MIL-HDBK-1110
rates. The compressor works by taking in, filtering, and
compressing a large volume of air by rotary or piston action and
then releasing it via the air hose into the blasting machine.
The capacity of a compressor is expressed in volume of air moved
per unit time (e.g., cubic feet per minute (cfm)) and is directly
related to its horsepower. The capacity required depends upon
the size of the nozzle orifice and the air pressure at the
nozzle. For example, a flow of 170 to 250 cfm at a nozzle
pressure of 90 to 100 psi is necessary when using a nozzle with a
3/8 to 7/16 inch orifice. This typically can be achieved with a
45 to 60 horsepower engine.
b) Air-Supply Hose. The air-supply hose delivers air
from the compressor to the blasting machine. Usually the
internal diameter should be three to four times the size of the
nozzle orifice. The length of the hose should be as short as
practical because airflow through a hose creates friction and
causes a pressure drop. For this reason, lines over 100 feet
long generally have internal diameters four times that of the
nozzle orifice.
c) Blasting Machine. Blasting machines or "sand pots"
are the containers which hold the abrasives. The capacity of
blasting machines varies from 50 pounds to several tons of
abrasive material. The blasting machine should be sized to
maintain an adequate volume of abrasive for the nozzles.
d) Abrasive Blasting Hose. The abrasive blasting hose
carries the air and abrasive from the pot to the nozzle. It must
be sturdy, flexible, and constructed or treated to prevent
electrical shock. It should also be three to four times the size
of the nozzle orifice, except near the nozzle end where a smaller
diameter hose is attached.
e) Nozzles. Nozzles are available in a great variety
of shapes, sizes, and designs. The choice is made on the basis
of the surface to be cleaned and the size of the compressor. The
Venturi design (that is, large throat converging to the orifice
and then diverging to the outlet, Figure 3) provides increased
speed of abrasive particles through the nozzle as compared with a
straight bore nozzle. Thus, the rate of cleaning is also
increased. Nozzles are available with a variety of lengths,
orifice sizes, and lining materials. The life of a nozzle
depends on factors such as the lining material and the abrasives
and varies from 2 to 1500 hours. Nozzles should be inspected
regularly for orifice size and wear. Worn nozzles result in poor
cleaning patterns and efficiency.
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