Heat exchanger double tube-sheet

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. 

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... 

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

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. 

The outer and inner tubes extend from separate stationary tube sheets. The process fluid is heated or cooled by heat transfer to/from the outer tube s outside surface. The overall heat transfer coefficient for the O.D. of the inner tube is found in the same manner as for the double-pipe exchanger. The equivalent diameter of the annulus uses the perimeter of the O.D. of the inner tube and the I.D. of the inner tube. Kem presents calculation details. 

Figure 8.4. Example of tubular heat exchangers (see also Fig. 8.14). (a) Double-pipe exchanger, (b) Scraped inner surface of a double-pipe exchanger, (c) Shell-and-tube exchanger with fixed tube sheets, (d) Kettle-type reboiler, (e) Horizontal shell side thermosiphon reboiler, (f) Vertical tube side thermosiphon reboiler, (g) Internal reboiler in a tower, (h) Air cooler with induced draft fan above the tube bank, (i) Air cooler with forced draft fan below the tube bank.

The single pass shell-and-tube heat exchanger can provide more surface area in a smaller length and volume. Tube diameter is usually smaller than for a double pipe exchanger, but the tube surfaces can still be chemically or mechanically cleaned. Tube sheets inherently make the design more complicated than the double pipe exchanger. 

---