Psychrometric Charts

Psychrometric Charts

A psychrometric chart is a graphical representation of the thermodynamic properties of moist air.

There are a number of different charts. They differ with respect to the barometric pressure, range of temperature, and the choice of coordinates.
The most popular charts in the U.S. have absolute humidity plotted against dry-bulb temperature.
Most published charts are for standard barometric pressure (14.696 psia).

Description of the psychromentric chart:

The horizontal axis form the dry-bulb temperature scaleand veritcal lines are lines of constant dry-bulb temperature.
The vertical axis is the humidity ratio scale and horizontal lines are lines of constant humidity. Note if the chart include vapor pressure, this will also be a vertical scale and the horizontal lines are lines of constant vapor pressure.
Curves lines are used to represent relative humidity. The line of saturation (100% RH) is used to provide the dewpoint temperature and the wet-bulb temperature scales. Lines of constant dewpoint are horizontal lines.
Constant wet-bulb temperatures form diagonal lines on the chart. A different set of diagonal lines represent lines of constant specific volume.
Lines of constant enthalpy are closely aligned to the lines of constant wet-bulb many charts use a single set of lines to represent constant wet-bulb temperature and constant enthalpy. The error is so minor that for most applications it is not important. An enthalpy scale is typically placed above the saturation line.

Reading the chart

The chart is used to determine the state of the air-water mixture when at least two properties are known. Since lines of constant values for most properties are not parallel or coincident, the point of intersection defines the state of the air. The exception is that dewpoint temperature, humidity ratio, and vapor pressure are all represented by horizontal lines thus they cannot be used without another property to define the condition of the mixture.

If dry-bulb temperature and wet-bulb temperature are know then the lines of constant values are found on the chart, interpolating as necessary. The point of intersection defines the state of the mixture and the other properties are determined by find the value of the line for that property that passes throught the defined point.

Availability of HARDCOPY of PSYCHROMETRIC CHART:

Psychrometrics charts may be obtained from ASHRAE 1-800-527-4723 or http://www.ashrae.org

Psychrometrics charts may be ordered from Carrier Packaged Training Literature Department.

1-800-644-5544. email: Contact.Carrier@carrier.utc.com (Contact Carrier)

Five versions of the chart are included in the following ASAE Standard: ASAE D271.2 Psychrometric Data, which is available from ASAE. http://asae.org/standards/

Psychrometric charts and software are also available from http://www.linric.com