Lesson 8 - Heat Exchangers

We have learned about the many measures taken in designing a heat exchanger to ensure that they are effective and reliable. However, the heat exchanger will only remain reliable and efficient over time if it is routinely maintained and properly operated. So, let’s discover more about good maintenance and operating practices so we can get the best out of our heat exchangers.

Like any equipment, heat exchangers are not maintenance free. Below are some of the key maintenance practices recommended for brewing heat exchangers:

  1.  Visual inspection

Heat exchangers should be externally inspected regularly for leaks. This is true for plate heat exchangers where small leaks developing from the plate pack can be an early warning to gasket failures.

  2.  Pressure testing

The shell feature of a shell and tube exchanger is a pressure vessel, and as such, it must adhere to the maintenance and inspection requirements of a certified pressure vessel.

While not regulated by pressure vessels standards, it is good practice for the tube side of a shell and tube exchanger and plate heat exchangers to be pressure tested at regular intervals. This ensures the integrity of the heat exchanger. It can easily be done by pressurising one side of the exchanger while the other is left open. If the pressure drops, it indicates a leak in the exchanger.

  3.  Gasket replacement

Over time, gasket material hardens and degrades, and gaskets will lose their ability to compress and form a seal. The lifespan of a gasket depends on the gasket material, type of fluids and operating temperature and pressures. Heat exchangers with highly corrosive fluids and extreme temperature and/or pressures will require more frequent gasket replacement. Gasket replacement is most applicable to plate heat exchangers as they have a large number of specialised gaskets. Shell and tube heat exchangers have far less gaskets that do not require as frequent replacement.

If gaskets are not replaced at the frequency recommended by the supplier, they will leak. This leak can be to the outside of the exchanger but worst still it could be utility fluid leaking into the product stream which will contaminate the product.

Gaskets must be precisely positioned in the plate channels when being replaced, otherwise they will leak. If you are interested in seeing how the gasket of a plate heat exchanger is replaced, please watch the following video. Note, this is for illustrative purposes only and is not a recommendation or endorsement of the supplier’s product. 

The reliability and performance of the heat exchanger are also affected by how it is operated. Below are recommended operating practices to ensure your exchanger remains efficient and reliable:

• Operating pressure and temperature

Heat exchanger components, in particular the plates and gaskets of plate heat exchangers, are sensitive to pressures and temperatures outside of their design conditions. Operating conditions should ensure that the temperature and pressure of the fluids in the exchanger never exceed the design parameters.

The lifespan of the gaskets is also affected by how the exchanger is operated.

As an example, and as a general rule of thumb, operating an exchanger at 10°C lower than maximum design temperature can give the gaskets a two year lifetime. By comparison, operating at a temperature of 10°C above this maximum will reduce the gasket lifetime to just six months.

• Product and utility flowrates

To avoid leaking gaskets due to the bending of the plates of the heat exchanger the product and utility flowrates should be as per the design specification of the heat exchanger. Variation from this design can happen when the utilities flowrate is adjusted to achieve the specified product outlet temperature.

• Pressure shocks

Heat exchangers, in particular plate heat exchangers, are sensitive to pressure spikes. All sources of pressure spikes across the system should be eliminated if the exchanger is to have a reasonable lifespan. Furthermore, the differential pressure between the utilities and product stream should not exceed the design specifications.

• Monitoring of shell-side fluid

As fouling is the main consideration, particularly on the shell side, which is more difficult to clean, ongoing quality monitoring of this fluid should be done to ensure its scaling and fouling potential is kept low.

• Temperature control

Temperature control through a plate and frame heat exchanger is very responsive, and control loops can be set up for fine control of the product fluid outlet temperature. Temperature control on the outlet of a shell and tube exchanger will be more sluggish than plate and frame, and the control loop should be tuned.