Matching a Vendor’s HRSG in

THERMOFLEX

Thermoflow Inc.

Contents

Part 1

Create The Drawing

Part 2

Model The HRSG At Design-Point

Part 3

Model The HRSG At Off-Design With Vendor Data

Part 4

Compare TFX With Vendor Model

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Create the DrawingPart 1

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There are two possible approaches to creating a THERMOFLEX drawing.Build model in THERMOFLEX Connect PEACE heat

exchangers, using mixer or splitter icons for inlets or outlets.

Use Water/Steam and Gas/Air Sources for streams entering HRSG.

Use Water/Steam and Gas/Air Sinks for streams leaving HRSG.

Build model in GT PRO and import into THERMOFLEX Save .TFX file as-is for later use

as a total system model.

Replace steam and gas connections with sources and sinks at inlet and exit conditions to create a single HRSG model.

Working with a single, isolated model of the HRSG speeds computation and reduces potential confusion.

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Create the Drawing in THERMOFLEXConnect the appropriate PEACE heat exchangers in TFX. Icons with green borders are PEACE icons.

Click icon to add picture

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Click icon to add picture

Import the Drawing from GT PROAfter pressing “COMPUTE” to calculate a plant heat balance that mimics the HRSG design point in GT PRO, select “Fully-Flexible Design” to import the cycle design into THERMOFLEX.

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Model the HRSG at Design PointPart 2

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Thermodynamic Design Mode

In Thermodynamic Design mode, a component’s performance is governed by defined thermodynamic characteristics.

In Edit Inputs mode, match the main cycle conditions perfectly, including steam conditions at the inlet of the steam turbine and at the exit of the HRSG.

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Entering Cycle Conditions in TD ModeIn Edit Inputs for the Gas inlet to the HRSG, match the GT exhaust entering the HRSG and the inlet temperature given for the vendor model. It is crucial to match both inlet and outlet gas conditions as well as gas composition.

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Entering Cycle Conditions in TD ModeIn Edit Inputs for the HP Steam outlet of the HRSG (HP Superheater), use Specify outlet steam temperature to match the HP steam specified in the vendor model. Be sure to use conditions for the outlet steam that are within immediate proximity of the HRSG.

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Engineering Design Mode

In Engineering Design mode, THERMOFLEX produces a hardware model of the component at design point using user-defined hardware constraints and the heat transfer requirements dictated by Thermodynamic Design mode.

When the model is computed in ED mode, THERMOFLEX will design and size a heat exchanger using internal algorithms. If we have vendor data, we will overwrite this computed physical profile later.

Thus it is not important to change any ED Inputs. Run the model and proceed to Off-Design Mode.

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Model HRSG at Off-Design with Hardware DataPart 3

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Off-Design Mode

In Off-Design mode, a component’s thermodynamic performance is governed by its size and physical characteristics.

To model the HRSG the next step is to specify hardware characteristics for each heat exchanger.

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Enter Hardware Data in THERMOFLEXPEACE components can be defined as hardware models in their “Edit Inputs > Hardware” tab, as shown above for the HP Superheater.

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Entering Vendor Data in THERMOFLEX

Vendor Fin Data: TFX Hardware Inputs:

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Entering Vendor Data in THERMOFLEX, Hardware Definitions

Pay attention to reconciling any non-standard definitions. For example, Thermoflow programs use "number of rows per water-side pass“ as an input, described in detail and illustrated by diagrams in Thermoflow's manuals.

Some vendors instead define the number of passes, which is clear (number of rows per pass = total number of rows/number of passes". Other vendors may define a "number of parallel circuits", or "number of banks", etc.

Make sure any ambiguity is resolved and the appropriate inputs used to mimic the vendor's flow pass configuration.

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Entering Data For Multiple HX in THERMOFLEX

In TFX, when an HRSG has many heat exchangers of the same type, with mostly similar hardware characteristics (tube length, transverse width, etc.), observe the following procedure to save time:

Make one HX icon, model it in Off-Design mode, and enter its given hardware characteristics (tube length, fin-tube type, etc.).

Return to Edit Drawing mode.

Make copies of it after entering the hardware data in OD mode and change the details.

Now you have multiple HX with the same default characteristics, and minor differences, and can change the few key differences, instead of entering every detail for each new HX.

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Entering Data For Multiple HX in THERMOFLEX

Click icon to add picture

Above is an example of this approach using one PEACE economiser that has been designed in OD Mode to make a series of economiser elements.

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Entering Data For Multiple HX in GT PROIf starting a model in GT PRO, set the tube length and duct width directly at GTP’s HRSG Inputs topic, within ‘HRSG Duct Sizing Criteria’ panel. This saves changing the values manually after importing the model into TFX.

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Compare THERMOFLEX and Vendor AssumptionsPart 4

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Comparing Results

Generally, if all details are entered correctly, heat recovery results should be within about 0.5% to 1% of vendor calculations.

If not? Several assumptions should be reviewed against those of the vendor to tune the THERMOFLEX model.

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Steam Properties

Remember to match the THERMOFLEX calculation engine with the vendor’s choice of steam properties.

The steam property formulation that THERMOFLEX uses can be selected in “Options > Current Settings”

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Calculation Options in “Other Inputs”The “Other Inputs” tab for each PEACE component contains several calculation assumptions that can be modified by the user. Shown is the Other Inputs tab for a superheater element.

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Modifying Calculation Assumptions

Vendors choose to include or not include several calculations which THERMOFLEX performs. The user may turn off these calculations if looking to match vendor data that does not incorporate them.

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Cross-Flow Correction

A heat exchanger with a single pass tube arrangement, or a high heat capacity ratio, will exhibit cross-flow behavior. THERMOFLEX accounts for this. If the user would like to override this calculation, they may turn on Disable HX cross flow correction in the heat exchanger's “Other Inputs” tab.

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HX Radiation Correction

If a HX sees strong radiation from the hot walls of an HRSG inlet duct, or from a duct burner, Compute radiation Q from DB or GT exhaust should be turned on in “Other Inputs”. This calculation can be turned off if you must to meet vendor data provided without it.

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Pressure Correction for Drum Elevation

The Hydrostatic correction for drum elevation accounts for the height of the drum. This calculation may be turned off by the user, if looking to match data produced without it.

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Differences in Gas-Side Pressure Drop

Gas-side pressure drop calculation models tend to have greater variations from vendor to vendor. It is not unusual to find differences of 10%-15% for individual heat exchangers. Also, different vendors may account for duct, catalyst, and miscellaneous gas side pressure drops in different ways. Stack losses, friction, leaving loss, and the offsetting hot-gas buoyancy can all be modelled different ways by different vendors.

Thermoflow's models without changing any correction factors should give good results, but if you wish to match a particular vendor, then you can change either the miscellaneous losses or the heat exchangers' gas-side pressure drop correction factor from its default value of 0.9.

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Differences in Steam-Side Pressure Drop

Evaporator pressures have a major effect on heat recovery, so it is vital to check superheater pressure drops to ensure evaporator pressures match at design conditions.

If there is any discrepancy, first make sure that the number of rows per pass in each HX is correct. Next, make sure that the row-to-row pressure drop inputs have been defined correctly (are they smooth 180 bends or header terminations?)

It is not unusual to still find some discrepancy due to either flow-distribution orifices or the unique geometry of each vendor's header arrangements. Enter an appropriate correction factor for steam-side pressure drop for each superheater to resolve any remaining difference.

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Reviewing Results

After implementing the level of detail described, heat transfer rates in the HRSG should match to within 0.5% - 1.0% at the Design Point, as well as at all Off-Design calculation points.

You can accept more variation in steam flow rates as long as they are offset by opposite variations in steam exit enthalpy, thereby minimizing effect on power generated.

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