Big Air, Part 2 (The Psychrometric Chart Unraveled)

wet BULB TEMPERATURE TYPICAL DRYING CURVE for industrial thermal drying — **Figure 1.** The wet bulb temperature scale is located along the curved upper left portion of the chart. The sloping lines indicate constant wet bulb temperatures.

saturation TEMPERATURE TYPICAL DRYING CURVE for industrial thermal drying — **Figure 2.** The saturation or dewpoint temperature shares the same scale as the wet bulb temperature (the curved upper left portion of the chart). However, the lines for constant saturation temperatures run horizontal.

relative humdity TYPICAL DRYING CURVE for industrial thermal drying — **Figure 3.** The relative humidity lines sweep from the lower left to the upper right of the psychrometric chart. Typically, each line is annotated and there is no axis with a scale.

absolute humdity TYPICAL DRYING CURVE for industrial thermal drying — **Figure 4.** The humidity ratio has its scale on the Y-axis and lines of constant moisture ratios are horizontal.

specific volume TYPICAL DRYING CURVE for industrial thermal drying — **Figure 5.** Specific volume does not have any specific scale axis. The lines for constant specific volume are each annotated with a value and run diagonally from the bottom right corner of the chart to the top left of the chart.

ehthalpy TYPICAL DRYING CURVE for industrial thermal drying — **Figure 6.** The enthalpy scale is marked on the axis and the lines run diagonally from the bottom right corner of the chart to the top left of the chart — usually considerably less steeply than the specific volume lines.

Before you can understand the complete psychrometric chart and how it is useful for sizing an industrial dryer, you first must understand the terms and the many parts that make up the whole psychrometric chart. (If you haven't read "Big Air, Part 1 (The Psychrometric Chart Unraveled)," start there.)

My "Tools of the Trade" series dealt with various instruments used in the acquisition of system parameters. For the data collected to have some value requires understanding and manipulation of the data, ultimately resulting in a model of your system. In this column, I will continue to will deal with the interpretation and manipulation of this data so that a better understanding of the operation can be gleaned.

Let's get back to the terms I was reviewing so that you can understand the "parts" before we take a look at the whole psychrometric chart.

Wet bulb temperature is the stable temperature that is obtained when evaporative cooling is ensuing (figure 1). It is determined by reading the constant temperature after wrapping a small piece of wet gauze over the thermometer bulb. This too is relatively simple to obtain if you practice how to measure it properly. Unfortunately, there are few electronic instruments that read wet bulb temperatures above 212^oF (100^oC).

The wet bulb temperature scale is located along the curved upper left portion of the chart. The sloping lines indicate constant wet bulb temperatures.

The saturation or dewpoint temperature is the temperature at which the air has no more capacity to retain more moisture (figure 2). Condensation of water from the air will begin immediately as the air is cooled. To obtain the readings, one would use a dewpoint indicator; unfortunately, again, these instruments only are effective at lower temperatures.

The saturation or dewpoint temperature shares the same scale as the wet bulb temperature (the curved upper left portion of the chart). However, the lines for constant saturation temperatures run horizontal.

The relative humidity is the ratio of the partial pressure for the water vapor to the saturation vapor pressure for the same dry bulb temperature. In English, it is a measure of how much moisture is present compared to how much moisture the air could hold at that temperature (figure 3). Common hygrometers will measure the relative humidity at lower temperatures, but again, it is difficult to get instruments to measure the relative humidity at temperatures above 212^oF (100^oC) with any reliability.

The relative humidity lines sweep from the lower left to the upper right of the psychrometric chart. Typically, each line is annotated and there is no axis with a scale. At 100 percent relative humidity, the gas is saturated and, therefore, this line provides the curved scale line for the wet bulb and the saturation temperature scale line. The 0 percent relative humidity coincides with the dry bulb temperature axis.

The humidity ratio or absolute humidity is a pure number and is the ratio of the weight of moisture (water) per unit weight of dry air (figure 4). The humidity ratio has its scale on the Y-axis, and lines of constant moisture ratios are horizontal. Humidity ratio is the moisture content of the air (dry basis).

The specific volume of the air is the volume that one pound of dry air occupies (figure 5), and it is the reciprocal of the density of the air. Specific volume does not have any specific scale axis. The lines for constant specific volume are each annotated with a value and run diagonally from the bottom right corner of the chart to the top left of the chart.

The specific enthalpy is the relative heat (energy) content per unit weight (mass) of air (figure 6). It is relative because the zero value is assigned -- commonly to zero degrees for air. For the enthalpy, a special axis is created along the curved upper left portion of the chart. The axis is a straight line that, on some charts, almost joins the top and bottom of the saturation temperature curve axis. The scale is marked on the axis and the lines run diagonally from the bottom right corner of the chart to the top left of the chart -- usually considerably less steeply than the specific volume lines.

In my next column ("The Psychrometric Chart, Part 1"), I will put all of the pieces together and discuss what a complete psychrometric chart can show you about your process.

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Darren A. Traub is executive vice president of Drytech Engineering, Irvine, CA, a company specializing in thermal drying and related system technologies. He has engineered and managed the project execution of numerous drying and bulk materials-handling systems in the United States, Africa, the Middle East and China. He can be reached at (949) 262-1222 or e-mail darren@drytecheng.com.