ANS:A - cold fluid is heated to a certain temperature by condensing steam (isothermal fluid).

When the cold fluid is heated to a certain temperature by condensing steam (which is an isothermal fluid), it means that the cold fluid gains heat from the steam until it reaches a specific temperature, while the steam remains at a constant temperature throughout the process. Here's an explanation of the process:

1. Cold Fluid: This refers to the fluid that is being heated by the condensing steam. It could be water, air, or any other fluid that needs to be heated. Initially, the cold fluid enters the heat exchanger at a lower temperature than the steam.
2. Condensing Steam (Isothermal Fluid): Steam is introduced into the heat exchanger at a high temperature. As it comes into contact with the colder surface of the heat exchanger (where the cold fluid flows), it releases heat energy to the cold fluid and starts to condense. During the condensation process, the steam remains at a constant temperature, known as the saturation temperature, because it is in thermal equilibrium with the liquid phase.
3. Heat Transfer: Heat is transferred from the condensing steam to the cold fluid through the walls of the heat exchanger. This transfer of heat causes the temperature of the cold fluid to rise until it reaches the desired temperature.
4. Isothermal Condition: Throughout the process, the steam maintains an isothermal condition, meaning it stays at a constant temperature (saturation temperature) until it completely condenses into liquid.
5. Outcome: Eventually, the cold fluid exits the heat exchanger at a higher temperature, having gained heat from the condensing steam. The steam, on the other hand, exits the heat exchanger as liquid at the same temperature it entered, having transferred its latent heat of vaporization to the cold fluid.