/ APPLICATION NOTE

Choice of outdoor humidity sensors critical to economical free cooling system

What Is Wet-Bulb Temperature?

The temperature of an air sample that has passed over a large surface of liquid water in an insulated channel is the thermodynamic wet-bulb temperature – it has become saturated by passing through a constant pressure, ideal, adiabatic saturation chamber.

Thermodynamic wet-bulb temperature is the minimum temperature that may be achieved by purely evaporative cooling Changing weather conditions can make measuring outdoor Wet-bulb temperature is the humidity a challenge, so selecting the right sensor is essential temperature of a volume of to the performance of a building’s HVAC system. air when cooled adiabatically (when no heat is transferred to HVAC systems often rely on outdoor relative humidity measurements, or or from the working fluid) to calculated parameters based upon humidity, to optimize the energy efficiency saturation at constant pressure of the cooling equipment. If the outdoor humidity sensor is inaccurate, building by evaporation of water into it, controls, energy efficiency, and human comfort are sub-optimized. Sensor all latent heat being supplied by selection is often overlooked, yet it is a critical decision that building owners the volume of air. must endure periodically throughout the life of a building. The wet-bulb temperature is a function of the dry-bulb When the outdoor air temperature is sufficiently cool and dry, a temperature- temperature and relative controlled air-side economizer can use outdoor air with no additional humidity or dewpoint. conditioning to cool the building. This is known as free cooling. An economizer consists of the following components: outside and return air dampers, damper Download a free program to actuators, outside air enthalpy (temperature & humidity) sensor or dry-bulb calculate wet-bulb temperatures and other parameters: (temperature only) sensor, discharge air temperature sensor, and economizer www.vaisala.com/ controller. Some economizers control enthalpy by utilizing outdoor air when its humiditycalculator. enthalpy level is lower than that of the air indoors. However, outdoor air with a low temperature or low enthalpy level can still carry a high moisture load. A better solution is to use only outdoor air with a lower dewpoint temperature than the desired indoor dewpoint level to prevent moisture build up in the building.

With water-side economizers, free cooling is achieved when the working liquid (chilled water) bypasses the chiller and is cooled directly by the cooling towers. Cooling towers use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature. The working fluid and the evaporated fluid are usually polymer is deposited between two both water. An outdoor wet-bulb conductive electrodes. The thin-film Recommended Instrument temperature reading is necessary to polymer absorbs and releases water Specifications determine a cooling tower’s efficiency vapor as the relative humidity of Full scale 0 to 100% RH and to initiate the free cooling cycle. the surrounding air increases and measurement range. This practice is suitable for cooler, decreases. The dielectric properties drier climates where the wet-bulb of the polymer film depend on the Minimum recommended temperature of the air is significantly amount of absorbed water. A change accuracy is ±3% RH (preferred lower than the dry bulb temperature. in the relative humidity of the is ±2% or better); acceptable surrounding environment changes variation in temperature is ±1°C Reliable Humidity the capacitance of the sensor. (±2°F). Measurements The electronics of the instrument Ensure instrument will work in Whether using an air- or water- measure the capacitance and convert the most extreme conditions side economizer, facility managers it into a humidity reading. the building experiences e.g. must rely on their outdoor sensors wide operating temperature to provide accurate information. Vaisala’s outdoor humidity range -40 to 60°C (-40 to Sensor drift and instability can transmitters incorporate this sensing 140°F). be detrimental, especially to high technology, resulting in accuracy Sensor is fully recoverable from energy consumption facilities such as and stability even under extremely saturation. data centers. A high quality outdoor harsh outdoor conditions. The humidity transmitter with tight Vaisala HUMICAP® sensors fully Electronics are encased in a weatherproof enclosure. tolerance is required for calculating recover from saturation. Optional other parameters such as dewpoint, heating to prevent condensation enthalpy, and wet bulb. A reliable is also available for high humidity outdoor sensor will recover from conditions. Outdoor transmitters Transmitter Placement saturated conditions, and some come with flexible installation Avoid areas near exhaust fans sensors even have built-in heating options and are available with solar or shaded areas that may affect cycles to prevent saturation. radiation and precipitation shields, measurements. which provide excellent ventilation Withstanding the while blocking both direct and Locate sensors away from heat Elements reflected solar radiation. For more and moisture sources. Vaisala introduced the Vaisala information visit: Avoid locations where air HUMICAP® thin-film capacitive www.vaisala.com/hvac circulation is obstructed by humidity sensor in 1973. Since then, structures or equipment. Vaisala has become the market leader in relative humidity measurements Utilize a mounting kit for and the thin-film capacitive humidity installation in inlet ducts, or a pole-mast installation kit for sensor has developed from one cooling towers or building roof company’s innovation into a global tops. industry standard. Use a radiation shield to protect Today, capacitive thin-film polymer the sensor from precipitation humidity sensors are widely used and solar radiation. in industrial and commercial applications. The sensor consists of a substrate on which a thin film of

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