I remember learning Psychrometrics in college. It was in one of my early chemical engineering classes and was really just a footnote to our main syllabus. The Professor was walking through it on the chalkboard, and explaining this and that; I thought that it seemed rather intuitive and unimportant to my career path (the irony!). I stopped taking notes and decided to sit back and try to reason through it. Somewhere mid-lesson I must’ve started daydreaming because suddenly the bell rang, and class was dismissed. I didn’t look further into it.
A few weeks later, we had our midterm. I had studied everything in my notes. So naturally, about 90% of the mid-term was based on psychrometrics. I thought, “no big deal, I remember enough of the lesson to do this”. Then, I looked at the psychrometric chart provided with the exam and it was unlabeled. I tensed up and kept snapping off the tip of my pencil. I decided to take ten minutes and think about the weather. Living in Wisconsin, I’d experienced almost every point on the psychrometric chart in the past 6 months, and I was able to deduce the labels on the chart. I got an A/B on the test, and thought, “Whew - at least I will never have to think about psychrometrics again”.
Fast forward 3 years, and I’m in Trane’s Graduate Training program, taking test after test… solving psychrometrics, with a very sharp pencil and plastic triangles. Six months later, I’m working in the support center, and a large percent of support calls require a detailed knowledge of pyschrometrics - especially calls related to HVAC nuances and building simulation. At that point, my relatively simple knowledge of psychrometrics helped me solve a number of convoluted problems.
By supporting every climate on earth (and one on Mars), there are several things I learned over the past decade that may not be common sense.
1) In really dry climates, energy-modeling defaults may need to change.
If for instance, you let certain energy-modeling software packages solve for the defaults in say Denver, Colorado, you might find that the solution yields near infinite airflow. This is because most of the populated world has dehumidification on the cooling coils and shoots for a 50% relative humidity. Shooting for 50% humidity in most of Colorado and the American southwest means INCREASING the relative humidity. In such climates, remember to either lock in your supply air dry bulb, or to change your target relative humidity.
2) Use a Psychrometric chart with appropriate elevation.
Elevation matters. This is because the atmospheric pressure changes, impacting the rate at which water evaporates relative to air (vapor pressure, etc). In an extreme example, if you drop a small ice cube on Mars, it will sublimate (i.e. vanish) before it hits this ground. This is because Mars is very dry and has very low atmospheric pressure.
The common pyschrometric chart is at sea level and that chart works for much of the world, but if you are at an elevated city.
3) Humid climates can really stink
Literally. Humid climates require extra attention to psychrometrics. A building with too high humidity easily acquires mold and mildew and develops a nasty odor, among other problems. For instance, in high rise residential buildings in Florida, standard practice is to have dedicated outdoor air with some form of dehumidification, often reheat (and you know it must be important if they are using extra heat in Florida)
4) Chilled beams
Don’t make it rain in your building! If you have exposed chilled beams, psychrometry is extra important. Solve it incorrectly in your design, and you might end up in a courtroom. This is nearly as true (thought not quite as extreme as chilled beams) for most of the latest buzzword airside systems such as UFAD (UnderFloor Air Distribution) and displacement ventilation.
5) Better Psychrometrics can yield efficiency without sacrificing comfort
Lowering the relative humidity to 40% or lower during the summer months can allow the increase of the dry bulb temperature without adversely affecting thermal comfort. Subsequently, increasing the relative humidity to as high as 60% in winter allows using lower temperatures without sacrificing comfort. In both scenarios, this approach yields a lower delta T, and thus less heat transfer through conduction.
It's simple to say that Psychrometrics is an unsung hero of HVAC and detailed knowledge is required to perform quality building design and analysis. To learn more about psychrometrics from an expert, check out this online course.
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