Channel head

A number of flume designs have been created specifically for use in partially fiUed circular conduits such as sewers. These are available in molded fiber glass and can be lowered through a manhole if required. As with all open-channel head-area meters, flumes must be sized to prevent submergence of the restriction. 

Galvanic corrosion can be controlled by the use of sacrificial anodes. This is a common method of controlling corrosion in heat exchangers with Admiralty tube bundles and carbon steel tube sheets and channel heads. The anodes are bolted direcdy to the steel and protect a limited area around the anode. Proper placement of sacrificial anodes is a precise science. 

J. Hydrodesulfurization unit effluent exchanger channel head and shell plate. (Hydrocarbon feed to unit and make-up hydrogen Bom ethylene unit)... 

Arseniev, A. S., Barsukov, I. L., Bystrov, V. F., Lomize, A. L. and Ovchinnikov, Y. A. (1985). 1H-NMR study of gramicidin-A transmembrane ion channel head-to-head right handed single stranded helices, FEBS Lett., 186, 168-174. 

The steam enters through the top of the channel head of the reboiler. Any superheat in the steam is quickly lost to the tubes. Superheated steam does very little in increasing heat-transfer rates in a reboiler. Actually, when considering the temperature difference between the steam and the process fluid, it is best to use the saturated steam temperature, as the real temperature at which all the heat in the steam, is available. For example, assume the following steam flow to a reboiler ... 

This improved heat-transfer rate, promoted by low velocity, applies not only for condensing steam but also for condensing other pure-component vapors. And since condensation rates are favored by low velocity, this permits the engineer to design the steam side of reboilers and condensers in general, for low-pressure drops. For example, if we measured the pressure above the channel head pass partition baffle shown in Fig. 8.1, we would observe a pressure of 100 psig. The pressure below the channel head pass partition baffle would typically be 99 psig. 

What would happen to a steam reboiler if the float in the steam trap became stuck in a partly closed position, or if the steam trap were too small Water—that is, steam condensate—would start to back up into the channel head of the reboiler, as shown in Fig. 8.3. The bottom tubes of the reboiler bundle would become covered with water. The number of tubes exposed to the condensing steam would decrease. This would reduce the rate of steam condensation, and also the reboiler heat duty. 

Figure 8.4 shows a control valve on the steam inlet line. The rate of steam flow to the reboiler is not really controlled directly, however, by this control valve. The actual rate of steam flow to the reboiler is controlled by the rate of condensation of the steam inside the tubes. The faster the steam condenses, the faster it flows into the channel head. The function of the control valve is to reduce the steam pressure in the channel head of the reboiler. For example, in case 1 ... 

Figure 8.4 Varying channel head pressure controls heat input.

To consider a third case, we wish to maintain the original 240°F shell-side temperature, but to increase the steam flow from 10,000 to 15,000 lb/h. This will force the steam inlet control valve to open. As the control valve opens, the pressure in the channel head rises from 100 psig to the full steam header pressure of 160 psig. At this pressure, steam condenses at 360°F. The new AT is then (360°F - 240°F) = 120°F. This new temperature driving force is 50 percent greater than the case one driving force of 80 percent. Hence the rate of steam condensation also increases by 50 percent, from 10,000 to 15,000 lb/h. 

Once the steam pressure in the channel head of Fig. 8.4 falls to the pressure in the condensate collection header, the steam trap can no longer pass condensate. Water will back up in the channel head, and water-log the lower tubes in the tube bundle. This will lead to unstable steam flow control. This is especially true if the steam supply pressure is less than 20 psig higher than the maximum condensate collection header pressure. 

It is better not to use a steam inlet control valve when using low-pressure steam. The channel head pressure will then always equal the steam header supply pressure. The flow of steam to the reboiler can then be controlled only by raising or lowering the water level in the channel head, as shown in Fig. 8.5. This sort of control scheme will work perfectly well until the water level drops to the bottom of the channel head. If the condensate drain control valve then opens further, in an attempt to increase steam flow into the reboiler, the condensate seal is blown, and the reboiler heat duty drops. 

A better design is shown in Fig. 8.6. In this scheme, a condensate drum is used to monitor the level in the channel head. As the drum level is drawn down, the number of tubes in the reboiler exposed to the condensing steam is increased. However, when the water level drops to... 

One important feature of Fig. 8.6 is the condensate drum balance line. Note, that this line is connected below the channel head pass partition baffle. This ensures that the pressure in the channel head, below the pass partition baffle, and the pressure in the condensate drum, are the same. If these two pressures are not identical, then the level in the condensate drum cannot represent the level in the channel head. For this reason, never connect the condensate drum vapor space to either the steam supply line or the top vent of the reboiler s channel head. 

Steam produced from demineralized water is free of carbonates. Steam produced from lime-softened water will be contaminated with carbonates that decompose in the boiler to carbon dioxide. As the steam condenses in a reboiler, the C02 accumulates as a noncondensable gas. This gas will be trapped mainly below the channel head pass partition baffle shown in Fig. 8.6. As the concentration of C02 increases, the C02 will be forced to dissolve in the water ... 

Venting the channel head through the balance line shown in Fig. 8.6 will prevent an excessive accumulation of C02. This is done by continuous venting from the top of the condensate drum. For every 10,000 lb/h of steam flow, vent off 50 lb/h of vapor through a restriction orifice, placed in the condensate drum vent. This is usually cheaper than controlling reboiler steam-side corrosion, with neutralizing chemicals. 

Varying the steam-to-condensate interface level to control the reboiler duty will promote steam leaks in the channel head-to-shell flanged closure. This is caused by the thermal cycling and stresses that result from constantly varying the level of condensate in the channel head. However, when low-pressure steam (<60 psig) is used, this becomes a minor problem, which may be safely ignored. 

When high-pressure steam (>100 psig) is used, rather significant leaks of hot condensate and steam can be caused by a variable condensate level in the channel head. For such higher-pressure steam sources, control of steam flow with condensate backup, as shown in Figs. 8.5 or 8.6, is best avoided. 

Operators who have problems with loss of reboiler capacity often attribute these problems to condensate backup. This is usually true. To drop the level of water out of channel head, either the steam trap or the... 

I had the front endplate on the cooling water side of the surface condenser (called the channel head cover) removed. Most of the tube inlets in the channel head tubesheet were plugged with crayfish (but in Louisiana where this story is set everyone calls these little creatures, crawfish). 

Referring to Fig. 19.1, we can see how a floating-head exchanger works. The tube-side flow enters the bottom of the channel head. This assumes the cold fluid to be on the tube side. The cold fluid may be on the shell side or the tube side of an exchanger. The convention is to put the cold fluid nozzle on the bottom of the exchanger. Sometimes this is necessary. Sometimes it does not matter, but it is still the convention. 

Inside the exchanger s channel head, we have the pass partition baffle, which divides the channel head into two equal portions. This baffle forces the total flow only through the bottom half of the tubes. The tubes themselves are pipes of either 3/4 or 1 in OD (outside diameter). The front end of each tube is slipped into a slightly larger hole drilled into the channel head tubesheet. This tubesheet is a disk about 2 in thick, slightly larger than the inner diameter of the shell (shell ID). 

But this is not the case with a floating-head exchanger. The tube-side fluid reverses direction in the floating head. It has to. There is no way to attach the tube-side outlet nozzle to the floating head. It is a mechanical impossibility. So we bring the tube-side fluid back to the top half of the channel head. So, half of the tubes are in countercurrent flow with the shell-side flow. And that is good. But the other half of the tubes are in concurrent flow with the shell-side flow. And that is bad. 

The center channel head pass partition baffle is cut out. 

Two off-center channel head pass partition baffles are welded in place, so that 25 percent of the tubes are above the upper baffle and 25 percent of the tubes are below the lower baffle. 

Both the channel head cover and the channel head tubesheet (see Fig. 19.1) must be remachined to accommodate the new baffles. 

Most of the heat exchangers in your plant are designed with the shell-side inlet and shell-side outlet at opposite ends of the shell, as shown in Fig. 19.6. However, you may have noted a few exchangers in which both the shell-side inlet and shell-side outlet are next to the channel head. This is a two-pass shell-side exchanger, of the type shown in Fig. 19.8. 

Venting inerts from the floating head end of a horizontal reboiler can be difficult. A novel method which successfully accomplished this (232) was installing a 1-in internal pipe that extended a top tube from the channelhead tubesheet to a vent in the channel head (Fig. 15.10c). The top tube was thus converted into a "vent tube. The internal pipe was coupled to the vent from inside the channel head, to permit removal. This technique cured a CO2 corrosion problem attributed to poor venting at the floating head end. 

Refinery Inability to vent accumulated CO2 from the steam (tube) side of a horizontal reboiler caused corrosion and tube leakage near the floating head. Problem was solved by extending an upper tube to make up a vent tube finm the floating head to a vent valve located at the channel head. A novel technique was developed to solve problem. 

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