HEAT EXCHANGER APPLICATIONS
DOE-HDBK-1018/1-93
Heat Exchangers
Because air is such a poor conductor of heat, the heat transfer area between the metal of the
radiator and the air must be maximized. This is done by using fins on the outside of the tubes.
The fins improve the efficiency of a heat exchanger and are commonly found on most liquid-to-
air heat exchangers and in some high efficiency liquid-to-liquid heat exchangers.
Air Conditioner Evaporator and Condenser
All air conditioning systems contain at least two heat exchangers, usually called the evaporator
and the condenser. In either case, evaporator or condenser, the refrigerant flows into the heat
exchanger and transfers heat, either gaining or releasing it to the cooling medium. Commonly,
the cooling medium is air or water. In the case of the condenser, the hot, high pressure
refrigerant gas must be condensed to a subcooled liquid.
The condenser accomplishes this by cooling the gas, transferring its heat to either air or water.
The cooled gas then condenses into a liquid. In the evaporator, the subcooled refrigerant flows
into the heat exchanger, but the heat flow is reversed, with the relatively cool refrigerant
absorbing heat from the hotter air flowing on the outside of the tubes. This cools the air and
boils the refrigerant.
Large Steam System Condensers
The steam condenser, shown in Figure 9, is a major component of the steam cycle in power
generation facilities. It is a closed space into which the steam exits the turbine and is forced to
give up its latent heat of vaporization. It is a necessary component of the steam cycle for two
reasons. One, it converts the used steam back into water for return to the steam generator or
boiler as feedwater. This lowers the operational cost of the plant by allowing the clean and
treated condensate to be reused, and it is far easier to pump a liquid than steam. Two, it
increases the cycle's efficiency by allowing the cycle to operate with the largest possible delta-
T and delta-P between the source (boiler) and the heat sink (condenser).
Because condensation is taking place, the term latent heat of condensation is used instead of
latent heat of vaporization. The steam's latent heat of condensation is passed to the water
flowing through the tubes of the condenser.
After the steam condenses, the saturated liquid continues to transfer heat to the cooling water
as it falls to the bottom of the condenser, or hotwell. This is called subcooling, and a certain
amount is desirable. A few degrees subcooling prevents condensate pump cavitation. The
difference between the saturation temperature for the existing condenser vacuum and the
temperature of the condensate is termed condensate depression. This is expressed as a number
of degrees condensate depression or degrees subcooled. Excessive condensate depression
decreases the operating efficiency of the plant because the subcooled condensate must be
reheated in the boiler, which in turn requires more heat from the reactor, fossil fuel, or other heat
source.
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