Skip to content

Common Mistakes

The same dozen errors account for most of the questions on the forum. Each one has a symptom, a cause, and (since version 10.2.9) a finding in the Flowsheet Check that names it. This page goes through them in the order a student usually meets them; the Diagnostic Codes page has the full text of every finding.

What you will learn

  • The errors that stop a solve, and the ones that let it finish with a wrong answer
  • How to read a finding: what it means, why it happens, how to fix it
  • A checklist to run before believing a result

Before the solve

A stream that goes nowhere

Symptom: the solver stops with "object not connected" or a unit is never calculated. Finding: STREAM_DANGLING, UNIT_UNCONNECTED, UNIT_NO_FEED. Cause: a stream was dropped on the canvas and never attached, or attached to the wrong port. Fix: open the unit's Connections panel; every inlet and outlet has a named port.

A feed with a hole in it

Symptom: "temperature not defined" or a flash error on the first unit. Finding: FEED_NO_TEMPERATURE, FEED_NO_PRESSURE, FEED_NO_FLOW, FEED_NO_COMPOSITION. Cause: one of the four numbers a feed needs is missing; see Degrees of Freedom. Fix: the Degrees of Freedom tab lists the empty slot by name.

A composition that does not sum to one

Symptom: flows come out slightly different from what was typed, or a compound is missing downstream. Finding: FEED_COMPOSITION_NOT_NORMALISED. Cause: fractions typed by hand that sum to 0.97, or a basis mix-up (mass typed as mole). Fix: use the Normalize button in the composition editor; check the basis selector.

An efficiency typed as a fraction

Symptom: a pump or compressor with absurd power, an outlet temperature far too high. Finding: EFFICIENCY_OUT_OF_RANGE. Cause: DWSIM reads efficiencies in percent; 0.75 means three quarters of one percent. Fix: type 75.

Split ratios that do not sum to one

Symptom: mass appears or disappears at a splitter. Finding: SPLITTER_RATIOS_NOT_NORMALISED. Fix: the ratios of the outlets in use must sum to 1; leave unused outlets at 0.

A reactor with no reactions

Symptom: the reactor solves and changes nothing. Finding: REACTOR_NO_REACTIONS. Cause: the reaction was created but not added to a reaction set, or the reactor points to an empty set. Fix: Reactions Manager; check the set assigned to the reactor and that the reaction is active.

A loop with no Recycle, or a Recycle with no estimate

Symptom: the solver never starts, or takes hundreds of iterations. Finding: RECYCLE_NO_ESTIMATE, and a blocker for a loop without a tear. Fix: see Recycles and Convergence.

After the solve

These let the solve finish and give a result that a real process could not produce. The Flowsheet Check runs the plausibility rules after every solve; Explain Result shows the numbers behind each.

A heater that cooled, a cooler that heated

Finding: HEATER_COOLED, COOLER_HEATED. Cause: an outlet temperature below the inlet on a heater, or a negative duty. The arithmetic is fine; the equipment is the wrong one. Fix: swap the unit, or fix the specification.

Pressure going the wrong way

Finding: PRESSURE_WRONG_DIRECTION. Cause: a pump with a negative rise, a valve with a negative drop, a compressor with an outlet pressure below the inlet.

A temperature cross in an exchanger

Finding: HX_TEMPERATURE_CROSS, HX_HEAT_FLOW_REVERSED. Cause: the specified outlet temperatures would need the cold stream to leave hotter than the hot stream enters (or the duty runs the wrong way). No area achieves this. Fix: Explain Result on the exchanger draws the T-Q diagram and marks the pinch; lower the duty, or change the flow arrangement, until the curves no longer cross.

A liquid below its freezing point

Finding: TEMPERATURE_BELOW_FREEZING. Cause: most property packages know nothing about solids and will happily report a liquid at 200 K. The finding compares the temperature with the compounds' melting points.

A column below its minimum reflux

Finding: COLUMN_REFLUX_BELOW_MINIMUM. Cause: the shortcut column computed a minimum reflux above the one specified; the stage count it reports is meaningless. The McCabe-Thiele diagram shows the pinch that causes it.

Mixing streams at different pressures

Finding: MIXER_PRESSURE_MISMATCH. Cause: two feeds at different pressures into one mixer. The simulator picks a rule (lowest by default); a real pipe would have back-flow.

A unit that did nothing

Finding: UNIT_HAD_NO_EFFECT. Cause: a heater with zero duty, a valve with zero pressure drop, a separator on a single-phase stream. Sometimes intended; often a specification left at its default.

Reading a finding

Every finding carries a code, one line on what is wrong, one line on what to do, and a link to the Diagnostic Codes page. In the Flowsheet Check window, select a finding to read the longer explanation (what it means, why it happens, how to fix it) and use Locate to jump to the object. The assistant and the MCP server return the same codes, so a script or an AI client can act on them.

A checklist before believing a result

  1. Flowsheet Check shows no blockers and no warnings you cannot explain.
  2. Every mass balance closes: Explain Result on a mixer, separator or column shows the inlet and outlet flows per compound.
  3. Every energy balance closes: the same reports write sum(mH)_in + Q = sum(mH)_out with the numbers.
  4. Temperatures go up in heaters and compressors, down in coolers, valves and expanders; pressures rise in pumps and compressors and fall everywhere else.
  5. No phase is where it should not be: a liquid at 200 K, a vapour at 20 bar and 300 K in a propane line.
  6. A second property package gives a similar answer on the pair that decides the process, or the Property Package Comparison says why it should not.
  7. Scenario Comparison after a change shows only the objects the change could reach moving.

Exercises

  1. Open Tutorial 02 and set the heater's outlet temperature below its inlet. Solve, run the Flowsheet Check, and read the finding. Then read Explain Result on the heater: what sign does the duty have?
  2. On Tutorial 05 - Heat Exchanger Design, raise the cold outlet temperature until the Flowsheet Check reports a temperature cross. Open Explain Result and find the pinch on the T-Q diagram. At what duty does the cross appear?
  3. Type 0.8 as the efficiency of the compressor in Tutorial 08. What does the power become, and which finding names the mistake?