ITG SUBJECT: HEAT EXCHANGERS TO AVOID CONTAMINATION

Subpart D, section 212.76 of the current proposed Good Manufacturing Practices (GMP) for manufacturing, processing, packing or holding of a Large Volume Parenteral (LVP) includes requirements for the use of heat exchangers. It states in part "Heat exchangers, other than the welded double-concentric-tube type or double-tubesheet type, must employ a pressure differential and a means for monitoring the differential."

Certain portions of a processing operation require the heating or cooling of one fluid by another, which is the function of a heat exchanger. Often times the quality of one of the fluids is much less than that of the other. For example, in the distillation process for the production of U.S.P. "Water for Injection" in the pharmaceutical industry, the distillate is cooled by feedwater that is classified as something less in quality than WFI. A feedwater to distillate contamination is possible in a leaky heat exchanger. It becomes important to detect this leakage so that corrective action may be taken or to make provisions to ensure that undetected leaks will not be a hazard. This is possible by constant monitoring of pressure differentials and by visual observation for the actual leaks as in the double-tubesheet heat exchanger. The double-concentric-tube type, because of its construction, has little or no chance for leakage to occur. There are many types of heat exchangers. No attempt will be made to describe or itemize them as it is beyond the scope of this guide. However, the bulk of the exchangers utilized are the shell-and-tube-type, which is the primary subject of this discussion.

Figure 1 shows one that is typical. The various components are identified.

FIG. 1



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[Legen: Shell    Baffle    Tube    Tube Sheeet   Bonnet]

The double-concentric-tube, sometimes known as the double-pipe, could be considered a simple shell-and-tube type with a single tube in a shell. This is depicted in Figure 2

FIG. 2



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One fluid flows through the inside pipe and the second fluid through the annular space between the outside and the inside pipe. The outside pipe is usually welded to the inside pipe but may be designed with a packing gland or stuffing box for sealing as shown in Figure 2. The important feature here is that the inside pipe is straight-through with no discontinuities to offer a source for leakage and intermingling of the two fluids. Such an exchanger may consist of several passes arranged in a vertical stack, such as the three pass as shown. Double-pipe exchangers are useful when not more than 100 to 150 ft 2 of heat exchange surface area are required. Beyond this range, it becomes more economical to bundle many tubes in a single shell and avoid the weight of metal required by adding larger pipes or additional passes in a series or parallel arrangement.

The shell-and-tube heat exchanger that utilizes many tubes in a single shell are usually classified by the number of passes that each fluid will make. For example, the 1-1 exchanger, as shown in Figure 3, has one shell-side pass and one tube-side pass. Another might be referred to as a 1-2 exchanger, that is, one shell-side pass and two tube-side passes. Other combinations are used, but the 1-2 and 2-4 types are most common.

FIG. 3



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The bundle of tubes in a shell-and-tube heat exchanger must be supported in some fashion within the shell so as to maintain the proper spacing for uniform and efficient heat transfer between the fluids. This is done by fastening the tubes to holes in a flat disc called a tubesheet. (See Figure 1) The usual methods for joining tubes to tubesheets are roller expanding; a combination of roller expanding and welding; welding only; roller expanding and heading or flaring the tube ends, followed by welding; and using a ferrule or similar device to interpose packing or seals between the outside of the tube and the inside of the tube hole. Regardless of the method, there is always the possibility of leakage at the joints of the tubes to the tubesheets. Joints that are tight when tested by the manufacturer may leak once installed and subjected to normal operating stresses or corrosive actions on welds. As stated previously, this leakage, if it is undetected, can contaminate the fluid from the second stream.

There are two methods for preventing contamination by leakage. One is to provide gauges to constantly monitor pressure differentials to ensure that the higher pressure is always on the "clean" fluid side. The other is to utilize the double-tubesheet type of heat exchanger.

Figures 4 and 5



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The double-tubesheet design is typically shown in Figures 4 and 5. It requires two tubesheets at the ends of the tubes. Adjacent tubesheets are joined to each other only by the tubes. The gap may be as small as 1/8-inch. Shrouding may be used to seal the gap between adjacent tubesheets so that the leaking fluid may be collected. This shrouding, with accompanying drain ports or holes, is an identifying feature during an Establishment Inspection when trying to determine if the double-tubesheet heat exchanger is being used. The shrouding is not an absolute necessity; and if not used by a firm, double tubesheet identification is much easier. It should be pointed out that if the shrouding is used, it is important that the drain ports be open and free to discharge any leakage that may occur. The double- tubesheet design does not eliminate leakage; but it does eliminate leakage of shellside fluid into the tubeside fluid.

References:

Perry's Chemical Engineers' Handbook, 4th Edition, McGraw-Hill, 1969.

McCabe & Smith, Unit Operations of Chemical Engineering, McGraw-Hill, 1956.

Yokell, Stanley. Double-Tubesheet Heat-Exchanger Design Stops Shell-Tube Leakage, Chemican Engineering, May 14, 1973.