What is the effect on the outlet temperatures in both the hot and cold streams, when the surface area (length and diameter) is reduced?

a - The outlet temperature of the cold stream decreases and the outlet temperature of the hot stream increases.

b - Both the outlet temperature increases

c - The outlet temperature of cold fluid increases and the outlet temperature of hot fluid decreases

The right answer is option A: The outlet temperature of the cold stream decreases and the outlet temperature of the hot stream increases.

Explanation:

In a heat exchanger, when the exchange area is reduced, and at the same condition for the rest of the variables, the amount of heat exchanged by the two fluids is reduced.

In the case of the hot fluid, it will rise at a higher temperature, compared to the temperature at which it would come out with a larger exchange area.

In the case of the hot fluid, it will come out at a lower temperature, compared to the temperature at which it would come out with a larger exchange area.

The correct answer is option A.

a - The outlet temperature of the cold stream decreases and the outlet temperature of the hot stream increases.

Explanation:

Hello,

In heat transfer processes, specific units such as heat exchangers are useful to either heat up a cold stream with a hot one or vise-versa. However, the heat transfer efficiency increases as surface area increases, it means that the higher the surface area the higher the heat exchanger's efficiency, therefore, if the surface area is reduced, the efficiency is reduced as well, due to the lack of more space for the heat transfer.

In such a way, the outer temperature of the cold fluid will be lower (less heating up) and the outer one of the hot fluid will be higher (less cooling down), thus, the outlet temperature of the cold stream decreases and the outlet temperature of the hot stream increases (a option).

Best regards.