The Psychrometric Chart, Part 1

TYPICAL DRYING CURVE for industrial thermal drying

Figure 1. Using psychrometric charts and empirical data obtained from the field will allow for a reasonably accurate model of a industrial thermal dryer to be built.

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.)

Now that we are somewhat familiar with all of the data ranges and curves on the psychrometric chart, let's figure how to use it. I will jump straight in.

Suppose you are at sea level. Taking a thermometer and measuring the ambient air temperature, you get 82^oF (28^oC). This is the dry bulb temperature. You take a reading with your hygrometer that tells us that the relative humidity is 60 percent. Looking at the chart in figure 1, you plot this condition at point A. From the chart, you can read off the remaining properties of this air as follows:

The wet bulb temperature is 72^oF (22^oC).
The dewpoint, or saturation, temperature is 68^oF (20^oC). That is, if you cool the air, the water will start to condense out at 68^oF.
The humidity ratio is 0.014 pounds of moisture per pound of air.
The specific volume is 14 ft³ per pound of dry air.
The enthalpy is 36 BTU per pound of dry air.

Next, imagine that you take this same air and heat it up indirectly to 200^oF (93^oC). In terms of the physical constituents of the air, you are changing nothing. The amount of water in the air remains the same, so you can follow the humidity ratio line until you reach the dry bulb temperature line of 200^oF. This is point B. Again, from the curve, you read off the corresponding properties of the air as follows:

The wet bulb temperature has increased to 96^oF (36^oC).
The dewpoint temperature has not changed.
The humidity ratio has not changed.
The relative humidity has dropped from 60 percent to 3 percent.
The air has expanded so that 17 ft³ of this air now weighs one pound.
The enthalpy, or energy of the air, has increased to 64 BTU per pound of dry air.

Suppose you now pass this heated air through a moist product. For this example, we'll make a huge assumption that all the energy in the air is used for evaporation. It never is, but I can take the liberty under guise of illustration. If this were the case, and we chose that the air, after passing through the product, was at 110^oF (43^oC), the air would follow the constant wet bulb temperature line (adiabatic because no heat is entering or leaving the system) until it reached the dry bulb temperature of 110^oF, which is point C. At this point, you obtain the following data:

The wet bulb temperature has remained the same.
The dewpoint temperature has risen to 94^oF (34^oC).
The humidity ratio has increased to 0.035 pounds of moisture per pound of dry air. It has picked up 0.021 pounds of moisture per pound of dry air from the product.
The relative humidity is back to 60 percent. Coincidence? Yes!
The air has densified so that 15.15 ft³ of air now weighs one pound.
The energy in the air has remained relatively constant. This is part of our huge assumption that the sensible heat in the air has been replaced by latent heat in the moisture.

Let's not make any more assumptions; instead, we will replace them with presumptions. Presume you take your dry bulb thermometer and measure the temperature of 110^oF above the product bed. And, by placing some wet gauze on the bulb in the same location, you measure a wet bulb temperature of 94^oF. This would be point D on the curve. For the air to follow path B to D, you obtain the following data:

The dew point temperature has risen to 88^oF (31^oC).
The humidity ratio has increased to 0.029 pounds of moisture per pound of dry air. It has picked up 0.015 pounds of moisture per pound of dry air from the product.
The relative humidity is at about 52 percent.
The air has densified so that 15.0 ft³ of air now weighs one pound.

The energy in the air has reduced to 58 BTUs per pound of dry air. This is a reduction of 6 BTUs per pound of dry air. Where has it gone? Losses! There are losses from radiation, conduction and convection. The product has increased in temperature. The skin of the dryer is hotter than ambient steel. The system is constantly losing heat.

In "The Pyschrometic Chart, Part 2," I'll take this example further by putting that air inside a piece of process equipment such as a dryer or oven.

Editor's Note: Use the link at the bottom of the page to receive a PDF document containing the article, which will show the graphic.

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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.

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