The expression “mobile compressor” refers to a compressor mounted on a chassis and supplied complete with an engine and all the necessary control systems, requiring only fuel or some other kind of primary power to generate compressed air. It can be mounted on wheels or skids. Usually it will also have a canopy to protect the working machinery from the weather and to reduce noise transmission. Figure 1 shows three examples of a two wheel rotary compressor capable of supplying air for a construction site. The pressure is up to 13 bar and the delivery up to 2801/s.
The expression “portable compressor”, although strictly a misnomer, is also commonly used; a compressor that can be easily lifted and moved from place to place is called “transportable”. Such compressors can be carried on the back of a truck or mounted inside a van.
Mobile compressors are frequently encountered by the roadside or on construction sites, where they are used to supply air for road breaking tools or drills and a wide range of contractor’s tools. When used on the road, the chassis and wheels have to meet the appropriate legislation for road-going trailers. When used only on construction sites or in quarries, a simpler and cheaper chassis can be used. For smaller sizes (less than about 80 1/s capacity), a two-wheel chassis is standard; for intermediate sizes up to about 300 l/s both two- and four-wheel types are available. For large sizes and for rugged sites such as in quarries four wheel types are standard. A four-wheel chassis can either have turntable or automotive type steering on the front wheels. Independent torsion-bar suspension may be fitted for road use. A lifting eye or bale protruding through the canopy roof is fitted as standard.
Construction-use compressors usually have a storage facility to hold one or more road breakers and lengths of hose. Depending on the application, a towable compressor may incorporate the following features:
• A straight tow bar with levelling leg (U.K. use)
• A cranked adjustable towbar with jockey wheel (European models)
• Towing lights and indicators
• Towing eye or ball coupling
• Cable operated parking brakes
• Overrun and pneumatic brakes
• Security features such as a lockable control panel door and the lifting bale accessible only from within the canopy
• Integral lubricator
• Electric generator
Most canopies are made of steel, with gull-wing or side and front doors to give access to the controls for starting and for servicing. If silencing is incorporated, the canopy is lined with sound deadening material. Smaller road-going compressors may have canopies made of glass fibre reinforced plastic. Portable compressors are also available skid-mounted as an alternative to a wheeled undercarriage.
Mobile compressor types are frequently used in the dirty environment of a construction site or quarry, far removed from the comparatively clean atmosphere of a factory, so special attention has to be paid to intake filtration for both compressor and engine. Cooling arrangements depend on the type of engine used. Air-cooled engines rely on fan cooling, water-cooled engines have a conventional automotive type radiator. The oil cooler (in an oil-flooded compressor) will probably be a conventional shell-and-tube cooler.
There will usually be found safety features such as automatic shut-down when the air delivery temperature, the engine coolant temperature or engine oil pressure exceeds a safe maximum. Indicators will give a visual warning when filters need changing.
A fuel tank with a capacity equivalent to one shift continuous operation is standard. Most of the canopies of portable units are lockable for security reasons.
The exigencies of the equipment hire market, into which many mobile compressors are sold, leads to their use in very adverse conditions, often with only rudimentary servicing.
The price is highly competitive, so the extra cost of sophisticated control systems would be neither necessary nor desirable.