Single Tubesheet/Double Tubesheet

In pharmaceutical or even food applications where you have a fluid on one side of the heat exchanger that must not be contaminated by the fluid on the other side, you must ensure that contamination can not take place.  This may be done by various methods but whatever method is chosen it must be reliable (as reliable as gravity).  For the sake of illustration let us consider WFI (Water For Injection) on one side as the fluid that must not be contaminated and on the other side is glycol for cooling the WFI.  Clearly the WFI must never be contaminated by the glycol because it would create a grave risk to an eventual user of an injectable or food product.  In this case it would satisfy regulations if the WFI was maintained at a pressure that was always above that of the glycol.  This pressure must be maintained at all times and under all circumstances which includes under fault conditions and failure modes.  If an electric pump is used to maintain the pressure differential then the regulations would not be satisfied as the pump could fail or the electrical supply to the pump could fail.  Under those conditions the pressure could be reversed and, if leakage occurred, it could contaminate the WFI.

If you are not able to guarantee that the pressure gradient will always protect your sensitive product then you should use a double tubesheet design.  In this design the failure of a tube to tubesheet joint will not result in contamination as the WFI or glycol will simply leak to the environment without causing a cross contamination.

In none critical applications or in an application where you can guarantee that you will maintain a proper pressure gradient, then a single tubesheet design will work well and is the most economical solution.  In applications where contamination could result in harm and it is not possible to guarantee that the pressure gradient will be maintained then you must use a double tubesheet design.  While double tubesheet designs cost more it is usually cheaper to use one than to try to engineer a 100% reliable pressure gradient.  (Scroll down for double tubesheet illustration)

Enerquip manufactures units with both single and double tubesheet designs and can help you in deciding which one you need.

Fully Drainable Heat Exchanger

In pharmaceutical and many food applications you should consider specifying a fully drainable heat exchanger if any of the following situations apply:

• If your heat exchanger is going to be left for periods of time essentially empty on the tubeside and there is a possibility of  bacteria growing in any pools of liquid that may be retained in the unit.
• If there is risk of product carryover and you are trying to reduce the "holdup" volume in your heat exchanger.
• If the unit is going to be left for periods in the "drained down" condition, then it is essential to drain all the fluid out of the unit to eliminate the corrosion risk at the liquid interface.

Enerquip's pharmaceutical designs ensure full drainability of the tubeside of the unit.  In most cases, by simply draining the unit from the lowest port on the head.  Specially designed channels inside the heat exchanger drain the upper chambers into the lower chambers to ensure an absolute minimum "holdup" volume is retained in the unit.

The proprietary "bow tie" design of the web in the waterbox of two pass unit gives greater flexibility during installation.  Even if rotational alignment is off by a few degrees the unit will still fully drain.  Holdup volume remains constant despite misalignment of up to plus or minus 5 degrees.  This is a feature that dramatically simplifies validation.  Scroll down for illustration.

Surface Finish

In 'high end' sanitary applications it is common to see very high quality surface finishes specified.  These are expensive to apply and are often unnecessary.  For example, in hot, recirculating WFI systems, there should be no source of carbon, so there should be no food for bacteria.  In addition, the temperatures should render most bacteria unable to multiply.  If this is so, then why specify a 10 Ra finish?  It will not improve performance of the unit and it will cost money to apply.  It would be much better and cheaper to specify a ground finish and electropolish than to specify a 10 Ra finish.  See the Technical Bulletins section for a paper on sanitary surface finishes.  Having said all that, we do our own electropolish so that we can control the quality of the finish.  We can provide almost any kind of finish you may need and we have a uniquely skilled electropolish department.  In short it does not matter what your specification says, we can provide it.  If you are looking for a high performance surface and need to save a little money we can help you with that as well.  We have scientific and engineering staff who have designed and built the production facilities of biopharmaceutical plants and have validated entire plants so we understand your problems and can speak your language.

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Double Tubesheet Configuration
  

This illustration shows the general arrangement of a double tubesheet heat exchanger.  This configuration is ideal where the product and the process fluid on the shell side must not be permitted to mix or cross contaminate.

Fully Drainable Water Box
  

Here you can clearly see the the features that make the water box super sanitary and permit free draining, even with a substantial degree of misalignment or rotation during installation.  These features are a huge aid to system validation as well as simplifying installation.