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Auxiliary port driven air compressor system considerations

Today many industrial engines come equipped, or can be ordered with, an auxiliary port (pump drive) that is used to power accessory equipment.  Quite often you will find a hydraulic pump or a standard piston type air compressor mounted there; what isn’t seen much is a rotary screw compressor mounted there.  We’ve had many inquiries asking if there is a compressor available that would meet the heavier duty cycle and air flow requirements for specific jobs.  These range from spray foam, pump priming, air gouging, to air tools and dust collection.  While VMAC does have direct drive, port mounted rotary screw compressors, there are several factors to consider before choosing to mount a rotary screw compressor to that auxiliary port.


Power
– Factors like HP required for CFM, thermal efficiency of the compressor and other parasitic loads on the engine help determine what size of engine you’ll require.  The available power at the auxiliary port is published by the engine manufacturer.  If you are running additional equipment from the same engine, you’ll also have power consumption from those items.  Do power and RPM curves of the engine match each piece of equipment you are using? A more in-depth look at these factors can be found in our article.

Torque Pulses – Torque pulses are caused by the firing order, angle and number of cylinders in an engine.  The high compression ratio of a diesel powered engine also amplifies the magnitude of these pulses.  Lovejoy Inc. has an informative article that goes into much more detail.  Using materials that are designed to handle these pulses is a consideration when designing port driven components.

Port limitations –Manufacturers publish specifications (ft.lb torque) and limits for the power and load that the port can support, however the initial specification may be adjusted (service factor) dependent upon the type of equipment being driven.  A smooth, low load blower puts less stress on the engine than a heavier loaded rotary screw compressor.  A piston compressor or crusher with load spikes puts a greater demand on the engine.  Inside the port, the gear train is designed to transmit power and handle the load of the equipment attached.  Working with the engine and equipment manufacturers to ensure they match is important.

Output vs. Engine RPM – Rotary screw compressor deliver air (CFM) in ratio to the speed of the rotors.  The more rotor rpm, the more air delivered.  Compressor gearing can help deliver more air at lower engine speeds.  When coupling a compressor to the auxiliary engine port, the port gearing also affects the speed the engine is required to run at.  Some ports have 1:1 gearing while others are designed as a speed increaser.  Knowing the air requirements of the job and the engine speed, port gear ratio, compressor gearing and compressor displacement will determine which compressor is required.

Direction of rotation – Engine ports may either turn clockwise or counter clockwise dependent upon the engine manufacturer, location on the engine and ordering options.  Rotary screw compressors are designed to turn in one direction, once again dependent upon gearing and the manufacturer.  A rotary screw compressor running in the wrong direction will quickly result in mechanical failure of the compressor.

Space constraints and clearances – Engine components, mounts, casting features and location of the engine port will determine how much space is available to mount a component to the engine port.  The size and shape of that component is also critical.  Some components have crankcase-257x300mounting flanges that can be rotated to increase the options for mounting.  Ultimately mounting the component to the engine, either in CAD or in reality, will determine fitment.

Port and input shaft types – Most auxiliary engine ports are manufactured to an SAE standard.  SAE A, SAE B and SAE C ports are all common in North America.  In Europe DIN ports are common.  Input shafts generally have a few options:  tapered shafts, keyway shafts and spline shafts are common.  The auxiliary component will need to match both the port and shaft type for it to fit.

Overhung load – Port specifications include how much load the port will support without the need of a brace to mount the auxiliary component.  If using a gear on the compressor to interface with the gear train in the port, an overhung load can cause issues.  Longer components are also likely to vibrate.  Unsupported radial and axial overhung loads may cause failures to your equipment.  The Gates Corporation defines overhung load as a force exerted perpendicular to a shaft beyond the outermost bearing. When that force exceeds the maximum rated capacity for the equipment, shafts and bearings become overloaded and wear out at a higher than normal rate. Although the article references belt driven components, the theory also applies to port drives.

Cooling – Another consideration is the cooling requirements of your compressor and engine.  We’ve written an article on that subject here.

Air intake – Mounting a compressor tight to the engine can optimize the available space, however, it can also introduce the challenge of ensuring your intake air is cool.  Routing your intake hose away from hot components, like the exhaust system, and drawing in air from the outside of an enclosure are considerations for your port driven compressor system.  We have an article on how inlet temperature affects air flow here.

Operating temperatures – Operating temperatures play a part in any compressor choice.  The location of the compressor and the location of the air intake can affect performance.  For more detailed information, we’ve written an article on how inlet temperature affects air flow here.

There are various ways of driving a compressor from your industrial engine and choosing the auxiliary port might be one of the easiest in terms of mounting the compressor itself.  If the power requirements of the engine and port meet the air compressor and air flow needs, then this might be a great option for you.  Remember to work with the manufacturer of the engine and the compressor to confirm the match of the components being considered.

If you have any questions about this article or anything mobile compressor related, please contact us.