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This article is reprinted from the March 1997 issue of
Plant Services, a Putman Publication, with permission from Gardner Denver Machinery Inc.A Putman Publication
Plant
Services THE BENCHMARK FOR SUCCESSFUL
PLANT MAINTENANCE & ENGINEERING
Reprinted From March 1997 Look beyond the sticker price when buying an air compressor It's the total cost of ownership that should guide you Bob Anderson, Engineering Manager, Rotary Products, Gardner Denver Machinery Inc., Quincy, Illinois When shopping for air compressors, it pays to look beyond the sticker price before you make your decision. The startling fact is that the initial price represents a mere 15 percent of compressor costs over a 5 to 7 year period. Savvy purchasing professionals know how to look under the hood, so to speak, when they make their choice. EnergyThe first place to kick the tires is in the area of energy efficiency. Buying an energy efficient compressor saves you tens of thousands of dollars in the long run. Compressors are not by their nature very energy efficient. In fact, it can take 7 to 8 horsepower of electric power to produce 1 horsepower of compressed air power. Energy costs for operating a compressor can exceed the initial cost of the compressor in the first year alone. So, if all you are comparing when choosing a compressor is the sticker price, you could end up paying a premium in the end. Look for a compressor that has the best energy saving features. The most efficient compressor on the market today is the double acting reciprocating compressor. However, it has been replaced in many plants by oil-flooded rotary screw compressors. Once the standard for plant air, the double acting reciprocating compressor is now used mostly for higher pressure or specialty applications. The demise of these compressors is due partly to high initial costs, complex installations, and the need for experienced maintenance crews. This leaves the majority of plant air being provided by rotary screw compressors. The best rotary machines have energy saving features built into their design. The most efficient rotary compressor is one that is fully loaded, providing the maximum flow-measured in cfm- per brake horsepower. When an application has long periods of little or no demand for compressed air you should have a compressor that can shut itself off. Or, at the very least, you can blow the oil reservoir down and operate at reduced horsepower usually between 18 and 22 percent of full load horsepower.
Efficient Operations When in this mode the compressor is either running full loaded, or, when system demand is satisfied, running completely unloaded and blown down. Of course, it's not wise having a compressor that is always unloaded. To avoid paying a premium for something that sits idle, ensure that your compressor is sized properly to match your needs-both present and in the near future. Most major reputable compressor companies have distributors that do a good job of sizing compressors. Although load-no load is touted as a high efficiency feature, there is a catch. Unless there is a sufficient amount of air receiver capacity-including the volume of air in the piping system-the feature can be energy inefficient. T here needs to be a minimum of 10 gallons of receiver capacity for every cubic foot per minute the compressor delivers. The larger the capacity of stored air, the longer the compressor can remain blown down and operating at a reduced horsepower.The startling fact is that the initial price represents a mere 15 percent of compressor costs over a 5 to 7 year period. Whenever a compressor’s system blows down, compressed air vents to atmosphere. In effect, you’re throwing money away by discharging air you paid money to compress. The key is ensuring the compressor stays blown down long enough for you to benefit from the lower horsepower consumption. If your system has long periods of unloaded operation, look for a compressor that also has a feature that turns the motor off after a preprogrammed time interval. This drops the horsepower to zero. If your system, has varying load cycles requiring different volumes at different times of the day and you do not have the required storage to run load-no load, you need to have a system that runs efficiently with partial loads. With partial loads the compressor produces compressed air at less than full capacity. The trick is to reduce compressor capacity while maintaining the minimum pressure required. Various methods of part load compressor controls achieve this end. One way of doing this with inlet valve modulation. This works by controlling the position of the inlet valve that opens and closes in response to system demand. Although extremely responsive, it is the least efficient way to run a compressor partially loaded. For example, running a compressor at 70 percent capacity still requires 91 to 93 percent of full load horsepower. The reason for this inefficiency is that, when at less than full capacity, a partially closed throttle restricts the opening and creates a vacuum at the inlet. This lowers the inlet pressure. At sea level, a partial vacuum of 4.7 pounds per square inch lowers the inlet pressure to 10 pounds per square in absolute from 14.7 pounds per square inch atmospheric. More work is now required to raise the inlet pressure to the final discharge pressure. Compressing this air to 100 psig-114.7 psia-requires a higher compression ratio-the ratio between the discharge and inlet pressures. The system had 114.47 divided by 14.7=7.8 compression ratio, and now at part load it is 114.7 divided by 10=11.5. Overall, less power is required to compress the smaller volume of air, but with the above inefficiencies, more energy must be expended per cubic foot of air. Alternate approaches Instead, the length of the rotor is shortened. At 70 percent capacity, a turn valve unit can achieve 78 percent brake horsepower-a 13 to 15 percent energy savings over inlet valve modulation. Other ways to control the capacity on positive displacement compressors is to control the speed-rpm-through a variable frequency drive on the motor or by driving a compressor with an engine. When buying an air cooled compressor, don’t forget to figure in the cost of the fan motor when calculating energy costs. A 100 horsepower compressor may use a 3 horsepower fan. Likewise, a 500 horsepower compressor uses a fan requiring 15 to 20 horsepower. High efficiency motors have a higher initial cost, but, over the long haul, save money. A standard motor has 91 to 92 efficiency rating. A high efficiency motor can operate at a high as 96 percent efficiency. However, keep in mind that efficiency vary with horsepower. For example, 50 horsepower high efficiency motors have ratings from 92.4 to 93 percent efficiency and 200 horsepower motors have efficiency ratings from 94.5 to 95 percent efficiency. Still, a savings of 4 to 5 percent is significant. Power CostsTo calculate the yearly power consists of a particular compressor, use the following formula:
Total BHP x 0.746x (Cost per kilowatt hour x hours of operation/motor efficiency)= annual power cost Compressor constructionThe very construction of a rotary screw air compressor is important when it comes to keeping energy costs lower. To get the utmost, state-of-the-art efficiency today, the rotors need to have an asymmetric profile. The asymmetrical rotor shape inherently provides a tighter seal between the grooves of the rotors to reduce slippage, eliminate vibration, and increase efficiency from 3 to 5 percent over standard rotor designs. Internal pressure drops For every 2 psi drop in pressure, you sacrifice 1 percent of brake horsepower. Pressure drops in a compressor occur in the aftercooler, the oil reservoir, air-oil separator, and also in the manifold piping.
Coolers If the oil temperature is too high, it preheats the incoming air. Operating a compressor with a discharge temperature of 190 degrees F versus 175 degrees F can cost 1 1/2 to 2 percent in overall efficiency, not to mention what happens to the oil. Oil loses half its life for every 10 degree rise in temperature. This means an 8,000 hour oil can become a 4,000 hour oil when the compress or discharge temperature reaches 190 to 200 degrees F. Oil life becomes 2,000 hours if the temperature climbs over 200 degrees F. This means your pay two to four times as much in lubricant costs when operating at increased temperatures. This equates to thousands of dollars over the life of the compressor. Keeping the oil cool and clean pays big dividends. Many people don’t think of cleaning their cooler, but by cleaning it with detergent and water, and blowing it out well, the discharge temperature could drop 20 degrees F. That doubles the life of your oil, not to mention the day-to-day efficiency of the operation. ViscositiesHigh oil temperatures mean lower oil viscosity. This results in more marginal lubrication for your bearings. The life of your bearings determines the life of your compressor. It’s best to have a regular oil analysis program as part of your maintenance program. Maintaining cool, clean oil pays dividends over the life of the compressor- much like the effectiveness of regular car oil changes. On the other side of the spectrum, oil that is too cool allows condensate to form in the system. Obviously, it is necessary to keep any water in a vapor state until it can be removed in the aftercooler-harmlessly. Discharge oil temperatures much below 170 degrees F may cause condensate to drop off in the oil. This increases the oil level in the system. It also causes harmful oil carryover. A thinner oil does not provide the lubrication necessary and internal parts can be damaged. Oils lose half their lives for every 10 degree rise in temperature. This means an 8000 hour oil can become a 4000 hour oil when the compress or discharge temperature reaches 190 to 200 degrees F. BearingsThe size and quality of the bearing can have an effect on the lift of a compressor. Most manufacturers choose a bearing with a L10 life-once known as B10 life-is the theoretical fatigue life of a bearing in which 10 percent of the population of bearings will fail under ideal conditions. Unfortunately, very few compressors operate under ideal conditions because of inconsistent or inadequate maintenance. The size of the bearing is determined by the size of the rotor. The larger the rotor, the larger the bearing. Also, as the rotor size increases, rotor speed decreases. A compressor running at a lower speed-1,800 rpm-tends to be more forgiving than one running at a high speed—3,600 rpm or higher. ContaminationIn the course of normal operation, it is not uncommon to have dirt or other contaminants enter the compressor. Dirt and debris cause more damage to a compressor with small bearings running at a higher speed, than to one with larger size bearings operating at a lower speed. Installations Installation costs are an issue when purchasing your compressor. Although rotary screw compressors need only a level floor beneath them, expenses can mount when connecting a compressor to your plant air system. Look for compressors that have piping and wiring connections near the edge of the frame. These are much easier and less expensive to install. It is also wise to consider the labor costs involved in working on compressors. Beware of compressors whose expendable parts are not easily accessible. Look for maintenance reducing design features like simplified access to an air-oil separator and filters, as well as spin-on filters. These design features can cut maintenance time by up to 90 percent. It’s best to have all electrical lines in conduit-tucked out of the way-where they won’t be damaged by liquids, oil spills, or chemicals found around compressors. These contaminants break down the wiring insulation, creating a safety hazard. Ensure that the manufacturer follows the National Electric Code closely. Price tagsCompressors with the lowest price tag can carry aftermarket parts with the highest price tags. When comparing compressors, look closely at the cost of aftermarket parts. Many compressor users face the dilemma of buying inexpensive replica or pirate parts for compressors or paying the premium for genuine compressor parts. The temptation can be strong for those focusing on short term expenses. Pirate parts are built to look and fit just like original parts, but that’s where the similarity ends. Economizing on air and oil filters, oil separators, and lubricants costs tens of thousands of dollars in damage to the compressor, oil carryover, and contaminated product. One of the biggest aftermarket expenses is compressor lubricants. Calculate the price of the lubricant with special attention to the lubricant change interval. Next, study the oil carryover level of each compressor. Find out where the oil carryover is measured. A compressor with an oil carryover of 2 to 3 parts per million (ppm) measured after the aftercooler, could have a carryover level of 10 ppm before the aftercooler. Whereas a compressor with an oil carryover of 2 ppm before the aftercooler has a 0.6ppm carryover after the aftercooler. You pay 4 times more in lubricant costs with the first compressor than with the second compressor. At prices over $50 a gallon, this is costly. Another expense comes in disposing of oil-contaminated condensate. Condensate cannot be put down the drain and must be disposed of properly. By making sure that the lubricant is fully demulsible, disposal costs are reduced drastically. Instead of paying to dispose of 50 gallons of oil-contaminated condensate, you dispose of a half pint of lubricant when you use an oil-water separator. Using a full demulsible lubricant with an effective oil-water separator saves thousands of dollars a year in condensate disposal costs. There needs to be a minimum of 10 gallons of receiver capacity for every cubic foot per minute the compressor delivers. MaintenanceWhen it comes to maintaining a compressor, make sure that you have a consistent maintenance program. If you do not have the personnel to perform the maintenance, obtain maintenance agreements through a distributor that sells the compressor. Nothing has a greater impact on the life of a compressor than a quality maintenance program. Who’s responsible This article has covered what the compressor manufacturer can offer to reduce the operating costs but the responsibility for reducing expenses must be shared. The user plays a major role in keeping costs in line with a regularly scheduled maintenance program. A compressed air audit shows how wastes such as leaks and unregulated air are reduced. When comparing compressors, be careful to compare apples to apples. Look beyond the sticker price to the rated efficiencies, oil carryover levels, cost of aftermarket parts, ease of installation, ease of maintenance, and service support. The initial price is important, but the real value is what you get for that price in terms of energy efficiency, reliability, and maintenance costs. PLANT SERVICES Magazine |
