Final answer:
Shell and tube condensers are mechanically cleaned when there is a performance issue such as a pressure drop, not on a strict time-based schedule such as weekly or annually.
Step-by-step explanation:
Shell and tube condensers are critical components in various industrial processes, including air conditioning systems, that function by exchanging heat and condensing process vapors into liquids. These condensers require regular maintenance to ensure efficient operation and longevity. The cleaning schedule for a shell and tube condenser is not strictly time-based such as weekly or annually. Instead, they are typically cleaned mechanically when there is an indication that they are not performing optimally, which can be evidenced by a drop in performance or pressure.
Upon the notice of pressure drops or other performance issues, a maintenance team would carry out a mechanical cleaning procedure. This is akin to the maintenance program implemented after the store reviewed its air conditioning system following infection concerns, ensuring that there was no growth of biofilms within the cooling tower. Safety officers might also request cleaning as a precautionary measure, particularly when public health is at risk. However, the cleaning frequency as per the given options is best addressed by 'C) When indicated pressure drops'.