Misting is based on the theory of Evaporative Cooling (or Desert Cooling), which uses the scientific principal of Thermal Dynamics, which states that water requires energy to evaporate, or 600 calories of heat per gram of water, and this heat is taken from the environment. To achieve this, water is forced through a very small orifice creating a very fine mist or fog of ultra fine 50 micron water droplets (smaller than the diameter of human hair). Once these very fine droplets of water are exposed to the hot atmosphere, they quickly absorb the heat present in the environment and evaporate in the process. This quick heat exchange is known as Flash Evaporation, it takes heat from the surrounding area, and drops its temperature by up to 35 degrees F, and consequently the air is cooled.
As a by product to cooling, there are three benefits:
Misting also humidifies the air, cleans it from pollutants such as dust, pollen and smoke, and repels flying insects.Evaporative cooling is not a new invention, it has been used since the dawn of civilization, by utilizing a small fountain in a courtyard, or water dripping at a window, and the wind blowing through that offered a cool breeze, that is why people feel cool and relaxed at locations where water is present, such as, beaches, lakes, rivers, waterfalls, and as fog spreads, temperature immediately drops.
Misting is Divided into Three Categories
- Low: 45 - 100 psi. Using city water pressure, no pump needed
- Medium: 100 - 250 psi. needs medium pressure booster pump
- High: 800 - 1200 psi. Needs high pressure booster pump
- Note: If water pressure is below 45 psi, as is the case with well water, then a booster pump is need even for low pressure applications.
- psi = Pressure in pounds per square inch.
Low Pressure misting is affected by the water pressure, and its flow rate. Normally in the USA, house water pressure ranges between 35 psi and 90 psi. High Pressure misting generates Fog, which is ideal misting, with finest mist. However the cost is way too high, because it uses high pressure pumps, stainless steel tubing, and metal fittings to handle the high pressure. The finer the mist, the more efficient is the system (due to the larger surface are), and the higher the pressure the finer is the mist produced.
Water Pressure Pressure Range vs. Estimated Temperature Drop
- Low: 45- 100 psi: ~25 degrees F
- Medium: 100 - 250 psi: ~30 degrees F
- High: 800 - 1200 psi: ~35 degrees F
Relative Humidity & its Effect on Misting
The ratio of the amount of moisture in air, to the maximum amount of moisture the air could absorb at that same temperature, is called the Relative Humidity (RH). The lower the relative humidity is, the more water could be vaporized. and the more heat could be removed, and the better a misting system would work. The nice thing about misting is that relative humidity is at its lowest when the temperature reaches its peak, making misting or evaporative cooling applicable to most geographical areas. Optimum conditions are achieved when relative humidity is below 80%. However when RH is over 80%, cooling can still be achieved by combining misting with fans to offer conductive cooling, and/or chilling the water to generate a chilled mist (by icing the supply water), and that works great even when it is really sticky outdoors.
Effect of Relative Humidity (RH) on Misting Temperature Drop
- Above 80% RH is poor: ~10 degrees F. (Adding a fan, and/or icing water improves that).
- Between 40% and 80% RH is good: ~25 degrees F.
- Below 40% RH is excellent: ~35 degrees F.
- Results vary depending on climate conditions, wind velocity, temperature, pressure, and the heigh of misters from ground level.