operation of the evaporative coolers(also called the swamp coolers). In
such coolers, hot and dry outdoor air is forced to flow through a wet cloth
before entering a building. Some of the water evaporates by absorbing heat
from the air, and thus cooling it. Evaporative coolers are commonly used in
dry climates and provide effective cooling. They are much cheaper to run
than air conditioners since they are inexpensive to buy, and the fan of an
evaporative cooler consumes much less power than the compressor of an air
conditioner.
Boiling and evaporation are often used interchangeably to indicate phase
change from liquid to vapor. Although they refer to the same physical
process, they differ in some aspects. Evaporationoccurs at the liquid–vapor
interfacewhen the vapor pressure is less than the saturation pressure of the
liquid at a given temperature. Water in a lake at 20°C, for example, evaporates
to air at 20°C and 60 percent relative humidity since the saturation pressure
of water at 20°C is 2.34 kPa, and the vapor pressure of air at 20°C and 60
percent relative humidity is 1.4 kPa. Other examples of evaporation are the
drying of clothes, fruits, and vegetables; the evaporation of sweat to cool the
human body; and the rejection of waste heat in wet cooling towers. Note that
evaporation involves no bubble formation or bubble motion (Fig. 3–64).
Boiling,on the other hand, occurs at the solid–liquid interfacewhen a liq-
uid is brought into contact with a surface maintained at a temperature Tssuf-
ficiently above the saturation temperature Tsatof the liquid. At 1 atm, for
example, liquid water in contact with a solid surface at 110°C boils since the
saturation temperature of water at 1 atm is 100°C. The boiling process is
characterized by the rapid motion of vapor bubblesthat form at the solid–
liquid interface, detach from the surface when they reach a certain size, and
attempt to rise to the free surface of the liquid. When cooking, we do not say
water is boiling unless we see the bubbles rising to the top.Chapter 3 | 151FIGURE 3–64
A liquid-to-vapor phase change process is called evaporationif it occurs at a liquid–vapor interface, and boilingif it
occurs at a solid–liquid interface.
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