ANS:D - all 'a', 'b' & 'c'

In a gas-liquid shell and tube heat exchanger:

1. The presence of a non-condensible gas decreases the condensing film coefficient because non-condensible gases create a barrier between the condensing liquid and the surface, hindering heat transfer.
2. Gases under high pressure are typically routed through the tube side. High-pressure gases can indeed be corrosive in nature, but the primary reason for routing them through the tube side is to prevent leaks. Tube side pressure is easier to contain than shell side pressure, reducing the risk of leakage.
3. Gases to be heated/cooled are normally routed through the shell side. This is because the corrosion caused by cooling water or steam condensate usually remains localized to the tubes, making tube replacement or maintenance easier and less costly.
4. Presence of non-condensible gas decreases the condensing film coefficient: When a non-condensible gas is present in a gas-liquid shell and tube heat exchanger, it creates a barrier between the condensing liquid and the heat transfer surface. This barrier hinders the condensation process by reducing the effective contact area between the liquid and the surface, leading to a decrease in the condensing film coefficient. As a result, heat transfer efficiency decreases.
5. Gases under high pressure are routed through the tube side: High-pressure gases can indeed be corrosive in nature, posing a risk to the structural integrity of the heat exchanger. However, one primary reason for routing high-pressure gases through the tube side is to contain the pressure more effectively. Tube side pressure is easier to contain than shell side pressure, reducing the risk of leakage and ensuring safety.
6. Gases to be heated/cooled are normally routed through the shell side: When gases are to be heated or cooled in a shell and tube heat exchanger, they are typically routed through the shell side. This is because the shell side is generally more accessible for maintenance and cleaning. Additionally, if corrosion occurs due to the cooling water or steam condensate, it tends to remain localized to the tubes. As a result, tube replacement or maintenance is easier and less costly compared to shell-side repairs.

So, the correct answer is (c) all 'a', 'b' & 'c'.

 

In summary, all of these factors contribute to the design and operation considerations in a gas-liquid shell and tube heat exchanger, ensuring efficient heat transfer while addressing safety and maintenance concerns.