Psychrometry: Moist Air Properties Calculations

Dry-Bulb Temperature:

Dry bulb temperature is what is usually meant by "air temperature". It is measured with a normal thermometer.

Dew Point:

Dew point is the temperature at which water vapor begins to condense out of the air.  Alternatively, it can be defined as the temperature at which air becomes completely saturated with moisture. In dehumidification by cooling and condensation, it is the temperature to which the moist air must be cooled to allow water removal.  

The lower the absolute amount of moisture in air, the lower the dew point of that air sample.

Dew points can be defined and specified for ambient air or for compressed air.  The higher the pressure of the air, the higher the "pressure dew point" will be.  Pressure dew point can be used as a proxy for allowable moisture content.  In many cases, pressure dew points are specified for air drying and handling equipment to avoid condensation in compressed air distribution lines exposed to low temperatures.

Wet-Bulb Temperature:

Wet bulb temperature is relatively easy to measure, but requires special equipment. 

The name "wet bulb" derives from the classic method of measuring this property - with a thermometer that has its bulb covered with a moistened piece of gauze or cloth, which is then placed into a flowing air stream or whipped around to speed up evaporation of the water in the bulb covering.  This process can be done relatively easily using a special tool known as a "sling psychrometer".  A sling psychrometer contains a normal (dry bulb) thermometer and a wet bulb thermometer.  They are mounted side-by-side and attached to a twirling rod or line.  The device is spun for ten or fifteen seconds and readings taken.  This process is repeated until consecutive readings stabilize.

The two temperature measurements, which have been produced at the same time in the same environment by the sling psychrometer, can be used to determine relative humidity with the aid of a "psychrometric chart" (which plots the relationship between many physical and thermodynamic variables for air at a given atmospheric pressure), a psychrometric slide rule, or their computerized equivalents.

Wet bulb temperature will never be higher than dry bulb temperature. The difference between the dry bulb and wet bulb temperatures is an indicator of the water content of the air.  If the two temperatures are equal (and the wet bulb was indeed wet), there was no cooling effect from evaporation from the moist thermometer bulb cover; which indicates the air is fully saturated with water vapor, and the relative humidity is 100%. 

Wet-Bulb Temperature has practical applications:  Wet-bulb temperature is the lowest temperature that water will reach by evaporative cooling, and is almost always lower than dry bulb. As a result, evaporative cooling can produce lower cooling water temperatures than cooling in "fin-fan" heat exchangers against dry bulb temperature.  Consequently, evaporative cooling is usually preferred over "dry" cooling methods in industrial applications. 

Wet bulb temperature is a critical parameter for sizing, and measuring the performance of evaporative-cooled cooling water systems.  Cooling towers are specified and designed to achieve an "approach to wet bulb" of a certain number of degrees while circulating a specified amount of cooling water.  Properly sized and maintained, many industrial cooling water towers can cool the circulating water to within about ten degrees Fahrenheit of the wet bulb temperature. Periodic checks of the actual "approach to wet bulb" allows plant operators to compare current versus historical cooling system performance, and helps them to determine when maintenance or system upgrades may be needed. 

Relative humidity, at dew point conditions, is 100%.  Otherwise, relative humidity is the ratio (expressed as a percentage) of the amount of water vapor actually present in the air, to the maximum amount that the air could hold under those temperature and pressure conditions. 

While useful as a general measure of the evaporative cooling potential of air, relative humidity must be translated into absolute measurements of water content to size industrial air handling and drying equipment. 

Absolute Humidity:

With the aid of a psychrometric chart, or its computerized equivalent, absolute values for water content such as weight fraction of ambient air, or weight-per-unit-volume of ambient air can be determined for any combination of dry bulb and wet bulb temperatures, or dry bulb temperature and relative humidity. For many purposes, the most useful measurements of water content are in relationship to the amount of dry air; because in humidification/ dehumidification processes, the amount of dry air will remain constant while the amount of water changes. 

With this information, water-removal (air drying) systems can be sized to absorb, condense or otherwise remove the necessary amount of water for specified combinations of atmospheric pressure, temperature, relative humidity and air flow rate.

Vapor Pressure:

At a given pressure and temperature the vapor pressure of water in fully saturated air will be equal to the molecular fraction water in the air/ water mix times the total pressure. It may be measured in any pressure measurement units. This is the same pressure as would be found in a closed container of liquid and gaseous water at the same temperature.  It will normally be no more than a few percent of the total atmospheric pressure. The vapor pressure line for pure water defines the 100% saturation or dew point line on the left side of a psychrometric chart. 

Heat Content (Specific Enthalpy): 

This thermodynamic measurement of the energy content of air is expressed in units such as BTU/ pound or kJ/kg.  It is used to compare two sets of temperature and humidity (and possibly pressure) conditions to determine how much energy must be added to or removed to heat or cool from the first to the second condition.

Specific Volume:

This is the unit volume of dry air per unit mass.  It is not the simple inverse of density, as it excludes the weight of water.  This measurement is useful for humidification/ dehumidification process calculations, as the amount of dry air will remain constant while the amount of water changes. 

Density:

Density is the the total mass of moist air per unit volume.  It is not the simple inverse of specific volume (see above). This measurement is often omitted from psychrometric charts to avoid confusion with specific volume.