Saving Energy with Psychrometric Calculations

Saving Energy with Psychrometric Calculations By: John Becker

Hands up, who’s heard of psychrometrics?  You may think you haven’t, but if you’ve ever listened to a weather forecast I guarantee you have… Psychrometrics are the way engineers (and weather forecasters) describe a mixture of air and water vapor.  The air all around us contains some water vapor, which makes certain calculations a little complicated.  So why do we need to calculate these things?  Read on to find out…

One of the ways a building automation system can save you energy is by using cool outside air at certain times of the year instead of mechanical cooling.  This is called economizing or free cooling.  There are two types of economizers, dry bulb and enthalpy (a.k.a. wet bulb).  This is where psychrometrics come into play.

A typical dry bulb economizer would bring in outside air below a certain dry bulb temperature.  But this doesn’t account for the total heat energy contained in the air.  By measuring only the dry bulb temperature, the heat energy resulting from the presence of water vapor is ignored.  This can make a significant difference.  For example, you might inadvertently bring in cool but very humid outside air, causing your building to feel muggy and uncomfortable.  Or you could miss out on using some very dry outside air that’s a little warmer but could help remove heat and moisture from your building.

Psychrometric Chart

This psychrometric chart illustrates the benefit of using an enthalpy economizer.

If you look at the chart on the left, you can see the difference.  The area shaded blue represents conditions that a dry bulb economizer would consider adequate for cooling.  The area shaded green represents the ADDED conditions that an enthalpy economizer would consider adequate.  In this example it doubles the available conditions for economizing.  This can result in higher energy savings than with a dry bulb economizer.

Implementing an enthalpy economizer requires measurement of outdoor relative humidity in addition to temperature.  The GCL+ program below illustrates how to calculate Enthalpy, Dew Point, and Wet Bulb Temperature given Dry Bulb Temperature and Relative Humidity (all temperatures in °F).

This GCL+ program calculates various psychrometric properties of air.

This GCL+ program calculates various psychrometric properties of air.

Here is a version of the program you can see better and copy into one of your own Delta controllers:

// Calculates psychrometric properties based on Dry Bulb Temp (deg. F) and Relative Humidity.

Variable A As Real
Variable B As Real
Variable P As Real

A = (‘Outside Temp‘ – ‘Dewpoint‘) / 10
B = (A1)
P = 12.2 // Atmospheric pressure in psi (Denver)

Enthalpy‘ = Enthalpy (‘Outside RH‘, ‘Outside Temp‘, °F)
Dewpoint‘ = DewPoint (‘Outside RH‘, ‘Outside Temp‘, °F)

// Equation from Naval Weather Service Technical Note #7
If ‘Outside Temp‘ >= 0 Then
Wet Bulb‘ = ‘Outside Temp‘ – (0.034 * A0.00072 * A * B) * (‘Outside Temp‘ + ‘Dewpoint‘ – 4 * P + 108)
Wet Bulb‘ = ‘Outside Temp‘ – (0.034 * A0.006 * 2) * (0.6 * (‘Outside Temp‘ – ‘Dewpoint‘) – 4 * P + 108)
End If

This is just one example of what you can learn by attending our free Delta Controls training classes.  We’ll be adding new session soon, so stay tuned!

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