Would like to know the correct procedure for flushing and cleaning stainless steel plate heat exchangers.

We have a WP24-14 plate GEA plate heat exchanger, operating now for 5 years as a condenser on a heatpump. Recently it has been noticed that head pressure has been increasing. It is suspected that a film of scale/scum have formed on the heat exchangers plate surfaces ( water side), this is inhibiting the heat transfer to the water. Water flow rate through the condenser is ok.

Have been told Citric Acid or Formic acid can be used to clean and flush stainless Steel plate heat exchangers.

Would appreciate any information on this topic.