Explanation:

Multi-pass heat exchangers are used to transfer heat between two fluids. The fluids flow through the exchanger in separate passages, which are arranged such that the hot and cold fluids flow in a counter-current or co-current manner to maximize the heat transfer rate. The following are the reasons why multi-pass heat exchangers are used:

High heat transfer coefficient:
Multi-pass heat exchangers are used to obtain a high heat transfer coefficient. The heat transfer coefficient is a measure of how efficiently heat is transferred between the two fluids. In a multi-pass heat exchanger, the fluids flow in a counter-current or co-current manner, which increases the turbulence and enhances the heat transfer rate. This results in a higher heat transfer coefficient compared to a single-pass heat exchanger.

Compact unit:
Multi-pass heat exchangers are used to get a compact unit. The compact size of the multi-pass heat exchanger is due to the increased heat transfer rate and the efficient use of the available surface area. The compact size makes it easier to install the heat exchanger in a limited space, which is especially useful in the chemical and process industries where space is a constraint.

Large temperature drop through the tube wall:
Multi-pass heat exchangers are designed to facilitate very large temperature drop through the tube wall. The temperature drop is facilitated by the counter-current or co-current flow of the fluids, which allows for a more efficient transfer of heat. This results in a larger temperature difference between the hot and cold fluids, which leads to a higher overall heat transfer rate.

Conclusion:
In conclusion, multi-pass heat exchangers are used to obtain a high heat transfer coefficient, a compact unit, and to facilitate a large temperature drop through the tube wall. These factors make multi-pass heat exchangers an ideal choice for various industrial applications, especially in the chemical and process industries.