To illustrate more clearly the effect of the increase of velocity on the friction loss, the actual vacuum in the hose has been computed for each 10 ft. of its length and curves drawn through these points. The results are shown in [Fig. 49]. The straight line indicates the vacuum which should exist were the velocity in the hose constant throughout its length, and the curved line shows the vacuum in the hose when the effect of the increasing velocity, due to the rarefaction of the air, is considered. The wide variation in the results shows clearly the error in the former assumption of a constant velocity in the hose throughout its length.
Another test, in which 44 cu. ft. of free air was passed through 100 ft. of 1-in. diameter hose, is shown graphically in [Fig. 50], which discloses that the assumption of a constant velocity in the hose produces an error of 35% in the results, indicating a loss of but 7.8 in., when the actual loss is 12 in. mercury.
Naturally, the lower the final vacuum at the hose cock, the less will be the error due to the assumption of constant velocity in the hose. Tests with 1¹⁄₂-in. hose gave results which agree substantially with the result given in tables already published, and it was this condition that led to the discovery of the error in the assumption stated.
FIG. 49. EFFECT OF INCREASE OF VELOCITY ON THE FRICTION LOSS.
Effect of Hose Friction.
—As any increase in the degree of vacuum necessary to be maintained at the vacuum producer over that maintained within the renovator requires a greater expenditure of power, without any increase in the efficiency or speed of cleaning, it is essential that the friction loss in the air conduit from the renovator to the vacuum producer should be made as small as possible. The friction loss in the hose is the greatest loss in any part of the system, being the smallest in diameter, and its reduction to the lowest figure possible is of vital importance.
Take, for example, the use of a Type A renovator with a vacuum within the renovator of 4¹⁄₂ in. mercury and with 29 cu. ft. of air passing through same. The friction loss, with varying lengths of different-sized hose, will be as follows:
TABLE 6.
Vacuum at Hose Cock with Type A Renovators
and with Varying Lengths of Different-Sized Hose.
| Size of Hose, In. Diameter. | Length, in Feet. | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| 100 | 75 | 50 | 25 | ||||||
| Vacuum at hose cock, in. hg. | |||||||||
| 1 | 10 | 8 | ¹⁄₂ | 7 | 5 | ¹⁄₂ | |||
| 1 | ¹⁄₄ | 6 | 5 | .7 | 5 | .25 | 4 | .85 | |
| 1 | ¹⁄₂ | 5 | .0 | 4 | .85 | 4 | .75 | 4 | .62 |