The basic idea
A top entry agitator hangs from the roof and needs a long shaft to reach the liquid. A side entry mixer comes in through a flanged nozzle on the lower shell. Its propeller pumps a jet of liquid across the tank floor. The jet hits the far wall, rises, returns over the top and comes back down to the propeller. In a well-designed installation the whole tank contents turn over this way, with no dead zones.
The motor, drive, bearings and seal sit outside the tank. Only the end of the shaft and the impeller are wetted.
Main parts
- Impeller. A hydrofoil or a propeller. Both are axial-flow designs, chosen for a lot of flow per kW rather than shear.
- Shaft. Short and overhung from the bearing frame. It is checked for bending from the hydraulic forces on the propeller and for critical speed.
- Bearing frame. Carries the shaft on a pair of bearings and takes the propeller thrust.
- Shaft seal. A mechanical seal, because the seal works below the liquid level. FMT uses a single mechanical seal, or a double seal for oil & gas and FGD where an extra barrier against leakage is needed.
- Drive. Either a gearbox, which is compact, enclosed and preferred for severe duty, or a single-stage V-belt drive. Both bring the motor speed down to 280 or 450 rpm at the mixer shaft.
- Motor. 2.2 to 37 kW, flameproof (FLP) for hazardous areas or non-flameproof (NFLP) where the area allows.
- Mounting flange. Bolts to the tank nozzle. The nozzle size and rating are set at design stage.
Mounting angle and position
On a round storage tank a side entry mixer is not pointed at the tank centre. It is aimed a few degrees to one side so the jet sets up a swirl as well as the floor sweep. Several mixers on one tank are usually grouped on one part of the shell and all aimed the same way, so that their jets add up instead of fighting each other.
On crude oil tanks the mixers are placed so the jets sweep as much of the floor as possible and stay clear of heating coils, swing lines and roof supports. On pulp stock chests the mixer is placed in relation to the chest outlet and, in rectangular chests, the end walls, so that stock keeps moving past the outlet.
What sets the size
Motor power and the number of units depend on:
- Tank volume and shape. Diameter, liquid height, and round or rectangular.
- Product. Specific gravity, viscosity at the working temperature, and the solids content or pulp consistency.
- Duty. What the mixer has to achieve. Typical duties are blending a tank to one quality, controlling sludge and BS&W in crude tanks, keeping pulp stock at constant consistency, or holding solids in suspension.
- Time. How fast the result is needed, for example a blend completed within a set number of hours.
Each duty has its own rules, so a rule of thumb from one industry gives the wrong answer in another. That is why we ask for the duty, not only the tank size.
Side entry vs top entry agitators
| Side entry mixer | Top entry agitator | |
|---|---|---|
| Best tank shape | Wide, low tanks (diameter ≥ liquid height) | Tall tanks and process vessels |
| Structure | No load on the roof | Needs a roof bridge or support beam |
| Shaft | Short and overhung | Long, may need a steady bearing |
| Maintenance | From the ground | From the roof |
| Seal | Below liquid level, so it must always seal | Above liquid level in most designs |
| Typical uses | Crude and product storage, stock chests, digesters, large slurry tanks | Reactors, small to medium process tanks, high-viscosity mixing |
Because the seal works under the full liquid head, seal choice and seal maintenance matter more on a side entry mixer than on a top entry machine. We tell you the seal type and the spares to keep in our offer.
Retrofitting an existing tank
A side entry mixer can be added to a tank already in service, as long as a suitable nozzle is fitted on the lower shell. Where the tank has no nozzle, one is added during a planned outage. We give the nozzle size, flange rating, elevation and orientation with our drawing, so your tank contractor can prepare it in advance.