What is psychrometrics? Probably one of the subjects you were least likely to study at the university level, psychrometric is the study of behaviors and principles related to moist air. It is important to understand that even though they are mixed in standard air, dry air and water vapor behave independently of each other. It is therefore important to understand the distinction between “air” and “dry air”. The two terms are often used in psychrometric problems and the difference between the two is important. “Air” is usually meant to mean a dry air/water vapor mixture whereas “dry air” is just the air component of the mixture. This is straightforward but is an easy mistake to make on the test.
One of the most important tools at your disposal regarding psychrometric problems on the PE exam will be the psychrometric chart. It can look a little messy, but it’s a treasure trove of useful information that will help shortcut many exam problems. In general, there are six air properties the psychrometric chart can display:
I will talk briefly about each of these properties and what they look like on the chart.
Dry bulb temperature is just the normal air temperature as read from a thermometer. When someone says “it’s over 100 degrees outside!” whether they know it or not, they are talking about dry bulb temperature.
Wet bulb temperature is the temperature a volume of air would be if it was cooled to saturation (100% relative humidity). Looking at this from the perspective of the psychrometric chart, if we were outside on a day with a 110°F temperature and no humidity, we could follow the wet bulb temperature line to the wet bulb axis to show a wet bulb temperature of 60°F:
Dew point temperature is the temperature at which air must be cooled to reach its saturated point at a given humidity ratio. This definition is very similar to that of the wet bulb temperature definition with the only major difference being the dew point temperature for a given state is the temperature assuming constant relative humidity. Let’s look at this graphically with a quick example:
Outside air is 90°F with a wet bulb temperature of 66°F. What is the air dew point temperature?
From this chart we can take the given wet and dry bulb temperatures and follow their respective lines to where they intersect. This is where your air mixture lies on the chart. The dew point of this state is the saturation point assuming a constant humidity ratio:
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The dew point temperature is approximately 53°F.
PE exam strategy: Dew point temperature and wet bulb temperature are similar yet easily confused concepts. Keep them straight for the test!
Relative humidity (φ) is essentially the percentage of how much water vapor is in air in relation to how much water vapor can be present in the air without condensing out. In mathematical terms this equates to:
Relative humidity (φ) runs across from the bottom left to the upper right region on the psychrometric chart:
The humidity ratio is the ratio of water vapor mass in the air and the mass of dry air. Mathematically, this ratio is usually expressed as:
Where mw is the mass [lbs] of water vapor and ma is the mass of dry air. The axis for the humidity ratio is on the right edge of the psychrometric chart:
Specific volume has units of [ft3/lb]. As such, it is just the inverse of density. Specific volume is read off the psychrometric chart from the following scale:
Enthalpy is essentially the energy content in the air due to heat. Units for enthalpy are [Btu/lbdry air]. Many heating and cooling problems on the PE exam will involve calculations using enthalpy as a prime state characteristic. For the purposes of the PE exam, the lines of enthalpy lie along the same lines at wet bulb temperature although they are on different scales. The enthalpy scale on the psychrometric chart is shown below:
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