Psychrometrics

Psychrometrics, or psychrometry, is the study of the physical and thermodynamic properties of gas-vapor mixtures, and the relationships between those properties. Of most common interest is the mixture of water vapor and dry air. The most frequent uses of psychrometric principles are for heating, ventilating, and air-conditioning and meteorology.

Psychrometric Ratio

The psychrometric ratio is an important property in the area of psychrometrics as it relates the absolute humidity and saturation humidity to the difference between the dry bulb temperature and the adibatic 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 adibatic 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 psychrometic relationships.

Psychrometric chart

File:Psychrometric chart simplified.png

A simple Psychrometric chart.

A psychrometric chart is a graph of the physical properties of moist air at a constant pressure or often equated to an elevation relative to sea-level. The chart graphically expresses how various properties relate to each other, and is thus a graphical 'equation of state'. The thermophysical properties found on most psychrometric charts are:

Dry-bulb temperature is that of an air sample, as determined by an ordinary thermometer, the thermometer's bulb being dry.

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.

Dew point temperature is that at which a moist air sample at the same pressure would reach water vapor saturation. At this saturation point, water vapor would begin to condense into liquid water fog or (if below freezing) solid hoarfrost, as heat is removed. The dewpoint temperature is measured easily and provides useful information, but is normally not considered an independent property. It duplicates information available via other humidity properties and the saturation curve.

Relative Humidity is the ratio of the mole fraction of water vapor to the mole fraction of saturated moist air at the same temperature and pressure. The notion that air "holds" moisture, or that moisture dissolves in dry air and saturates the solution at some proportion, is an erroneous (although widespread) concept.

Humidity Ratio is the proportion of mass of moisture present in a unit mass of air at the given conditions (DBT, WBT, DPT, RH, etc.).

Specific Enthalpy symbolized by h, also called heat content per unit mass, is the sum of the internal energy of a thermodynamic system. These values correspond to the saturated state and are to be read parallel to DPT values.

Specific Volume, also called Inverse Density. Volume per unit mass.

The versatility of the psychrometric chart lies in the fact that by knowing three independent properties of some moist air, one of which is the chart's pressure, the other properties can be determined. Changes in state, such as when two air streams mix, can be modeled easily and somewhat graphically using the correct psychrometric chart for the location's air pressure or elevation relative to sea level. For locations at or below 2000 ft (600 m), a common assumption is to use the sea level psychrometric chart.

Willis Carrier, considered the 'father' of modern air-conditioning, rearranged the Mollier diagram for moist air (its T-s chart) to allow these graphical solutions. Many variations and improvements to the psychrometric charts have occurred since, and most charts do not show the specific entropy (s) like the Mollier diagram. ASHRAE now publishes what are considered the modern, standard psychrometric charts, in both I-P and SI units, for a variety of elevations or air pressures.

How to read the chart

The most common chart used by practitioners and students alike is the "w-t" (omega-t) chart in which the Dry Bulb Temperature (DBT) appears horizontally as the abscissa and the humidity ratios (w) appear as the ordinates.

In order to use a particular chart, for a given air pressure or elevation, at least two of the six independent properties must be known (DBT, WBT, RH, Humidity Ratio, Specific Enthalpy, and Specific Volume). This gives rise to 6C2 or 15 possible combinations.

DBT : This can be determined from the abscissa

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 Ordinate 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

A hygrometer is an instrument used to measure 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 is not saturated water will evaporate from the wick causing the temperature reading to be lower. The accuracy of the wet-bulb temperature depends on how fast air passes over the bulb. 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 is 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.

External links

Psychrometric Chart - Detailed psychrometric chart including curves for enthalpy, air mass, and water mass.
100 Years of Innovation - Carrier Corporation on the introduction of psychrometrics by Willis Carrier
American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
Universal Industrial Gases, Inc. webpage - Links to physical properties tables, to psychrometric charts that depict inter-relationships among the various physical properties of air, to online psychrometric properties calculators for easy calculation of properties, and to individual gas information pages.
Corwin's Calculators Calculator for humidity, dew point.