Hot Tip #159 Waste Water Heat Exchanger Comparision

For any heat exchanger to approach 100% exchange efficiency it must be in a COUNTER-flow condition.

The Maximum exchange efficiency while in a CO-flow state is 50%.

Most shell & tube exchangers have a reversing valve. A correctly operating reversing valve takes the flow out of COUNTER-flow & places it into a CO-flow condition. The OEM’s recommendation is to run in reverse (CO-flow) about 10% of total run time.

I have found several facilities were the air lines had been reversed and it was in reverse flow 90% of total run time! Plate & frame exchangers are never in a CO-flow condition.

Chapter 12 of TRSA’s Hand Book provides a spreadsheet that shows the lost energy from a heat exchanger in a CO-flow condition. It can be tens of thousands of dollars per year! 

Heat exchanges are more efficient if both water streams are flowing turbulently (Reynolds number above 4,000). I find that most shell & tube exchanges have both water streams in a laminar flow condition.

It is very expensive to increase the size of a shell & tube exchanger if your facility water needs increase. Additional plates can be added to plate & frame exchangers easily (affordably).

Cleanliness of the heat exchange area is by far the greatest detriment to exchange efficiency of any heat exchanger! Most shell &tube exchangers have no GOOD pre-filter installed with them. All working plate & frame exchangers have some type of pre-filter, generally a shaker screen. Plate & frame exchangers will not work long without good pre-filtering.

There is no easy way to clean shell & tube exchanges. The wastewater side, (the dirtiest side) can be cleaned by removing the end caps and pushing a long brush though each tube; the same as cleaning a rifle barrel. There may be more than a hundred tubes. The city waterside of the tube (the cleanest side) cannot be cleaned at all! Both sides of the plates of plate & frame exchangers can be cleaned much easier by loosening the compression nuts, spreading the plates apart, and washing them with a high pressure washer.

Installed costs of both types of exchangers are about the same.

 

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