Question

In which of the following type of heat exchanger the heat exchange between the two fluids occur by their complete physical mixing?

• A. Direct contact heat exchanger
• B. Indirect heat exchanger
• C. Recuperator
• D. Regenerator

Hint:

Direct contact heat exchangers allow physical mixing of fluids, enabling both heat and mass transfer, unlike indirect types like recuperators or regenerators.

Answer 1

The Correct Option is A

Step 1: Understand types of heat exchangers. 
Heat exchangers transfer heat between two fluids, and their classification depends on whether the fluids are physically separated or allowed to mix: Direct contact heat exchanger: The two fluids come into direct physical contact and mix, allowing heat transfer through convection and, in some cases, mass transfer (e.g., cooling towers where air and water mix directly).
Indirect heat exchanger: The fluids are separated by a solid wall (e.g., a tube or plate), and heat is transferred through the wall without mixing (e.g., shell-and-tube heat exchangers).
Recuperator: A type of indirect heat exchanger where heat is transferred continuously through a separating wall, typically in a counterflow or parallel flow arrangement (e.g., plate heat exchangers).
Regenerator: A heat exchanger where heat is transferred indirectly via a storage medium (e.g., a solid matrix). The hot fluid heats the medium, and then the cold fluid absorbs heat from the medium (e.g., rotary regenerators). The fluids do not mix.
Step 2: Analyze the requirement for complete physical mixing. 
The question asks for a heat exchanger where heat exchange occurs by complete physical mixing of the two fluids. This implies that the fluids must come into direct contact and mix together, rather than being separated by a wall or medium:
In a direct contact heat exchanger, the fluids mix directly (e.g., hot water sprayed into cold air in a cooling tower). Heat exchange occurs through direct contact, and there may also be mass transfer (e.g., evaporation).
In indirect heat exchangers, recuperators, and regenerators, the fluids are kept separate, either by a wall or a storage medium, so there is no physical mixing. 
Step 3: Evaluate the options. 
(1) Direct contact heat exchanger: Correct, as this type involves complete physical mixing of the two fluids, allowing direct heat (and often mass) transfer. Correct.
(2) Indirect heat exchanger: Incorrect, as the fluids are separated by a solid wall, preventing physical mixing. Incorrect.
(3) Recuperator: Incorrect, as a recuperator is a type of indirect heat exchanger where fluids are separated, so there is no mixing. Incorrect.
(4) Regenerator: Incorrect, as a regenerator uses a storage medium to transfer heat indirectly, with no mixing of the fluids. Incorrect. 
Step 4: Select the correct answer. 
In a direct contact heat exchanger, heat exchange between the two fluids occurs by their complete physical mixing, matching option (1).