the NCF, are governed by a pressure control system,adjusted to compress air to a maximum pressure of100 pounds per square inch (psi). Compressor units areavailable in reciprocating, rotary, or screw design.Compressors are classified as either single-stage ormultistage. A single-stage compressor has onecompressing element that compresses the air from theinitial intake pressure to the final discharge pressure in onestep. The multistage compressor has more than onecompressing element. The first stage compresses the airto an intermediate pressure, then through one or moreadditional stages to final discharge pressure. Themultistage system is more efficient than the single-stagesystem, because the air cooling that occurs between stagesreduces buildup of pressure due to a temperature rise.All military construction compressors are governedby a pressure control system. In a reciprocatingcompressor, this control system causes the engine toidle and the suction valve to remain open when thepressure reaches a set maximum. When the air pressuredrops below a set minimum, the pressure control systemcauses the engine to increase speed and the suction valveto close, starting the compression cycle again. Therotary compressor output is governed by varying theengine speed. The engine operates automatically at thespeed needed to compress enough air to supply thedemand at a fairly constant pressure. When the enginehas slowed to idle because of low demand, a valvethrottles the amount of free air that may enter thecompressor.The screw compressoroutput is controlledautomatically and provides a smooth, uninterruptedcapacity from full load to no load in response to thedemand for air. This capacity is achieved by a floatingspeed engine control and a variable inlet compressor.COMPRESSOR CAPACITYThe capacity of an air compressor is determined bythe amount of free air (at sea level) that it can compressto a specified pressure, usually 100 psi per minute, underthe conditions of 68°F and a relative humidity of 38percent. This capacity is expressed in cubic feet perminute (cfm) and is usually included in thenomenclature of the compressor.The number of pneumatic tools that can be operatedat one time from an air compressor depends on the airrequirements of each tool; for example, a 55-pound classrock drill requires 95 cfm of air at 80 psi. A 210-cfmcompressor can supply air to operate two of the drills,because their combined requirements is 190 cfm.However, if a third such drill is added to the compressor,the combined demand is 285 cfm, and this conditionoverloads the compressor and the tools and results inserious wear.NOTE: When the pressure and volume of air to apneumatic tool drops 10 percent below the designedminimum, the tool efficiency is reduced 41 percent.Compressor LocationInstall the compressor unit so it is as close to levelas possible. Compressor design permits a 15-degreelengthwise and a 15-degree sidewise limit onout-of-level operation. The engine, not the compressor,is the limiting factor. When the unit is to be operatedout of level, you should be sure to do the following:1. Keep the engine crankcase oil level on the fullmark with the unit level.2. Ensure the compressor oil gauge shows full withthe unit level.Compressed Air SystemA compressed air system consists of one or morecompressors, each with the necessary power source,regulation, intake air filter, aftercooler, air receiver,connecting piping, and a distribution system to carry theair to points of use.The object of installing acompressed air system is to provide enough air at thework area at pressures adequate for efficient operationof pneumatic tools.Many construction jobs require more cfm than onecompressor can produce. Also, terrain conditions oftencreate problems of distance from the compressor to theoperating tool. Since the air line hose causes a loss ofpressure (friction loss) at distances farther than 200 feet,a system has been devised for efficient transmission ofcompressed air over longer distances. This system is airmanifolding (fig. 14- 10).Figure 14-10.—Methods of manifolding compressors.14-8
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