Humidity and Saturation Questions and Answers

Answer: A

Correct answer: the relative humidity

Heating at constant pressure adds no water and removes none, so the absolute humidity, the water partial pressure and the dew point all stay exactly where they were. What changes is the *saturation* pressure, which rises steeply with temperature - so the same partial pressure becomes a smaller fraction of it and the relative humidity falls. That is why heating a room dries the air without removing any water from it.

Answer: B

The first two are correct and describe the standard cool then reheat arrangement. The third is wrong, since reheating raises the temperature and lowers the relative humidity but adds no water. The absolute humidity is unchanged by the reheat.

Answer: C

The ratio of the two humidities is one half. The relative humidity of the same air would be a slightly different figure, since it is a ratio of pressures rather than of humidities. The two agree only at zero and at full saturation.

Answer: D

Saturation fixes the water partial pressure at the vapour pressure of water, which is a function of temperature alone. The total pressure then converts that into a mass ratio, which is why the chart is drawn for one pressure.

Answer: A

The latent heat of the small evaporated fraction removes far more energy than the sensible heat transfer. The lowest temperature the water can approach is the wet bulb temperature of the air. A few per cent of the flow evaporating does the work.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. A cooling tower cools by evaporation, not by conduction to the air, so its limit is the wet bulb temperature rather than the dry bulb - and on a hot dry day the wet bulb may be ten or fifteen degrees below the dry bulb. The approach, the gap between the leaving water and the wet bulb, is therefore the measure of a tower’s performance.