Psychrometrics is the study of the physical and thermal properties of air and water vapor mixtures. The air’s capacity to absorb heat and moisture depends on its characteristics. Seven physical and thermal characteristics are used to describe air and water vapor mixtures. An understanding of these characteristics and their relations to each other will help to better understand ventilating principles.

The seven physical and thermal properties are:

Dry-bulb temperature°F
Humidity ratiolb H20/lb dry air
Relative humidity%
EnthalpyBTU / lb dry air
Dew-point temperature°F
Wet-bulb temperature°F
Specific volumeft3/lb dry air

The interrelationship between air and the moisture it holds provides the basis for maintaining a suitable environment. The above seven properties are used to describe ventilation principles. A psychrometric chart is a convenient way to graphically describe the interrelationships between the seven physical and thermal properties. Knowing any two of the psychrometric values defines the other five values.

Psychrometric chart

WHAT IS A 'BTU'?

British Thermal Unit (BTU) is the amount of heat energy needed to raise the temperature of one pound of water by 1°F. This is the standard measurement used to state the amount of energy that a fuel has as well as the amount of output of any heat generating device, including chickens in a poultry house.

All combustible materials have a BTU rating. For instances, propane has about 15,000 BTUs per pound and wood (dry) has abut 7,000 BTUs per pound.

Although it is still used 'unofficially' in some metric English-speaking countries (such as Canada, the U.S. and the United Kingdom), its use has declined or has been replaced in other parts of the world. In scientific contexts, the BTU has largely been replaced by the international system of units (abbreviated SI from the French Le Système International d'Unités) of energy, the joule (J), though it may be used as a measure of agricultural energy production (BTU/kg).

Dry bulb temperature is the regular temperature measured using either a common thermometer or other temperature sensor. It describes how hot or cold the air is. Temperature is commonly measured in degrees Fahrenheit (°F) or degrees Celsius (°C) (see Figure 7.2 below for conversions). Dry-bulb is sometimes abbreviated ‘db’.

Figure 7.2 - Comparing Celsius and Fahrenheit temperature scales on a dry bulb thermometer

Figure 7.2 - Comparing Celsius and Fahrenheit temperature scales on a dry bulb thermometer

Humidity ratio is a very important air characteristic even though it is not commonly used outside of engineering. The humidity rati describes the moisture holding capacity of the air. There is no common way to directly measure humidity ratio. Values are very small and can range from 0 to 0.044319 lb H2O/lb dry air for saturated air at 100°F db.

Saturated air is air that is holding the maximum amount of moisture possible to hold in the air. The common rule of thumb is that the moisture holding capacity of saturated air doubles for every 20°F increase in temperature. Fifty degree air holds 0.0077 lb H2O/lb dry air which is slightly more than double the moisture holding capacity at 30°F, 0.0035 lb H2O/lb dry air. Similarly, 70°F air holds 0.0158 lb H2O/lb dry air, which is about double the moisture holding capacity of 50°F air.

Relative humidity is a term commonly used to describe how much water vapor is in the air as a percent. Saturated air is at 100% relative humidity (RH). Air with a 50% RH and 100°F contains half the water vapor of saturated air at 100% RH and 100°F.

Enthalpy describes the heat energy content (BTU/lb dry air) of the air and water vapor mixture. The air’s energy content changes if either or both the dry-bulb and humidity ratio change. Therefore enthalpy (energy) is important not only in heating and cooling processes but also in humidifying and dehumidifying processes.

Dew-point temperature is the temperature at which moisture starts to condense from the air at a constant humidity ratio. Dew-point temperatures are commonly reported in weather reports to indicate the amount of moisture in the air. It is directly related to the humidity ratio. Surfaces (i.e., sides of cold drinks with ice, inside building surfaces) at temperatures below the air’s dew-point temperature will have condensation forming on them. Frost is condensation on surfaces at temperatures below freezing. In poultry houses insulation is needed in cold weather to keep the walls and ceilings above the dew-point temperature to prevent either condensation or frost formation.

Wet-bulb temperature is a temperature measured by a thermometer with the bulb or sensor covered with a water moistened wick in a moving air stream (see Figure 7.3). The wet-bulb temperature is always below the dry-bulb temperature. The difference between wet and dry-bulb temperatures is important in evaporative cooling.

Figure 7.3 - Comparing dry bulb (on left) and wet bulb (on right) thermometers

Figure 7.3 - Comparing dry bulb (on left) and wet bulb (on right) thermometers

Specific volume is the volume in cubic feet occupied by a pound of dry air at a specific dry-bulb temperature and pressure, expressed as cubic feet per minute (CFM) to mass (pounds) of dry air being exchanged during ventilation.

Sensible heat is heat that produces a change in the dry-bulb temperature. It takes approximately 0.24 BTU to raise one lb of dry air 1°F. Supplemental heaters are used to add sensible heat to the air to maintain a desired temperature.

Latent heat (of vaporization) is heat used to evaporate water. Evaporation changes liquid water to water vapor. The air’s latent heat changes if and only if there is a change in the air’s humidity ratio. The amount of heat energy needed to evaporate a pound of water does vary with temperature but it is common to use a value of 1,044 BTU/ lb H2O for processes involving agricultural animals.