Heat Recovery from Air Compressors

The oil separator element doesn't rank high on a screw compressor's consumable list. Air filter, oil filter, lubricating oil, purchasing and maintenance people keep a close eye on those. The oil separator element hides inside the oil-air tank. Can't see it, can't touch it. Often nobody thinks about it until the pressure differential alarm goes off.

Inside the screw compressor's compression chamber is a mix of oil and air. Ratio roughly 1:50 to 1:100. Air needs to go out for use. Oil needs to stay and keep circulating. The oil separator element is the part that separates these two. Atlas Copco, Ingersoll Rand, Sullair, these brands' machines all have essentially the same separator element structure. Oil-air mix passes through a layer of fiberglass or synthetic fiber material. Oil coalesces into droplets on the fibers, runs down the element wall to the bottom for recovery. Air continues forward.

New separator element outlet oil content can be held to 2 to 3 ppm. Some premium models can do below 1 ppm. Over time the fiber layer gets coated with contaminants. Separation efficiency declines. Oil content gradually creeps up. This process is gradual. Most of the time there won't be a sudden major problem.

High differential means the compressor is doing useless work. Air has to overcome extra resistance to get through the element. All that energy converts to heat and dissipates. Differential going from 0.3 bar to 1 bar, assume 7 bar discharge pressure, that's roughly a 10% increase in working pressure. Power is approximately proportional to pressure. Electricity bill goes up with it. A 75 kW machine running 6,000 hours a year, the extra electricity cost is enough to buy several separator elements.

Differential keeps climbing. Pressure on the separator element keeps increasing. The fiberglass layer itself has no real strength. Relies entirely on the outer metal cage for support. Differential exceeds the design limit, the fiber layer bulges, tears, and eventually the airflow punches a hole through it.

Meeting time, blame game. Equipment department says we maintained by the book. Purchasing says the element was the model you specified. Production says we just use the air, we don't manage the machine. Separator element costs a few hundred dollars. Something goes wrong and nobody wants to take the hit.

The Oil Return Line

At the bottom of the separator element there's a thin tube. Internal diameter usually 1.5 to 2 mm. Connects back to the compression chamber or oil-air tank bottom. Coalesced oil returns through this tube.

A lot of maintenance people don't know what this tube is for. Some people changing the separator element don't even reconnect the oil return line. Figure a tube that thin, what could it possibly do. After a while they find oil carry-over. Assume it's a separator element quality problem.

Tube is thin. Easy to clog. Clogged, oil can't return. Accumulates at the bottom of the separator element. Liquid level rises past a certain point, airflow carries the oil out. Symptoms identical to a ruptured separator. Outlet oil content spikes. Open it up, separator element is perfectly intact.

Fusheng and Kaishan machines, this problem shows up a bit more. Not saying these two brands are bad. Might be related to oil circuit routing. Might also just be that there are a lot of them out there so more cases are encountered. Atlas Copco's machines have a somewhat more reasonable oil return line design. Lower clogging probability. But not zero.

Oil return line blockage is mostly caused by oil sludge. Lube oil at high temperature oxidizes and cokes. Generates gummy deposits. Sticky stuff plugs the thin tube. Oil used past its change interval has a noticeably higher tendency to coke. Some factories run lube oil to ten thousand hours without changing. Save a little on oil, spend a lot more on machine repairs later.

Checking the oil return line is simple. Remove it, blow through with compressed air. Clear is clear, blocked is blocked. Poke with a thin wire. Clear it out, put it back. This step takes a few minutes. But a lot of people changing the separator element don't even check. Next time a problem comes up, still clueless.

Separator Clogging Fast, Check Upstream

Normal separator element lifespan is 3,000 to 6,000 hours. Depends on brand, operating conditions, and oil quality. Some machines hit differential alarm before 1,000 hours. At that point you can't just swap the element and call it done.

Air filters are cheap. A few dollars each. No reason not to replace them. But some factories just don't. Or buy the absolute cheapest ones. Save a few dollars, separator element fails early. Not worth it.

High ambient dust environments. Cement plants, mines, woodworking shops. Air filter working normally still can't catch all fine particles. Below 5 microns can pass through the air filter into the system. Over time the quantity adds up. Under these conditions separator element lifespan can be cut in half or more. Some factories add a pre-filter stage before the compressor intake. Works well.

Separator element quality itself is a factor. Some small manufacturers' replacement elements use inferior materials. Fiber layer density is uneven. Thin spots clog first or rupture first. Cheap ones last a few hundred hours before problems. Count in the downtime losses and you're worse off. More on this later.

These factors often stack up. Air filter not changed often enough. More dust gets in. Oil gets dirty faster. Separator clogs faster. Only watching the separator element is useless. The source is upstream.

Can You Wash and Reuse Old Separator Elements

Every year someone gets this idea. The old element looks fine. Just the pressure differential is a bit high. Wash it, can it keep going? Soak in detergent, blow with high-pressure air, dry it out. Go through the process, put it back in. Differential does come down some.

Doesn't last long. Short end, a few hundred hours. Long end, about a thousand. Then the differential is back up. And outlet oil content is way higher than a new element. Oil carry-over is serious.

The logic is simple to explain. But every year people still don't believe it. The temptation of saving a few hundred on an element is too strong. Some small factory owners personally supervise the washing. Wash it, install it, runs a few hundred hours, problem. Swap in a new one. The whole exercise saved nothing.

There are even people making a business of this. Collecting old separator elements, cleaning and refurbishing them, reselling. Price is half a new one. Some buy them. Use them, have problems, don't even know it was a refurbished element. Think they just got unlucky with a counterfeit.

OEM Parts vs Aftermarket

OEM separator elements are expensive. That's a fact. Atlas Copco's 75 kW machine separator element, OEM part typically quoted at $200 to $300. Run into a greedy dealer, can go to $350. Aftermarket ones, cheap end $70 to $80, higher end $120 to $130.

Price gap is right there. Aftermarket market is big. Pall and Mann are established filter manufacturers. Supply to all kinds of equipment. Quality is stable. Their screw compressor separator element replacements perform close to OEM level. 30 to 40 percent cheaper. This tier of aftermarket, use with confidence.

Problem is the market also has massive quantities of products from unknown small manufacturers. Search online and the same model separator element runs from $40 to $200. Can't tell the difference from the pictures. Is $40 the same as $200? Materials are definitely different. Process is definitely different. Cheap ones have thin fiber layers, uneven density, weak cages. Shorter lifespan. Higher chance of problems.

Buying aftermarket, watch the dimensions. Outer diameter, height, thread connection. Must match exactly. Off by a millimeter or two and it won't fit. Or fits but doesn't seal. Bad seal is the same as no separator element. Oil-air mix leaks straight through the gap. Buying online, pictures don't tell you. Receive the goods and find the dimensions don't match. Returns and exchanges are a hassle.

Cage strength matters too. When differential is high, the cage is all that's holding. Cage too thin or welds not solid, element deforms or even collapses. Some cheap ones look fine from outside. Problems only show up late in the element's life. Squeeze it with your hand and you can feel the stiffness. Too soft, no good.

Buying online is convenient now. A lot of factory purchasing departments just order from e-commerce sites. Go cheap with off-brand stuff. Problem comes, accountability time, turns out the supplier doesn't even have proper credentials. Recommendation: go through proper channels. Brand-name dealers for Pall, Mann, that sort of thing. Or local parts suppliers with a good reputation. Spend an extra fifty or a hundred dollars. Peace of mind.

If you absolutely must chase value, find the factory that makes for the OEM. Atlas Copco, Ingersoll Rand don't manufacture their own filter elements. They outsource to specialized filter manufacturers. The contract manufacturers also sell directly. Price is below OEM but quality is the same. This channel takes connections to find. Not everyone has the access.

When to Replace

Pressure differential is the core indicator for separator element condition. Most screw compressors have differential display or alarm function. Alarm threshold usually set at 0.8 to 1 bar. Some older machines lack this function. Need manual measurement with a differential gauge.

Dragging past 1 bar, on one hand you're paying extra electricity, on the other hand rupture risk increases. Some factories insist on waiting for the alarm. Alarm threshold set high too. Doesn't save much money. Risk is being carried the whole time.

When changing the separator element, check the oil return line while you're at it. Mentioned earlier. A lot of oil carry-over faults trace back to the return line. Checking takes just a few minutes. Models with a screen at the return line inlet, clean the screen too. That screen is very fine. Clogs faster than the return line.

Old element taken out, worth cutting open and looking. The color and form of the clogging material provides information. Gray-black powder, mostly dust. Means air filter or environment has a problem. Brown sludge means lube oil has severely degraded. Metal particles mean something inside the machine is wearing. Look carefully to find out where.

Cutting open and looking. Most people don't bother. Too much hassle. Pull the old one out, throw it away. Actually takes a few minutes and you can spot a lot of issues. Of course, for factories with tons of machines and not enough people, they genuinely can't attend to these details. Replacing on schedule is already good enough. Asking them to also cut open and inspect, that's expecting too much.