Relative Humidity
Relative humidity (RH) expresses how close moist air is to saturation at a specified temperature. It is defined as the ratio of the actual partial pressure of water vapor to the saturation vapor pressure of water at the same temperature. RH is therefore a temperature-dependent quantity and does not directly represent the absolute amount of moisture in the air. For a fixed water-vapor partial pressure, increasing air temperature increases the saturation vapor pressure and therefore decreases RH. Conversely, cooling moist air can increase RH until the air reaches saturation. The temperature at which RH reaches 100% is the dew-point temperature. If a surface temperature falls below the dew point, water vapor can condense on that surface. Relative humidity is widely used in HVAC, building design, drying processes, storage, electronics, and environmental control because excessive humidity can promote condensation, corrosion, and biological growth, while very low humidity can cause drying and electrostatic problems. For many psychrometric calculations, RH is combined with dry-bulb temperature, pressure, and humidity ratio to fully describe the moist-air state.
Knowledge Check
1.What does relative humidity represent?
2.Air has a water-vapor partial pressure of 1.5 kPa and a saturation vapor pressure of 3.0 kPa at the same temperature. What is the relative humidity?
3.Why can relative humidity decrease when air is heated even if no moisture is removed?
4.What happens when moist air is cooled to its dew-point temperature?
5.A room contains moist air at 25°C with a dew-point temperature of 15°C. A window surface reaches 12°C. What is the likely result?
6.Which statement correctly distinguishes relative humidity from humidity ratio?
7.At a fixed water-vapor partial pressure, what generally happens to relative humidity when air temperature increases?
8.What does 100% relative humidity indicate at a given temperature?
9.Why is relative humidity important in building HVAC design?
10.If air is cooled below its dew point in a cooling coil, what process normally occurs?
11.An air sample has a water-vapor partial pressure of 2 kPa and a total pressure of 100 kPa. Using W = 0.622 p_v / (P - p_v), what is the approximate humidity ratio?