Enthalpy, Dew Point and Wet Bulb Calculations in CBAS
We previously covered the topic of enthalpy and dew point in our article Enthalpy as it relates to free cooling in HVAC. In today’s article we will go over how to create these points in CBAS.
There are 3 calculated point types we will be working with: wet bulb, dew point, and enthalpy. We will need two physical points to calculate these – dry bulb temperature and relative humidity. To recap:
Dry Bulb – usually referred to as “air temperature” – measured in Deg F, this is the temperature as sensed with a standard sensor that is freely exposed to the air, but shielded from radiation and moisture.
Relative Humidity – Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100 percent relative humidity (%RH) means that the air is totally saturated with water vapor and cannot hold any more.
Dew Point – the temperature at which the air is completely saturated with moisture and water vapor starts to condense out of the air. Measured in Deg F and calculated using Dry Bulb and %RH.
Wet Bulb – represents the effect of evaporative cooling on the temperature sensor. This temperature will be lower than Dry Bulb but higher than Dew Point. Measured in Deg F and calculated using Dry Bulb and %RH.
Enthalpy – value used to describe the total amount of heat present in an air-vapor mix. Measured in BTU/lb.
Point programming in CBAS
First, you will need to program your hardware points for temperature and humidity.
The steps to add the various calculation points are identical. To begin, you will add a Software Analog Input point:
In the new point’s Program Screen, select Program Calc, Insert Row, Set Value of Software Point, Calculation.
At this step, you will select the type of calculation you wish to create – Enthalpy, Dew Point, or Wet Bulb.
Once you’ve selected the desired calculation type, CBAS will guide you through the process of selecting the hardware temperature and humidity points needed for the calculation.
You can then repeat the above steps for each type of calculation you wish to create.