CHE3166 Chap.6 Heat Exchangers and Mechanical Design
Heat Exchangers and Mechanical Design
Heat-exchanger design starts with duty, temperature feasibility and an energy balance, then iterates area, overall coefficient and pressure drop. Mechanical design converts credible normal and upset cases into pressure, temperature, material, corrosion allowance and load assumptions. Calculations, datasheets and drawings must remain consistent under revision. Close the energy balance before estimating metal area.
Identify sensible heating, cooling or phase change and verify that the inlet and outlet temperature program is feasible for the proposed flow arrangement. The overall heat-transfer coefficient combines film, wall and fouling resistances; it depends on properties, velocity, geometry and cleanliness, so record its basis and revisit it after preliminary sizing. Thermal and hydraulic design interact.
Higher velocity can improve heat transfer while raising pressure drop, pumping cost and erosion risk. More area can reduce approach temperature while increasing capital, plot space and fouling surface. Plan which side is easier to clean, how the equipment is isolated and opened, what signal triggers maintenance and whether layout preserves bundle-removal space.
Mechanical design begins with operating and upset cases, not a copied normal pressure. Define design pressure and temperature, external loads, cyclic service, material, corrosion allowance and relevant protection. Avoid an impossible combination of maxima, but do not omit credible blocked-in or utility-loss conditions. Carry governing values into the calculation, datasheet and drawing.
A vendor deviation in geometry, pressure drop, material or nozzle arrangement must be assessed and recorded. Independent checking should trace at least one thermal result and one mechanical case from source data through every controlled deliverable.
What this chapter covers
- 01
Energy balance and thermal duty
- 02
Temperature driving force
- 03
Overall heat-transfer coefficient
- 04
Area, velocity and pressure-drop iteration
- 05
Design pressure and temperature
- 06
Datasheets, drawings and independent checks
Worked example · free
Set a mechanical design basis from an upset case
- +2Document normal operation and the credible blocked-in scenario.
- +3Select a design-pressure basis that covers the governing case under the applicable design method.
- +3Carry design conditions, material and allowance into the calculation, datasheet and drawing, then independently check them.
Key terms
- Overall heat-transfer coefficient
- A combined measure of film, wall and fouling resistances to heat transfer.
- Design temperature
- The documented temperature basis used to evaluate material and mechanical capability.
Heat Exchangers and Mechanical Design FAQ
Why close the energy balance before estimating area?
Area sizing depends on the duty and feasible temperature program. Extra area cannot repair an inconsistent energy balance or an impossible temperature approach. Both checks must precede detailed mechanical selection.
What must be reviewed after a nozzle moves?
Review access, piping loads, layout, the mechanical calculation, the equipment drawing and affected process documents under controlled revision. Confirm the new revision across the complete package.
What does Assignment 3 add to the heat-exchanger design?
The mechanical-design task adds design pressure and temperature, corrosion allowance, end-cover selection, construction materials and minimum wall thicknesses. It also requires the earlier thermal design spreadsheet to be updated when mechanical choices change the exchanger configuration, with old and new calculated values retained. Treat that update as an interface check between thermal sizing and containment design.
Assessment move
Solve in layers: duty, temperature feasibility, coefficient basis, area and pressure drop, then mechanical conditions and document consistency. Revisit earlier layers whenever geometry or service changes. Start each thermal problem by drawing hot and cold temperature paths and checking the closest approach. Close the duty with explicit mass flow, heat capacity or phase-change data and units.
State the basis for the overall coefficient, estimate area and then ask whether the implied velocities and pressure drops are practical. Add fouling and cleaning access before declaring the geometry complete. For mechanical review, list normal, start-up, shutdown, blocked-in and utility-loss cases, then identify the governing pressure and temperature without combining conditions that cannot coexist.
Build a consistency table containing design case, material, allowance, nozzle basis, document revision and checker. When vendor information arrives, compare it against every required field and route accepted deviations back into the PFD, P&ID, layout, datasheet and report. Finish by recomputing one result from the final printed values. Develop an original exchanger case from two inlet streams and target outlet temperatures.
Close both sides of the energy balance, identify the closest temperature approach and decide whether the arrangement is feasible before choosing a coefficient. State which film or fouling resistance dominates and how velocity affects both heat transfer and pressure drop. Iterate a preliminary area with a pressure-drop limit, then add cleaning method, isolation and bundle-removal space.
Build a mechanical case table covering normal operation, start-up, shutdown, blocked-in and utility loss. Select governing pressure and temperature without combining unrelated extremes. Carry those values, material and allowance into a mock datasheet and drawing. Introduce one vendor deviation in geometry or nozzle orientation and trace its consequences through performance, piping loads, layout and maintenance.
Ask an independent checker to recompute one thermal result and one mechanical basis from the final printed inputs. Finish with a revision audit that distinguishes required values, supplier guarantees, process estimates and open confirmation items so the package remains usable after design changes. Add an operability pass after sizing: identify start-up heating or cooling, venting, draining, thermal expansion and isolation.
These transient needs can change nozzle arrangement and mechanical loads even when the steady thermal duty remains unchanged. Keep the final recommendation tied to the stated basis, and record the observation that would require a revised engineering decision.
