PSYCHROMETRIC CHART FUNDAMENTALS:EXAMPLES OF AIR MIXING PROCESS

EXAMPLES OF AIR MIXING PROCESS

The following examples illustrate use of the psychrometric chart to plot values and determine conditions in a ventilating system. The examples also show how to obtain the same results by calculation. Example A requires only Chart No. 1. Example B requires both Charts No. 1 and 2 since the outdoor air temperature is in the range of Chart No. 2.

In this example:

1. A fixed quantity of two-thirds return air and one-third outdoor air is used.

2. The return air condition is 75F dry bulb and 62.5F wet bulb.

3. Outdoor air condition is 10F dry bulb and 50 percent rh.

To find the mixed air conditions at design:

1. Plot the return air (RA) condition (Point A) and outdoor air (OA) condition (Point B).

2. Connect the two points with a straight line.

3. Calculate the mixed air dry-bulb temperature:

(2/3 x 75) + (1/3 x 36) = 62F dry bulb

4. The mixed air conditions are read from the point at which the line, drawn in Step 2, intersects the 62F dry-bulb line . (Point C).

EXAMPLE B:

Plotting values when both Chart No. 1 and Chart No. 2 are required.

In this example, a ventilating system (Fig. 7) is used to illustrate how to plot data on Chart No. 2 and transfer values to Chart No. 1. Chart No. 2 is similar to Chart No. 1 except that it covers the –40 to 50F temperature range. This is the temperature range immediately below that of Chart No. 1. Note that there is an overlap of temperatures between 35F and 50F. The overlap is important when transferring values from one chart to another.

This example illustrates mixing two different air conditions with no change in total heat (enthalpy). Any changes in the total heat required to satisfy space conditions are made by heating, cooling, humidification, or dehumidification after the air is mixed.

In this example:

1. A fixed quantity of two-thirds return air and one-third outdoor air is used.

2. The return air condition is 75F dry bulb and 62.5F wet bulb.

3. Outdoor air condition is 36F dry bulb and 40 percent rh.

To find the mixed air condition:

1. Plot the outdoor air (OA) condition on Chart No. 2, Fig. 8

2. Plot the return air (RA) condition on Chart No. 1, Fig. 9.

3. Calculate the mixed air enthalpy as follows:

a. For the return air, project a line parallel to the enthalpy line from Point A to the enthalpy scale on Figure 9. The value is 28.2 Btu per pound of dry air.

b. For the outdoor air, project a line parallel to the enthalpy line from Point B to the enthalpy scale on Figure 8. The value is 3.1 Btu per pound of dry air.

c. Using the determined values, calculate the mixed air enthalpy:

(2/3 x 28.2) + (1/3 x 3.1) = 19.8 Btu per

pound of dry air

4. Calculate the mixed air moisture content as follows:

a. For the return air, project a line from Point A horizontally to the moisture content scale on Figure 9. The value is 0.0094 pounds of moisture per pound of dry air.

b. For the outdoor air, project a line from Point B horizontally to the moisture content scale on Figure 8. The value is 0.00065 pounds of moisture

per pound of dry air. Also, project this value on to Chart No. 1 as shown in Figure 9.

c. Using the determined values, calculate the mixed air moisture content:

(2/3 x 0.0094) + (1/3 x 0.00065) = 0.00648 pounds of moisture per pound of dry air

5. Using the enthalpy value of 19.8 and the moisture content value of 0.00648, plot the mixed air conditions, Point C, on Chart No. 1, Figure 9, by drawing a horizontal line across the chart at the 0.00648 moisture content level and a diagonal line parallel to the enthalpy lines starting at the 19.8 Btu per pound of dry air enthalpy point. Point C yields 53.3F dry-bulb and 49F wet-bulb temperature.

6. Read other conditions for the mixed air (MA) from Chart No. 1 as needed.

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