The slot centerline velocity in the circular slot hood is expressed in equation in which the centerline velocity can be calculated by inserting the specific Xs value into the term for the slot centerline distance form the slot hood opening. Table 2 shows the observed and calculated values for centerline velocities. Since the calculated centerline velocities from equation were approximately equal to the observed ones from the laboratory experiment for any Xs values, equation is properly adapted to a regression equation. The equation by Silverman gave approximately the same values as the observed velocities for a circular slot hood with a cylinder, but the velocities for a circular slot hood without a cylinder were apparently different from those given by Silverman. The calculated velocities obtained by Dalla Valle's equation were smaller than those from equation or observed velocities, and the difference increased with the increase in Xs. This difference might be caused by the difference in the velocity measuring method. The velocity in front of the slot hood was measured with a hot-wire anemometer in the present study, while Dalla Valle measured the velocity pressure with a special Pitot tube and a Whalen guage and calculated the velocities from the measured velocity pressure. It is known that with the Pitot tube and Whalen guage, error often occurs in the low velocity range below 5 m/sec.
Deviations in the plots form the regression line became relatively large in the
175-250mm range in Xs. This is because the direction of airflow at the center of the
round opening becomes perpendicular to the centerline of the slot hood.
As shown in Figure 3, the entry loss factor for the circular slot hood was in inversely
linear proportion to the aspect ratio of the slot, but according to the ACGIH manual, the
Fh value for a rectangular slot hood is constant at 1.78 when the W/L is below 0.2.
The Fh values ranging from 0.1 to 0.6 obtained and the plenum box to the Fh values in
the circular slot hood. In a previous study on the characteristics of pressure loss, the
author reported that round openings with flanges reduced the Fh value by half of those
without a flange. The plenum box is thought to function like a flange at the opening.
As shown in Table 1, the concentrations of fine airborne particles and three organic
solvents were reduced remarkably to levels below the threshold limit values during
operation of the local exhaust system with the circular slot hood. Concentrations of total
airborne dust and chromium oxide were particularly reduced at the sampling points close
to the mixing vessel. This indicates that the circular slot hood effectively eliminated the
dispersed fine particles when putting the raw materials into the mixing vessel. The
concentrations of airborne MEK exceeded the TLV of 200ppm at many sampling points
before operation of the local exhaust system with the circular slot hood, but the
concentrations of MEK were also effectively reduced to far below the TLV. The
average concentrations of MEK, MIBK and toluene at all the sampling points were
10.12, 2.74 and 4.2ppm, respectively, during operation of the local exhaust system. The
saturated vapor pressures for MEK. MIBK, and toluene were 71, 16 and 22mmHg at 2
0℃, respectively. The most volatile MEK of the three gave the highest airborne
concentrations before and during operation of the local exhaust system. The
concentrations of MIBK and toluene were below or close to each TLV even before
operation of the local exhaust system. This is due to their lower volatility.
Since the Fh value for the circular slot hood is far smaller than that of a rectangular
slot hood reported previously, a circular slot hood can be operated under conditions of
smaller pressure loss and less power for a fan giving a higher cost performance.
In conclusion, a circular slot hood properly designed can be applied effectively to many
batch manufacturing processes in reactors, mixing vessels and hoppers equipped with
round opening at the top.