Aside from that, the other versions of the psychrometric chart out there are mostly specialty charts and curiosities. Richard Mollier from Germany made an important contribution in 1923, as you’ll see in the next section. Willis Carrier was the first, with what he called the hygrometric chart in 1904.
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On top of all that, having nine different variables means you’ve got a lot of options for how to show them in a chart.Ī lot of folks have been involved in the development of the psychrometric chart over the past century. It also can take a while to figure it all out. As you might expect, taking nine variables and putting them into one chart puts a lot of information at your fingertips. Now we’re going to delve into how we can combine those quantities and create the psychrometric chart. Adapted from Moore, 1993.To this point, we’ve taken a look at what exactly psychrometrics is and defined the top nine psychrometric quantities. HVAC systems usually change both the sensible and latent heat of an outdoor air stream by running it through hot and cold (and sometimes additional hot) coils to alter the sensible heat - and subsequently the humidity-of the air.įigure 6: Mechanical cooling typically involves cooling air below the dew point until the desired humidity ratio is achieved, and then reheating the air to achieved the desired dry-bulb temperature.
However, this is representative of the typical mechanical air-conditioning process. This may seem like a roundabout and inefficient process. As the the air-vapor mix is reheated, the condition moves horizontally to the right on the chart ( Figure 6). In order to lower the relative humidity, or to bring the conditions into the human thermal comfort zone, sensible heat will need to be added while the moisture content (latent heat) is maintained. This is the dew point-the critical temperature at which condensation occurs as the dry-bulb temperature of an air-vapor mix is lowered.Īfter being cooled beyond the dew point temperature, the humidity ratio of the air-vapor mix will be reduced and the air will remain at 100 percent relative humidity. water) starts to form on the cool window surface. As the temperature of the air adjacent to this surface is pulled beyond the saturation line, its limit for holding moisture is reached and "dew" (i.e. (See Defining Humidity Ratio and Relative Humidity for a more in-depth explanation.)Īn example of this condensation occurrence would be when hot-humid indoor air comes into contact with a cool window surface during the winter. This concept of the relative humidity (see the red lines on Figure 1) of an air-vapor mix at any given dry-bulb temperature helps one determine the saturation level of the air-vapor mix relative to its current capacity. As the air temperature drops, it has less capacity to hold moisture. As air gets warmer and expands, it has the ability to hold more moisture. Also, air can only hold so much moisture. Proportionately, it is a very small amount of moisture relative to the overall air mass, but it is in there.
This is called the humidity ratio.Īir always has some amount of moisture in it. Latent heat is expressed on the psych chart in terms of pounds of moisture per pound of dry air. (See Thermal Dynamics: Visualizing Sensible Versus Latent Heat for a more in-depth explanation.) On the other hand, latent heat is the “wet” heat captured in the air as water undergoes phase change from liquid to vapor via evaporation or boiling. This is the heat that one can "sense" with a conventional thermometer, as the term suggests. Sensible heat is the “dry” heat in the air and relates directly to the dry-bulb temperature. How much latent (moisture-related) heat is in the air? (See the cyan lines on Figure 1.).How much sensible (dry-bulb) heat is in the air? (See the green lines on Figure 1.).