How evaporation and evaporative cooling works: Evaporation is a natural process that is constantly happening all around us. For example, when our bodies heat up because of activity or hot weather, we release sweat that evaporates, helping our bodies cool. Another example of evaporative cooling can be seen near waterfalls, rivers, lakes and oceans. When dry air passes over bodies of water, some water evaporates, causing the relative humidity to rise. Liquid water molecules become vapor in the dry air through a process that uses energy to change its physical state. Heat is transferred through convection from the higher temperature of the air, to the lower temperature of the water, resulting in cooled air. The heat transferred to the water is consumed by the process of evaporation, so the water stays at a relatively constant temperature. Once the air becomes saturated, it can no longer hold more water, and evaporation stops. For this reason, water in high-humidity environment does not evaporate as quickly as that in a low-humidity climate, and hence swamp coolers are much more effective in dry climates.

Mechanical components of a typical swamp cooler. click for copyright

The reason we feel much warmer in a climate with high relative humidity is because we are not able to cool our bodies by sweating nearly as much. The high level of humidity prevents effective evaporation because the air is not able to take up much more vapor than it already holds. For the same reason, swamp coolers cannot work effectively in climates with high relative humidity because they depend on the evaporation of water to cool air. They also introduce more humidity into the air, which is undesirable in a climate that is already quite moist.

Difference between a swamp (evaporative) cooler and an air conditioner: The main difference between a swamp cooler and an air conditioner is that a swamp cooler uses evaporative cooling, while an air conditioner uses a heat pump and the vapor-compression refrigeration cycle. Swamp coolers have a simpler design, involving only a fan that pulls air through a medium saturated with water, raising the humidity and lowering the temperature of the air. An air conditioner uses a heat pump to circulate a liquid and transfer heat from a cooler place (the indoors) to a warmer place (outdoors). This vapor-compression refrigeration cycle is the same way that kitchen refrigerators operate. With an air conditioner, heat is transferred out of the indoor air without affecting its humidity at all, making it more desirable to use in humid climates. Air conditioners also require much more power to run the cycle.

Psychrometrics: The way we analyze the conditions of water-vapor mixtures is through the use of a psychrometric chart (see attached, and Figure 1). With known dry bulb and wet bulb air temperatures, we can determine important information such as the percent relative humidity, and the dew point temperature. Using values from the chart, engineers analyze the conditions of air confined within a building or home, and use HVAC systems to regulate these values to a comfortable level. We may also use the values to estimate the effectiveness of a particular system. For example, the greater the difference in wet bulb and dry bulb temperatures, the lower the relative humidity, resulting in conditions more suitable for the use of a swamp cooler.

How to use a psychrometric chart: To use the chart, we must be given two values to determine our point of measure. For example, if given wet bulb and dry bulb temperatures, we can find the point of measure by observing where they line up. From here, we can read directly to the left to determine the dew point temperature, and read the curved lines to determine relative humidity (see Figure 1). We may also look up values such as humidity ratio, specific volume of air, and enthalpy, although for the purposes of this activity these are not useful. Note that along the curved line farthest to the left, the relative humidity is 100%, and the dry bulb and wet bulb temperatures are equal. As these temperatures differ more and more, the relative humidity decreases.

Figure 1. An example simplified psychrometric chart for temperatures and relative humidities, used by engineers to regulate heating, ventilation and air conditioning systems to provide comfortable indoor air conditions for people. click for copyright

Cooling load calculation for swamp coolers: To calculate the cooling load of a room, we use a method that helps determine the correct size cooler. Swamp coolers are rated by CFMs, or cubic feet of air flow per minute. To keep a room cool, swamp coolers must displace the entire volume of a room approximately every two minutes. So, to determine the cooling load of a room, divide its volume (in cubic feet) by two minutes.

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Example: For a 200 sq. ft. room, with 9-foot ceilings, the CFM rating needed would be.

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• Gather materials.
• Make copies of the Cooling Load Analysis and Computation Worksheet and Psychrometric Chart (best printed in color), one each per student.

1. Divide the class into groups of two or three students each.
2. Hand out worksheets and charts, one each per student.
3. Once students have completed the first two parts of the worksheet, discuss their answers as a class to make sure the work was done correctly.
4. Guide students through the next steps to design/build their own swamp (evaporative) cooler prototypes: Cut out the bottom of your cardboard box, leaving a rim (about 1-inch [2.5 cm] thick) around the edge. This becomes the back of the cooler. See Figure 2.

Figure 2. Back of the cooler. click for copyright

5. Tear off three sheets of paper towels large enough to span the box floor. Staple them together to create one thick paper towel sheet.
6. Glue or staple the paper towel sheet to the inside rim of the box floor (see Figure 3).

Figure 3. (left) Lay the paper towel sheet in the bottom of the cooler box, attaching it to the box bottom rims on four sides (right). click for copyright

7. Insert a small electric fan into the box with the front of the fan facing the top of the box. Then stand the box on its side. See Figure 4.
8. Use a pencil to mark cut lines on the box flaps to make it clear what cardboard to remove from the front of the box so as to not block air flow to/from the fan. For round fans, use a compass to draw cut lines that result in a neat circle shape (see Figure 4).

: Figure 4. (left) Place the fan inside the cooler casing, facing away from the paper towel side. (right) Use a compass with the point placed on the center of the fan to draw box flap cut lines at the outer fan edge. click for copyright

9. Cut out the marked areas on the box flaps (see Figure 5).
10. Tape or glue the flaps down so that they form a seal around the fan. Use packing tape to seal up any other gaps in the cardboard casing so that air may only enter from the back (see Figure 5).

Figure 5. (left) Cardboard cut along marked lines. (right) Flaps attached to one another and sealed, closing off the front of the cooler. click for copyright

11. Cut a flap on the top or the side of the box to permit access to the fan control (see Figure 6). Make sure you are able to close the flap again.

Figure 6. Make a closeable flap on the box casing to provide access to fan controls. click for copyright

12. Test the swamp coolers: On a hot day, have teams operate and test their swamp coolers to see if they can feel a temperature difference in air coming out from device. Have students feel the air coming out of the fan before and after they saturate the paper towels to see if they notice a difference.
13. As students test and analyze their prototype devices, have them answer the worksheet questions and problems, completing their worksheets.
14. Once testing and worksheet problems are competed, discuss results as a class.
15. Conclude by conducting the engineering re-design and problem-solving exercises as described in the Assessment section.

Discussion: Solicit, integrate and summarize student responses.

• Have you ever stood in front of a fan on a hot day to cool down? How does a fan cool down a room or a person without lowering the temperature of the air blowing through it?

Worksheet: Have students complete the activity worksheet; review their answers to gauge their mastery of the subject.

Question/Answer: Ask the students and discuss as a class:

• Why is it important to completely close off all parts of the device except the outflow? (Answer: To work, we want the device to pull air through the saturated water filter. If the fan were able to pull air from elsewhere, the air would not get cooled by passing by the water.)

Engineering Re-Design: Have students think about their swamp cooler prototype design. What worked? What didn't work? What could you do to improve your design to work over a larger area? Have students think about the trade-offs involved in their design. Then have them make a sketch or write a description about any design changes they would make.

Problem Solving: Have students complete the following two-part exercise:

Problem 1: Sketch a psychrometric chart and draw the path of air being cooled by a swamp cooler. Indicate the starting and ending points as well as a direction. Label the dry bulb and wet bulb temperatures, as well as the relative humidity at each state. Use the initial conditions of 30% relative humidity and a room temperature of 85^o F. Assume we want to stop cooling when the room gets to be 72^o F, and that the wet bulb temperature remains constant.

Problem 1 answer. #1 is the starting point; #2 is the ending point. click for copyright

Problem 2: A car has been out in the cold all night, but as the sun comes up the ambient temperature begins to rise. Suppose the surroundings warm up more quickly than the car windows, whose surface temperature is measured to be about 50^o F. Assume the relative humidity is 60% and the dry bulb temperature is 65^o F. Would water begin to condense on the surface of the car? If so, at what point would it stop? (Answer: Yes, water would begin condensing because the glass surface is below the dew point temperature (≈52^o F) of the surrounding air. The condensation would stop when the glass surface temperature exceeds the dew point temperature.)