Transcript Matching a Vendor’s HRSG in THERMOFLEX

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 Drawing
Part 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.
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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.
Create the Drawing in THERMOFLEX
Connect the appropriate PEACE heat exchangers in TFX. Icons with green borders are PEACE icons.
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Import the Drawing from GT PRO
After pressing “COMPUTE” to calculate a plant heat balance that mimics the HRSG design point in GT PRO, select “FullyFlexible Design” to import the cycle design into THERMOFLEX.
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Model the HRSG at Design Point
Part 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 Mode
In 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 Mode
In 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 Data
Part 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 THERMOFLEX
PEACE 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:
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TFX Hardware Inputs:
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
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 PRO
If 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
Assumptions
Part 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.
Thermoflow Inc.