Introduction:
In a steam power station, a surface condenser is used to convert the exhaust steam from the turbine back into liquid form, which can be reused in the boiler. The condensate is typically cooled using a cooling water system, which removes heat from the condenser. The undercooling of condensate refers to cooling it below its saturation temperature, which is the temperature at which it would normally condense at a given pressure.

Explanation:
The undercooling of condensate in a surface condenser is undesirable because it reduces the efficiency of the power plant. This can be understood by considering the following points:

1. Effect on heat transfer: Undercooling the condensate reduces the temperature difference between the condensing steam and the cooling water. This decreases the overall heat transfer rate in the condenser. As a result, more cooling water is required to achieve the desired condensation of the steam. This increases the cooling water requirements and the associated pumping power, which leads to a decrease in the overall efficiency of the plant.

2. Effect on turbine efficiency: Undercooling of condensate increases the backpressure on the turbine. The backpressure is the pressure in the exhaust steam path after it passes through the condenser. The increase in backpressure reduces the pressure difference across the turbine, which in turn decreases the turbine efficiency. This results in a lower power output from the turbine and a decrease in the overall efficiency of the plant.

3. Effect on thermal stresses: Undercooling of condensate can also lead to increased thermal stresses in the condenser. When the condensate is cooled below its saturation temperature, it can cause thermal contraction and differential expansion between the condenser tubes and the tube sheets. This can result in increased thermal stresses, which can lead to tube failures and damage to the condenser. Repairing and maintaining the condenser due to such failures can be costly and time-consuming.

Conclusion:
In conclusion, undercooling of condensate in a surface condenser used in a steam power station is undesirable because it reduces the efficiency of the plant, increases the cooling water requirements, and can lead to increased thermal stresses in the condenser. Therefore, it is important to design and operate the condenser in such a way that the condensate is cooled to its saturation temperature, ensuring optimal performance and efficiency of the power plant.