Explosion Proof Air Compressors for Hazardous Environments

A rotary screw compressor that keeps tripping on high temperature is telling you something is wrong. Once discharge temperatures push past 230°F the oil starts breaking down, and from that point the damage accelerates because degraded oil transfers heat worse, makes the oil degrade faster. A machine that's a little warm this month will be a lot warm next month if nobody does anything about it.

The machines are designed to run between roughly 180°F and 225°F at the discharge, depending on manufacturer and model. What matters for troubleshooting is the sustained reading under steady load, not the peak during a demand surge. A machine that held 190°F six months ago and now parks at 208°F under the same conditions has lost eighteen degrees of margin. The facilities that catch this early are the ones logging discharge temperatures regularly. Everybody else finds out when the alarm trips.

The temperature probe on most compressors sits at the airend outlet or on the separator tank, which understates the peak. The injected oil absorbs the thermal spike inside the airend before the mixture reaches the sensor. By the time the gauge reads 230°F, conditions inside the airend have been worse than that for a while.

Oil

People think of the oil cooler as the cooling system. It isn't. The oil does the cooling. It picks up compression heat inside the airend and carries it out to the cooler, which just provides the surface area for the oil to dump the heat into the airflow. When the oil can't do its thermal job, the cooler probably can't compensate no matter how clean it is.

Low sump is the simplest oil problem and the easiest one to miss on an overheating call. When the sump drops even a quart or two on a 50-horse machine, the remaining volume absorbs the same heat load and every gallon comes back to the airend hotter. On an Atlas Copco GA37 or GA55 the sight glass is on the separator tank. On most Ingersoll Rand machines it's on a remote reservoir or a sight tube off the sump. On older machines that have had aftermarket oil systems added over the years, there might be more than one sight glass and reading the wrong one gives a misleading level. Always read with the machine shut down and the oil settled. Running readings are unreliable because the oil is distributed between the sump, the separator, the cooler, and all the connecting lines.

A level that keeps dropping with no puddle on the floor points to the condensate drain. Check it for an oily film. A worn separator element passes oil into the air stream faster than normal. The sump drops, the machine runs hot, and there's no external evidence of where the oil went. Pulling the separator differential usually finds it. Normal differential with oil still disappearing means the shaft seal and the hose connections under the enclosure panels need a closer look. External leaks on enclosed packages can sometimes drip onto hot surfaces and evaporate or run down the inside of the enclosure panels where they're invisible from outside. Sometimes the only evidence is a slight oily smell near the machine. On machines with remote-mounted aftercoolers or dryers, check the connecting hoses too. A weeping connection at the separator tank outlet can lose oil slowly enough that it never pools on the ground, just coats the fitting and evaporates.

External leaks should be fixed. Obvious advice, but it's surprising how many machines run with known slow leaks because the leak is in a hard-to-reach spot and nobody wants to shut the machine down long enough to repair it.

Overfilling is a different problem. Too much oil gets whipped into foam by the rotors. Foam transfers heat poorly and the excess carries over into the air stream. The correct fill is model-specific and on a smaller machine the difference between correct and overfull can be less than a gallon. The Atlas Copco GA series service manual (section 5, lubricant specifications) lists the exact sump capacity and acceptable range for each frame size. Kaeser's SFC/CSD manuals have the same information in their maintenance chapter. Without the manual, call the distributor with the serial number. They can look it up. The sight glass markings are only valid at the specified reading condition, which is typically machine off, oil settled for five to ten minutes, at operating temperature. Reading the glass on a cold machine or a running machine gives a number that may not correspond to the actual marks on the glass.

Oil oxidation is where the real money gets spent on unnecessary diagnostics. Fresh oil is amber. As it degrades it darkens through brown to black and the byproducts are acidic. They eat copper components in the circuit and deposit varnish on hot metal surfaces. The thermal valve and airend injection ports get it first. Once the valve can't open fully under heavy load, the cooler doesn't get enough flow and temperatures climb. The machine might run fine on a light shift and overheat after lunch when production ramps up.

Once the valve is restricted, the machine runs hotter, and hotter oil makes more varnish. That feedback can run away in weeks on a machine that was already close to its thermal limit, though it depends heavily on the oil type and the load profile. On a machine with plenty of headroom the same process takes much longer. Replacing just the valve without changing the oil guarantees the new valve fails on the same schedule. A lot of shops make this mistake because the valve is cheap and the oil change isn't, and the service writer doesn't want to quote both on the same invoice. The callback two months later costs more than the oil change would have.

The smart shops replace the thermal valve on a schedule somewhere between 8,000 and 16,000 hours, depending on what the oil looks like. On mineral oil, closer to 8,000. On clean synthetic, you can push it. The valve itself costs $30 to $60 depending on the manufacturer. The labor to pull it depends entirely on the machine. On Kaeser, it's nothing. On an older IR SSR it might be a couple hours of disassembly. Either way, the cost of scheduled replacement is less than one diagnostic visit to chase a stuck valve you didn't know was stuck.

The rate of varnish formation depends on how close the machine runs to its thermal ceiling. A compressor that's five degrees under its shutdown setpoint has almost no margin for any degradation in the oil, the cooler, or the room before it trips. A machine running thirty degrees under the setpoint can tolerate some oil degradation before it becomes a problem. Knowing where your machine sits in that range tells you how urgently the oil condition matters. Most operators don't know because they've never compared the current discharge temperature to the shutdown threshold. The controller displays it. Nobody looks.

How fast the oil goes bad depends on the base stock. A good synthetic goes 8,000 hours and comes out looking like dark honey. Mineral oil in the same machine will be black at 2,000. The per-gallon price difference is real but the synthetic lasts three to four times longer and doesn't varnish the internals. On any machine with chronic thermal issues, synthetic is the only sensible option.

The argument against synthetic usually comes from procurement, not from the people who actually work on the machines. Purchasing sees $40/gallon versus $12 and questions the spend. The people changing the oil know the $40 product saves thousands in avoided thermal valve replacements and cooler flushing over its life. This is one of those arguments that gets relitigated every year when the purchase orders come through. Having the oil analysis reports that show clean internals at 6,000 hours on synthetic makes the case better than any cost spreadsheet. The analysis report is the hard evidence. Without it, the conversation is just opinions about oil pricing, and procurement wins that argument every time because they have the numbers and maintenance doesn't.

A lot of plants run 125 PSIG because that's what the setpoint was when the machine was commissioned and nobody has revisited it. Nothing in the plant needs 125. The farthest endpoint needs maybe 85 PSIG and there's 30 PSI of pressure drop in undersized piping and leaking fittings between the compressor and that endpoint. Fixing the piping and the leaks and dropping the setpoint to 100 PSIG reduces the thermal load on the cooling system and extends oil life at the same time. But that's a compressed air system project, not a compressor repair, and it requires someone to map the distribution system and measure the pressure drops, which doesn't happen unless somebody asks for it specifically. The DOE's Compressed Air Challenge program publishes a system assessment guide that walks through this process. Most plants that go through it find enough inefficiency to pay for the assessment several times over, but getting someone to do the initial assessment is the bottleneck.

Emulsified oil loses a lot of its thermal conductivity. The milky appearance from water contamination is easy to spot in a sample. How much thermal capacity is lost depends on the concentration, but even moderate emulsification noticeably affects heat transfer. On overheating calls this gets missed because the work order says "high temperature" and the tech goes straight for the cooler and fan. The milky appearance is obvious if you look at a sample.

Where the water comes from: compressors that short-cycle without reaching full operating temperature accumulate moisture. The typical pattern is a machine that runs fine during the production week on long loaded cycles, or at least seems fine. Saturday the plant runs light, compressor short-cycles all day, never gets hot enough to cook moisture out. Monday morning the sump is milky and the machine runs hot from the first loaded cycle. People assume something broke over the weekend. Nothing broke. The duty cycle changed.

Cold-weather startup compounds the moisture problem. A machine in an unheated building sits over a weekend with its oil at 25°F. Monday morning the cold oil moves sluggishly. Moisture condenses inside the cooler tubes and mixes into the oil before the machine even reaches loaded operation. The compressor starts the week with a thermal handicap that doesn't clear on its own, and if the cycle repeats through the winter without anyone catching it, the oil degradation accumulates. A $100 sump heater prevents this entirely. Atlas Copco and Kaeser both list sump heaters as a standard accessory in their cold-weather installation guides (Atlas Copco reference document 2935 0119 01 covers cold climate recommendations for the GA series). For some reason a lot of northern installations don't have them. The price is trivial compared to an oil change, let alone the damage from running emulsified oil for weeks before someone notices.

The fix for emulsified oil is drain, flush, refill, and run at full temperature long enough to cook residual moisture out of all the passages and dead spots in the circuit. Takes most of a shift on a larger machine, maybe a couple hours on a smaller one. Machines that short-cycle by design, such as backup units or trim machines, need a weekly full-temperature run or a coalescing separator on the oil return. Otherwise it comes back every time the machine sits idle in humid weather or cold weather, and you're doing the same oil change every few months. The cost of a coalescing separator is probably less than one unplanned oil change when you factor in the labor.

Mixed oils in the same sump cause expensive misdiagnosis because the evidence hides inside the cooler. PAG and PAO in the same system can react and form a gel that coats cooler tubes from the inside. External fins look spotless. Machine overheats. Fresh oil dilutes the gel temporarily and the temperature usually drops for a couple weeks. Then it reforms and the temperature comes back. By this point multiple service calls have been billed and the customer is frustrated and starting to question whether they need a new compressor.

There's no way to find the gel without pulling the cooler apart or cutting the oil filter open. Once you've seen it you learn to ask about oil history early. Has anyone changed the brand in the last year or two? Did they flush the system? The answer is usually "we switched to a cheaper oil" or "I don't know." Either answer points at incompatibility. It also catches plants where a well-meaning maintenance worker bought "compatible" oil from the local distributor based on the viscosity grade number without checking the base stock chemistry. Same viscosity, completely different chemistry, gel deposits within a few hundred hours.

For plants running multiple brands on the same floor, separate labeled drums for each machine. Otherwise the second-shift operator grabs whatever's closest. I've seen plants solve this by putting the drums on different sides of the building and color-coding the fill caps. Whatever works. The goal is to make it harder to use the wrong oil than to use the right one.

Oil analysis every thousand to two thousand hours catches most of this before the temperature gauge shows anything. Three or four samples over time tell you what's degrading and how fast. The value is in the trend line, not any individual sample. Viscosity trending downward across consecutive samples says the oil is thinning from thermal stress. The wear metals panel shows which components are taking damage, and combined with the acid number you get a picture of the whole oil circuit in one report that you can't get from a temperature reading or a visual inspection.

Having the reports on file also changes the conversation when something does fail. Clean oil analysis at 4,000 hours followed by a thermal valve failure at 4,500 points to a parts failure and potentially a warranty issue. Degraded oil at 3,000 hours that nobody changed, followed by a thermal valve failure at 4,500, points to a maintenance failure. The lab reports establish the timeline.

A lab charges $20 to $40 per sample. What kills most programs isn't the cost, it's the discipline of pulling consistent samples on schedule. A tech who pulls one sample during operation and the next one twenty minutes after shutdown gets viscosity readings that aren't comparable, and the trend line is garbage. Some shops standardize on pulling the sample within five minutes of shutdown, which is close enough to operating temperature to be consistent. Some service contracts include sampling, which costs more monthly but solves the consistency problem. Polaris Laboratories and Blackstone Laboratories both do standard compressor fluid panels with turnaround inside a week. Kaeser offers their own Fluid Analysis Service through their distributors.

Restricted oil flow from line blockages, sludge, kinked hoses, or a pump starting to cavitate produces the same temperature rise as a low sump. Oil flow rate doesn't appear on any standard control panel. Temperature is the only indicator and it looks identical to five other causes. A 10% restriction builds so slowly that the temperature rise over the first couple months gets written off as ambient conditions or aging oil. By the time the restriction is severe enough to trip a shutdown, the tech is working backward from 230°F with no idea it started at 205°F three months ago. This is where temperature logging pays off. Somebody writing down the discharge temperature at the start of each shift would see the gradual rise as a clear trend, and the diagnosis would point toward flow restriction rather than all the other possibilities.

A filter in bypass sends unfiltered oil through the bearings, thermal valve, cooler tubes, and separator. By the time the machine runs hot, every component is borderline degraded but nothing is clearly failed. These are the frustrating calls where three or four parts get swapped before the temperature comes down, and nobody can say afterwards exactly what fixed it. The service bill on these is painful and the customer usually thinks they got overcharged because they can't see why five parts were necessary. From the tech's side, there wasn't a better way to do it. When contamination has degraded the whole circuit, you can't isolate a single failure.

On machines that have been in service for fifteen years with various repairs, the oil piping sometimes doesn't match the factory layout anymore either. A fitting rerouted during a previous repair can create a restriction. Comparing current routing to the service manual diagram is tedious but occasionally turns up something. On older machines a sticking check valve in the oil return can let hot compressed air back into the sump. Temperature rises and nothing in the oil circuit explains it. Rare, but memorable when you find it.

What to Check

Check the room first. Measure ambient at the height of the compressor's cooling air intake with a handheld thermometer. Anything more than about 15°F above outdoor ambient means a ventilation problem, and nothing done to the machine matters until the room is fixed. On a mid-range machine the cooling fan pulls over 7,000 CFM through the oil cooler (Atlas Copco lists exact airflow values per model in their GA technical specifications, and Kaeser publishes the same in their CSD/CSDX datasheets). That air has to come in and leave without recirculating.

Compressor rooms get worse over time. Storage in front of louvers. Dust in ductwork. A second machine crammed in. The approach temperature through the cooler, meaning the difference between air entering and oil leaving, is fixed by design. When outdoor ambient swings from 30°F in winter to 100°F in summer, discharge temperature follows that swing. A room that works in March can easily be fifteen degrees too hot in August. ASHRAE Handbook, HVAC Applications, Chapter 49 covers general ventilation for equipment rooms but doesn't give compressor-specific CFM values, so you end up cross-referencing the manufacturer's heat rejection spec with the room volume. Nobody does this calculation at most facilities. The compressor went into whatever room had space and power, and the ventilation is whatever was already there.

The seasonal aspect catches people because a compressor that ran fine all winter starts tripping in June and the immediate assumption is that something broke. Nothing broke. The ambient went up 40 degrees and the room's ventilation can't handle it. This is a room problem, not a machine problem, but try telling that to a plant manager who's losing production to shutdowns and wants the compressor fixed, not a lecture on building ventilation. Measuring the room temperature at intake height and showing the number is the fastest way to redirect the conversation.

A $200 exhaust fan solves most room problems. In hot climates the room may need an evaporative cooler on the intake, or a supply fan ducted from a cooler area of the building. The manufacturer's rated maximum ambient is usually 40°C to 46°C but the manual doesn't tell you how to achieve that in a given room. That engineering work falls on the facility, and it doesn't always get done. The service tech explains to the plant manager that the fix is a building modification, not a compressor repair. That conversation usually needs to happen two or three times before anyone acts on it. Having a thermometer reading from the room at compressor intake height makes the argument more convincing than trying to explain approach temperatures.

After the room, pull an oil sample. Look at color and clarity. Check level. Inspect the cooler by blowing from the inside face out, not the other way. Blowing from the outside pushes contamination deeper into the core, which makes it look cleaner without actually improving airflow. Internal fouling is harder. Comparing oil temperature at the inlet versus the outlet is the only way to assess it without pulling the cooler apart. On Kaeser plate-type coolers the passages are narrow and plug easily. Flushing with compatible solvent at major service intervals helps. Cooler cores with horizontal fins collect contamination on the upper surfaces where it cements into a hard cake. Vertical fins shed debris better under gravity. In dusty or fibrous environments the cooler may need external cleaning monthly regardless of orientation.

Fan belt. Stretches during break-in, probably loose by 500 hours. Enough airflow at idle, not at load. The overheating tracks the demand cycle and disappears whenever someone checks the machine during a quiet period. Ten minute fix with a wrench and a straightedge. Experienced techs check the belt first. Inexperienced ones check it last, after three hours of chasing other things. The difference in the customer's bill is substantial.

A three-phase motor that's lost a phase keeps spinning but slower. The fan turns, nobody suspects it, airflow drops by a third. Check per-phase amps against nameplate. On enclosed packages where the fan isn't visible without removing panels, run the machine open for a few minutes and watch. Listen for bearing noise in the motor while you're there.

Thermal valve, if you can get to it. Some shops just swap it on a schedule rather than pulling it to test. The cost of the valve is trivial compared to the diagnostic labor of pulling it and the risk of putting it back if it tests borderline.

Separator differential over the manufacturer's limit needs to be fixed before judging anything else. A loaded separator adds backpressure and pushes oil through the cooler faster than designed, cutting heat transfer per pass. Everything downstream tests worse than it is when the separator is loaded. Most Kaeser and Atlas Copco controllers display it. A lot of older Sullair and IR machines don't have the readout, and the element gets changed on running hours that may not match the actual loading rate. A machine on clean synthetic might get 8,000 hours from an element. Same element on degraded mineral oil can load in 3,000. Install a differential gauge if the machine doesn't have one.

Pressure. A machine set higher than anything in the plant needs should be brought down. Every extra PSI costs energy that becomes heat. For the one endpoint that needs higher pressure, a booster at the point of use costs less than running the whole system hot. Whether it actually gets reduced depends on whether someone can convince operations.

Intake filter in dusty environments like cotton mills, woodworking shops, and foundries. A loaded filter raises compression ratio. On enclosed packages where the same filter serves the cooling circuit, a clogged filter increases heat generation and reduces cooling airflow at the same time. Install a differential gauge on the filter housing. Costs next to nothing and takes the guesswork out of filter replacement timing.

When everything else checks out, the airend needs evaluation. Bearing clearances, rotor surfaces, vibration analysis. Internal clearances open over tens of thousands of hours. Compressed air leaks back across the rotors, gets recompressed, generates extra heat. Bearing wear lets the rotors shift. At several thousand RPM even light contact produces intense localized heat. Multiple symptoms usually appear together: rising temp, declining output, more oil consumption, louder operation.

A machine that is mechanically fine but runs full load all day is undersized. The plant outgrew it or leaks are eating capacity. Most older plants without a leak survey are losing more than anyone thinks. CAGI publishes data on typical compressed air system leakage rates, and the numbers are bad. A plant that hasn't been surveyed in a few years is typically losing 20% to 30%. The Compressed Air Challenge program, which is DOE-affiliated, puts the average even higher in plants with extensive pneumatic tool use. An ultrasonic survey finds leaks fast. The repairs are mostly fittings and hose replacements that any maintenance crew can do. In a plant that hasn't been surveyed, fixing the leaks sometimes recovers enough headroom to take the compressor back from chronic full-load operation to a duty cycle with thermal margin. The payback in energy savings alone usually comes within a few months, and the reduced thermal stress on the compressor is a bonus that doesn't get captured on any financial report but shows up as fewer service calls.

Some machines were undersized from day one. The original demand estimate used optimistic diversity factors or didn't account for future expansion. The compressor has run near 100% since commissioning and trips every summer. The techs keep the cooler clean and change the oil on schedule and the machine still overheats in July. There's nothing wrong with it. It just can't make enough air for the facility. Adding a second machine or replacing with a larger unit is the fix, and that's a capital expenditure conversation, not a maintenance conversation. Getting capital approved for compressed air equipment when the current machine "still works" is its own challenge. The machine does still work. It just works at 100% load all day and overheats because of it.

Costs

Airend rebuild kits run $8,000 to $12,000 in parts for a 100-horse machine. 20 to 40 hours shop labor, oil flush, maybe a cooler if it's contaminated with bearing debris. Seals rated for 10,000 hours at normal temp crack at 3,000 under chronic heat. Bearings rated for 40,000 hours fail at a fraction of that when the oil can't maintain film thickness. By the time the shop opens the airend, the scope usually expands beyond what was quoted because there's more damage inside than the symptoms suggested.

Energy waste doesn't show on work orders. Overheating compressor leaks air internally, runs longer. In manufacturing, compressed air is typically 20% to 30% of the electric bill. A few percent efficiency loss shows up monthly but nobody traces it to the compressor. Maintenance doesn't see it because it's not parts or labor. Finance sees it but has no way to connect it to a machine running fifteen degrees over spec.

The oil changes, belt adjustments, filter replacements, and cooler cleanings that prevent overheating cost a fraction of what the rebuild costs. An oil analysis program that catches degradation before it becomes damage costs even less than the preventive maintenance.

PM on a 100-horse machine, including analysis, oil, cooler cleaning, belts, filters, and valve swaps, runs maybe $3,000 to $5,000 per year. Single airend failure costs five to ten times that plus downtime. Math is obvious. Getting budget approved is always the harder part.