Psychrometrics
Psychrometrics is the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures.
History
With the inventions of the hygrometer and thermometer, the theories of combining the two began to emerge during the sixteenth and seventeenth centuries. In 1818, a German inventor, Ernst Ferdinand August, patented the term “psychrometer”, from the Greek language meaning “cold measure”. The psychrometer is a hygrometric instrument based on the principle that dry air enhances evaporation, unlike wet air, which slows it.Common applications
Although the principles of psychrometry apply to any physical system consisting of gas-vapor mixtures, the most common system of interest is the mixture of water vapor and air, because of its application in heating, ventilation, and air-conditioning and meteorology. In human terms, our thermal comfort is in large part a consequence of not just the temperature of the surrounding air, but the extent to which that air is saturated with water vapor.Many substances are hygroscopic, meaning they attract water, usually in proportion to the relative humidity or above a critical relative humidity. Such substances include cotton, paper, cellulose, other wood products, sugar, calcium oxide and many chemicals and fertilizers. Industries that use these materials are concerned with relative humidity control in production and storage of such materials. Relative humidity is often controlled in manufacturing areas where flammable materials are handled, to avoid fires caused by the static electricity discharges that can occur in very dry air.
In industrial drying applications, such as drying paper, manufacturers usually try to achieve an optimum between low relative humidity, which increases the drying rate, and energy usage, which decreases as exhaust relative humidity increases. In many industrial applications it is important to avoid condensation that would ruin product or cause corrosion.
Molds and fungi can be controlled by keeping relative humidity low. Wood destroying fungi generally do not grow at relative humidities below 75%.
Psychrometric properties
Dry-bulb temperature (DBT)
The dry-bulb temperature is the temperature indicated by a thermometer exposed to the air in a place sheltered from direct solar radiation. The term dry-bulb is customarily added to temperature to distinguish it from wet-bulb and dew point temperature. In meteorology and psychrometrics the word temperature by itself without a prefix usually means dry-bulb temperature. Technically, the temperature registered by the dry-bulb thermometer of a psychrometer. The name implies that the sensing bulb or element is in fact dry. WMO provides a 23-page chapter on the measurement of temperature.Wet-bulb temperature (WBT)
The thermodynamic wet-bulb temperature is a thermodynamic property of a mixture of air and water vapor. The value indicated by a wet-bulb thermometer often provides an adequate approximation of the thermodynamic wet-bulb temperature.The accuracy of a simple wet-bulb thermometer depends on how fast air passes over the bulb and how well the thermometer is shielded from the radiant temperature of its surroundings. Speeds up to 5,000 ft/min are best but it may be dangerous to move a thermometer at that speed. Errors up to 15% can occur if the air movement is too slow or if there is too much radiant heat present.
A wet bulb temperature taken with air moving at about 1–2 m/s is referred to as a screen temperature, whereas a temperature taken with air moving about 3.5 m/s or more is referred to as sling temperature.
A psychrometer is a device that includes both a dry-bulb and a wet-bulb thermometer. A sling psychrometer requires manual operation to create the airflow over the bulbs, but a powered psychrometer includes a fan for this function. Knowing both the dry-bulb temperature and wet-bulb temperature, one can determine the relative humidity from the psychrometric chart appropriate to the air pressure.
Dew point temperature
The saturation temperature of the moisture present in the sample of air, it can also be defined as the temperature at which the vapour changes into liquid. Usually the level at which water vapor changes into liquid marks the base of the cloud in the atmosphere hence called condensation level. So the temperature value that allows this process to take place is called the 'dew point temperature'. A simplified definition is the temperature at which the water vapour turns into "dew".Humidity
Specific humidity
Specific humidity is defined as the mass of water vapor as a proportion of the mass of the moist air sample ; it is closely related to humidity ratio and always lower in value.Absolute humidity
The mass of water vapor per unit mass of dry air containing the water vapor. This quantity is also known as the water vapor density.Relative humidity
Is a ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air was saturated.Specific enthalpy
Analogous to the specific enthalpy of a pure substance. In psychrometrics, the term quantifies the total energy of both the dry air and water vapour per kilogram of dry air.Specific volume
Analogous to the specific volume of a pure substance. However, in psychrometrics, the term quantifies the total volume of both the dry air and water vapour per unit mass of dry air.Psychrometric ratio
The psychrometric ratio is the ratio of the heat transfer coefficient to the product of mass transfer coefficient and humid heat at a wetted surface. It may be evaluated with the following equation:The psychrometric ratio is an important property in the area of psychrometry, as it relates the absolute humidity and saturation humidity to the difference between the dry bulb temperature and the adiabatic saturation temperature.
Mixtures of air and water vapor are the most common systems encountered in psychrometry. The psychrometric ratio of air-water vapor mixtures is approximately unity, which implies that the difference between the adiabatic saturation temperature and wet bulb temperature of air-water vapor mixtures is small. This property of air-water vapor systems simplifies drying and cooling calculations often performed using psychrometric relationships.
Humid heat
Humid heat is the constant-pressure specific heat of moist air, per unit mass of the dry air.The humid heat is the amount of heat required to change the temperature of unit mass of a water vapor - air mixture by 1 °C.
Pressure
Many psychrometric properties are dependent on pressure concept:- vapor pressure of water;
- atmospheric pressure at the location of the sample.
Psychrometric charts
Terminology
A psychrometric chart is a graph of the thermodynamic parameters of moist air at a constant pressure, often equated to an elevation relative to sea level. The ASHRAE-style psychrometric chart, shown here, was pioneered by Willis Carrier in 1904. It depicts these parameters and is thus a graphical equation of state. The parameters are:- Dry-bulb temperature is that of an air sample, as determined by an ordinary thermometer. It is typically plotted as the abscissa of the graph. The SI units for temperature are kelvins or degrees Celsius; other units are degrees Fahrenheit and degrees Rankine.
- Wet-bulb temperature is that of an air sample after it has passed through a constant-pressure, ideal, adiabatic saturation process, that is, after the air has passed over a large surface of liquid water in an insulated channel. In practice this is the reading of a thermometer whose sensing bulb is covered with a wet sock evaporating into a rapid stream of the sample air. When the air sample is pre-saturated with water, the WBT will read the same as the DBT. The slope of lines of constant WBT is the ratio between the heat of vaporization of water and the specific heat of dry air, roughly 0.4.
- Dew point temperature is the temperature at which a moist air sample at the same pressure would reach water vapor "saturation." At this point further removal of heat would result in water vapor condensing into liquid water fog or, if below freezing point, solid hoarfrost. The dew point temperature is measured easily and provides useful information, but is normally not considered an independent property of the air sample as it duplicates information available via other humidity properties and the saturation curve.
- Relative humidity is the ratio of how much water vapor there is to how much water vapor there would be at saturation under the same temperature and pressure. RH is dimensionless, and is usually expressed as a percentage. Lines of constant RH reflect the physics of air and water: they are determined via experimental measurement. The concept that air "holds" moisture, or that moisture "dissolves" in dry air and saturates the solution at some proportion, is erroneous ; see relative humidity for further details.
- Humidity ratio is the proportion of mass of water vapor per unit mass of dry air at the given conditions. It is also known as the moisture content or mixing ratio. It is typically plotted as the ordinate of the graph. For a given DBT there will be a particular humidity ratio for which the air sample is at 100% relative humidity: the relationship reflects the physics of water and air and must be determined by measurement. The dimensionless humidity ratio is typically expressed as grams of water per kilogram of dry air, or grains of water per pound of air.
- Specific enthalpy, symbolized by h, is the sum of the internal energy of the moist air in question, including the heat of the air and water vapor within. Also called heat content per unit mass. In the approximation of ideal gases, lines of constant enthalpy are parallel to lines of constant WBT. Enthalpy is given in joules per kilogram of air, or BTU per pound of dry air.
- Specific volume is the volume of the mixture containing one unit of mass of "dry air". The SI units are cubic meters per kilogram of dry air; other units are cubic feet per pound of dry air. The inverse of specific volume is usually confused as the density of the mixture. However, to obtain the actual mixture density one must multiply the inverse of the specific volume by unity plus the humidity ratio value at the point of interest:
In the ω-''t'' chart, the dry bulb temperature appears as the abscissa and the humidity ratio appear as the ordinate. A chart is valid for a given air pressure. From any two independent ones of the six parameters dry bulb temperature, wet bulb temperature, relative humidity, humidity ratio, specific enthalpy, and specific volume, all the others can be determined. There are possible combinations of independent and derived parameters.