Dew Point – Tdp
The Dew Point is the temperature at which water vapor starts to condense out of the air, the temperature at which air becomes completely saturated. Above this temperature the moisture will stay in the air.
If the dew-point temperature is close to the air temperature, the relative humidity is high, and if the dew point is well below the air temperature, the relative humidity is low.
The Dew Point temperature can be measured by filling a metal can with water and ice cubes. Stir by a thermometer and watch the outside of the can. When the vapor in the air starts to condensate on the outside of the can, the temperature on the thermometer is pretty close to the dew point of the actual air.
The dew point temperature can be read by following a vertical line from the state-point to the saturation line. Dew point is represented along the 100% relative humidity line in the Mollier diagram.
Dry-Bulb Temperature – Tdb
Dry bulb temperature is usually referred to as air temperature, is the air property that is most common used. When people refer to the temperature of the air, they are normally referring to its dry bulb temperature. Dry-bulb temperature – Tdb, can be measured by using a normal thermometer. The dry-bulb temperature is an indicator of heat content and is shown along the left axis of the Mollier diagram. The horizontal lines extending from this axis are constant-temperature lines.
Wet-Bulb Temperature – Twb
Wet bulb temperature is associated with the moisture content of the air. Wet bulb temperature can be measured with a thermometer that has the bulb covered with a water-moistened bandage with air flowing over the thermometer. Wet bulb temperatures are always lower than dry bulb temperatures but they will be identical with 100% relative humidity in the air (the air is at the saturation line). On the Mollier diagram, the wet-bulb lines slope a little upward to the left (dotted lines).
Heating of Air
Cooling and Dehumidfying Air
Mixing of Air of different Conditions
Adding Steam or Water (liquid)
The psychrometric chart is a variant of the Mollier diagram used in some parts of the world.
The process transforming a Mollier diagram to a psychrometric chart is shown below. First it has to be reflected in a vertical mirror, then rotated 90 degrees.
Evaporation from Water Surfaces
The amount of evaporated water can be expressed as:
Problem 6 (page 22)
An indoor pool evaporates a certain amount of water, which is removed by a dehumidifier to maintain +25ºC, f=70% RH in the room (state 1 in figure). The dehumidifier, shown in figure, is a refrigeration cycle in which moist air flowing over the evaporator cools such that liquid water drops out, and the air continues flowing over the condenser. The air after the evaporator (state 2) has a temperature of +14ºC. For an air flow of 0,10 kg/s dry air the unit has a coefficient of performance COPR =3,0.
Total pressure in the room is constant 101325 Pa.
a) the amount of water that evaporates from the pool ( steady state)
b) the compressor work input
c) the absolute humidity and enthalpy (kJ/kg of dry air) for the air as it returns
to the room (state 3 in figure)
Problem dryer with heat pump