Resumo de Air- Cooled Condensers

Air- Cooled Condensers

Air- cooled condensers may be favored where water is in short supply. Condensation occurs inside tubes that are transversally finned on the Air side for a low air-side heat transfer coefficeint. Fig 1.8 illustrates a typical air-cooled heat exchanger. The unit shown is a forced draft unit because the air is blown across.The alternative design includes a fan at the top that is called induced draft. Air-cooled condensers can also become economical if condensers take place at temperature that are about 20 c higher than the ambient temperatures.

Disadvantages of the air-cooled condensers are their requirements of a relatively large ground area and space around the unit. With multipass condensers, a problem arises with redistributing the two-phase mixture on entry to the next pass. This can be overcome in some cases by using U-tubes or by having separate tube rows only for the condensate (fig. 1.9 ). There is only arrangement practically possible: each successive pass must be located below the previous one to enable the condensate to continue forward.

Direct Contact Condensers

Direct contact condensers are inexpensive and simple to design, but have limited application because the condensate and coolant are mixed. The main advantages of these condensers besides there low cost is that they cannot be fouled and they have very high heat transfer rates per unit volume. There are generally three types of direct contact and condensers : pool, spray and tray.

In pool condensers, vapor is injected in a pool of liquid. The coolant may be a process fluid to be heated. There may be some operational problems with this type of condenser. The first is that the condensation front may move back into the vapor inlet line, causing the liquid to be periodically ejected, often with some violence.

The second is that a very large vapor bubble may form in the liquid pool, and this may collapse suddenly causing damage to the vessel. These problems may be avoided by having the vapor injected through large number of small holes or by using special ejectors that mix the incoming vapor with liquid in a special mixing tube.

The most common type of direct contact condenser is one in which subcooled liquid is sprayed into the vapor in a large vessel. This arrangement is illustrated in Fig 1.10 . Very often these units are used for condensing steam using water as a coolant. In these cases, the mixing of water with condensate presents no major problem.

With condensing a vapor whose condensate is immiscible with a spray liquid, however, a separator is required after the condenser to recover the produt. Alternatively, the condensate product may be cooled in a single-phase exchanger and some recycled as coolant spray.

At first observation, there then seems little benefiting using a combination of a direct contact condenser and a conventional single-phase exchanger instead of only one shell-and-tube condenser.

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