Psychrometrics Totally Explained

Everything about Psychrometrics totally explained

Psychrometrics or psychrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. The term derives from the Greek psuchra (cold

) and metron (means of measurement

Common applications

The principles of psychrometry apply to any physical system consisting of gas-vapor mixtures. The most common system of interest, however, are mixtures of water vapor and air because of its application in heating, ventilating, and air-conditioning and meteorology.

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:
ť

r = frac 
ight) = 15

possible combinations.
   DBT: This can be determined from the abscissa on the x-axis, the horizontal axis
   DPT: Follow the horizontal line from the point where the line from the horizontal axis arrives at 100% RH, also known as the saturation curve.
   WBT: Line inclined to the horizontal and intersects saturation curve at DBT point.
   RH: Hyperbolic lines drawn asymptotically with respect to the saturation curve which corresponds to 100% RH.
   Humidity ratio: Marked on the y-axis.
   Specific enthalpy: lines of equal values, or hash marks for, slope from the upper left to the lower right.
   Specific volume: Equally spaced parallel family of lines.

Dry-bulb temperature

Common thermometers measure what is known as the dry-bulb temperature. Electronic temperature measurement, via thermocouples, thermistors, and resistance temperature devices (RTDs), for example, have been widely used too since they became available.

Wet-bulb temperature

The thermodynamic wet-bulb temperature is a thermodynamic property of a mixture of air and water vapor. The value indicated by a simple wet-bulb thermometer often provides an adequate approximation of the thermodynamic wet-bulb temperature.
   A wet-bulb thermometer is an instrument which may be used to infer the amount of moisture in the air. If a moist wick is placed over a thermometer bulb the evaporation of moisture from the wick will lower the thermometer reading (temperature). If the air surrounding a wet-bulb thermometer is dry, evaporation from the moist wick will be more rapid than if the air is moist. When the air is saturated no water will evaporate from the cloth wick and the temperature of the wet-bulb thermometer will be the same as the reading on the dry-bulb thermometer. However, if the air isn't saturated water will evaporate from the wick causing the temperature reading to be lower.
   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 (60 mph) are best but dangerous to move a thermometer at that speed. Errors up to 15% can occur if the air movement is too slow or if there's too much radiant heat present (sunlight, for example).
   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.

Further Information

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