How to Choose the Right Parameter for Humidity Control in HVAC

Application Note www.vaisala.com

The Dew point temperature (Td) indicates at which temperature you will see the onset of condensation. A low dew point temperature indicates dry conditions and a high dew point indicates high humidity. The dew point cannot exceed the ambient temperature. When the dew point is the same as the ambient temperature, you have reached saturation and the RH is 100%. The advantage of using dew point in HVAC controls is that it is not affected by temperature changes. This is an advantage if you need really tight and stable control for both temperature and humidity. The control circuits are independent of each other, so changing the temperature does not change the dew point in the Most people working in the HVAC field are familiar with relative controlled space and vice versa. humidity (RH) as a parameter used to measure and control Dew point and temperature buildings. But RH is not always the best choice as a control control is used in spaces with the parameter. In some cases there are options that will make the highest requirements for stability, conditions more stable or the system commissioning easier. like labs, museums, and data Let´s take a look at the different options. centers.

Wet Bulb Temperature

Relative Humidity in the summertime and lower The wet bulb temperature temperatures during the winter indicates the temperature to which Relative humidity is treated as heating season. It is also directly a water surface can be cooled by the default humidity measure in related to human comfort and evaporation. This cooling effect HVAC. It is the best choice for many biological processes such varies with the relative humidity controlling office and other spaces as mold growth. A drawback is of the ambient air. When the air where human comfort is the main that very tight temperature and is saturated with water there is purpose, RH is a quite good choice humidity control is difficult to no evaporation and no available as a control parameter. It allows achieve as the temperature affects cooling. The wet bulb temperature flexibility in the temperature the relative humidity as well. Thus, is used to control cooling towers settings without changing RH two controls may start fighting which can give access to low-cost control settings, for instance each other causing oscillation in cooling especially in hot and dry allowing higher temperatures the control loops. climates. If the humidity is too

VIM-G-How-to-choose-the-right-Applications-Note-B211751EN-A.indd 1 01/10/2018 17.31 high and the wet bulb temperature kJ/kg. If you know the enthalpy All of these humidity measures can approaches the ambient of return air and make up air, you be calculated from measured RH temperature it does not make can decide directly if you should and temperature. You can do this sense to run the cooling towers as re-condition return air or replace in your control system, but many the available cooling effect is too it with outdoor air. This is not modern humidity transmitters can small. immediately evident just from do the job for you. For instance, temperature measurements as the Vaisala HMD62 duct humidity Enthalpy the humidity of the air impacts sensor allows you to select the the enthalpy more than the desired humidity output in the Enthalpy indicates how much temperature. Enthalpy is thus the field with a DIP-switch. Vaisala energy needs to expended to measure of choice when your also offers a free of charge online get to the measured state from a target is to maximize energy humidity calculator that makes reference state, usually dry air at savings. parameter conversions easy. 0 °C. The most common unit is

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