Oilfield Air Compressor Types Demystified

Not sure whether you want a screw or a scroll? When do you want a recip and when should you go with a continuous duty cycle compressor? There are a lot of options available for compressor technologies in the oilfield and they all have their unique advantages and challenges. We&#;ve put together a bit of a summary below (and of course there are exceptions to every rule). Feel free to reach out to if you have more questions!

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Screw Compressors:

Pro&#;s and Con&#;s

Pros:

&#; Reliable

&#; Quiet

&#; Simple maintenance

&#; Simple design with few moving parts

Cons:

&#; Expensive

&#; Oil injected options require oil filtration

&#; Inefficient/Ineffective in on/off operation

Screw Compressors are a reliable, long-lasting and low maintenance compressor typically found in higher flow rate and horsepower applications (10+, although smaller sizes do exist). The screw compressor works by spinning two interlocking screws together. The screws rotate in opposite directions, pulling air from the intake side where there is a relatively large gap between the lobes. As the screws spin together, the gap between the lobes narrows, compressing the gas and pushing it to the discharge nozzle located on the opposite side of the intake.

Screw compressors work best in a 100% duty cycle (continuous operation) system with steady loads. Recycle or slide valves allow air to be recirculated around the compressor but decrease compressor efficiency. In most applications you will want to look at an aftercooler to reduce the air temperature before drying the air. You can split the screw compressor range into two broad categories, oil injected screws and oilless screws.

The real advantage of oilless screws are places where you absolutely can&#;t have any oil in the downstream compressed air, such as medical grade air or the food and beverage industry. These compressors have better short-term maintenance (no oil changes), however they typically wear out faster than oil injected screw compressors due to increased friction and heat generation. They also tend to be less efficient, suffering from internal air leakage &#; high pressure air recirculating back to the low-pressure side due to the inability to seal between the edge of the screw and the casing.

Oil injected screw compressors tend to be more common in the oilfield and, although they have a shorter maintenance interval, the lifecycle cost and reliability of a oil injected screw compressor makes it a clear win for most applications. The oil forms a thin film around the outside of the screw, minimizing the amount of air leakage and increasing compressor efficiency. It also allows for additional cooling of the compressor. Modern filtration and oil capture technologies make removing oil from the air stream, for the purposes of oilfield use, simple and easy.

Tips for Screw Compressors

• Excellent option for those high horsepower, high air flow applications.
• From a maintenance and lifecycle cost basis, we recommend going with an oil injected screw compressor. Oil-less compressors are nice in the short term, but, like all rotating equipment, lubrication will increase the lifespan and decrease overall maintenance.
• Look into aftercoolers and intercoolers to keep your air charge temperature down.
• The greatest wear and losses in efficiency tend to be when starting and stopping. Try to minimize start/stop operation.

Reciprocating Compressors:

Pro&#;s and Con&#;s

Pros:

&#; Cost Effective

&#; Reliable

&#; On/Off Duty Cycle

&#; Simple maintenance

Cons:

&#; Loud

&#; More frequent oil changes than screws or scrolls

&#; Requires filtration for oil carryover

Probably the most common type of oilfield air compressor out there, the reciprocating air compressor operates in a similar fashion to its much larger natural gas compression brethren. A piston (or more) attached to a crank shaft cycles up and down in a cylinder. As the piston moves down a check valve lets air flow into the cylinder. As the piston comes back up it compresses the air through a second check valve.

The sweet spot for reciprocating air compressors (for oilfield air usage) tend to be 10 hp or less. They are a partial duty cycle compressor, meaning the compressor runs in an on/off fashion, compressing air typically into a large receiver and then turning off until that volume is depleted. This allows the air compressor to handle a large range of flows easily, and as maintenance is a function of run time, low duty cycle air compressors can go a long time between maintenance. In many cases this allows them to exceed their continuous duty cycle brethren (screws, scrolls and rotary vanes) for lifecycle costs.

Tips for Reciprocating Compressors

• Try to oversize the compressor a bit if you can. The larger the compressor, the less it runs, the longer the lifespan and maintenance interval.
• Fork out the money for a duplex unit. They tend not to cost that much more (especially in the 5 hp range due to the commonness of those units) and they can double your maintenance interval as well as adding redundancy.
• If you&#;re consistently requiring over 125 psig discharge consider a pressure lubricated over a splash lubricated unit.

Scroll Compressors

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Pro&#;s and Con&#;s:

Pros:

&#; Inexpensive initial cost

&#;Longer maintenance intervals then recips

&#; Low inrush current

&#; Quiet

Cons:

&#; Very expensive maintenance

&#; Often requires aftercooler

&#; Relatively low maximum pressures

&#; Low inrush current

The scroll compressor uses a rotating and a stationary scroll to progressively compress air from the larger volumes at the outside of the compressor to a smaller volume at the center. The fairly simple design with few moving parts makes the scroll compressor easy to manufacture and provides a fairly high volumetric efficiency. They also tend to be oil free, eliminating the need for oil changes (though you often pay in down the road for new tips seals).

Scroll compressors in the oil industry tend to fit well in the <10 hp range with reciprocating compressors. Because of the lack of oil and filters they can go a long time between maintenance cycles, however the tip seals are high wear areas and tend to be expensive to pull apart and rebuild. For this reason, most scroll compressors are considered &#;throw away&#; units once their major maintenance cycle comes up (typically around 10,000 hours of run time).

Tips for Scroll Compressors

• The air discharge from these can be hot. Make sure to cool it off before sending it to a dryer (aftercooler).
• Similar to a reciprocating compressor, look for the duplex units to take advantage of the time between maintenance intervals and have that redundancy.
• Really good fit for low hp applications (2 hp and less) or where you can&#;t have any chance of oil (not common in oilfield applications)

Rotary Vane Compressors

Pro&#;s and Con&#;s:

Pros:

&#; Compressor gets more efficient as it runs

&#; Quiet

&#; Works well for continuous high flow applications

Cons:

&#; High maintenance costs like scrolls

&#; Air leakage at higher pressures

&#; Not as efficient with varying loads

A rotary vane compressor consists of a rotor, a stator and blades. The slotted rotor is arranged eccentrically within the stator. The blades are located in slots in the rotor and are able to slide in and out. As the rotor spins, the blades make contact with the stator. Air enters at the more open side of the stator (opposite where the rotor sits). As the space between the blades decreases with the spinning of the rotor, the air is compressed.

Rotary vane compressors tend not to be overly common in the energy industry. Similar to screw compressors, there are lubricated and oil free compressors. The lubricated compressors tend to have a higher efficiency and longer lifespan however they require oil changes and require a coalescing filter to capture the oil on the discharge. Oil free versions reduce maintenance intervals in exchange for shorter lifespans.

Tips for Rotary Vane Compressors

• These compressors honestly aren&#;t very common in oilfield applications, however they are a good compressor and are very common in agriculture and other industries. Make sure you have good access to components and a service technician.
• Use in applications where you have fairly low pressure requirements but require high volumes.

In Summary:

If you&#;re high horsepower (>10 hp) or utilizing fairly high and consistent air flow volumes, screw is the way to go. While often too expensive in the lower horsepower ranges, they have great maintenance and reliability characteristics for the greater flow ranges. A great option for gas plants or batteries.

If you&#;re looking for reliability and longevity in the lower horsepower flow rate range, go reciprocating. While they will need more frequent maintenance than a scroll, the oil changes are far cheaper than the tip replacement on scrolls and the compressors will last longer and cost you less in the long run. They have an excellent price point too.

If you&#;re really remote and can&#;t get around to do an oil change in every 6-12 months, look into scroll compressors. Their life cycle costs can be high and they tend not to be as efficient, but they can go long periods of time without any maintenance.

There are many types of compressor options to choose from; it is crucial to consider the size you will need as the size will relate to what volumes of air you need for your operations. You should consider the pressure you need for your application and if there needs to be a storage tank (also known as an air receiver) to manage your demand for compressed air. When selecting the right screw compressor, you also need to consider if you need a fixed or variable speed drive.

Also, consider the service and operation of a compressor and what it might mean to your business. Do you have enough resources? Do you have the proper service contracts in place? The best step is to speak to a compressed air expert first.

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