Double tube-sheets

Common practice is to specify exchanger surface in terms of total external square feet of tubing. The effective outside heat-transfer surface is based on the length of tubes measured between the inner faces of tube sheets. In most heat exchangers there is little difference between the total and the effective surface. Significant differences are usually found in high-pressure and double-tube-sheet designs. 

The price of a shell and tube exchanger depends on the type of exchanger, i.e., fixed tube, U-tube, double tube sheets, and removable bundles. The tube side pressure, shell side pressure, and materials of construction also affect the price. If prices cannot be obtained from endors, correlating in-house data by plotting /fr vs. number of ft with correction factors for the variables that affect price will allow estimating with fair accuracy. If not enough in-house data is available to establish good correlations. it will be necessary to use the literature, such as References 16. 17. and 18. 

The tube sheet forms the barrier between the shell and tube fluids, and where it is essential for safety or process reasons to prevent any possibility of intermixing due to leakage at the tube sheet joint, double tube-sheets can be used, with the space between the sheets vented Figure 12.16c. 

Welded double concentric tube or double tube sheet with tri-clamp sanitary fittings (tube side)... 

Component (auxiliary equipment) selection should be made with the assurance that it does not create a source for contamination intmsion. Heat exchangers should be double tube sheet or concentric tube design. 

Heat exchangers, other than the welded double-concentric-tube type or double-tube sheet type, must employ a pressure differential and a means for monitoring the differential. The pressure differential shall be such that the fluid requiring a higher microbial quality shall be that with the greater pressure. Written records of the pressure differential monitoring shall be maintained as required in 212.183... 

Inside the shell sits the tube bundle, in which the tubes are either brazed, welded, or rolled to a circular single or double tube sheet. Typical tube flow patterns are either one, two, four, or six passes. 

If mixing of the shell-side and tube-side fluids cannot be tolerated, then use the double tube-sheet design shown in Figure 4.4 for extra protection. Because... 

Figure 4.4 Double-tube sheet heat-exchanger design. From Ref. 18 with permission.

In the ICI gas-heated reformer (GHR) (Figure 49) the reformer tubes consist of an outer scabbard tube with an open ended bayonet tube inside, and the annular space between the tubes is filled with the reforming catalyst. The steam/natural gas mixture enters the tubes via a double tube sheet and flows downwards through the catalyst, and the reformed gas leaves through the bayonet tubes. To enhance the heat transfer from... 

The WFI storage tank water is usually maintained at about 80°C. This water may be temperature controlled (heated) at the return end of the WFI loop via a WFI heat exchanger (shell and tube double tube sheet) or a hotjacketmaybeused. A heat exchanger is the preferred method. Wherecool WFI is required, point of use coolers (double tube sheet) or a cool WFI loop is provided. Considerable control effort is needed for the point of use cooler design to meet the continuous flow non-stagnancy standards. 

The second method provides a double barrier within the steam generator itself. The double barrier has conventionally been in the form of concentric tubes and double tube sheets with the intermediate space filled with a static third fiuid to assist in the transfer of heat. The third-fluid system was equipped to detect any leaks which might occur in either of the single barriers. Under the doublewall philosophy as originally developed, if a leak occurred in either barrier, the heat exchanger was taken out of service for repair or replacement. 

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