Look beyond the sticker price when buying an air compressor

It’s the total cost of ownership that should guide you.

By Bob Anderson

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% 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.

Energy

The 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% of full load horsepower.

Efficient operations

One way to run a compressor efficiently can be to operate the compressor in a load-no load mode. 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, this feature can be energy inefficient.

There 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.

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 is 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% capacity still requires 91 to 93% 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

A better way to part load a compressor is by controlling the rotor length. A turn-valve or poppet-style valve unit opens windows up as demand decreases. The inlet valve remains fully open so the compression ratio is not affected.

Instead, the length of the rotor is shortened. At 70% capacity, a turn valve unit can achieve 78% brake horsepower, a 13 to 15% 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 a 91 to 92% efficiency rating. A high efficiency motor can operate at as high as 96% efficiency. However, keep in mind that efficiency rating vary with horsepower. For example, 50 horsepower high efficiency motors have ratings from 92.4 to 93% efficiency and 200 horsepower motors have efficiency ratings from 94.5 to 95% efficiency. Still, a savings of 4 to 5% is significant.

Power costs

To calculate the yearly power consists of a particular compressor, use the following formula:

Compressor construction

The 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% over standard rotor designs.

Internal pressure drops

Most compressor operators understand pressure drops in a plant air system. But, not everyone pays attention to the pressure drops within a compressor package itself. Pressure drops mean more than just a loss of pressure, it also means a loss of dollars.

For every 2 psi drop in pressure, you sacrifice 1% 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

The size of a compressor's cooler also plays a role in reducing costs. Controlling oil temperature in an oil-flooded rotary screw compressor is a real balancing act. If the oil temperature is too high, it preheats the incoming air. Operating a compressor with a discharge temperature of 190 degrees Fahrenheit versus 175 degrees Fahrenheit can cost 1-1/2 to 2% 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 compressor discharge temperature reaches 190 to 200 degrees Fahrenheit. Oil life becomes 2,000 hours if the temperature climbs over 200 degrees Fahrenheit. This means you 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 Fahrenheit. That doubles the life of your oil, not to mention the day-to-day efficiency of operation.

Viscosities

High 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 a 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 Fahrenheit 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.

Bearings

The size and quality of the bearing can have an effect on the life of a compressor. Most manufacturers choose a bearing with a L10 life of 100,000 hours. L10 life, once known as B10 life, is the theoretical fatigue life of a bearing in which 10% of the population of bearings will fail under ideal conditions. Unfortunately, very few compressors operate under ideal conditions because of inconsistent or inadequate maintenance.

Experience has shown that a compressor with an L10 life of 100,000 hours needs to be overhauled every 30,000 to 32,000 hours, 4 years of round the clock service.

The size of the bearing is determined by the size of the rotor. The larger the rotor, the larger the bearing. Also, as 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 more.

Contamination

In 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.

Also there is a direct relationship between noise and higher speed compressors. Noisy compressors require either sound enclosures, a separate room, or both. These are added expenses.

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%.

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 tags

Compressors 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 replicator 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.

The inherent inefficiency of inlet valve modulation is seen in the shape of the above curve. As capacity decreases, power requirements drop only slightly. 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 0.6 ppm 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 fully demulsible lubricant with an effective oil-water separator saves thousands of dollars a year in condensate disposal costs.

Maintenance

When 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 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.

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