Can Side Entry Agitators Be Used in A Tank That Is Less Than Half Full?

When selecting mixing equipment for industrial tanks, one very common question is whether a side entry agitator (sometimes called a side-mounted tank mixer or side entry mixer) will perform adequately when the tank is operating at less than half full. This question is especially pertinent in industries where fill levels vary, batches change size, or interim operations run at lower levels.

 

How Side Entry Agitators Work

Side entry agitators are mounted through the sidewall of a tank rather than from the top (top entry) or bottom (bottom entry). Their impellers or mixing blades are inserted horizontally (or at an angle) into the liquid and generate a flow path across the tank volume—typically sweeping the tank floor and creating circulation and turnover. They are often used in large storage tanks (petroleum, chemical storage, oils) or process tanks where large diameter and low height make top entry less desirable. Their advantages include lower structural supports (since motor/gearbox is outside the tank), easier maintenance access, and the ability to retrofit existing tanks via side nozzles.

In a typical cycle or process for side-entry mixing, the blades induce a flow that travels horizontally across the tank floor, lifting settled solids, entraining fluid, and sweeping upwards and outwards in a spiral or jet pattern. This helps achieve homogenisation, suspension of solids, heat transfer, or blending duties.

 

The Influence of Fill Level on Mixing Performance

One of the key factors in tank mixing design is how much liquid is present relative to the tank geometry—particularly height, diameter, and the ratio of height to diameter (H/D). When a tank is less than half full, several aspects change:

• Reduced fluid depth: With less liquid above the impeller region, the vertical circulation path is shorter and the impeller may not engage a large mass of fluid.

• Altered flow patterns: The jet or sweep generated by impeller may reach the free surface quickly and may create more surface turbulence rather than engaged bulk flow.

• Free surface shear and air entrainment: At lower fill levels, the impeller may disturb the surface, causing vortexing or entrainment of air, reducing effective mixing.

• Potential for dead zones: With less fluid volume, there's a higher risk that some regions—especially near the far wall or the bottom opposite the entry—may experience weaker circulation or remain stagnant.

• Power utilisation inefficiency: The mixer may run at designed power for a larger volume, but with less fluid, the fluid resistance is lower, which can lead to higher speeds, higher shear, and potential erosion or undesirable shear-sensitive product damage.

Many guidelines for side entry mixers assume a certain minimum fill, or at least a tank geometry and mix duty designed for full volume or a high fill scenario. For example, documentation from major side-entry mixer manufacturers highlight their use in large storage tanks with deep volumes and broad diameters; the design often assumes full or near-full conditions for optimal fluid turnover.

 

Can They Be Used When the Tank Is Less than Half Full?

The short answer is: yes—side entry agitators can be used in tanks that are less than half full, but with caveats. It depends on how the mixer is designed, whether you accept limitations, and how you adapt the system. Here are the main considerations:

When it's feasible:

• If the mixing duty is less demanding (for example minor blending, mild circulation, temperature uniformity rather than full turnover), then even at lower fill you may achieve acceptable results.

• If the side entry mixer is designed with adjustability in speed (variable frequency drive) or impeller geometry optimized for shallow liquid depth, it can still generate sufficient flow.

• If the tank geometry (diameter, height, aspect ratio) is favourable (relatively wide, shallow, with the mixer located to sweep the available volume) the lower fill may not be a critical limitation.

• If multiple mixers are installed (reducing the volume each has to handle) then each can operate efficiently even in partial fill conditions.

When you must be cautious:

• If the mixing duty requires full top-to-bottom circulation (for example, suspending settled solids, homogenising density differences, or strong heat transfer), operating below half fill may compromise performance.

• If the impeller is sized for the full volume but the fill is low, the flow may skim along the free surface rather than engage the full bulk, resulting in dead zones.

• If the liquid is viscous or has high density differences (settling solids, layers), the mixing power and geometry may not provide the needed shear or entrainment with shallow depth.

• If the mixer cannot be controlled (fixed speed/impeller) and was selected for full tank conditions, the risk increases of under-mixing or inefficient energy use.

In fact, some technology providers explicitly note that "side entry agitators can be used when the tank is less than half full, if an optional variable speed drive is installed to reduce RPM and adjust to the lower volume" thereby adapting to the changed conditions. This implies that with the right adaptation the system can still be effective.

 

Design and Operational Guidelines for Low-Fill Side Entry Mixing

If your situation involves tanks frequently operating at less than half full, here are key guidelines to ensure you still get acceptable mixing results:

• Check geometry and clearance: Ensure the impeller has sufficient clearance from the tank bottom and sidewalls even at lower fill, and that the entry angle allows the jet to sweep the available fluid rather than hitting the surface prematurely.

• Use variable speed drives: A VFD allows you to reduce impeller speed when volume is lower, reducing undesirable shear or surface disturbance and maintaining flow patterns appropriate for the actual fill level.

• Select impeller type for shallow depth: Consider impeller styles that generate horizontal and upward flow efficiently in shallow fluids—such as large diameter axial flow blades, high entrainment types, or jet impellers.

• Install baffles or flow-directing internals if needed: If the tank is wide with low level, adding baffles or flow guides can help avoid dead zones, redirect flow, and enhance mixing uniformity.

• Monitor mixing performance: Use empirical testing or computational fluid dynamics (CFD) to verify mixing time, circulatory flow paths, and homogeneity under the lower fill scenario. Differences compared to full-fill conditions may be significant.

• Consider multiple mixers or adaptive placement: A single side entry mixer may struggle in shallow fill if the tank is large; installing two mixers (either opposite or at 90°) or selecting a swivel/movable angle mixer improves coverage.

• Plan for depth changes: If the tank's fill level varies widely (sometimes full, sometimes low), the mixer should be sized for the worst case (lowest fill) or have adjustable features (e.g., impeller can be raised/lowered, or multiple speeds).

• Maintenance and seal considerations: Side entry mixing often supports "under full tank condition" maintenance by using shut-off devices. At lower fill levels this can still work, but ensure the mechanical seal is appropriate and the mounting flange remains sealed even if the fluid height is low.

 

Practical Example (Hypothetical)

Let's say a chemical processing tank has a diameter of 6 m and a full-fill depth of 4 m (volume ~113 m³). The process often operates at only 40% fill (~45 m³). The mixing duty is to maintain homogeneity and moderate heat transfer, but only shallow fill is present for long periods.

A side entry agitator is installed through the side wall about 1.2 m above the floor. Initially it was sized for full-fill (with impeller diameter ~1.8 m, speed ~150 RPM). After commissioning, operators discover when fill drops to 0.4 full, the mixing time increases, and product layering appears.

To adjust, the team fits a variable frequency drive to allow speeds as low as 90 RPM, modifies impeller pitch to increase axial lift, and installs a second side entry mixer opposite the first. The result: mixing time returns to acceptable levels even at low fill. Key take-aways: the mixer worked at low fill, but only with adaptation.

 

Summary

In conclusion: Yes, side entry agitators can be used in tanks that are less than half full—but successful application depends on thoughtful design and adaptation. The key factors are tank geometry, impeller selection, variable speed capability, and operational monitoring. If your tank operation includes frequent low-level periods, you should explicitly specify the mixer for that scenario. Otherwise you run the risk of wasted energy, poor mixing, layering, or longer mixing times.

For procurement, always ask the supplier: "What is the minimum fluid depth (or fill ratio) at which this mixer has been proven to perform?" and "What controls (VFD, impeller geometry) are included to support shallow-fill operation?"

If you are looking to source reliable industrial mixers for these conditions, consider reaching out to KeHeng, specialises in side entry agitators. Engaging with a manufacturer that understands both process demands and export logistics will help you align equipment, mixing performance and supply chain needs.