How to Calculate Latent Heat of Vaporization

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Latent heat of vaporisation describes the amount of heat that must be added to an amount of liquid at its boiling temperature in order to solely and completely convert the liquid to its vapour form.

This vaporisation heat is usually a number of times greater than the heat required to raise the liquid's temperature from ambient to the boiling point, which is why foods cook far more quickly when steamed than when boiled. You can calculate the latent heat of vaporisation for any liquid from published data about its physical properties.

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Define the application. In this example, boiling tap water at 15.6 degrees C-Fahrenheit (deg-F) is vaporised in a distillation unit until 1-quart has been distilled. Assuming that the distiller has an electrical-vaporising efficiency of 0.8, you can calculate how much heat in Btu (British Thermal Units) and kW-hr (kilowatt-hours) must be added to the 1-quart of boiling water to convert it to steam.

Look up the enthalpy value of 1-pound of boiling liquid water at 212-deg-F, and saturated steam at 212-deg-F in published steam tables. The difference between the two values, 180.17-Btu/pound (Btu/lb), and 1150.5-Btu/lb, respectively is the latent heat of vaporisation for 1-pound of water. Subtracting 180.17 from 1150.5 yields 970.33-Btu/lb.

Scale the latent heat of vaporisation for the 1-pound of water up to indicate the latent heat of vaporisation for the 1-quart produced by the distiller. One quart of water weighs approximately 0.943kg, so multiplying the 970.33-Btu/lb by 2.08 equals 2020.43-Btu to vaporise the quart of water.

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Define the application. In this rubbing alcohol (isopropyl alcohol) example, you can calculate the amount of Btu of cooling (latent heat of vaporisation) that is available by applying an entire 473ml bottle of rubbing alcohol (60-per cent concentration in water) to feverish patients in a hospital. Since the alcohol evaporates so easily, the heat of vaporisation is extracted from the patient's skin it is in contact with, which is what imparts the sensation of coolness.

Look up the latent heat of vaporisation data for propanol in a physical data compendium. The Material Data Safety Sheet for isopropyl alcohol shows this value to be 286-Btu/lb. The specific gravity is 0.785.

Correct the value from Step 2 for 16-fluid ounces of 60-per cent concentration rubbing alcohol. The 454gr at 60-per cent concentration by volume constitute 272gr of IPA, with a specific gravity(S.G) of 0.785. Therefore there are only 9.6-fluid ounces x 22.3gr/454gr/pound = 214kg of IPA in the 454gr bottle. The latent heat of vaporisation for a 454gr bottle is therefore only 0.214kg x 286-Btu/lb = 134.7-Btu. This is only about 14-per cent of the value of water, which is why the cooling time, while dramatic, is so short-lived with IPA.