Dollars & Sens

Air ends and rotor diameter determine bearing size. Large air ends allow for the use of larger, more durable bearings, as much as 170% larger than bearings on high speed air ends.

Why Should Air End Size Be A Concern?

Minaturization has many advantages in automobiles, computers and other products. Just the opposite is true in stationary rotar screw compressors. Bigger is better! In fact, the only advantage in downsizing a rotary compressor air end (the machinery that compresses the air) is reducing the manufacturing cost.

Durability and Efficiency of a compressor package are largely a function of the size of the compressor air end. Although smaller air ends can be manufactured at a lower cost, over the life of a compressor package, larger air ends are more cost-effective. The greater operating efficiency of a larger air end reduces energy costs year after year, while the added durability reduces troublesome maintenance and the costs associated with downtime.

How Does Size Affect Durability?

In rotary screw air ends, two intermeshing rotors compress an air/oil mixture. The volume per minute of air delivered by an air end is determined by the length, diameter and speed of the rotors. For a small air end to deliver the same volume as a larger air end, rotor speed must be increased.

Companies that downsize their air ends user gears to increase the speed of rotors to meet volume requirements. The gears on high speed air ends will inevitably wear out and require replacement. Gardner Denver designs larger air ends for its 40 through 150 HP Electra-Saver II and 100 through 150 HP Electra-Saver compressors so they can be direct driven at the drive motor speed (1780 RPM). These larger, slower running Gardner Denver air ends have no gears.

How Does Size Affect Durability?

In addition to being more durable, larger air ends that have slower rotor speeds also deliver compressed air more efficiently than smaller air ends. On an air end, the clearnances between the rotors can be thought of as leakage areas. A smaller rotor set has a higher leakage area per unit displacement than a larger rotor set (displacement is measured in cubic feet of air per revolution). In addition, the oil that is used to lubricate the rotors creates "drag." Efficiency losses due to drag are increased when rotor speed is increased. Gear driven units suffer further efficiency losses as a result of gear friction. The bottom line is that the inefficiencies of smaller air ends show up as increased power useage.

The most important compressor efficiency measure to consider is the volume of compressed air delivered each minute and the corresponding brake horsepower required to produce that volume.The Gardner Denver compressor, with it's larger air end, uses less brake horsepower per volume, and outperforms outperforms other brands.

The superior efficiency of larger, slow speed air ends most often results in substantial savings in power bills each year. When compared to smaller, high speed air end designs, slow speed Gardner Denver compressors deliver the energy savings that make them the best value on the market.

The Gardner Denver Advantage

The air end is the heart and soul of your compressor package. It is the most expensive component to replace and determines the basic operating costs. Competitors will claim their compressors are durable and efficient because they know you value these product attributes. Gardner Denver does more than just claim in their literature that their compressors are durable and efficient. When considering compressors, compare the air ends. If you want efficiency and durability, the size and speed are your primary concerns.

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Buyer's Guide

Why It Pays to Look Under the Sticker When
Buying an Air Compressor


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.



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 best rotary machines have energy savings 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 resevoir down and operate at a reduced horsepower usually between 18 and 22 percent 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 need. 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.

When 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 effeciently 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 acheive 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. This method is an extremely responsive way to run a compressor partially loaded.

A better way to part load a compressor is by controlling the rotor length. A turnvalve 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 percent capacity, a turn valve unit can achieve 78 percent brake horsepower. A 13 to 15 energy savings over inlet valve modulation.

High efficiency motors have a higher initial cost, but, over the long haul, save money. A standard motor has a 91 to 92 percent efficiency rating. A high efficiency motor can operate as high as 96 percent efficiency. Over the run a savings of 4 to 5 percent is significant.

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 assymmetrical 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.


For more information please contact our extremely knowledgeable sales stafff at Ontario Compressor Supplies. They can answer all your questions and give you the help you need to make a smart investment.

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