Vahterus Plate & Shell Heat Exchangers are individually designed for each client’s specific application. As part of the design, fouling is taken into consideration, as well as the customer’s knowledge and experience of the process. A correctly designed Plate & Shell heat exchanger has a highly turbulent flow. This turbulence creates a high wall shear stress, which helps to keep the exchanger free from fouling, and most of the heat exchangers produced over years at Vahterus have never required cleaning. In rare cases, fouling cannot be avoided, but several Plate & Shell cleaning methods can be used. The method is chosen based on the type of fouling. For the most difficult fouling challenges, Vahterus has developed new methods. Deeper access can be achieved through removable flow director design and the capacity to stretch the plate pack, which opens up the channels for easier cleaning. The fouling product can be removed using a chemical bath, high pressure water cleaning or soda blasting.

If there is a risk of fouling, the unit’s performance should be monitored by measuring temperature and pressure loss. When losses are greater than permitted, the heat exchanger should be cleaned.

Methods for cleaning

• Back flushing
• Chemical cleaning
• Steam cleaning
• Mechanical cleaning
• Soda blasting
• Pyrolysis
• Ultrasonic cleaning

The requirement for, and regularity of cleaning the heat exchanger depends on the type of media being processed. If there is a risk of fouling, the performance of the unit should be monitored by measuring temperatures and pressure losses. When pressure losses reach an unacceptable level or temperatures indicate poor heat transfer, the heat exchanger should be cleaned. There is no need to open the heat exchanger in order to effect cleaning. There are also two methods to clean the heat exchanger:

1. back flushing
2. chemical cleaning

Back flushing

In most cases, loose materials, such as organic compounds gathered on the surface of the plates, can be removed by reversing the flow or with back flush washing with warm water or cleaning medium. Warm water or cleaning medium is flushed at high velocity on the primary and/or secondary side in the opposite direction to that of normal operation (velocity approx. 2–3 times the normal velocity). The valves in the connection pipes must be closed and the drain valves mounted on the pipelines opened. The dirty medium must be discharged and disposed with in accordance with local regulations.

Description of the back flush system:

1. close valves 1.1 and 1.2. Temperature of the heat exchanger must be between 10°C and 30°C;
2. drain off the liquid from the primary or secondary side (the side that must be cleaned) by opening valve 2.2;
3. connect a hose to valve 2.1;
4. water should flow through the heat exchanger for 10–15 minutes;
5. check that organic compounds and dirt are removed;
6. stop the water flow, remove cleaning water and close valve 2.1;
7. fill the heat exchanger with system liquid in accordance with the start-up procedure (see operating manual). Close valve 2.2.

If it is not possible to back flush the unit or if the results are ineffective, a specific detergent must be used.

Cleaning with chemical materials

A company familiar with cleaning heat exchangers should be entrusted with chemical cleaning. The cleaning process should involve the use of common cleaning brands, which normally contain additives that make cleaning more effective and prevent corrosion. If scale cannot be removed from the heat exchanger by washing, soda solution, 2% caustic and/or 0.5% nitric acid or 5% sulphamic acid (NOT sulphuric acid) may be used up to 60o C. Phosphoric acid at 5% concentration can also be used.

Notes:

• Before cleaning the shell side of the heat exchanger with chemicals, check the resistance of the flow directors to that material. See also materials and the technical data sheet of the heat exchanger;
• After chemical treatment, the plates absolutely must be rinsed carefully with clean water;
• When cleaning on the coolant side a high temperature can occur in the heat exchanger. Precautions must be taken to ensure that the pressure on the refrigerant side does not increase excessively;
• Before cleaning, remove the refrigerant or be sure the refrigerant side is not trapped within the exchanger.

Example of the procedure used to clean the secondary side with chemicals:

1. close the valves 1.3 and 1.4. The temperature of the heat exchanger must be between 10°C and 30°C;
2. connect the hoses of the cleaning system with chemicals to valves 2.3 and 2.4;
3. fill the tank with the chemical solution;
4. switch on the pump (4) and let the pump run for 30-40 minutes. During cleaning, check the concentration and activity of the chemicals on inlet and outlet side of the heat exchanger at certain intervals. Stop cleaning when there is no further change;
5. when fouling has been removed, drain the chemicals from the heat exchanger and the tank. Neutralize applied chemicals;
6. wash the plate surface with clean water in back flow;
7. after shell side cleaning pay special attention to the washing of the dead spaces behind the flow directors if they exist.
8. start up the heat exchanger into normal operation according to start up directions.

Analysis after cleaning

Analyse whether fouling might be avoided by:

• installing strainers;
• decreasing operating temperatures (installing desuperheater);
• increasing system pressure;
• refreshing the medium more frequently;
• bringing the medium to the correct concentration;
• increasing the flow between the plates;
• changing the capacity control into a system with constant flow.