Return Tube Header

The tubes of a heater are connected in series by return bends. When cleaning will not be required, welded return bends may be used. Generally, however, each tube will have a header at one end with a removable plug for cleaning. If much cleaning is required, removable plugs will be provided at both ends of each tube. 

The temperatures at various places in the heater are computed step-by-step as follows  

Take as a basis one square foot of radiant absorbing surface in the heater. This should be a square foot of the equivalent plane surface. The rate of heat absorption on this surface will be the same as that on the most exposed element of the 

Maximum Permissible Radiant Absorption Rate Rate Of Heat Release, Btu/Hr.Ft.  

Pseudo Flame Temperature Temperature Of Combustion Space Tube Temperature 

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Figure 1-1 a. The "mule ear" type return tube header is the most commonly used in the process industry. 

There are several hundred pressure tubes, each containing bundles of 28 fuel rods, 50 cm long. The coolant is at a pressure of around 10 MPa (1450 psia) and the D2O is at 310°C. Headers on each side of the vessel collect and return coolant from all the tubes. The 4-mm wall-thickness zirconium—4.5%... 

Water hammer can also occur in steam mains, condensate return lines, and heat exchange equipment where steam entrapment can take place (Fig. I). A coil constructed and installed as shown here, except with just a steam trap at the outlet, permits steam from the control valve to be directed through the center tube(s) first. Steam then gets into the return header before the top and bottom tubes are filled with steam. Consequently, these top and bottom tubes are fed with steam from both ends. Waves of condensate are moved toward each other from both ends, and steam can be trapped between the waves. 

The tube-side fluid now flows into the floating head, which acts as a return header for the tubes. The tube-side flow makes a 180° turn and flows back through the top hah of the floating-head tubesheet. The floating head is firmiy attached to the floating-head tubesheet. But why is it that one end of the tubes must be left free to float The reason is thermal expansion—or, more precisely, the differential rate of thermal expansion between the tubes and the shell. 

Return headers are available with minimum center-to-center distances of the tubes from 1-1/2 o 2 diameters, depending on the tube size and... 

For a given working pressure the cost of a tube varies approximately as the square of the diameter. Return headers, being geometrically similar, vary in weight and cost approximately as the cube of the diameter. For a given total surface, smaller tubes are obviously cheaper. On the other hand, the pressure drop per unit length varies as the inverse of the diameters, 1/T>. To use smaller diameter tubes and have a reasonable pressure drop, it is usually necessary to have some passes in parallel. 

The results show that six rows of finned tubes can do the work of 18 rows of bare tubes. Finned tubes are more expensive than bare tubes, but other costs must also be considered such as additional steel structure, refractories and return headers. 

The HCIHX for the HTTP is a vertical helically-coiled counter flow type heat exchanger in which the primary helium gas flows on the shell side and the secondary in the tube side as shown in Fig.2. The major specification is shown in Table 1. The primary helium gas of the maximum 950°C enters the HCIHX through the inner tube In the primary concentric hot gas duct. It is deflected under a hot header and discharged around the heat transfer tubes to transfer primary heat to the secondary cooling system. It flows to the primary circulator via an upper outlet nozzle and returns between the inner and outer shell in order to cool the outer shell. 

The one danger in frequent steam spalling is erosion lo the last few tube return bends. These bends should be retrofitted with mule-ear return type plug headers which are highly resistant to erosion. 

Header boxes Most ends of tubes are connected by U-bends that make inspection difficult. Therefore, special return fittings with plugged connections are provided that permit inspection and cleaning. Because the removable plugs are subject to leaking, all headers must be completely enclosed in header boxes. Snuffing steam is used to suppress fire in this area. 

Coolant is supplied to tho procoaa tubes via the inlet nosale from tho inlet coolant supply header through the individual inlet connectors. Coolant discharged from tho tubes is returned to outlet... 

For the shell and tube exchangers, assume 12 ft, 1.25" tubes on a 1.5" square pitch and assume 2 ft additional length at either end of the exchanger for tube return and feed header. 

As a rule, 5 Cr-0.5 Mo steel components have sufficient resistance to all but severe cases of droplet impingement in transfer lines. Higher alloys should be used for furnace tubes and associated components, such as headers and return bends. 

The composition, and physical and mechanical properties of materials for headers and return bends, irrespective of whether they are cast or wrought, rolled or welded in, should be compatible with those tubes to which they will be connected and be of a weldable quality. The use of cast alloy steel parts should require approval. 

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