If one end of a "U" tube is connected to the discharge of a forced draft fan, what happens to the liquid levels?

When one end of a "U" tube is connected to the discharge of a forced draft fan, the behavior of the liquid levels within the tube is influenced by the pressure difference created by the fan. The forced draft fan generates a pressure differential as it pushes air through the system. As the fan discharges air, it creates a lower pressure at the inlet side of the fan which affects the liquid levels in the "U" tube.

The connection end, being under the influence of the fan discharge, will experience a relative reduction in pressure compared to the open end of the tube. This difference in pressure causes the liquid to be pushed higher in the side that is open to the atmosphere. Consequently, the open end of the "U" tube will have a higher liquid level than the connection end.

This principle relies on fluid mechanics, specifically how pressure differentials can influence fluid levels in a column. The lower pressure at the fan's discharge pulls the liquid down at that end, resulting in a higher level in the open end of the tube. Therefore, when a forced draft fan discharges, it contributes to this fluid dynamic effect, demonstrating why the correct answer states that the open end will have a higher liquid level than the connection end.