Contributed by: Integrated Teaching and Learning Program,
College of Engineering, University of Colorado at Boulder
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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
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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.
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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
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small electric fan adjustable spray bottles scissors, to cut
cardboard compass, to mark curved lines to cut in the cardboard
adjustable water spray bottles
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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?
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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?
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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. *
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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 doesnt evaporate. This is why it feels hotter when its
humid outside.
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Psychrometrics: The way we analyze the conditions of water-
vapor mixtures is through the use of a psychrometric chart.
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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.
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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.
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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.
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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.
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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.
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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).
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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).
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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)
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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.
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What are the pros and cons of using this type of cooling system
for your home?
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Why is it important to completely close off all parts of the
device except the outflow?
