Look beyond the sticker price when buying an air compressor
It’s the total cost of ownership that should guide you.
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.
Smart & savvy purchasing professionals know how to look under the hood, so to speak, when they make their choice.
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 hp. of electric power to produce 1 hp. of compressed air power. Energy costs for operating a compressor can exceed the initial cost of the compressor in the first year alone.
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.
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 compressed.
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 are used.
Inlet valve modulation, 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.
Another 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 percent 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 electric motor.
When buying an air cooled compressor, don't forget to figure in the cost of the fan motor when calculating energy costs. A 100 hp. compressor may use a 3 hp. fan. Likewise, a 500 hp. compressor uses a fan requiring 15 to 20 hp.
High efficiency motors have a higher initial cost, but, over the long run, 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, the efficiency rating vary with horsepower.
For example, 50 hp. high efficiency motors have ratings from 92.4 to 93% efficiency and 200 hp. motors have efficiency ratings from 94.5 to 95% efficiency.
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° F versus 175° F can cost 1-1/2 to 2% in overall efficiency, not to mention what happens to the oil.
Keeping the oil cooled 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° F. That doubles the life of your oil, not to mention the day-to-day efficiency of operation.
High oil temperatures mean lower oil viscosity. This results in reduced lubrication for your bearings. The life of your bearings determines the life of your compressor.
On the other hand, oil that is too cool allows condensate to form in the system. It is important & necessary to keep any water in a vapour state until it can be removed in the aftercooler--harmlessly.
Discharge oil temperatures much below 170° F may cause condensate to drop off in the oil. This increases the oil level in the system. It also causes the increase oil carryover. A thinner oil does not provide effective lubrication and internal parts can be damaged.
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 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.
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--1,500 rpm--tends to be more forgiving than one running at a high speed--3,000 rpm or more.
In the course of daily operation, it is common 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.
Low speed[rpm]=Low Noise; High speed[rpm]=Loud Noise.
Noisy compressors require either sound enclosures, a separate room, or both.
Installation costs may need to consider 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 labour costs involved in working on compressors. Beware of compressors whose consumable 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.
Price of consumable parts
Compressors with the lowest price tag may carry aftermarket - consumable parts with the highest price tags. When comparing compressors, look closely at the cost of aftermarket - consumable parts.
When it comes to maintaining a compressor, make sure that you have a consistent maintenance program. Including regular briefing & training of your technicians.
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.
Finally, in conclusion
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 - consumable parts, ease of installation, ease of maintenance, easy availability of aftermarket parts 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.