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Evaporative cooling uses the principle of water evaporation to cool, no environmentally dangerous refrigerants, just cool clean water. Just like that cool feeling you got when you were a kid running through the sprinkler on a windy day.
Evaporative cooling has many factors, which are driving interest in this technology. Among these are:
 | Environmentally friendly Evaporative Cooling versus environmental concerns of
refrigerants used in mechanical refrigeration. |
| Improved indoor air quality through the air washing nature of evaporative cooling. |
| Low energy usage versus mechanical refrigeration enhances your energy management and
saves money. |
| Low operational costs makes evaporative cooling a great alternative to traditional
ventilation-only systems. |
| Transportable Evaporative Cooling can reduce overall cooling requirements on systems
which are struggling with capacity due to increased outdoor air or internal load
requirements. |
The Evaporative Cooling Principle
Water applied to an extensive, corrugated media surface and mixes with air which passes through the media. Energy is required to change the liquid water into a vapor, this process called "Evaporation" absorbs heat from the air stream which decreases the air temperature while increasing the relative humidity.
Press Release:
Evaporative Cooling, nature’s most natural, low cost cooling process has just been improved. Recent developments by Indirex, a Colorado based company, have brought equipment advancements in indirect cooling (evaporative cooling without moisture addition) and portable cooling.
Finally … A Quantum Leap for Evaporative Cooling!
Over 4,000 years ago an ancient Egyptian inventor hung a mat over his doorway wetted with water. As the water was evaporated off of the mat by the mild breeze that blew into the dwelling, the air was cooled and evaporative (or swamp) cooling was born.
Since then, evaporative cooling has changed significantly, but the basic theory of hot air giving up energy to convert water in a liquid form to a gas form (evaporation) has remained the same. This form of cooling is the original environmentally friendly, clean, and low cost cooling process.
Through the years many materials from hog’s hair and shredded aspen wood to high efficiency, rigid cell-honeycomb paper material, have replaced the Egyptian wetted mat. Figure 1 illustrates the basic arrangement of the high efficiency material. The media is wetted from either a drip-type or spray-type water distribution system to saturate the media. The media is corrugated to maximize the contact area between the air and water. The hot air is passed through the media and is cooled as it evaporates the water into the air stream. In this process, called direct evaporative cooling, the air’s temperature is lowered but the moisture content is increased. In a typical application in Phoenix, AZ summer design conditions are 108° F with 70° F wet bulb temperature, 16% relative humidity and 48 grains of moisture per pound of dry air.
As the air is passed through traditional portable evaporative coolers the air is cooled to 78° F with 70° F wet bulb temperature, 68% relative humidity, and 96 grains of moisture. Obviously, the temperature is significantly cooler and the humidity level is much higher. All of this cooling is achieved with relatively low energy consumption since the cooling uses only a water pump and a little more horsepower for air movement. Typically, an evaporative cooling system will utilize approximately 80% less energy than a mechanical refrigeration system.
In many areas, the increased humidity levels are a welcome advantage of evaporative cooling as a source of humidification during both summer and winter seasons.
However, in many regions and applications higher ambient humidity levels make direct evaporative cooling less effective and comfortable. Indirect evaporative cooling, a relatively new concept, utilizes the same process of direct evaporative cooling to cool a secondary independent air stream which cools some sort of medium. This medium then cools the primary independent air stream with no moisture addition to that primary air stream. Common processes include utilization of cross flow plate exchangers, heat pipes, and a system involving a cooling tower and a cool water coil.
The indirect cooling process is shown in figure 2 as a cross flow plate exchanger. Water is distributed over the exchanger core at the secondary air outlet location.
Utilizing the Indirect exchanger as a precooler for a direct evaporative cooling media is illustrated in figure 3.
These cooling processes are shown graphically by a psychrometric chart in figure 4.
This psychrometric chart is used to show the relationship between temperature and humidity of air. Point ‘A’ shows a typical design day in Phoenix, AZ where the design condition is 108ºF and 48 grains of moisture. Point ‘B’ shows that conventional portable evaporative coolers provide air at 78ºF and 96 grains of moisture. Point ‘C’ illustrates the ability of a two-stage cooler to provide air at 67ºF and only 76 grains of moisture. It is apparent that indirect coolers produce more cooling (line 1-5 vs line 1-3) with less moisture content (line 2-6 vs line 2-4). An indirect cooler can be operated without the direct stage to provide cooling with no moisture added.
Indirect/Direct evaporative cooling can be applied in a much greater climatic region as illustrated on Table 1A.
Direct EC |
2-Stage EC |
|||
| Location | Design |
Supply Air |
Supply Air |
|
o F |
o F |
o F |
||
| Albuquerque, NM | 93 |
66.9 |
57.4 |
|
| Boise, ID | 94 |
69.5 |
60.5 |
|
| Denver, CO | 93 |
66.1 |
56.3 |
|
| Detroit, MI | 87 |
75.2 |
70.8 |
|
| El Paso, TX | 98 |
71.1 |
61.3 |
|
| Las Vegas, NV | 106 |
74.4 |
62.8 |
|
| Los Angeles, CA | 81 |
67.6 |
62.6 |
|
| Oklahoma City, OK | 96 |
78.6 |
72.2 |
|
| Phoenix, AZ | 108 |
78.0 |
67.0 |
|
| Pittsburgh, PA | 86 |
73.4 |
68.7 |
|
| Sacramento, CA | 97 |
74.9 |
66.8 |
|
| Salt Lake City, UT | 94 |
68.7 |
59.4 |
|
| San Antonio, TX | 97 |
78.8 |
72.2 |
|
| Temperatures do not include fan/motor heat gain | ||||
Indirect evaporative exchangers can also be applied in commercial and residential equipment. Indirect exchangers act as precoolers for the evaporator coils or chilled water coils in mechanical refrigeration systems as illustrated in Figure 5. These indirect precoolers significantly reduce operational expenses by lowering inlet temperatures to the coil and reducing chiller run times.
These new developments in evaporative cooling would make our ancient Egyptian inventor proud. Now the concept as simple as water evaporation can be applied in many new applications. This means energy savings, cleaner indoor air, a cleaner environment, and cool air for you.
Karl Egbert P.E.