Evaporation, latent heat

High-pressure condensate forms at the same temperature as the high-pressure steam from which it condenses, as the enthalpy of evaporation (latent heat) is transferred from it. When this condensate is discharged through a steam trap to a lower pressure the energy it contains is greater than it can hold while remaining as liquid water. The excess... 

The boiling point of water is 100 °C (212 °F) at atmospheric pressure, and at this same point the enthalpy of evaporation (latent heat of vaporization) is 970 Btu/lb. 

Since methanol is chosen as refrigerant, evaporation latent heat of methanol is smaller than that of water, and unit quahty refiieerating capacity of methanol is smaller than thatofH20/IiBr. 

Let us now look more closely at an element of superheated liquid mixture at the liquid/vapour interface during surface evaporation. The spent Uquid, after evaporating a methane rich vapour, has increased in density due to both evaporative latent heat of cooling and an increase in more dense ethane composition. The spent element is convectively unstable and sinks away from the surface. 

Single-stage evaporators tend only to be used when the capacity needed is small. It is more usual to employ multistage systems which recover and reuse the latent heat of the vaporized material. Three... 

As with distillation, the dominant heating and cooling duties associated with an evaporator are the vaporization and condensation duties. As with distillation, there will he other duties associated with the evaporator for heating or cooling of feed, product, and condensate streams. These sensible heat duties will usually be small in comparison with the latent heat changes. 

When an atom or molecule receives sufficient thermal energy to escape from a Hquid surface, it carries with it the heat of vaporization at the temperature at which evaporation took place. Condensation (return to the Hquid state accompanied by the release of the latent heat of vaporization) occurs upon contact with any surface that is at a temperature below the evaporation temperature. Condensation occurs preferentially at all poiats that are at temperatures below that of the evaporator, and the temperatures of the condenser areas iacrease until they approach the evaporator temperature. There is a tendency for isothermal operation and a high effective thermal conductance. The steam-heating system for a building is an example of this widely employed process. 

For the air—water system, the humidity is easily measured by using a wet-bulb thermometer. Air passing the wet wick surrounding the thermometer bulb causes evaporation of moisture from the wick. The balance between heat transfer to the wick and energy requited by the latent heat of the mass transfer from the wick gives, at steady state,... 

The saturation temperature of a vapor rises when it is mechanically compressed and its latent heat is available at a higher temperature. AppHcation of this heat to an aqueous stream evaporates part of the water, producing a distillate of pure water. AppHcation of vapor compression has grown significantly since 1960. 

Single-Effect Evaporators The heat requirements of a singleeffect continuous evaporator can be calculated by the usual methods of stoichiometry. If enthalpy data or specific heat and heat-of-solution data are not available, the heat requirement can be estimated as the sum of the heat needed to raise the feed from feed to product temperature and the heat required to evaporate the water. The latent heat of water is taken at the vapor-head pressure instead of at the product temperature in order to compensate partiaUv for any heat of solution. If sufficient vapor-pressure data are available for the solution, methods are available to calculate the true latent heat from the slope of the Diihriugliue [Othmer, Ind. Eng. Chem., 32, 841 (1940)]. 

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