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Two stage Evaporative Cooling System

Two-stage evaporative coolers do not produce humidity levels as high as that produced by traditional single-stage evaporative coolers.

In the first stage of a two-stage cooler, warm air is pre-cooled indirectly without adding humidity (by passing inside a heat exchanger that is cooled by evaporation on the outside). In the direct stage, the precooled air passes through a water-soaked pad and picks up humidity as it cools. Because the air supply to the second stage evaporator is pre-cooled, less humidity is added to the air (because cooler air can’t hold as much moisture as warmer air). The result, according to the manufacturer, is cool air with a relative humidity between 50 and 70 percent, depending on the climate, compared to a traditional system that produces about 80 percent relative humidity air.

An advanced two-stage evaporative cooler uses 100 percent outdoor air and a variable speed blower to circulate cool air. Two-stage evaporative coolers can reduce energy consumption by 60 to 75 percent over conventional air conditioning system

A 2-stage evaporative cooling system (aka "indirect/direct" cooling) represents the most advanced cooling system utilizing evaporative cooling technology. This cooling option is much more energy efficient than cooling with refrigerant (standard air conditioning), but it's not suitable for all cooling applications.

Let's explain 2-stage evaporative cooling in terms of the stages of cooling that occur. In the first stage, hot outside air passes inside a heat exchanger that is cooled by evaporation on the outside. During this initial cooling phase, the incoming air stream doesn't pick up any extra moisture. In the second stage, the same air stream passes through a water-soaked pad where the additional cooling takes place and the air picks up some additional humidity.

A two-stage evaporative cooling system can deliver cool indoor air with a relative humidity in the 50%-65% range, while direct evaporative cooling systems typically condition the air at around 70% relative humidity.

Direct evaporative cooling system
A direct evaporative cooling system is the simplest type of cooling system available. You can duplicate the function of a direct evaporative cooling system by wrapping a wet handkerchief around your neck on a hot day, and standing in front of a fan. It takes heat energy to turn water (or any liquid) from a liquid into a vapor. As the water in the wet handkerchief evaporates, your body gives up heat and you feel cooler.

A direct evaporative cooling system has just a few basic parts, as shown in the drawing. A large fan pulls hot air through a dampened sponge-like pad, and distributes the resulting cooler air to the interior space through a ductwork system.

A direct evaporative cooling system works best in a hot, dry climate where it's desirable to add humidity to air while cooling it. In hot, dry areas throughout the Southwest, this type of cooling system is a popular alternative to refrigeration-based air conditioning. Evaporative cooling systems consume less energy than refrigerant-based systems because a fan can operate on less power than a compressor.

If you live in the Southeast, where summer weather is hot and humid, a direct evaporative cooling system is not a wise choice. Instead, consider an indirect evaporative cooling system a, two-stage evaporative cooling system, or a conventional refrigerant-based AC system.

Indirect Evaporative Cooling
Indirect evaporative cooling works on the same principle as direct evaporative cooling lowering air temperature by causing water to evaporate. The main difference with an indirect system is that a heat exchanger is used to cool the air supplied to the living space. The evaporative cooling cycle occurs in the heat exchanger.

Here's a sequential explanation of what happens in an indirect evaporative cooling system:

  1. Hot outside air is blown through a heat exchanger that is supplied with water. One design for this type of heat exchanger features a series of metal tubes that are kept wet on their outside surfaces. As hot air passes over these tubes, the water evaporates and the tubes are cooled. After passing over the tubes, the cool, moist air is exhausted to the outside.

  2. As cooling happens on the heat exchanger's exterior surfaces, hot exterior air is drawn through the tube interiors. This air is cooled, but without gaining any extra humidity, before it is blown through ductwork to the building interior.
Indirect evaporative cooling provides cool air to interior spaces without as much humidity as direct evaporative cooling. This, cooling method is more suitable for areas where additional humidity isn't desirable for interior air. Because indirect evaporative cooling requires two fans rather than one, it consumes more electricity than direct evaporative cooling.