Introduction
Back Calc Input in FluidFlow (v3.54+) solves for the input value on one element that will produce a specific result at another point in your model. Instead of trial-and-error, you define a target result (e.g., temperature, pressure, flow) and let FluidFlow iterate the selected input until the target is met.
Back Calc automates the "reverse solve" step in your workflow—ideal for setpoint tuning, control strategy design, and rapid what-if testing.
Key Benefits
Eliminate manual guesswork and reduce design iterations
Hit precise setpoints for temperature, pressure, or flow with minimal effort
Reduce engineering hours spent on system tuning and optimization
Simplify workflow by removing the need for external reverse calculations
When to Use Back Calc
You know the required result (e.g., temperature at a specific pipe) and need the input that achieves it
You're tuning a control valve, pump, or heat exchanger to meet a downstream target
You want to test feasibility quickly before running broader optimization (e.g., via Multi Calc)
How Back Calc Works
You define a Target (element and result property) and a Source (element and input property to vary).
FluidFlow iteratively adjusts the Source input within your defined range and increment.
The closest match to your target is reported, with visual and tabular outputs available.
Using Back Calc Input
Access the tool
Go to Tools → Back Calc Input
Define the Target
Target element: Choose where the result will be measured (e.g., a pipe)
Result property: Select what you want to achieve (e.g., In Static Temperature, Pressure, Flow)
Target value: Enter the desired setpoint
Define the Source
Source element: Choose the component to adjust (e.g., a control valve, pump, heat exchanger)
Input property: Select which input to vary (e.g., Design Flow, % Open, Speed)
Configure search parameters
Range: Enter start and end values to calculate
Change increment: Set the step size (larger = faster, smaller = more precise)
Run the calculation
Click Run to iterate through candidate inputs; the flowsheet updates in real time
Review results
See the closest match on completion
Use the Report and Chart tabs for detailed data and visualization
Apply and validate
Enter the suggested value (”Closest Match”) into the element's input editor
Click Calculate to validate the result across the full model
Worked Example: Temperature Control Valve
This example from the Back Calc Tutorial video shows how to determine the control valve flow required to meet target temperatures at a downstream pipe.
System setup
Inlet conditions: Water at 25°C and 2.5 bar(g)
Outlet pressure: 1.5 bar(g)
Heat exchanger: Fixed temperature increase of 80°C
Target location: Pipe-4 (In Static Temperature)
Source to vary: Temperature Control Valve (Flow Control Valve 4) → Design Flow
Scenario A: Achieve 85°C at Pipe-4
Tools → Back Calc Input
Target
Element: Pipe-4
Result property: In Static Temperature
Target value: 85°C
Source
Element: Flow Control Valve 4
Input property: Design Flow
Range: 15 to 40 m³/h
Change increment: 0.5 m³/h
Run and review
Closest match: 38.5 m³/h
Review the Report/Chart tabs for details
Apply and validate
Enter 38.5 m³/h into the valve's Design Flow
Click Calculate and verify Pipe-4 temperature ≈ 85°C
Scenario B: Achieve 95°C at Pipe-4
Optionally duplicate the model (to keep Scenario A)
Tools → Back Calc Input
Target
Element: Pipe-4
Result property: In Static Temperature
Target value: 95°C
Source
Element: The relevant control valve
Same range and increment as Scenario A
Run and apply
Closest match: 16.5 m³/h
Enter 16.5 m³/h as the valve Design Flow and recalculate
Pipe-4 temperature updates to approximately 95.1°C
Need tighter accuracy?
Reduce the change increment and re-run
Interpreting the Results
Closest match: Displayed on completion of the run
Report tab: Tabular view of the evaluated inputs and corresponding results
Chart tab: Visualize the relationship between the varied input and the target result
Example Applications
Scenario | Back Calc Use | Outcome |
Control valve tuning | Solve for Design Flow or % Open to hit a downstream temperature or pressure | Faster setpoint commissioning |
Pump setup | Find pump speed to achieve required system flow or head | Optimized energy use and performance |
Orifice plate selection | Calculate required orifice diameter to achieve specific pressure drop | Accurate flow restriction |
Bypass line configuration | Determine bypass flow rate needed to maintain mixed stream temperature | Reliable temperature control |
Best Practices
Define precise target parameters before starting Back Calc to avoid unnecessary recalculations.
Use realistic search ranges to reduce computation time. Narrow ranges produce faster, more accurate results.
Adjust change increments for precision — larger increments for quick estimates, smaller increments for fine-tuning.
Verify results by recalculating the model after applying the determined input value.
Duplicate models for testing when running multiple Back Calc scenarios, so you can compare results without overwriting your original setup.
FAQs
Q: What properties can I target with Back Calc?
A: Temperature, pressure, flow rate, and other supported thermodynamic or hydraulic properties.
Q: What if my Back Calc result is close but not exact?
A: Reduce the change increment and re-run. Consider narrowing the search range around the best candidate.
Q: Does Back Calc work on complex models?
A: Yes, but complex networks may require broader ranges and longer run times.
Q: Can I save Back Calc configurations?
A: Presets aren't stored. For reuse, duplicate a configured model and adjust as needed.
Conclusion
The Back Calc Input feature in FluidFlow 3.54 streamlines system fine-tuning by eliminating trial-and-error methods. Simply specify your desired outcome, and the software calculates the exact input required. This powerful tool delivers precise results for temperature control, pressure settings, and other variables, integrating seamlessly with your workflow. Engineers can make faster decisions, minimize iterations, and optimize system efficiency from the outset.
Leverage Back Calc Input to achieve precise outcomes quickly, removing guesswork and accelerating design optimization.