Turnover Rate (TOR)
Impeller sizes are determined by calculating the TOR (sometimes called the time of rollover) for each compartment. This is the time, in seconds, required to completely move the fluid in a compartment (Table 10.1) and can be calculated by knowing the tank volume and impeller displacement:
TOR = (Vt⁄D)×60
. Vt=tank volume, in gallons or liters
. D=impeller displacement, in GPM or LPM (as displayed in Table 10.2).
For flat and canted impeller applications, TOR should range between 40 and 85 seconds. As the TOR approaches 40 seconds, the chance for vortex formation and possible air entrainment increases. At values greater than 85 seconds, the proper suspension may be jeopardized and solids will begin to settle.
Table 10.1 Typical Turnover Rate Values, in seconds
60-Hz Impeller Displacement D Values
|Diameter Flat Canted Contour|
For contour impeller applications, values must be significantly faster (i.e., smaller numbers) to achieve the same results, but because of the impeller design, air entrainment is less probable. In symmetrical compartments, the fluid has a nearly equal distance to travel from the center of the impeller shaft or from the impeller blade tip before it contacts the vessel wall. Agitators should be placed where the shaft is centered in the tank or compartment.
When defining the area in which to mix, it is best to work with symmetrical shapes like squares or circles (as viewed in a plan drawing or overhead view of the tank layout). Rectangular tanks should be converted to nearly square compartments if possible. Maximum fluid working volumes in compartments should not be higher than 1 foot (about 3⁄10 m) from the top of the tank. This will allow for a little extra capacity in emergencies, slightly out of level installations, and/or fluid movement on floating rigs.
Working volume for square or rectangular tanks is calculated by knowing dimensional values for length (L), width (W), and height (H; in feet for gallons, in meters for liters):
Vt = L × W(H− 1)× 7.481
The working volume for round tanks with flat bottoms is:
Vt = Π r²(H−1)× 7.481
Vt = Π r²(H−0.3)× 1000
For round tanks with dish or cone bottoms, calculations for working fluid volume are based on straight wall height (i.e., this height is measured from the tank top to where the tank joins the cone or dish at the bottom). This leaves adequate free space above the maximum fluid operating level. In all cases, if H<5 feet (1.5 m), a radial flow impeller should be specified.