Blog - Mechanical PE Exam

Psychrometrics for the Mechanical PE Exam [Part 1]

One of the most important tools for the potential PE Exam test taker is the psychrometric chart. Understanding this chart and how to navigate the chart will definitely be a huge advantage to the test taker.
[February 28, 2012]

     The psychrometric chart describes the various properties of moist air. Moist air is defined as an air-water mixture. Moist air on a psychrometric chart ranges from Dry Air to Saturated Air. Dry air is defined as having no water vapor and is located on the X-axis of the psychrometric chart. Saturated air is defined as air-water mixture at equilibrium between the liquid and vapor phase. A simple way to think of saturated air is - Moist Air in Balance with its liquid and vapor phases. This saturated air is defined by the exponential curve - called the saturation curve, see Figure 1.


     The psychrometric char does not account for variations in pressure. As you can see, pressure is not shown on any axes, because it is constant. For the PE exam and for this guide, it is assumed that the psychrometric chart is based on atmospheric pressure (14.696 psia or 1 atm or 29.921 in. Hg). The psychrometric chart for the PE exam also only shows the typical temperatures encountered by a typical HVAC and Refrigeration Engineer.


Now that you understand the basics of a psychrometric chart, we will now go into detail on the following (3) Pscyhrometric Chart Topics:


Outline

  • Air Properties on the Psychrometric Chart: What does each property tell of the air-water mixture?
  • Movement on the Psychrometric Chart: What causes each type of movement on the chart (Right to Left, Up and Down)
  • Typical Psychrometric Questions: Mixtures of Moist Air, Humidification/Dehumidification, Heating/Cooling Questions

1. Psychrometric Chart - Properties

The psychrometric chart as previously discussed, graphically shows the theromdynamic properties of Moist Air (air-water mixture, humid air, etc.).

  • Dry Bulb Temperature - This is the temperature most people are familiar with, as they are always shown on thermometers and thermostats. It is called dry because the temperature is not affected by the moisture in the air.
  • Wet Bulb Temperature - This temperature takes into account the moisture in the air. A wet bulb temperature that is equal to the dry bulb temperature, indicates saturated air. A wet bulb temperature much lower than the dry bulb temperature is an indication of dry air.
  • Relative Humidity - Relative humidity is the ratio of the amount of water vapor in the air mixture - compared to the maximum amount of water vapor the air can hold at that dry bulb temperature. For example, an air mixture at 100% relative humidity is shown on the saturation curve because the amount of water vapor in the air mixture is equal to the maximum amount. Dry air is shown as the x-axis, which has a relative humidity of 0%, since there is no water vapor in the air mixture.
  • Humidity Ratio - This property describes the amount of water content in the air, expressed as the water vapor per unit weight of dry air[lbm of water vapor/lbm or air] or [kg of water vapor/kg of air]. Sometimes the humidity ratio is expressed as grains of water vapor per 1 lbs of dry air. Symbol = [W].
  • Enthalpy - The amount of energy in moist air [Btu/lb of dry air]. Symbol = [h].
  • Specific Volume - The cubic feet of the mixture per pound of dry air.[ft^3/lb]

Psychrometrics for the Mechanical PE Exam [Part 2]