What To Consider When Purchasing a Rotary Air Compressor?
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What do you consider when purchasing a rotary air compressor? Let’s say you buy a Mercedes, BMW or a new Tesla (considered three of the best brands on the market today) and you drive it 95 mph and get a speeding ticket. You pull over to the side of the road and scrape the passenger side door against the side rail. You now have an expensive car with side damage and an expensive speeding ticket…What do you do? Blame the police officer for pulling you over and ruining your expensive auto? Why not blame the car? Or, better yet, why not blame Elon Musk for inventing the electric Tesla that can go from zero to sixty miles per hour in less than five seconds. I mean, let’s face it, the odometer says that the car can go 190 miles per hour and you just wanted to make sure that the car could actually obtain this speed! You were simply fact checking the odometer. Heck, you should be given a metal, not a ticket!
Well, that same sort of logic applies to the equipment that you buy for your facility. The OEM has certain specifications and recommendations and believe it or not, you should adhere to those guidelines like the posted speed limits on every roadway in America. If you chose not to adhere to those OEM guidelines, your compressed air equipment will not be reliable and not experience the stated longevity that you thought it would. In a nutshell, it may consume more electrical energy and may not produce the compressed air quality required for your daily production.
It’s also important to have the right size for your manufacturing needs. Having an oversized compressor is just as bad as having too small of a compressor. You don’t need to impresses your competitors or neighbors here. Maybe you don’t need a Mercedes Benz or a Tesla for your business. Perhaps a Honda Pilot or a Ford F150 would be a better fit. The important thing to consider before buying a rotary screw compressor is the CFM (cubic feet per minute). How much air do you really need to run all the machines at once. This is also true when you are considering what size refrigerated air dryer best compliments your rotary screw compressor. Same is true here. Having too big of a dryer is just as bad as having too little of a dryer!!
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THE AIR DRYER CFM AND THE COMPRESSOR CFM ARE ALMOST THE SAME.
FOR INSTANCE: THE NXB06 SERIES IS A 7.5HP ROTARY SCREW COMPRESSOR
THE 7.5HP ROTARY SCREW PRODUCES AROUND 28CFM
THE DRYER (BLUE MACHINE ON TOP OF THE TANK CAN HANDLE AROUND 30-35CFM)
We could talk about rotary screw horsepower and size all day long, but it really comes down to the CFM. Here are some important things you will need to consider when installing your brand new Curtis NXB rotary screw air compressor:
Electrical
1) Have your system voltage measured. The most common electrical utility voltages in the United States are 200 volt, 208 volt, 230 volt, 460 volt.
2) Electrical utilities guarantee a voltage supply tolerance of + 10%. For example, if the voltage stated supply is 230 volts it could measure at any given time from 207 up to 243 volts.
3) Most electrical utilities attempt to hold the supply voltage to + 5%. Operating electrical motors off design or away from their nameplate voltage will generate additional heat leading to reduced motor life expectancy.
4) The majority of induction motors, 5 hp through 600 hp supplied to the U.S. market, are 230 & 460 volts; however, 200 & 208 volt systems are becoming more popular. If a 230 volt name plated motor is operated on a 200 to 208 volt system it will operate in an elevated heated condition leading to extremely reduced motor life. This is not a recommended practice. There are motors specifically designed for 200 & 208 volt operation. For these applications, motors name plated as such should be used.
2) Many air compressors operate into the motor service factor. As such, request the package full load operating amp draw from the manufacturer to properly size the electrical utility supply equipment (i.e., wire, electrical quick disconnect, overloads, transformers). Seek out a qualified electrical contractor in your area to assist in proper electrical utility modeling and sizing. I reiterate, find a GOOD electrician in your area to properly diagnose and install the correct equipment for your facility.
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Placement
1) Proper placement and mounting is vital. Request a General Arrangement (GA) drawing from the OEM for each piece of compressed air equipment purchased. The GA drawing should illustrate the weight and recommended clearances to be maintained around the equipment. The floor will need to support the static weight of each piece of equipment. The equipment should be secured to the floor to prevent it from shifting around imparting stress on connected utilities such as piping, electrical and ducting. In addition, many states have adopted seismic codes mandating equipment be structurally secured to the floor. Many floors appear flat to the eye but are not flat enough to meet accepted equipment mounting practices. Many floors can be crowned (bowed) or have undulations. When securing equipment to a floor it is important to identify floor imperfections filling the air gaps with shims. When properly shimmed, the tightening of the mounting bolts will not bend or distort the equipment frame. Frame distortion will lead to excessive equipment vibration. Excessive vibration causes premature component failure which reduces machine reliability, increases the cost of ownership and could potentially create an unsafe situation and void your warranty.
Proper clearances should be adhered to. These recommended clearances provide boundary area around the machine for safe, effective operation and servicing of the machine. This boundary area also allows adequate cooling airflow around the machine.
Piping
All piping should be properly supported as to not impart stress on the equipment. When connecting piping to the equipment pipe connector, it should be supported with proper tooling to prevent the equipment connector from turning during the piping tightening process. If the equipment receiving connector is allowed to rotate it very well could damage internal components to the equipment. Air piping should be properly sized and configured to prevent short cycling of the air compressor controls. A rule of thumb is to always use piping at least the size of the supply connection. If the dryer connections are larger don’t worry, the reason could be that the dryer is over sized due to rerating or simply the fittings are larger to slow down the compressed air velocity through the dryer. The piping exiting the dryer can be the same size as the air compressor connection or the outlet of the dryer, which can be used to establish the piping downstream of the dryer.
Ambient Air
Ambient air temperature plays a crucial role in not only human and animal performance, but on the life of your equipment. If it’s too hot or too cold, your equipment may not function properly.
If the ambient air condition drops below 32°F, measures should be taken to prevent the compressed air equipment from being exposed as it will freeze. During air compressor operation the most effective way to condition the room temperature is to use the heat generated by the operating air compressor(s). Excessive heat can be discharged from the room by way of a properly sized wall fan. The wall fan can be thermostatically controlled to keep the room at a constant temperature. If there are times when all of the compressed air equipment is not operating, an auxiliary heating system should be considered.
Air-cooled rotary screw air compressors give off 2545 BTUs/hr per horsepower.
Example: A 50 horsepower air-cooled rotary screw air compressor discharges 127,250 btus/hr into its surrounding environment.
That is enough heat to keep a 1500 square foot home comfortable in the dead of winter with outdoor temperatures below 32°F. Many of these air compressors are installed with little thought of eliminating heat build-up around the air compressor. The heat discharged into the ambient air surrounding the air compressor is continuously cycled through the heat exchangers elevating the temperature. Unchecked, the temperature will rise to an unacceptable level, eventually overheating the air compressor & shutting it down. Ducting can be attached to the air compressor to directly route the cooling air discharge outdoors or away from the rotary air compressor. Wall or ceiling fans can also be strategically placed in the room to exhaust the hot cooling air outside. Either way, an adequate amount of replacement cooling air must be brought into the room.
All installations have contaminants in the ambient air. Contaminants can negatively impact an air compressor in many ways, two of which are indigestion into the air compressor itself and or fouling of the air-cooled heat exchangers. The best way to determine if the air compressor is digesting an abnormally high amount of contaminants is to frequently perform oil analysis. The oil analysis should find components not normally part of the oil make-up. Important to note: religiously blow out the radiator and clean the filters once a day or at least once a week!!
In the case of a lubricated rotary air compressor, all of the ambient air drawn into the machine for compression flows right past or through the airend bearings. Too much ingestion of particulate will damage the bearings. The oil not only cools the air during compression, but it also washes the contaminants out of the air. Over time these contaminants will build up continuing to be cycled through the bearings.
This doesn’t account for multiple particles clumping together, which will damage the bearing surfaces. Two solutions for reducing digestion of contaminants are to install an air filter with a lower micron rating and higher efficiency rating. The ratings should be adjusted until the oil analysis indicates the level of contaminants is satisfactory. Remember, as the rating of the inlet air filter is increased the surface area should also be increased or the air filter may have to be changed out too frequently from excessive fouling. The second solution is to relocate the inlet air filter to a remote environment, which is cleaner than the immediate area around the air compressor. The piping used to remotely mount the air filter should be non-ferrous. PVC works great as it is light, easy to assemble and inexpensive. Connect it to the air compressor airend with an easily removable joint in case machine repairs or troubleshooting is required. The piping i.d. should be no smaller than the i.d. opening in the OEM supplied inlet filter mounting assembly. For every 10 feet of inlet pipe run, the pipe diameter should increase one size or 1″. If the inlet air filter is relocated outdoors it should be protected from digestion of rain or snow.
Relocation of the inlet air filter is a recommended solution when the surrounding area around the air compressor contains hazardous vapors or fumes. There are air filters, which can remove hazardous fumes or vapors; however, they are costly to purchase and maintain.
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