cen58933_ch10 - cen58933_ch10.qxd 12:37 PM Page 515 CHAPTER...

BOILING AND
CONDENSATION
W
e know from thermodynamics that when the temperature of a liquid
at a specified pressure is raised to the saturation temperature T
sat
at
that pressure, boiling occurs. Likewise, when the temperature of a
vapor is lowered to T
sat
, condensation occurs. In this chapter we study the
rates of heat transfer during such liquid-to-vapor and vapor-to-liquid phase
transformations.
Although boiling and condensation exhibit some unique features, they are
considered to be forms of convection heat transfer since they involve fluid
motion (such as the rise of the bubbles to the top and the flow of condensate
to the bottom). Boiling and condensation differ from other forms of convec-
tion in that they depend on the latent heat of vaporization h
fg
of the fluid and
the surface tension at the liquid–vapor interface, in addition to the proper-
ties of the fluid in each phase. Noting that under equilibrium conditions the
temperature remains constant during a phase-change process at a fixed pres-
sure, large amounts of heat (due to the large latent heat of vaporization re-
leased or absorbed) can be transferred during boiling and condensation
essentially at constant temperature. In practice, however, it is necessary to
maintain some difference between the surface temperature T
s
and T
sat
for ef-
fective heat transfer. Heat transfer coefficients h associated with boiling and
condensation are typically much higher than those encountered in other forms
of convection processes that involve a single phase.
We start this chapter with a discussion of the boiling curve and the modes of
pool boiling such as free convection boiling, nucleate boiling, and film boil-
ing. We then discuss boiling in the presence of forced convection. In the
second part of this chapter, we describe the physical mechanism of film con-
densation and discuss condensation heat transfer in several geometrical
arrangements and orientations. Finally, we introduce dropwise condensation
and discuss ways of maintaining it.
515
CHAPTER
10
CONTENTS
10–1 Boiling Heat Transfer 516
10–2 Pool Boiling 518
10–3 Flow Boiling 530
10–4 Condensation
Heat Transfer 532
10–5 Film Condensation 532
10–6 Film Condensation Inside Hori-
zontal Tubes 545
10–7 Dropwise Condensation 545
Topic of Special Interest:
Heat Pipes 546
cen58933_ch10.qxd 9/4/2002 12:37 PM Page 515

10–1 BOILING HEAT TRANSFER
Many familiar engineering applications involve condensation and boiling heat
transfer. In a household refrigerator, for example, the refrigerant absorbs heat
from the refrigerated space by boiling in the evaporator section and rejects
heat to the kitchen air by condensing in the condenser section (the long coils
behind the refrigerator). Also, in steam power plants, heat is transferred to the
steam in the boiler where water is vaporized, and the waste heat is rejected
from the steam in the condenser where the steam is condensed. Some elec-
tronic components are cooled by boiling by immersing them in a fluid with an
appropriate boiling temperature.