Continuing my discussion of the psychrometric chart and how to use it (see "The Psychrometric Chart, Part 1"), I showed a psychrometric chart, plotting four points. Allow me to proceed with the topic by picking up our ongoing example and taking it further, by putting that air inside a piece of process equipment such as a dryer or oven.

Suppose you have a fan that can deliver 10,000 standard cubic feet per minute (scfm). That is 10,000 cfm at sea level and 70oF (21oC). This translates to a specific volume of 13.33 ft3 of air per pound. You would, of course, verify this volume by conducting a pitot traverse with a suitable manometer across a duct, or using an anemometer at the fan inlet; performing both a suitable number of diameters downstream of turbulence.

The density of standard air is 0.075 lb/ft3. At our condition, 82oF and 60 percent relative humidity, the specific volume is 14 and the density is 0.0714 lb/ft3. Thus, 10,000 ft3/min of air at this density has a mass flow rate of 714 lb/min of air.

We do not change the mass flow rate of air, and hence at 200oF, you have energy available of

714 lb/min x 64 BTUs/lb = 45,700 BTUs/min

The amount of moisture in the inlet air is

714 lb/min x 0.014 lbwater/lbair = 10 lb/min of water

The amount of moisture in the exhaust is

714 lb/min x 0.029 lbwater/lbair = 20.7 lb/min of water

This system is evaporating approximately 10 lb/min of water. If the inlet and discharge moisture content of the product are known, data relating to rate can be developed.

Things get more complicated when there is recycled air, the dryer is multistage, or a direct heat source is used. There is obviously moisture carryover in recycled air, and one of the byproducts of combustion is water.

It is useful to trace the dryer. Take measurements of the wet and dry bulb temperatures at various points along the air path. Perform pitot traverses in ducts to obtain volumetric flow rates and convert these to mass flow rates. Use the chart shown in the September column to define the operating conditions in the dryer. Compare these with the theoretical and specified conditions.

Using psychrometric charts and empirical data obtained from the field will allow for a reasonably accurate model of a dryer to be built. I must reiterate that psychrometric charts are a graphical representation of known formulae and that exact calculations using those formulae can be performed if desired.

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