Rotary Screw Compressor Tips
The oil flooded rotary screw compressor is typically very dependable and relatively inexpensive to purchase and maintain. These are a few reasons why they have become so popular.
The following are a few tips that can help you get the most out of your oil flooded rotary screw compressor.
Liquid contamination can be devastating to the oil flooded rotary screw. Consider that less than 1% of water in the lubrication can reduce bearing life by as much as 40%.
It is possible for condensation to form inside the compressor. This can be prevented if you keep the operating temperature of the compressor above the pressure dew point.
Another area of concern is the discharge air piping of an oil flooded rotary screw compressor. The piping can act as an aftercooler causing condensation to develop in the piping.
It is essential to take precautionary steps during installation to keep the condensation from running back into the compressor. The following are some tips for installation:
a) Always install an air separator or a drain leg immediately between the compressor and the discharge piping.
b) Always install an automatic condensation drain on the separator or drain leg to make sure the liquid is removed from the system. The drain must be reliable and it is a good idea to pick one with a clear bowl for easy inspection.
c) Always slope the piping away from the compressor.
Air Oil Separator Overview
As the air and lubricant mixture is discharged from the air end of the rotary screw, it is directed into the separator tank. This mixture is directed through baffles that are positioned to cause the lubricant to have an inertial action from continuous change of direction.
The lubricant, having more weight than air, begins to form droplets that fall to the bottom of the separator tank or oil sump. The air, still containing a mist of lubricant, enters a separator element.
As the air and lubricant mist enters the first stage of the separator element, the mist of lubricant coalesces on the fibers and forms droplets, which fall to the sump of the separator.
The air and lubricant mist then flows through the second stage of the separator element where more lubricant droplets are formed and fall inside the element. Then, the lubricant is directed back to the compressor inlet by use of a scavenge or lubricant return line from the bottom of the separator element.
Finally, the separated compressed air leaves the separator tank through a terminal check valve.
Oil carryover refers to the oil that makes it past the separator filter and into the discharge piping.
Excessive carryover can get expensive when you consider the high cost of lubrication. Also, the oil will contribute to problems in the air delivery system.
The oil carryover will mix with any dirt, rust and water from condensation that is present in the piping system. This mixture tends to build up a sludge that will ultimately jam or clog production equipment, air tools and drains.
The separator filter is designed to pull the oil from the compressed air and return it to the lubrication system. The air that leaves the separator tank, under normal conditions, will have approximately 2 to 3 ppm of lubricant carryover.
An increase in oil carryover might be an indication that it is time to replace the separator filter. However, there are other factors, such as the following, that can cause oil carryover.
a) The scavenging line or scavenging line filter may be plugged with contamination.
b) The scavenging line may not be correctly installed in the separator filter. It should reach the bottom of the separator element and the end should be cut to a 45 degree angle.
c) Excessive oil carryover can also be caused by simply overfilling the oil sump.
Some air systems have both reciprocating and rotary screw compressors. The pulsations generated by the reciprocating compressors can impact the service life of the rotary screw compressors.
If a rotary screw compressor is connected to the discharge air lines of a reciprocating compressor, the pulsations through the piping can cause damage to many of the components of a rotary screw compressor. This includes the separator filter, gauges, compressor controls and check valves.
The solution is to install an air receiver tank between the reciprocating compressor and the rotary screw compressor. Also, if rotary screw and reciprocating compressors are operated together, it is always best to establish the rotary screw as the base load machine. This is the most energy efficient way to operate both types in the same system.
Hi Temp Damage
We often hear the oil flooded rotary screw referred to as a “water cooled” or “air cooled” compressor. The correct description would be “oil cooled” because it is the lubricant that removes the heat of compression.
The heat of compression refers to the heat released when the air is compressed. As an example, a single 100 horsepower rotary screw compressor will typically generate from 250,000 to 400,000 BTUs per hour of running time.
Oil flooded rotary screw compressors typically operate in the range of 160 to 180 degrees F. Shut down settings vary, but most will not shut down due to high temperatures until reaching 230 degrees F.
The damage from high temperatures can begin, long before the compressor shuts down. The following are a few examples.
a) Excessive heat kills most compressor oils. It is a well accepted rule of thumb that the life expectancy of oil is reduced by half for every 20 degree rise above normal operating temperature.
Long oil change intervals must be monitored with frequent lube analysis when operating the compressor in an environment with high ambient temperatures. This is your protection against the damaging effects of oil breakdown.
b) Air compressors operating in elevated temperatures will pass more water into your compressed air system than compressors operating at normal temperatures.
c) Operating at elevated temperatures will decrease the efficiency of the air compressor. The airend is slowly opening its clearances and accelerating wear which will reduce the service life of the compressor.
Things To Check
A high temperature problem indicates that you are having trouble cooling the oil because the heat is in the oil that was injected into the compressor. Here are a few things to consider.
a) The compressor needs to have the correct amount of lubricant. The compressor oil serves as coolant in oil flooded rotary compressors. In most modern rotary screws, the compressor oil is circulated 7 or more times per minute through the machine. This makes the oil level a critical issue during high ambient temperature conditions.
b) A thermal bypass valve is used to regulate the oil flow to, and around the cooler. Increased operating temperatures could be caused by a malfunction in the thermostat element of the bypass valve.
c) The heat generated by the process of compressing air is related to the ratios required to achieve compression. A compressor with a properly sized and clean inlet air filter will have a ratio of 7 when compressing 14.7 psi ambient air to 100 psi. A pressure drop in the air that enters the compressor will increase the compression ratio. This will generate more heat and use more energy. A dirty or obstructed inlet air filter can cause the pressure drop. And, violating rules for inlet pipe size and length can also create a pressure drop.
d) It is the job of the air or water cooler to remove heat. However, the coolers found on most rotary screw compressors have very little excess capacity to handle increases in oil temperatures.
This makes it extremely important to keep the inside and outside of the coolers clean. You might consider installing a larger cooler if the problem can be isolated to the cooler during the times of high ambient temperature.
e) There are some factors unique to the oil flooded rotary screw compressors that have air coolers. An air cooler will use the air in the immediate environment to cool the compressor oil. It must be designed and installed with the proper ventilation and air flow.
Most rotary screw compressors with air coolers will operate 100 to 110 degrees F above the ambient temperature. If the temperature near the compressor is over 100 degrees F (for example, in a small compressor room during the summer), the compressor oil will be in trouble.
An understanding of the oil flooded rotary screw compressor will help you extend the service life of this equipment. It will also prevent you from being at the mercy of a compressor dealer for advice and service.
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