When the heat exchanger heads are opened during maintenance, circumferential cracks are often found at the connections between the tube bundle and the shell. Tracing the operating records, these are mostly related to repeated thermal stress caused by excessive temperature differences. Whether the structural selection of a shell-and-tube heat exchanger matches the actual operating conditions directly determines the probability of such problems. The following provides a Q&A-style explanation from a technical comparison perspective, covering fixed tubesheet versus expansion joint structures, wall thickness grading, and pressure differential cleaning criteria.
In the fixed tubesheet structure, the tubesheets at both ends are rigidly connected to the shell, offering simple fabrication and the tube side can withstand higher pressure. However, when the metal wall temperatures of the shell side and tube side differ significantly, the thermal expansion of the shell and tube bundle differs, generating thermal stress at the connections between the tubesheet, tubes, and shell. When the design temperature difference exceeds 60°C, an expansion joint should be installed on the shell to absorb part of the thermal elongation and relieve thermal stress. The expansion joint structure increases shell-side resistance and manufacturing cost, and if the shell-side medium tends to foul, cleaning at the expansion joint bellows is inconvenient. Selection requires comprehensive weighing of the actual operating temperature difference, shell-side medium cleanliness, and maintenance cycle: for conditions with small temperature differences and clean shell-side media, the fixed tubesheet structure is sufficient; for occasions with large temperature differences or frequent startup and shutdown, the expansion joint structure is more reliable.
The wall thicknesses of the shell and tube bundle are determined by grading according to design pressure; the higher the design pressure, the correspondingly greater the wall thickness. Design temperature is used to verify the allowable stress and expansion of the material at that temperature, and the two must not be confused. Wall thickness calculation also needs to consider corrosion allowance, negative deviation of steel plate, and machining thinning. For the tube bundle, in addition to strength requirements, attention should also be paid to the connection method between tubes and tubesheet; the composite connection of expansion plus welding helps to some extent with sealing reliability under fluctuating temperature conditions.
During operation, the readings of inlet and outlet pressure gauges and thermometers should be recorded periodically. If the inlet-outlet pressure differential increases by more than 0.05 MPa compared with the initial clean state, it usually indicates scaling or blockage in the tube-side or shell-side flow passages, and cleaning should be scheduled. The cleaning method should be selected based on the nature of the fouling layer: mechanical cleaning is suitable for hard scale, chemical cleaning for soft scale, but attention must be paid to the corrosive effect of the cleaning solution on the tube material and shell. After cleaning, the pressure differential should be re-measured and recorded as the baseline for the next cleaning criterion.
