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Cooling – Evaporative Cooling

Evaporative cooling capitalizes on the cooling effect produced by the evaporation of water. Cooling towers used with water-cooled chillers employ the principles of evaporation to reject heat from chiller systems.

Three types of evaporative cooling are commonly in use.

  • Direct evaporative cooling – Changes water to vapor using the heat in the air to evaporate the water. Warm dry incoming air is changed to cool moist air delivered to the space to be conditioned.
  • Indirect evaporate cooling – Uses a heat exchanger to transfer the heat of the space to be conditioned to the cool moist air. The cool moist air never enters the conditioned area.
  • Indirect-Direct or Two-Stage evaporative cooling – Combines indirect cooling by using a heat exchanger to pre-cool incoming air (first stage) with direct cooling, whereby the pre-cooled air is further cooled as it picks up moisture as it passes through a water-soaked media (second stage). Since cooler air holds less moisture, less humidity is added to the air that enters the conditioned space.

Because evaporative coolers turn warm dry air into cooler moist air, they are best suited to areas or purposes where the air is hot and the humidity is low or moderate. In humid areas, the additional moisture added to the air by evaporative cooling may increase humidity levels to a point that is too high for comfort. Dry, hot areas with sufficient water supply such as Denver, Phoenix, El Paso and other desert, southwestern and/or mountainous states can more effectively employ evaporative cooling. The following industries can also benefit from the cost-effectiveness of evaporative cooling:

  • Industrial plants
  • Commercial kitchens
  • Laundries
  • Dry cleaners
  • Greenhouses
  • Spot cooling for loading docks, warehouses, factories, construction sites, workshops, garages and kennels
  • Confinement farming such as poultry and hog ranches and dairies

How It Works
The typical evaporative cooler consists of a fan or blower, electric blower motor with pulleys, a water pump and evaporative cooling pads. Cooling pads can be made of wood wool or fiber, plastics or melamin paper. Greater efficiency is achieved with thicker pads.

Energy or heat is required to transition water from its liquid state to a vaporous condition. As hot, dry outside air passes through the water-soaked cooling pads, it evaporates the water from the pads, which lowers the temperature of the air and produces a cooling effect.

Evaporative coolers can be mounted on the roof or sidewalls of a building. Inside air is not recirculated through the system so large vents are required to move inside air to the outside.

Advantages/Disadvantages of Evaporative Systems
In the appropriate application, evaporative cooling can be a real cost saver. Estimated cost for installation is 1/8 to 1/2 that of vapor compression systems and operating costs can be 1/4 the cost. Evaporative systems require only minimal power to operate the fan and water pump.

Evaporative systems can improve air quality due to the constant, high flow of outside air. However, adequate filtering is needed to prevent the intake of pollen, odors and other outdoor contaminants.

Care must be taken to avoid the pitfalls of any water intensive system such as mineral deposits on system parts, freezing of water lines and components, and water borne bacteria such as Legionnaire’s disease.

Evaporative cooling produces humidity levels of 80% to 90%, which in hot, dry climates can improve comfort for building occupants. However, higher humidity levels can also accelerate corrosion and create condensation that can damage building components, electronic equipment, and wood and paper products.