Fouling in oil cooler
What is Fouling?
In the context of an oil cooler, fouling is the accumulation of deposits on either the inner surfaces of the tubes (oil side) or the outer surfaces of the fins (air/water side). This layer of deposit has very poor thermal conductivity, creating a barrier between the hot oil and the cooling medium (air or coolant).
Types & Causes of Fouling
Fouling is categorized based on what the deposit is and where it occurs.
1. Oil-Side Fouling (Internal Fouling)
This occurs on the inside of the tubes where the oil flows.
- Carbon & Varnish Deposits: Caused by oil degradation due to excessive heat (“thermal breakdown”). The oil oxidizes and forms hard, coke-like carbon deposits and sticky varnish.
- Sludge Formation: Results from contamination of the oil with water, coolant, fuel, or soot. This creates a thick, mayonnaise-like substance that clogs passages.
- Contaminant Buildup: Dirt, metal wear particles, and other abrasives circulating in the oil can settle and accumulate in the cooler’s small tubes.
- Wax Precipitation: In some applications, the paraffin in oil can solidify at lower temperatures, causing waxy deposits.
2. Coolant/Air-Side Fouling (External Fouling)
This occurs on the side of the cooler exposed to the cooling medium.
- Air-Cooled Coolers: Dirt, dust, pollen, insects, leaves, and other airborne debris get lodged in the fins, blocking airflow.
- Water-Cooled Coolers: Scale formation from dissolved minerals in the coolant (like calcium and magnesium) can coat the surfaces. Corrosion products from the cooling system and biological growth can also cause fouling.
Effects and Consequences of Fouling
The primary effect is a drastic reduction in thermal efficiency, leading to:
- Increased Operating Temperatures: The oil cannot reject heat effectively, causing it to run hotter. This creates a vicious cycle: hotter oil degrades faster, leading to more fouling.
- Reduced Oil Life: High temperatures accelerate oil oxidation and breakdown, meaning you must change the oil more frequently.
- Elevated Engine/Machine Temperature: The entire system runs hotter, increasing wear on bearings, pistons, rings, and other critical components.
- Increased Pressure Drop: Deposits restrict the flow passages. The oil pump must work harder to push oil through the clogged cooler, leading to:
- Lower Oil Pressure: downstream of the cooler.
- Increased Pump Load: which can slightly reduce engine power.
- Cooler Failure: Severe restriction can lead to a bypass valve (if equipped) opening constantly, sending hot, unfiltered oil directly to the engine. In worst-case scenarios, it can cause the cooler to rupture internally or externally.
How to Clean and Prevent Fouling
Cleaning Oil-Side Fouling
This is challenging and often requires professional service.
- Chemical Solvents: Specialized hydrocarbon solvents or alkaline cleaners are used to dissolve carbon, varnish, and sludge. The process involves:
- Removing the cooler.
- Flushing with solvent, often with agitation.
- Soaking for several hours.
- Thoroughly rinsing and drying.
- Ultrasonic Cleaning: The most effective method for severe internal fouling. The cooler is submerged in a heated cleaning tank where high-frequency sound waves create cavitation bubbles that scrub deposits off the internal surfaces without damaging the metal.
- Mechanical Cleaning: Less common, but tools like “gun bore” brushes can sometimes be used on larger tube designs.
Cleaning Coolant/Air-Side Fouling
- Air-Cooled (External Fins):
- Compressed Air: Blow out debris from the back side (engine side) towards the front.
- Water Rinse: Use a low-pressure stream of water and a mild detergent. Never use a high-pressure washer, as it will bend the fins and worsen the problem.
- Fin Straightening: Use a fin comb to restore airflow after cleaning.
- Water-Cooled (External Tubes):
- Chemical Descaling: Circulating a descaling acid (like citric or phosphoric acid) through the coolant passages dissolves mineral scale. This is a highly specialized procedure.