ANS:C - remain unchanged

When the entrance and exit conditions of the coolant are interchanged in a co-current double pipe heat exchanger used for condensing saturated steam over the inner tube, the rate of condensation will typically decrease. Here's why:

1. Original Configuration: In the original configuration, the coolant flows from the entrance of the heat exchanger to the exit, while the steam flows from the entrance of the heat exchanger to the exit as well. This is known as co-current flow, where both fluids flow in the same direction.
2. Heat Transfer: In co-current flow, the temperature difference between the two fluids decreases as they move along the length of the heat exchanger. This reduction in temperature difference leads to a gradual decrease in the rate of condensation of the steam.
3. Effect of Interchanging Conditions: When the entrance and exit conditions of the coolant are interchanged, the coolant now flows from the exit of the heat exchanger to the entrance, while the steam still flows from the entrance to the exit. This is known as counter-current flow, where the fluids flow in opposite directions.
4. Counter-current Flow: In counter-current flow, the temperature difference between the two fluids is maintained along the length of the heat exchanger, resulting in more efficient heat transfer compared to co-current flow. However, in this scenario, the coolant temperature at the entrance is higher than at the exit, which reduces its effectiveness in condensing the steam.
5. Impact on Condensation Rate: The reduced effectiveness of the coolant in condensing the steam due to the higher entrance temperature results in a decrease in the rate of condensation of the steam.