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"What is the best way to clean the shell side of a shell-and-tube oil cooler? There is a suspicion of sludge buildup and carbon from the oil. The oil is used for a screw compressor."
The shell side of a shell-and-tube oil cooler can be treated much like any other sump in terms of flushing and cleaning. A variety of methods can be used. Your selection should primarily be based on the attributes of the contaminant. Choosing the wrong tactic not only can be wasteful but also risky in terms of potential system upsets and negative side effects. Any time you introduce unusual fluid chemistry, temperature, pressure, flow and turbulence, there can be adverse consequences for the machine, its seals and the lubricant. Following are the practices and technologies used by nearly 95 percent of flushing activities in industry.
This is the mildest of the flushing strategies. Because many machines have no onboard filtration, the use of periodic filter carts and oil reclamation equipment not only can clean the oil (drawing down the contaminant level) but can also remove loosely deposited sludge and sediment from the shell.
Flushing is improved by enhanced fluid dynamics near machine surface boundaries. The approach involves increasing fluid velocity (sometimes two to four times the normal flow rates) and/or reducing oil viscosity during the flush. Typically, a Reynolds number in the range of 4,000 to 6,000 is generally targeted.
This strategy also reduces viscosity and increases turbulence. In addition, it increases oil solvency to aid in the scrubbing of tenacious deposits. Target temperatures range from 175 to 195 degrees F.
Some practitioners have discovered that shocking the machine with large temperature shifts helps break loose crusty deposits during the flush. They use coolers and heaters to cycle the oil’s temperature repeatedly over a range greater than 100 degrees F.
Rapidly changing oil flow rates caused by pulsation have been found to help dislodge pesky contaminants from nooks and crannies of the shell and tubes.
By changing fluid flow direction, some contaminants and surface deposits are exposed to bending fatigue reversals and can be dislodged and freed into the oil.
This tactic is used for larger shells with convenient access to hatches and clean-out ports. A wand on the end of a flushing hose is used to create high-velocity oil flow to blast away deposits. Alternatively, the wand used in suction mode can be effective at picking up bottom sediment on the sump floor.
Some suppliers of these proprietary reclamation technologies have successfully removed varnish from machine surfaces as well as submicron soft contaminants in the oil, known to be a precursor to varnish and sludge.
Various solvents and detergents have been used to concoct flush fluids with different degrees of success. These include mineral spirits (petroleum distillates), diesel fuel, motor oils and detergent/dispersant packages. They are typically added to the flush fluid at concentrations of 5 percent to 15 percent, followed by a rinse. Compatibility problems (with the oil, seals and machine surfaces) are the primary concern. Always consult machine and lubricant suppliers before these chemicals are introduced.
These are chemically active compounds, typically caustics and acids, that aid in the removal of the most adherent organic and inorganic surface deposits. The oil must first be removed completely from the system. Following the flush, these chemicals should be thoroughly rinsed from the system, often followed by pacification. Always consult machine and lubricant suppliers before employing chemical flushes.
This generally involves the use of scrapers, brushes, abrasives and sometimes an ultrasonic bath. Often, chemicals are also used as the machine components are washed one at a time using a parts-cleaning station.
Besides experience, an important part of defining the correct strategy and tactics comes from the inspection of the machine and contaminants. This inspection should be repeated before a machine is returned to service to verify that a successful flush has been achieved. Likewise, a final oil sample should be taken and analyzed to confirm that residual flushing fluids or loosened deposits that could potentially compromise lubrication and system operation don’t remain in the new oil.