Tag: impeller

Turnover Rate (TOR)

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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

where

. 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

Impeller Type	Removal	Addition	Suction	Reserve	Pill
Canted/flat	50-75	50-75	65-85	50-80	40-65

Table 10.2
60-Hz Impeller Displacement D Values

Diameter Flat Canted Contour
In	Mm	Gpm	1pm	Gpm	1pm	GPM	1pm
20	508	1051	3978	909	3441	N/A	N/A
24	610	1941	7374	1645	6226	N/A	N/A
28	711	2839	10746	2468	9341	5861	22185
32	813	4635	17543	3764	14247	N/A	N/A
38	965	7342	27789	6343	24008	10604	40136
40	1016	8411	31836	7284	27570	N/A	N/A
42	1067	N/A	N/A	N/A	N/A	13940	52762
44	1118	11300	42771	9928	37577	N/A	N/A
45	1143	N/A	N/A	N/A	N/A	16812	63633
48	1219	14401	54508	12512	47358	20020	75776
52	1321	18630	70515	16100	60939	24852	94063
54	1372	N/A	N/A	N/A	N/A	27602	104475
56	1422	N/A	N/A	N/A	N/A	30353	114887
60	1524	N/A	N/A	N/A	N/A	36567	138404
64	1626	N/A	N/A	N/A	N/A	43533	164771

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):

For gallons:

Vt = L × W(H− 1)× 7.481

The working volume for round tanks with flat bottoms is:
For gallons:

Vt = Π r²(H−1)× 7.481

For liters:

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.