As you may have seen in one of my earlier posts, graphs, charts and tables are extremely helpful tools for a chemical engineer. With so many different combinations of temperature, humidity, pressure, mass fraction, etc; charts efficiently organize data and provide an engineer with a quick method to determine a quantity. In ChE 031, we frequently use charts and tables to determine values for our system balance problems. Recently, the class was introduced to psychrometric charts, which are imperative in solving humidity problems.

The chart looks complicated, but once you understand how to use it, it is a great asset in solving questions. On the right y-axis there are mass fractions for (kg water/kg dry air). Given humid air, there is always a specific ration of water to dry air. On the x-axis, there is the dry bulb temperature, which is basically the absolute temperature that the humid air is at. Those are the simple components, the real fun starts on the inside of the chart. The vertical bowed lines indicate the relative humidity of the air. So, if given the relative humidity and temperature, you could determine the desired point on the graph and then trace across to find the mass fraction of water. If given any two values, you can always find the third. The upper left boundary of the chart is a curved line that contains important information. There are perpendicular lines to the boundary which are used to determine the values provided by the upper left boundary. Along the boundary exists the wet bulb temperature, which deals with the temperature of the evaporation at 100% saturated air. Extended past the boundary on the stair-like straight lines are enthalpy values. If given conditions for humid air, you can plot the point on the chart, and then determine all these quantities explained above. This chart is extremely useful when given a humid air system.

Above is an example of a dehumidifier. Humid air is cooled so water condenses and then a stream of lower humidity (or none) is released. If faced with a question like this, there are many different parameters that could be given. With the help of the psychrometric chart, you could determine the heat released to produce a certain amount of liquid water or determine the leaving conditions, among many other possibilities. There process for humidifying air is similar, but in this case liquid water is heated until it becomes a vapor and then it is put into an air stream in order to increase the humidity. The psychrometric chart makes life much easier when dealing with humidity balance problems.