We are in the business of selling rotary screw compressors, and we sell quite a few to users that have outgrown their two-stage reciprocating/piston compressors in the 5 to 20 hp range. Sometimes demand or duty cycle has increased beyond the practical range of their piston compressors. Or they need higher air quality. Sometimes noise and vibration are the issue. But there are many cases where a reciprocating compressor is still a very good, economical fit for the shop, but service issues lead them to think they need a different solution.
Heat is often the enemy
Most small shop recips are not designed to handle 100% duty cycle. In other words, they cannot run flat out for long lengths of time without sustaining heat-related wear or damage. Generally, these small two-stage units operate at relatively high temperatures (275-350°F), so they need to stop and cool down periodically. (This is why they are typically set at 145-175 psig, even though most tools only need 90 psig.) Duty cycles vary — we’ve seen 50% to 80% –depending on the design and quality of construction.
There are a number of heat-related problems, but first let’s talk about what can cause them.
First, the compressor’s environment plays a critical role in its reliability. If the room is too hot, or doesn’t get enough ventilation it will run hotter than designed. To reduce noise, many recips are installed in out of the way locations (e.g. utility closets). Ventilation is often poor, creating more heat and higher discharge temperature.
Second, excessive run time can result in heat-related problems. There are several reasons for excessive run time, and a system can suffer from any or all of them:
- The compressor is undersized for the productive demand.
- More users or larger tools have been added to the demand.
- Leaks have developed (leaks in fittings, hoses and tools are just another type of air user—even if completely unproductive).
- Lack of storage in the tank due to water. The air leaving the compressor pump is hot and contains moisture in vapor state. In the tank, the air cools and moisture condenses into liquid. Condensate can build up quickly, especially in warmer, humid climates (gallons per day). If the tank is not routinely drained, it will fill with water leaving less room for air. Less air storage => more run time => more heat =>more problems.
Potential heat-related issues
Below are some of the mechanical issues caused by overheating. Generally, these can be repaired economically.
Piston rings no longer seal properly against cylinder walls, thus losing compression. When this happens the pump may have to run longer (and even hotter) to meet demand. Lubricating oil breaks down faster and gets past the rings more easily, requiring more make-up oil to prevent further mechanical issues (and degrading air quality).
FAILED INTAKE / EXHAUST VALVES
Oil carry-over builds up and may prevent valves from properly seating, creating blow-by through valves. This can cause the intercooler safety relief to release, and also cause the voltage supply breaker / fuses to trip due to stalling out the pump. Over time this can burn out the drive motor.
FAILED CHECK VALVES
Recips tanks have check valves to make sure they don’t start under load. Over time the elevated temperatures along with oil carry-over can distort the nylon piston in the valve, so the piston can’t seal properly. When this happens, back-pressure from the tank will create head pressure on the pump. When the compressor tries to start, the extra amps drawn by the motor can trip breakers and burn the motors out. If you have issues with belts breaking prematurely, it could be from a failed check valve on the tank. If the compressor pump is trying to start against head pressure, the crankshaft may not move even though the motor is. Motor goes, pump won’t > belts slip/wear/break.
MOTOR BURN OUT
Motors generate heat in normal operation but will cool themselves adequately unless they are in too hot an environment or are energized and try to turn something that doesn’t want to be turned, such as a pump with head pressure (see above) or one that is not properly lubricated. Over time, the insulation on motor windings will degrade and the motor will need to be rewound or replaced.
By design, reciprocating compressors vibrate. Vibration affect many things. Vibration can loosen piping connections, as well as any threaded nut or bolt. It can loosen up electrical connections and create electrical drop-out, sparking, tripping out breakers and blowing fuses. Pressure switches can also fail due to excessive vibration. Vibration can create cracks in welds and joints at the tank feet, platform and saddles. Excessive vibration also increases noise levels, loosens safety guards, and can even break up a concrete floor.
Replace missing or cracked vibration pads. While you are at it, if the discharge piping from the tank is hard pipe, swap it out for flexible steel braided hose. Same goes for electrical supply from the wall disconnect to the starters. Make sure the belt guard is secure. A missing or loose belt guard is not just a safety issue but is an OSHA violation.
Noise can often be abated with well-placed, insulated stud walls. The key is not to restrict airflow. If you are contemplating constructing a separate room to isolate a hot, noisy compressor, it is worth doing the math to see if a rotary compressor makes sense. The quieter rotary unit may cost less than permitted construction and almost certainly take less time to install. This assumes you have a good place with good ventilation and access, and that the unit will be run enough to gain some of the energy advantage.
A note about tanks
As explained above, storage is vital to the longevity of the compressor. It’s also important for meeting demand and system performance. Tanks don’t need much maintenance but you want to keep them dry. Not just for the storage, but to minimize rust. Over time, rust will build up in the tank and plug up the drain port. This makes the case for a quality automatic condensate drain that won’t get gunked up by the oil-water-rust mixture.
An ounce of prevention
The piston style compressor is simple and requires relatively little service, but it cannot be ignored. Here are some tips, whether you are installing new or want to keep ol’ faithful going:
- Ventilate the compressor room to maintain positive air flow. If the compressor is in a confined space, install louvers and thermostatically controlled fans as needed.
- Routinely drain the tank. Better yet, install an automatic drain (with test function)
- Check oil levels routinely. Add make up oil as needed and perform oil changes on schedule with an oil recommended by the manufacturer. Avoid automotive motor oils, which contain a lot of detergents that leave deposits.
- Replace the air inlet filter routinely. You may be able to vacuum it out to extend the replacement interval. Plugged air filters restrict the performance of the compressor and increase operating temperature.
- Find and fix leaks on the compressor and in the system all the way to the fittings, hoses and tools at point of use. Listen for leaks and hissing sounds while the unit is off. On the compressor, check the intercooler and its SRV, pressure switch, and the liquid drain on the tank (which some people leave cracked open to avoid liquid build up).
- The belt life on most recips can be very long if you take care of them, but excess heat will reduce belt life. Look for wear and cracks that might cause them to come apart. Damaged belts can create more vibration.
- Check duty cycle. If the unit is running more than it used to, you could be using more air or there could be water build-up in the tank and you have less storage. Another possibility is ring wear. A pump up test will tell you if the machine is still making air to specification and point you toward the cause.