Terminal velocity..…Don’t let it kill your heat exchanger!

Ensuring the correct velocity of cooling medium through the heat exchanger is vital to the long term health of the unit. Get it wrong and results could be terminal.

The velocity (or flow rate) of the fluid that circulates through a typical shell and tube heat exchanger is extremely important. Not only can it have a direct bearing on the heat transfer efficiency of the heat exchanger, in instances where the incorrect flow rate is used, it is often the cause of premature failure of the unit, but for very different reasons. In this article, we will look at some of the reasons why.

Every heat exchanger is designed to operate at a maximum recommended flow rate for the cooling medium being used and manufacturers, such as Bowman, expect this to be adhered to. Unfortunately, real life shows us that this isn’t always the case and in instances where a given flow rate is exceeded, this often results in the premature failure of the unit.

The problem is quite simple. As the flow rate of the cooling medium is increased – beyond the manufacturer’s recommendation – the speed with which the fluid enters the heat exchanger increases and the resulting velocity with which it hits the unit’s tube stack can damage both the tubes and the tube plate securing them in position inside the heat exchanger. Considering that the tube stack is usually the most expensive component in a heat exchanger, it simply doesn’t make sense to ignore the manufacturer’s maximum flow rates, as this premature erosion is costly, both in terms of replacing the unit itself and the resulting downtime due to failure.

But that’s only part of the story. Low flow rates can create a number of other issues, too!

It is widely accepted that a velocity of less than 1 metre per second (1m/s) reduces the efficiency of the heat exchanger. It’s also a fact that using a heat exchanger that is too large for the application could reduce the heat transfer performance of the unit, as it effectively reduces the velocity of the cooling medium.

And if flow rates are very low, or where the water inside the heat exchanger is allowed to stagnate, this can lead to other problems, which are covered in a separate article ‘Is stagnant water killing your heat exchanger?’ So, for different reasons, both low and high flow rates of cooling fluid can have a serious, detrimental effect on the service life of a heat exchanger.

However, in some applications, low or high water flow rates are simply unavoidable. What do you do in those instances?

At Bowman, we recommend a maximum flow rate of 3m/s for standard heat exchangers using cupronickel tubes, when cooling with fresh water and 2m/s when using seawater. For applications where higher velocity is important, alternative materials, such as titanium, are available, which offer extended service life.

Additionally, Bowman also offer a multitude of end cover options allowing cooling water to travel through the tubes in 1, 2 or 3 pass configurations. This enables the same unit to be suitable for a variety of different flow rates, whilst ensuring the velocity is controlled within the set parameters.

But if you are in any doubt about the optimum flow of an existing Bowman unit, or the correct unit for your requirements, use the link to download the manual, which will give more information on the flow rates and velocities that are suitable for each heat exchanger in the Bowman range. Still unsure? Then contact our technical sales team on +44 (0)121 359 5401 or email them on [email protected].

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