Many different designs and applications of compact heat exchangers are used in military, commercial, and business aviation applications as well as ground support equipment.
They primarily encompass water, oil, fuel, and air coolers as well as evaporators and condensers used in vapor compression systems (air conditioning and refrigeration).
New heat exchangers are usually cleaned of raw material preservative oils as well as manufacturing debris and fluxes. Many times, after being in service, it is clearly specified to flush and clean a used heat exchanger after being contaminated from a component failure or as preventive maintenance service.
The ultimate goal for both the technician and the customer is to be sure the job is done right the first time, in the most economical and professional manner; and that the repaired system will operate properly. Regardless of the heat exchanger application, what equipment, method, and chemical you choose to use, it is ultimately the technician’s responsibility that the flushing project has a positive, rather than a negative effect on the system. Flushing done right is a proven way to reduce the expense of comebacks and warranty repairs.
Unfortunately, we also find little or no references on how to flush or what to use; and definitely no warnings that some methods are expensive, time consuming, and ineffective on certain heat exchanger designs.
So the questions are:
- Can we clean them?
- How do we clean them?
- What works, what does not, and why?
It is globally accepted that industry standard methods to clean heat exchangers requires some form of appropriate cleaning chemical, velocity, and at times agitation. To meet the goal of a clean component, it is critical that you understand and evaluate the failure, the debris load that may be present, and how it entered the component; as well as any other debris binding residues that will need to be removed. If you cannot identify or determine what is in there how can you select the proper solution? To do this properly requires some basic understandings of the application, component designs, as well as some simple chemistry and physics principles.
Look at the design
Some applications were listed in the first paragraph and they can be as far reaching as component designs, so for the sake of brevity with this article let’s just talk basics.
First we have the “tube and fin,” which is nothing more than one long tube weaving back and forth; with one inlet, one outlet, and one common single pathway, which by design pose no real issues to clean. However, this tried and true “tube and fin” design has been somewhat abandoned for more intricate designs that produce more surface area for improved thermal transfer while maintaining a smaller and more compact size. These are “parallel flow” and other “multi pass” designs as well as, open web, cross flow, stack plate, multiple tube “tube and fin,” and many more.
The commonality here is the multiple paths and passageways that bring the “path of least resistance” rule into play when considering effective cleaning methods. If not familiar, this is a fluid flow principle that basically states that fluid will flow around an obstruction when provided with an alternative “path of least resistance.”
Most lubricating, fluid, and gas systems using heat exchangers are designed for smooth linear flow without cavitations or vibration. This allows for swirls, eddies, and pools to occur in the corners and crevices of the multiple path heat exchangers, where residues will accumulate and trap or bind debris. This is why many times we see that simple circulating equipment will not get heat exchangers completely clean, even after flowing for hours and hours.
Selecting a cleaner
So now we know what the system is (application — oil, air, water, A/C, etc.), and the challenges facing us given the type of heat exchanger being used (design); now we must select a chemical cleaner. The chemical properties of a solvent or cleaner will displace, dissolve, or in some way chemically alter and or displace the contamination on a surface; and must be selected for its known effectiveness on the known residues.
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