air, air conditioning, convection, cooling, cooli ng load, efficiency, energy, evaporation, e vaporative cooling, heat, heat transfer, house, housing, HVAC, indoor air, prototype, radiation, swamp cooler, thermal energy, water
Explain the process that cools air during the evaporation of water. Calculate a room's cooling load (with a swamp cooling system). Explain how the engineering concepts in this design project can be applied to solve a real-world problem.
cardboard box (to fit around a small electric fan) paper towels, three-layers thick to cover box bottom hot glue Cooling Load Analysis and Computation Worksheet, one per person Psychrometric Chart, one per person
small electric fan adjustable spray bottles scissors, to cut cardboard compass, to mark curved lines to cut in the cardboard adjustable water spray bottles
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?
Have you ever wondered why a fan can cool you down without lowering the temperature of the air blowing through it? Or, when you add a water-misting device to the fan, why it cools you down even more?
This can all be explained by the concept of evaporative cooling. Consider the way our bodies regulate comfortable temperatures. We tend to shiver when we are cold, and sweat when we are too hot. *
Evaporation of sweat cools our bodies. The more the sweat evaporates the cooler we get. If there is water already in the air our sweat doesn’t evaporate. This is why it feels hotter when its humid outside.
Psychrometrics: The way we analyze the conditions of water- vapor mixtures is through the use of a psychrometric chart.
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.
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.
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.
Pass out materials Complete the first two parts of the worksheet. 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.
Tear off three sheets of paper towels large enough to span the box floor. Staple them together to create one thick paper towel sheet. Glue or staple the paper towel sheet to the inside rim of the box floor (see Figure 3).
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. 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).
Cut out the marked areas on the box flaps (see Figure 5). 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 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)
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.
What are the pros and cons of using this type of cooling system for your home?
Why is it important to completely close off all parts of the device except the outflow?