Humidity and Saturation Questions and Answers

Answer: C

The first two are correct. The third is inverted, since a large difference means dry air with rapid evaporation and the gap closes to nothing as saturation is approached. The depression is what a psychrometric chart is entered with.

Answer: D

Correct answer: contacted with recirculating water

Adiabatic saturation is defined by an insulated chamber in which air is contacted with water recirculated at a steady temperature: the air leaves saturated, and the water settles at the temperature where the latent heat it gives up to evaporation is exactly balanced by the sensible heat the air gives up in cooling. For the air-water system that temperature nearly coincides with the wet bulb temperature, which is why the two are used interchangeably in practice.

Answer: A

Both are true and the reason gives the condition: the coincidence is a property of air and water specifically, and for other vapours such as organic solvents the two temperatures differ appreciably. Solvent drying calculations must keep them separate.

Answer: B

The chart deals only with the air and water system at a fixed total pressure, and a combustion calculation has nothing to do with it. The other three are exactly what the chart is drawn to provide and can be read from a single point on it.

Answer: C

Correct answer: total pressure

A psychrometric chart relates humidity, temperature, enthalpy and saturation, and every one of those relations depends on the total pressure - so a chart is drawn for one pressure, usually one atmosphere, and is invalid at any other. Working at altitude or on a pressurised system requires a chart drawn for that pressure or a calculation in its place.

Answer: D

The construction works because the wet bulb line joins states of equal adiabatic saturation temperature. Every other property of that air is then read off at the same point. One point on the chart carries every property of that air.

Answer: A

The dry air is unchanged, so subtracting the two humidities gives the water gained directly. This is precisely why the dry air basis is used rather than a moist air one. The dry air acts as the tie component through the drier.

Answer: B

The water content per kilogram of dry air is unchanged, so only the relative humidity falls as the saturation pressure rises. This is why heating alone is an effective way of drying something. The air can then hold more water before it saturates.

Answer: C

Keeping the dry air basis throughout means the humid volume, humid heat and enthalpy all refer to the same kilogram of dry air. The consistency is what makes the chart internally coherent. Every property on it shares the same basis.

Answer: D

It combines the heat capacity of the dry air with that of the water it carries, weighted by the humidity. Its value therefore rises with humidity, which matters in sizing an air heater. Moist air takes more energy to heat than dry air does.