Water has many unique molecular properties that make it extremely important. One application that takes advantage of these properties is a so-called evaporative cooler (or swamp cooler, https://en.wikipedia.org/wiki/ Evaporative_cooler), which is a device that cools air through the evaporation of water. Liquid water is changed into water vapor, whereby the energy used to evaporate the water (i.e. latent heat of vaporization) is consumed from the internal energy (i.e. temperature) of the air brought in from outside. The result is a lowering of the outside air temperature as it is brought inside. The process tends to also increase the humidity (vapor content) of the air. Hence it is an attractive option in some semi-arid regions where outside air is very hot and dry. a) What is the latent heat of vaporization for water? Is this value generally large or small compared to other substances? What are some of the implications of this? b) Suppose you have an evaporative cooler that has an air volume of 1 m3. In other words, at any instant in time it contains 1 m3 of outside air that it will cool and transfer indoors. Suppose the evaporative cooler has a pump that injects 5 mL of water into the cooling system. How much energy input would be required to completely evaporate the water? c) Assuming all of the energy is taken from the internal energy (temperature) of the air, what would be the air temperature indoors? Assume the water to be at the outside air temperature of 30 degrees Celsius. Hints: Keep in mind that the air, which is being cooled, has a heat capacity about 24% of water (0.24 x 4177 Jkg-1K-1). The density of air at 30 degrees Celsius is 1.16 kg/m3.
Water has many unique molecular properties that make it extremely important. One application that takes advantage of these properties is a so-called evaporative cooler (or swamp cooler, https://en.wikipedia.org/wiki/ Evaporative_cooler), which is a device that cools air through the evaporation of water. Liquid water is changed into water vapor, whereby the energy used to evaporate the water (i.e. latent heat of vaporization) is consumed from the internal energy (i.e. temperature) of the air brought in from outside. The result is a lowering of the outside air temperature as it is brought inside. The process tends to also increase the humidity (vapor content) of the air. Hence it is an attractive option in some semi-arid regions where outside air is very hot and dry.
a) What is the latent heat of vaporization for water? Is this value generally large or small compared to other substances? What are some of the implications of this?
b) Suppose you have an evaporative cooler that has an air volume of 1 m3. In other words, at any instant in time it contains 1 m3 of outside air that it will cool and transfer indoors. Suppose the evaporative cooler has a pump that injects 5 mL of water into the cooling system. How much energy input would be required to completely evaporate the water?
c) Assuming all of the energy is taken from the internal energy (temperature) of the air, what would be the air temperature indoors? Assume the water to be at the outside air temperature of 30 degrees Celsius. Hints: Keep in mind that the air, which is being cooled, has a heat capacity about 24% of water (0.24 x 4177 Jkg-1K-1). The density of air at 30 degrees Celsius is 1.16 kg/m3.
Given questions are related to thermodynamics and refrigeration and air-conditioning.
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