The "Tools of the Trade" series (links at the bottom of the page) has 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. The next few "Drying Files" will deal with the interpretation and manipulation of this data so that a better understanding of the operation can be gleaned.

I have been careful over the years to ensure that I refer to the heat carrier in convection dryers as a gas. This inference has not been as opposed to a liquid or a solid, but rather to be nonspecific as to the composition of the carrier gas. Some dryers use an inert gas such as nitrogen or work in a modified environment such as an oxygen-depleted environment; hence, they do not use "air" as the carrier. However, most of the dryers in service today are atmospheric and use air as the carrier gas. Air demonstrates properties that are unique because it is composed of numerous gaseous elements, some solid contaminants and some liquid vapors. It is fundamental to understand the properties of the "gas" in drying systems, and none more than the air in convective dryers.

Psychrometrics involves the thermodynamic (heat and mass) properties of moist air. Psychrometric charts are the graphical representations of the properties and are very useful tools in understanding drying systems. This file will explain how to read and use a psychrometric chart.

Before I get carried away, some basics. Air is not air! Atmospheric air is the stuff we deal with on a day-to-day basis. It has all the necessary gaseous elements plus some water vapor and contaminants. Dry air is pure air -- if such an animal exists in today's polluted environment. To make dry air, you remove the water vapor and contaminants from atmospheric air. Moist air is a two-part mixture of dry air and infinite different levels water vapor. How moist the moist air is is a function of how much water vapor there is in the mixture. For all intent and purpose, atmospheric air and moist air are the same critters for our applications.

I do need to clarify that although I will be referring to a generic psychrometric chart in this series (one that I have created and has no reference use), the psychrometric chart used for a specific application must be for the local atmospheric pressure and obviously needs to cover the desired temperature range. Getting your hands on some of these can be a challenge. Because most dryers do not operate at atmospheric pressures or specified pressure increments, adjustment for barometric pressures differences will be required to obtain precise information. For our purposes, a chart in the general atmospheric pressure range is sufficient.

Also, charts are published with different coordinate systems. For this series, I will be using the standard ASHRAE practice of a uniform but slanted enthalpy scale on rectangular coordinates of humidity ratio vs. dry bulb temperature. This will all make sense as we progress.

To most. a psychrometric chart looks complicated and foreboding. In actuality, it is a simple chart made up of (an infinite number of) lines, each of which defines one property. If you know any two of the properties of the air, you can read off the chart a tremendous amount of additional data about that same air.

Let's Begin

The first few terms may be a review for regular readers of "Drying Files."

The dry bulb temperature scale is listed on the bottom of the chart, or the X-axis (figure 1). Remember, the dry bulb temperature is the temperature of the air, the day's highs and lows, or the reading on your thermometer when placed into a duct on your dryer. This is the simplest property to measure and certainly will be one of the two needed to use a psychrometric chart. Constant dry bulb temperatures are made up of vertical lines on the chart.

Next month, I'll continue with more terms, including wet bulb temperature, dewpoint temperature and relative humidity.