Air compressor test apparatus
BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to an apparatus for use in testing the efficiency of an air compressor system (consisting of compressor and drive) and as a related aspect a method for the testing of the efficiency of an air compressor. The preferred embodiment of the invention is particularly adapted for on-site testing of air compressors in existing installations. 
     Pneumatic systems (i.e. compressed air systems) are in widespread use, such as in manufacturing plants and service stations. Compressed air is used as a power source for many applications, such as for penumatic tools and equipment and for pressurizing inflatable devices such as tires and the like. 
     In many typical installations an air compressor is associated with a compressed air storage tank, and the compressor operates whenever the storage pressure drops below a certain level. By using a storage tank, the compressed air system has an adequate supply available for peak demands, and yet during periods of lower demand, can be replenished by operation of the air compressor. An air compressor is typically sized on the basis of expected demand, and therefore, there is a wide range of compressor sizes in actual use, each one&#39;s size depending upon the expected demands for the particular installation. 
     As is the case for virtually all mechanical devices, continuing usage of an air compressor eventually results in the compressor becoming less efficient. The extent of the inefficiency may not be readily apparent because the increased costs incurred for powering the compressor are often lumped into general power costs for the plant or service station. In other words, while a compressor may continue to perform adequately, the power input to run the compressor may progressively increase with time and in this way constitute a significant hidden cost of operating the plant or service station. 
     The present invention is directed to a novel apparatus for testing of the efficiency of an air compressor system, and it has the particular advantage of not requiring the compressor to be disconnected and transported to a test facility. In other words the testing can be done on-site and simply involves connecting the test apparatus in the on-site installation and then running the compressor for a brief amount of time to acquire information sufficient to calculate the efficiency of the compressor system. On the basis of the information obtained through use of the test apparatus, a calculation of the compressor&#39;s efficiency is made. The owner of the compressor can then decide whether the efficiency has become low enough to warrant replacement or rebuilding of the compressor. 
     In addition to the ability of the present invention to provide on-site testing of a compressor, a single test apparatus is adapted to testing a wide range of compressor sizes. Moreover, the apparatus can be used regardless of the type of power used to drive the compressor. Typical compressors are driven either by electric motors or by gas powered engines. Therefore the power input to a compressor during the test procedure can be readily ascertained by measuring either the electric power input in the case of an electric motor driven compressor or by the fuel input to the engine in the case of an engine driven compressor. 
     The apparatus of the present invention also has the advantage that it can be fabricated entirely from standard parts and/or parts which can be readily fabricated. The preferred embodiment of the invention disclosed herein comprises a number of shut-off valves, pipes and fittings, all standard parts, and it also includes a set of orifice plates which can be readily fabricated. 
     The foregoing features, advantages and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at the present time in carrying out the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of test apparatus embodying principles of the present invention, 
     FIG. 2 is an enlarged fragmentary view, partly in section, of a portion of the test apparatus shown in FIG. 1 and enlarged, and 
     FIG. 3 is an enlarged cross sectional view of one of the component parts of the structure shown in FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates an exemplary presently preferred embodiment of test apparatus 10 according to the present invention. Apparatus 10 comprises an inlet 12 for connection to the outlet of an air compressor to be tested. 
     A shut-off valve 14 and an oil and water trap 16 are connected in a pipe line 18 which extends from inlet 12. A tee 20 comprises a removable plug 21 to provide a tap for connection to a pressure gauge which measures main line pressure. The test apparatus can therefore be conveniently connected in most conventional pneumatic systems. 
     A pipe line 22 forms a continuation of line 18 extending from tee 20 to a tee 24. Tee 24 provides for the branching of line 22 to a pressure regulator 26 and a by-pass control valve 28. The two branches rejoin at a tee 30. With this arrangement it can be appreciated that by-pass control valve 28 and pressure regulator 26 are connected in a parallel circuit relationship. 
     A cross 32 extends the circuit to a pipe line 34 and also provides two taps for sensing conditions during test. One tap 36 is used for sensing temperature and the other tap 38 is used for sensing total pressure. The taps are shown closed by removable plugs, but during test they are removed and appropriate sensing devices are threaded into the taps. 
     Pipe line 34 leads to a series of four tees 40, 42, 44 and 46. A corresponding branch 48, 50, 52 and 54 is associated with each of the four tees 40, 42, 44 and 46, respectively, and the four branches are essentially identical. The last tee 46 is plugged at 47. 
     Each branch 48, 50, 52, 54 contains a corresponding shut-off valve 56, 58, 60 and 62, respectively, and a corresponding orifice plate assembly 64, 66, 68 and 70, respectively. As can be seen from consideration of FIG. 1 each branch has a shut-off valve upstream of the corresponding critical flow orifice plate assembly. This means there is the velocity of sound at the orifice. 
     The orifice plate assemblies 64, 66, 68, and 70 in turn connect to corresponding tees 72, 74, 76, and 78, respectively, and these tees are in common in a pipe line 80 which leads to a tee outlet 82 at which air is exhausted. Tees 78, 82 are plugged at 81, 83. 
     FIGS. 2 and 3 illustrate representative details of orifice plate assembly 64. The branch contains two confronting pipes 84, 86. Around the outside of each of the confronting pipe ends is a circular ring. 88 is the ring on pipe 84 and 90 is the ring on pipe 86. The rings are affixed to the pipes in any suitable manner, and comprise circular radial flanges flush with the ends of the pipes. 
     A circular orifice plate 92 is captured between the two rings 88 and 90. The OD of plate 92 corresponds to the OD of the ring flanges and there are suitable aligned holes provided in the rings and orifice plate for fasteners 94, nuts and bolts for example, which securely clamp the orifice plate between the two confronting pipes and rings in a sealed manner. Sealing may be accomplished by placing gaskets 96, 98 respectively on opposite faces of the orifice plate in the manner shown in FIG. 2. The illustrated pattern for the fasteners comprises four fasteners arranged 90 degrees apart. 
     Orifice plate 92 comprises a central orifice 100. The illustrated orifice comprises a frusto-conical taper 102 leading to a circular cylindrical portion 104. The orifice is located on the axis of the two confronting pipes 84, 86. The orifice is made of a known precise size, particularly the portion 104. 
     The construction of the four orifice plates assemblies 64, 66, 68 and 70 in the preferred embodiment are exactly the same except for the sizes of the orifices. Each orifice plate assembly has a different sized orifice. For example each orifice plate assembly is adapted for a particular flow rate range, and the following chart illustrates exemplary ranges for examplary size orifices in 3/8&#34; orifice plates, preferably brass or other non-corrosive metal. 
     
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                      Diameter of Flow Rate                               
Orifice Plate                                                             
         Length of Orifice                                                
                      Orifice Portion                                     
                                  Range                                   
Assembly Portion 104  104         (SCFM)                                  
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64       0.125&#34;       0.100&#34;       6-16                                   
66       0.125&#34;       0.150&#34;      14-36                                   
68       0.125&#34;       0.200&#34;      25-64                                   
70       0.125&#34;       0.300&#34;       56-145                                 
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     The typical testing procedure is as follows. The compressor is connected to inlet 12, and tee 20 is connected to the mail line pressure gauge. Appropriate pressure and temperature instrumentation is connected to cross 32. 
     Valve 14 is opened and the compressor is operated. The main line pressure is set by regulator 26 and is monitored by pressure sensing instrumentation connected to tap 21 in cross 20. Appropriate adjustments of pressure regulator 26 and by-pass control valve 28 are made to produce a substantially steady measurable main line pressure reading with the compressor continuously running. In other words the apparatus is &#34;balanced&#34; to the compressor. The appropriate orifice selection is made singly or in parallel by the selective opening and closing of the shut-off valves 56, 58, 60, and 62 such that the pressure reading at tap 21 is at least 1.9 times the pressure at discharge tap 82, normally atmospheric pressure. 
     Since it is known which shut-off valves 56, 68, 60 and 62 are open and closed, the particular orifice selection is known, and hence also the specific restriction imposed on the compressed air delivered by the running compressor. Having taken measurement of the resulting line pressure at cross 32, and compressure inlet pressure and temperature, the ideal power required to deliver the measured rate of compressed air is determined. This is because the ideal power is related to the pressure increase and flow rate. 
     Concurrent with the pressure measurement is a measurement of the power input to the compressor-drive. This can be done for example by a wattmeter connected to the electrical power line to the electric motor in the case of an electric motor driven compressor, and in case of an engine driven compressor, the fuel flow rate to the engine can be measured. 
     It is also necessary to take the temperature measurement at one of the taps across 32 so that an accurate determination of air density at cross 32 can be determined. Once the appropriate measurements have been taken, the efficiency of the compressor can be determined by relating the ideal power to the compressor-drive input power. On the basis of this calculation the owner of the compressor can determine whether it is appropriate to repair or replace the compressor. 
     For any given compressor, one test may be sufficient. Alternatively it may be deemed desirable to perform plural tests so that readings can be obtained at different main line pressure settings, and these may include readjustment at different settings. The actual amount of testing will depend upon the particular compressor installation. 
     In adjusting the test apparatus the objective is to seek a balanced condition during which the compressor runs continuously at a fixed main line pressure and for a particular combination of open and closed shut-off valves 56, 58, 60 and 62. If a particular setting of main line pressure and of shut-off valves 56, 58, 60 and 62 is incorrect, this fact will become known either by the inability to maintain a measurable pressure (i.e. orifice selection too large) or by the compressor shutting off (i.e. orifice selection too small). The measurements which are obtained may be processed manually or by means of a computer containing a suitable program. In the case of multiple readings, use of a computer will enable the efficiency calculations to be most quickly obtained. The calculations can be performed by use of conventional formulae relating to the various parameters. 
     The illustrated embodiment comprises the various adjustable component parts (i.e. shut-off valves, by-pass control valve, and pressure regulator) manually operable by individual manual operators. A more elaborate embodiment could provide for automatic operation from a master panel. Also, use of barometric pressure measurement to account for variations of atmospheric pressure, and temperature measurement of compressor inlet air can be added to implement ideal power calculation. 
     The foregoing has disclosed a unique apparatus for testing the efficiencies of air compressors on site. While a preferred embodiment has been disclosed, it will be appreciated that principles are applicable to other embodiments.