Short answer: a strong shell-and-tube design report shows a clear chain: a heat balance that fixes the duty, a justified temperature profile, an LMTD with the right correction factor, a tube-side coefficient from a recognised correlation, and an area check that is compared honestly with your software output. Markers reward reasoning and cross-checks, not just a table of final dimensions.
Working on a heat-exchanger design report and unsure how it will be marked? Send us your brief and we will walk through the structure.
What the task is really testing
Courses on heat exchangers and heat transfer, including modules coded B50TE, usually pair a software design (HTRI Xchanger or similar) with a short manual verification. The software finds a workable geometry. The manual part proves you understand the physics behind it. A report that only pastes software tables misses most of the available credit. You can read a typical version of this task on our B50TE heat exchangers Assignment #1 question page.
The typical scenario is a process-industry heat-recovery problem: a hot waste stream pre-heats a cold feed, both stay liquid, and a pressure-drop limit applies to each side. The question is rarely “can it be done” but “which design is best and why”.
Where marks usually sit
| Report section | What markers look for | Common way to lose marks |
|---|---|---|
| Design and justification | Stated criteria (area, pressure drop, velocity, fouling, length), several geometries compared | One geometry presented with no alternatives |
| Parameter effects | Individual and combined effect of two changes, explained physically | Quoting software numbers without explanation |
| Manual duty and hi | Heat balance, property choice, correlation, comparison with software | Ignoring that one stream is a mixture |
| Area check | LMTD with F, or ε-NTU, using the software’s coefficients; commentary on the gap | Using LMTD without the F correction |
| Presentation | Concise report, legible drawings, reports in appendices | Unlabelled screenshots, no worked example |
For a worked illustration of the calculation chain, see our reference extract on duty, LMTD and tube-side coefficient (for reference only, not for submission).
A step-by-step plan
- Write the assumptions first. Liquid streams, fluid allocation, allowable pressure drop and fouling resistances.
- Do the heat balance. Convert volumetric flow to mass flow with a stated density, then compute the duty and the unknown outlet temperature.
- Check the temperature cross. If the hot outlet is cooler than the cold outlet, one shell pass may be unable to deliver the duty at all.
- Calculate R and P and test whether a single shell is feasible. If not, consider shells in series or relaxing the target.
- Estimate tube-side velocity and Reynolds number and pick a correlation that suits the regime.
- Combine resistances into an overall coefficient, then compute the required area.
- Compare with the software and explain every significant difference.
The calculation points that decide the grade
Duty and the temperature profile
Start from mass flow, specific heat and temperature change. State where each property comes from and at what temperature. For a mixture such as a water–ethanol wash, say how you estimated specific heat and why a pure-water value would be a simplification.
LMTD and the correction factor
A U-tube exchanger is a 1-2 shell, so the counter-current LMTD must be reduced by an F factor. Designers generally avoid operating where F falls much below about 0.75, because the area penalty grows quickly. When R and P are high, a single shell can be thermodynamically impossible, and the honest answer is to use shells in series or accept a lower outlet temperature.
Tube-side coefficient
Dittus–Boelter is the familiar quick estimate; Gnielinski is generally preferred for a wider range of Reynolds numbers and usually gives a somewhat higher value. Whichever you use, show the Reynolds and Prandtl numbers, name the heating or cooling exponent and compare the result with the software figure.
Area and commentary
Do not stop at a number. State whether the manual area is larger or smaller than the software area and why: different shell-side coefficient, baffle spacing, fouling treatment, property data or rounding of tube count. A short, reasoned paragraph earns more than a precise but unexplained match.
How to justify a design: choosing your criteria
The first mark band is about justification, so choose your criteria before you run any software case, and keep them fixed. Typical criteria are required area, tube-side and shell-side pressure drop against the allowance, tube velocity, tube length, number of shells and ease of cleaning. Then compare at least three geometries against those same criteria.
| Design lever | Usually improves | Usually costs you |
|---|---|---|
| More tubes per pass (lower velocity) | Lower tube-side pressure drop | Lower coefficient, more fouling risk with solids |
| Fewer tubes per pass (higher velocity) | Higher tube-side coefficient, less fouling | Higher pressure drop, longer tubes |
| Longer tubes | Less shell diameter and fewer tubes for the same area | Handling and pull-out space, higher friction loss |
| More shells in series | Higher F, closer approach, bigger recovery | Extra cost, piping and floor space |
| Lower outlet target | Smaller area, easier design | Less heat recovered, more supplementary heating |
Write one sentence under each row explaining why it applies to your case. A design that scores slightly worse on area but much better on fouling can still be the best answer if you argue it with the evidence in front of you.
Explaining parameter changes
When the task asks what happens if two parameters change, individually and together, treat it as a physics question. Say what the change does to velocity, Reynolds number, coefficient, temperature difference, duty and pressure drop, in that order. Then explain whether the second change amplifies or offsets the first. Combined effects are rarely the sum of the separate ones, because one change often alters the conditions under which the other acts. Software can confirm your reasoning, but the reasoning is what is marked.
Planning your time
Many students spend most of the available time in the software and leave the manual calculations to the last evening. Reverse that. Do the manual duty and a rough area first, because they give you a target for the software to match. Then iterate geometries, record each trial in a short table, and write the justification while the results are fresh. Keep the final day for checking units, labelling drawings and trimming the text.
Want a second pair of eyes on your calculation layout before you finalise it? Our team can explain the method and common checks.
Mark-maximising checklist
- Every property has a value, a temperature and a stated source.
- Mass flow, not volumetric flow, is used in the heat balance.
- R, P and F are shown, not just quoted.
- At least three geometries are compared against stated criteria.
- Velocity is discussed against both fouling and pressure drop.
- Each parameter change is explained physically before any software result is quoted.
- Differences between manual and software values are explained.
- Appendices hold the full software reports and the drawings are legible.
Common mistakes
- Applying the pure-water density to the wash and not saying so.
- Skipping the F factor, which understates area.
- Choosing the hot fluid placement without a reason.
- Presenting a lower-velocity design without commenting on suspended solids and fouling.
- Exceeding the pressure-drop allowance and not noticing.
- Treating the software result as proof instead of as one data point to check.
If your course restricts the use of generative AI or outside help for the assessed piece, follow that policy. Reference material, including our worked extract, is there to help you understand the method so you can do the calculations yourself. Learn how we work on our about us page.
Frequently asked questions
Do I need to do the heat exchanger design by hand if I use HTRI?
Usually both are expected. The software supplies the geometry, while the manual calculation checks duty, a tube-side coefficient and the area. Markers read the manual part as evidence that you understand the result.
Which correlation should I use for the tube-side coefficient?
Dittus–Boelter is quick and widely taught. Gnielinski is better across a wider range of Reynolds numbers. Use either with clear reasoning, show the intermediate numbers and compare with the software value.
Why does a U-tube exchanger need an F correction?
Because flow is neither purely counter-current nor co-current. The 1-2 arrangement gives a lower effective mean temperature difference than a true counter-current unit, and F quantifies that loss.
What if the F factor comes out very low or cannot be calculated?
That usually signals a temperature cross that one shell cannot handle. Options are shells in series, a different flow rate, or a lower outlet target. Explain which you chose and why.
How should I treat a stream that is a mixture?
Estimate density, specific heat, viscosity and conductivity at the mean bulk temperature using sensible mixing rules or a property package, and state the limits of the estimate. Do not average pure-component viscosities linearly.
How long should the report be?
Brief reports are normally preferred. Use tables, short paragraphs, labelled drawings and put full software output in the appendices.
Ready to turn your calculations into a clean report? Our report-writing team can explain structure and method, and you can read student reviews first.
About the expert
Reviewed by Nada Ahmed. Her profile lists Thermodynamics & Heat Transfer as her field and a Bachelor’s in Mechanical Engineering from Jumeirah College.