component 89 in the downstream zone.

The behaviour of component 89 concerning heat transfer and radiation resembles that of component 90 (combustion zone), but there is no reaction, especially no post-combustion.

 

Extensions for the Pressure Drop Calculation of the Flue Gas Side (for Components 88 and 89)

In Component 89, the pressure drop calculation of the flue gas side can be specified as Kernel expression, EDPO. As geometrical details of the heating surface are specified in Component 89, it is possible to access the corresponding specification values (like e.g. pipe diameter, partition, details on fins, etc.) and to implement an own pressure drop calculation (e.g. according to manufacturer information) in the Kernel expression EDPO.

The pressure drop on the flue gas side is calculated in Component 89 and transferred to Component 88. In Component 88 there is a flag for this, FDP, which switches between the previous pressure drop calculation according to Bernoulli’s principle and EDPO of Component 89.

For the flue gas zone are additionally as result values

• the flow velocity in free cross-section as VELF

• the volume of the VOLFG flue gas zone

• the wall surface of the smoke zone AWS

output.

 

MODEL:

The calculation model first balances the mass flow, momentum and energy flows

• (1)      M1 =  M2
• (2)      P1 = P2 + DP12
• (3)     Q3+Q4+Q5+QR_6+QR_7+VERL= M2*H2 - M1*H1

with the indices

• 1 for flue gas inlet
• 2 for flue gas outlet
• 3, 4, 5 for heat flow to the main (3) or to the auxiliary surfaces (4,5) in the flue gas zone
• 6,7 for the heat flow to the flue gas side component upstream (7) and component downstream(6) respectively.

Concerning enthalpy, mass flow and pressure, the component assumes, that H1 or H2, M1 or M2, P1 or P2 are given on the attached lines. In each case the missing value is calculated.

Pressure drop limitations in off-design (Extras --> Model Options--> Calculation -->Relative pressure-drop maximum) :
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. For this reason, pressure drop limitations have been installed.

 

Note: Improvements of Convergency:

In certain cases, there were cycles that required 1000 or more steps to convergency. Therefore, certain steps were performed to improve the convergency behaviour.

One step was a test to include the lower calorific value and the material composition into matrix solution. By default, the matrix equations are restricted to mass flow, enthalpies and pressures. The test was performed with the components 4, 15, 18, 25, 33, 60, 80, 88 und 90. The result was an improvement of convergency in some cases, but a worsening in other cases.

Therefore, this feature will not be used by default. If anyone likes to test it, you may activate it by setting the level of integration of material equations to " in simulation only" (“Extras”, ”Model options”, “Iteration” ).

 

CALCULATION RULES

The heat flow to main and auxiliary heating surface in the flue gas zone are calculated in the components 89 and 91 and  transferred to component 88 via the logic lines. If upstream or downstream main heating surfaces are attached, in component 88 the corresponding radiation flow are calculated:

QR_6 = PHI6_I*CS*EBS3_I*(1-EBS6_I)*LAMBDA*(TRADH6**4-TRADL6**4)*BEW3

QR_7 = PHI7_I*CS*EBS3_I*(1-EBS7_I)*LAMBDA*(TRADH7**4-TRADL7**4)*BEW3

Form factors PHI, emissivities EBS and evaluations BEW are calculated in the respectively connected modules. The radiation temperature of the emitting zone 88 is the average of in- and outlet temperature.

TRADH6=TRADH7=(T1+T2)/2+273.15

The radiation flows are always positive, that is there is no radiation into the flue gas originating from surfaces with a possibly higher wall temperature. The radiation temperature of the heating surfaces corresponds to the average wall temperature.

TRADL6 = TWAND6

TRADL7 = TWAND7

 COMPONENT IDENTIFICATION

Currently there is no identification mode for component 88.

AUXILIARY HEATING SURFACES

Besides the main heating surfaces there frequently are smaller heating surfaces in the form of water or steam cooled parts of the construction (supporting tubes, cooled walls) in a flue gas zone. They have the same flue gas in- and outlet conditions as the main surface, but usually are integrated in another part of the water / steam path. These auxiliary surfaces are represented by component 91.

Main and auxiliary heating surfaces contribute to the heat exchange within the zone 88, but radiation exchange with neighbouring zone only affects main surfaces, component 89.

An auxiliary surface should be used only if a main surface is already attached.

GEOMETRIC DESCRIPTION OF FLUE GAS ZONES

The arrangement of inlet and outlet area of the flue gas zone essentially influences the calculation of the form factor PHI. To represent the different types of steam generator design (single-pass boiler, double-pass boiler) with or without direction changes in the flue gas path the specifications values OUTH and FTYP can be used. Their use is shown in the diagram given below:

 

User Input Values

 

The identification values marked in blue are reference values for off-design mode. These values are referred to for the actual off-design values used in the equations respectively. If these identification values are stream data, then these values are often taken from attached pipes or calculated values.

The inputs for nominal values are also black in a Sub profile for a given component, in case the user selected the local off-design option for that component and in the nominal values were saved into that component. 

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Characteristic Lines

No characteristic lines are used.

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Physics Used

Equations 

 

 

 

Component Displays

Display Option 1

Example

Click here >> Component_88 Demo << to load an example.