ANS:C - water velocity can be increased conveniently to increase the overall heat transfer co-efficient because of its lower specific volume compared to steam.

The statement "water velocity can be increased conveniently to increase the overall heat transfer coefficient because of its lower specific volume compared to steam" refers to a principle known as velocity effect in heat exchangers, particularly in the context of condensers used in thermal power plants. Here's an explanation of this principle:

1. Lower specific volume of water: Water has a lower specific volume compared to steam. Specific volume is the volume occupied by a unit mass of a substance. In other words, water is denser than steam for the same mass. Due to this lower specific volume, water can flow through tubes at higher velocities compared to steam for the same mass flow rate.
2. Velocity effect: In heat exchangers, such as condensers, the heat transfer coefficient is affected by the velocity of the fluid flowing over the heat transfer surface. Higher fluid velocities result in increased turbulence and mixing near the surface, leading to enhanced heat transfer rates. This effect is known as the velocity effect.
3. Increasing water velocity: Since water has a lower specific volume compared to steam, it can be conveniently pumped at higher velocities through the tubes of the condenser. By increasing the water velocity, the velocity effect enhances the convective heat transfer coefficient on the water side of the condenser. This leads to improved overall heat transfer performance.
4. Optimizing heat transfer: By increasing the water velocity, the overall heat transfer coefficient in the condenser is increased, allowing for more efficient heat transfer from the steam to the cooling water. This optimization is crucial for maximizing the thermal efficiency of the power plant and ensuring effective condensation of the steam.

In summary, the lower specific volume of water compared to steam allows for higher water velocities to be conveniently achieved in condenser tubes. This increased water velocity enhances the convective heat transfer coefficient, leading to improved overall heat transfer performance and optimized thermal efficiency in thermal power plants.