Tube bundle fouling

As the fan airflow is pretty constant, the fan s head is also constant. Another way of stating this is to say that, as a tube bundle fouls, the resistance to airflow increases. This reduces the airflow through the bundle, but the pressure loss of the airflow through the tube bundle does not change. 

To prove this to yourself, find the electric circuit breaker for a fan s motor. The amp (amperage) meter on the circuit breaker will have a black needle and a red needle. The black needle indicates the actual current, or amp load. The red needle is the amperage load that will trip the motor, as a result of overamping. Over time, as the tube bundle fouls and airflow through the bundle is restricted, the black needle never moves. 

An induced-draft fan (see Fig. 14.1) is a different story. As the tube bundle fouls... 

In a forced-draft air cooler, cool air is blown through the underside of the fin tube bundle. In an induced-draft air cooler, cool air is drawn through the underside of the fin tubes. Either way, road dust, dead moths, catalyst fines, and greasy dirt accumulate along the lower row of tubes. As the tubes foul, they offer more resistance to the airflow. However, note that... 

If one of the air coolers begins to experience tube-side fouling, the fluid flow will be reduced. But the tube-side pressure drop will remain the same. The pressure drop across all five air-cooler bundles, shown in Fig. 14.6, is 10 psig. 

The hotter, fouled tubes must grow. But their horizontal expansion is constrained by the cleaner, colder tubes, since the colder tubes do not allow the hotter tubes to grow, the hot tubes bend. This, then, is the origin of the twisted tubes we see when an improperly designed tube bundle is pulled from its shell during a turnaround. 

Note, also, how there is a rapid increase in the pressure drop in the tube bundle, as the tubes foul and plug, because of low velocity. The increase in AP stops only when the terminal-tube velocity is reached in the unplugged tubes. 

The resulting four-pass tube bundle will have a tube-side velocity twice as high as it did when it was a two-pass exchanger 3 ft/s. Experience has shown that in many services, doubling this velocity may reduce fouling rates by an order of magnitude. That is fine. But what about pressure drop ... 

The third example shown in Fig. 19.7 is my favorite for sensible-heat transfer in fouling service. This consists of 1-in tubes set on lV2-in rotated square pitch. The pitch layout is the same as that of the square pitch. It is just the tube bundle that is rotated by 45°. The shell-side fluid cannot flow without interference between the tubes. Hence, the tubes promote turbulence, and improve heat transfer. The large tubes,... 

Actually, retrofitting a tube bundle with low fin tubes often reduces heat-transfer capacity. This happens when the controlling resistance to heat transfer is shell-side fouling. The fouling deposits get trapped between the tiny fins. This acts as an insulator between the shell-side fluid and the surface of the tubes. In severe shell-side fouling services, I have replaced fin tubes with bare tubes, and doubled the heat-transfer duty on the exchanger. 

A floating head-type shell-and-tube heat exchanger is recommended for this application because of the need to provide capacity for thermal expansion of the tube bundle. The floating head also enables easy withdrawal of the tube bundle for cleaning purposes. This factor may be very advantageous, not because the streams are subjected to fouling, but because of the possibility that platinum carryover from the reactor will be deposited on the walls of the tubes. Platinum recovery is improved by providing easy access. 

Calculate the heat-transfer coefficient for a fluid with the properties listed in Example 7.18 if the fluid is flowing across a tube bundle with the following geometry. The fluid flows at a rate of 50,000 lb/h (22,679.5 kg/h). Calculate the heat-transfer coefficient for both clean and fouled conditions. 

Fouling in petroleum-refinery service has necessitated rough treatment of tube bundles during cleaning operations. Many refineries... 

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