Latent heat of evaporation

Condensation is the process of reduction of matter into a denser form, as in the liquefaction of vapor or steam. Condensation is the result of the reduction of temperature by the removal of the latent heat of evaporation. The removal of heat shrinks the volume of the vapor and decreases the velocity of, and the distance between, molecules. The process can also be thought of as a reaction involving the union of atoms in molecules. The process often leads to the elimination of a simple molecule to form a new and more complex compound. 

L, = latent heat of evaporation of steam at flash pressure, Btu/lb... 

T = temperature of tube heating surface, °R Tj = saturation temperature of liquid, °R Bl = coefficient of Figure 10-99 hfg = latent heat of evaporation, Btu/lb... 

A recent development in heat recovery has been the heat tube. This is a sealed metal tube which has been evacuated of air and contains a small quantity of liquid which, for boiler applications, could be water. When heat from the flue gases is applied to one end of the heat pipes the water in the tube boils, turning to steam and absorbing the latent heat of evaporation. The steam travels to the opposite end of the tube which is surrounded by water, where it gives up its latent heat, condenses and returns to the heated end of the tube. Batteries of these tubes can be arranged to form units, usually as a water jacket around a section of a flue. 

This is the device where the air or water being cooled gives up its heat to provide the latent heat of evaporation to the refrigerant. Superheat is also added to the refrigerant at this point to prevent damaging liquid forming on the way to the compressor. 

Water is injected into the air stream in a fine mist by pumped jets or spinning disc. For practical purposes, the psychrometric plot follows a wet bulb line. The air provides the latent heat of evaporation, resulting in a fall in dry bulb temperature. If water were to be supplied at up to 100°C the humidified condition would be at a correspondingly higher total heat of 420 kJ per kg water supplied. 

A liquid boils and condenses - the change between the liquid and gaseous states - at a temperature which depends on its pressure, within the limits of its freezing point and critical temperature. In boiling it must obtain the latent heat of evaporation and in condensing the latent heat must be given up again. 

If by water spray or washer, the necessary heat must be put into the air first to provide the latent heat of evaporation. This can be done in two stages, A to T to C, or three stages A to Hto Jto C, if reheat is required to get the exact final temperature. The latter is easier to control. 

Moisture can be removed from any material which is to be dried, by passing air over it which has a lower water vapour pressure. Also, in removing this moisture, the latent heat of evaporation must be supplied, either directly by heating, or by taking sensible heat from the airstream which is carrying out the drying process. 

Transfer to the hot reservoir and expand isothermally till a mass m of liquid has been evaporated. The heat absorbed along BC is mLe where Le = latent heat of evaporation at T°. 

Le — latent heat of evaporation. v2 — Vi = Ar = volume change accompanying unit mass of phase transition at the pressure p. 

According to Clausius the latent-heat of evaporation of a liquid is approximately a linear function of the temperature, and diminishes with rise of temperature (cf. 94) ... 

Since the latent heat of evaporation always decreases, the value of a" for all substances increases (algebraically), with rise of temperature if negative, then at a certain temperature it becomes zero, and then positive. Thus, above 127° chloroform shows the phenomenon of compressive cloud-formation observed with ether. 

Mathias (1896) found, in his experiments on the latent heat of evaporation of liquid C02, S02, and N20 up to the critical points, that the curve Lc = /(T) cuts the T axis at right angles at the point T = TK. Thence at the critical point... 

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