ANS:B - U-tube

Tube expansion allowances exist in U-tube heat exchangers. Explanation:

• Tube expansion allowances refer to the space provided within the heat exchanger design to accommodate thermal expansion and contraction of the tubes.
• In U-tube heat exchangers, the tubes are bent into a U-shape configuration. As the fluid inside the tubes heats up or cools down during operation, the tubes expand or contract.
• Tube expansion allowances are necessary to prevent damage to the heat exchanger components, such as the tube sheet and the tubes themselves, due to excessive stresses caused by thermal expansion.
• In multipass fixed tube sheet and single pass fixed tube sheet heat exchangers, the tubes are typically securely attached to the tube sheet, allowing limited movement. However, in U-tube heat exchangers, the U-bend design allows for more significant tube movement, necessitating tube expansion allowances.
• Therefore, tube expansion allowances exist specifically in U-tube heat exchangers to accommodate thermal expansion and contraction of the tubes.