Clearance is not taken into consideration in the foregoing figures, but clearance is very much more of a bete noir in theory than in practice. The early designers, as shown in the "Dubois-Francois" illustrations, Figs. 3 and 4, regarded clearance loss as a very serious matter. Even at the present time some air compressor manufacturers admit water through the inlet valves into the air cylinder, not so much for the purpose of cooling as to fill up the clearance space. A long stroke involving expensive construction is usually justified by the claim that a large saving is effected by reduced clearance loss. Let us see what the effect of this clearance is. Assuming that we have an air compressor which shows an isothermal pressure line, there would be some loss of power due to clearance space, because we would have a certain volume of air upon which work was done and heat produced, that heat having been absorbed and the air being retained in the cylinder and not serving any useful purpose. But let us assume that we have a compressor which shows an adiabatic pressure line. We now have the air in the clearance space acting precisely as a spring, compressed at each stroke, retaining its heat of compression, and giving it out against the air piston at the point when the stroke is reversed.

There is no loss of power in such a case as this, but, on the contrary, the air spring is useful in overcoming the inertia of the piston and moving parts. The best air compressors give a result about midway between the isothermal and the adiabatic, and the net loss of power directly due to clearance is so small as to be practically unworthy of consideration.

It must not be inferred from the preceding remarks that the designer of an air compressor may neglect the question of clearance. On the contrary, it is a very important consideration. If we assume a large clearance space in the end of an air cylinder of a compressor which is furnishing air at a high pressure, we may readily conceive that space to be so large, and that pressure so high, that the entire volume of the cylinder would be filled by the air from the clearance space alone, and the compressor would take in no free air and would, of course, produce no compressed air.

Loss in capacity of air compressors by clearance is in direct proportion to the pressure.

Owing to the loss of capacity by clearance space at high pressures, it is important that compound air cylinders should be used for furnishing air at high pressure. With compound air cylinders the air is compressed to alternate stages of pressure in the different cylinders, and the clearance loss is thus reduced because of the reduced density of the air in the clearance spaces. In ordinary practice air compressors deliver the air at less than 100 pounds pressure, so that with a properly designed air cylinder the clearance space is so small that the capacity of the compressor is not materially affected.

Two systems are in use by which the heat of compression is absorbed, and the difference between one and the other is so distinct that air compressors are usually divided into two classes (1) wet compressors, (2) dry compressors.

A wet compressor is that which introduces water directly into the air cylinder during compression.

A dry compressor is that which introduces no water into the air during compression.

Wet compressors may be subdivided into two classes.

(1) Those which inject water in the form of a spray into the cylinder during compression.

(2) Those which use a water piston for forcing the air into confinement.

The injection of water into the cylinder is usually known as the Colladon idea. Compressors built on this system have shown the highest isothermal results, that is, by means of a finely divided spray of cold water the heat of compression has been absorbed to a point where the compressed air has been discharged at a temperature nearly equal to that at which it was admitted to the cylinder. The advantages of water injection during compression are as follows:

(1) Low temperature of air during compression.

(2) Increased volume of air per stroke, due to filling of clearance spaces with water and to a cold air cylinder.

(3) Low temperature of air immediately after compression, thus condensing moisture in the air receiver.

(4) Low temperature of cylinder and valves, thus maintaining packing, etc.

(5) Economical results, due to compression of moist air (see table 3).

Table 3

SHOWING THE RELATIVE QUANTITY OF WORK REQUIRED TO COMPRESS A GIVEN VOLUME AND WEIGHT OF AIR, BOTH DRY AND MOIST - ALSO RELATIVE VOLUMES WITH AND WITHOUT INCREASE OF TEMPERATURE FROM COMPRESSION.

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| | |

|Compression at |Compression |

|a Constant |with |

|Temperature. |Increase of |

|Mariotte's Law. |Temperature. |

__|________________|__________________________________|________________________________

| | | | | | | | | | | | | | |

1|0.1 | | | | | | 20 | 68 |1.0 | | | 68 | | |

2|0.5 | 7199|1468|0.612| 7932|1618| 85.5|186 |1.222| 733|0.092|111 |3.0|23500|22500

3|0.333|11356|2316|0.459|13360|2725|130.4|267 |1.375|2004|0.150|135.5|4.0|37000|35000

4|0.25 |14260|2909|0.374|17737|3618|165.6|330 |1.495|3477|0.196|153.5|4.8|48500|45000

5|0.200|16580|3383|0.320|21209|4326|195.3|384 |1.595|4629|0.213|167 |5.4|58500|52500

6|0.167|18475|3768|0.281|24310|4959|220.5|429 |1.681|5835|0.240|179 |6.0|67000|60000

7|0.143|20038|4087|0.252|27048|5517|243.2|470 |1.758|7040|0.260|190 |6.4|75000|66000

8|0.125|21422|4370|0.229|29518|6021|263.6|506.5|1.828|8096|0.274| | | |

9|0.111| | |0.210| | |282 |539.6|1.891| | | | | |

10|0.100| | |0.195| | |299 |570.2|1.950| | | | | |

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