DESCRIPTION OF THE PSYCHROMETRIC CHART

Psychrometric Chart

 The ASHRAE Psychrometric Chart is a graphical representation of the thermodynamic properties of air. There are five different psychrometric charts available and in use today:

Chart No. 1 – Normal temperatures, 0 to 50⁰ C 

Chart No. 2 – Low temperatures, -40 to 10⁰ C 

Chart No. 3 – High temperatures, 10 to 120⁰ C

Chart No. 4 – Very High temperatures, 100 to 200⁰ C

Chart No. 5 – Normal temperatures at 750 meters above sea level, 0 to 50⁰ C

Chart No. 6 – Normal temperatures at 1500 meters above sea level, 0 to 50⁰ C

Chart No. 7 – Normal temperatures at 2250 meters above sea level, 0 to 50⁰ C

Chart No. 1 can be used alone when no freezing temperatures are encountered. Chart No. 2 is very useful, especially in locations with colder temperatures. To apply the lower range chart to an HVAC system, part of the values are plotted on Chart No. 2 and the resulting information transferred to chart No. 1. This will be discussed in future posts (Refer to Next post with the title “Examples of Air Mixing Process”. These two charts allow working within the comfort range of most systems.

THE ABRIDGED PSYCHROMETRIC CHART

The chart shown above is an abridged form of Chart No. 1. Some of the scale lines have been removed to simplify illustrations of the psychrometric processes. Smaller charts are used in most of the subsequent examples. Data in the examples is taken from full-scale charts.

The major lines and scales on the abridged psychrometric chart identified in bold letters are:

— Dry-bulb temperature lines

— Wet-bulb temperature lines

— Enthalpy or total heat lines

— Relative humidity lines

— Humidity ratio or moisture content lines

— Saturation temperature or dew point scale

— Volume lines in cubic meters per kilogram of dry air

The chart also contains a protractor nomograph with the following scales:

— Enthalpy/humidity ratio scale

— Sensible heat/ total heat ratio scale

When lines are drawn on the chart indicating changes in psychrometric conditions, they are called process lines.

With the exception of relative humidity, all lines are straight. Wet-bulb lines and enthalpy(total heat) lines are not exactly the same, so care must be taken to follow the correct line. The dry-bulb lines are not necessarily parallel to each other and incline slightly from the vertical position. The purpose of the two enthalpy scales (one on the protractor and one on the chart) is to provide reference points when drawing an enthalpy (total heat) line. The protractor nomograph, in the upper left corner, is used to establish the slope of a process line.

The various properties of air can be determined from the chart whenever the lines of any two values cross even though all properties may not be of interest. For example, from the point where the 21⁰C dry-bulb and 15.5⁰C wet-bulb lines cross (As shown in image below, Point A), the following additional values can be determined:

— Relative humidity is 56 percent (Point A)

— Volume is 0.845 cubic meters per kilogram of dry air (Point A)

— Dew point is 12⁰C (Point B)

— Moisture content is 8.75 grams of moisture per kilogram of dry air (Point C)

— Enthalpy (total heat) is 43.5 kilojoules per kilogram of dry air (Point D)

— Density is 1.163 kilograms per cubic meter (reciprocal of volume)

Below is another plotting example. This time the dry-bulb temperature line and relative humidity line are used to establish the point. With the relative humidity equal to 60 percent and the dry-bulb temperature at 25⁰C (In the image below, Point A), the following values can be read:

— Wet-bulb temperature is 19.5⁰C (Point A)

— Volume is 0.86 cubic meters per kilogram of dry air (Point A)

— Dew point is 17⁰C (Point B)

— Moisture content is 12.0 grams of moisture per kilogram of dry air (Point C)

— Enthalpy is 56.0 kilojoules per kilogram of dry air (Point D)

— Density is 1.163 kilograms per cubic meter (reciprocal of volume)

Below image is same as above image, but is used to obtain latent heat and sensible heat values. 

The above two images indicate that the enthalpy(total heat) of the air is 56.0 kilojoules per kilogram of dry air (Point D). Enthalpy is the sum of sensible and latent heat (Line A to E + Line E to D, above image). The following process determines how much is sensible heat and how much is latent heat. The bottom horizontal line of the chart represents zero moisture content. Project a constant enthalpy line to the enthalpy scale (from Point C to Point E). Point E enthalpy represents sensible heat of 25.5 kilojoules per kilogram of dry air. The difference between this enthalpy reading and the original enthalpy reading is latent heat. In this example 56.0 minus 25.5 equals 30.5 kilojoules per kilogram of dry air of latent heat. When the moisture content of air changes but the dry-bulb temperature remains constant, latent heat is added or subtracted.

            The psychrometric graph and its value is turning out to be progressively forgotten among the designer profession. It is proper that understudies of engineering, in a time of ecological mixture molding of structures acknowledge how accommodating and what's more significant, the diagram is helping them with understanding what possibilities there are to molding their structures. The

plotting of psychrometric information of a specific environment is the initial phase in becoming mindful of the plan conditions or limitations. It should be a forerunner to any plan approach and might turn into a plan idea generator when a comprehension of our outer climatic circumstances is completely perceived and appreciated. Many authors of psychrometric topics have felt that there is a need among engineering specialists to take single or various focuses and plot them onto the psychrometric outline. A specialty has  been filled to give clients a basic instrument for plotting main informative items and the safe place onto a psychrometric outline. Hence, to get a proper outcome one should know how to use the psychrometric chart and be particular about the values to find out , in order to carry out their air conditioning calculations. More importantly, the selection of proper psychrometric chart is one of the most important aspect before applying their principles to find out the values.

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