Why Does Specific Gravity Matter in Mixer Design? 

What Is Specific Gravity and Why Is It Important in Mixing? 

 

Specific gravity (SG) is a dimensionless measure that compares the density of a substance to the density of water. In mixer design, it plays a critical role because it directly influences the energy needed to mix a fluid and affects how fluid layers interact within a tank. Understanding SG helps engineers select the right mixer setup, including impeller type, size, and drive system. 

 

Specific gravity is calculated as: 

 

SG  = _(P)fluid_ 

 (P)water 

 

Where:

(P)fluid is the fluid density (kg/m^3) 

  • (P)water is typically 1,000 kg/m^3 at 4°C 

 

A specific gravity greater than 1 means the fluid is denser than water. Many industrial fluids—like slurries, oils, and emulsions—fall into this category, requiring specialized mixing strategies. 

Learn More On How To Calculate SG

Why SG is it important in Mixing?  

 

A: It is commonly the reason mixing is required. In particular, it is the reason why many products or processes fall out of specification. If the product includes two or more immiscible fluids. Those fluids will have a tendency to Separate into layers. This is driven by Gravitational Force (downward) as well as the buoyant force (upward).  

 

Net force difference causes fluid movement until a hydrostatic equilibrium is reached:  

P = pgh 

 

Where: 

 P = Pressure at a depth, 

p = Fluid density 

g = Gravitational acceleration 

h = Height (or depth) 

 

This equilibrium is the natural state of the fluids, and the reason mixing is required. The common term for this in industry is Separation, or Phase Separation. Separation is a large challenge for product and process control.  

 

A simple example is Oil and Water.

  • Oil: SG = 0.85
  • Water: SG = 1.00 
  • Result: Oil floats above the water. 

The Role of Specific Gravity in Mixer Design 

  • Impeller Load and Torque Requirements: Fluids with higher specific gravity exert heavier loads against the impeller. This increases torque demands and can influence shaft design, motor sizing, impeller design and gearbox ratios. 
  • Impeller Selection: Denser materials often settle quickly, requiring impellers with greater axial flow (e.g., hydrofoil or pitched-blade turbines) to suspend solids and maintain homogeneity. 
  • Stratification Prevention: Mixing systems must be able to manage fluids with different SGs to prevent stratification or layer separation. 
  • Startup Load: Starting a mixer in a high-SG environment increases the load. Gearboxes must be rated not just for operating torque, but also startup torque. 

How Specific Gravity Interacts With Viscosity and Reynolds Number

 

While SG measures density, it often intersects with viscosity and Reynolds number in defining fluid behavior. Higher SG can reduce the Reynolds number, shifting flow from turbulent to laminar. In turn, this influences power number (Np) and mixing efficiency. 

Use the following equations to evaluate system design: 

 

Reynolds Number (Re) for Mixing: 

 

Re  = _pND2__ 

      µ 

 

Where:  

  • p = fluid density (kg/m3) 

  • N = impeller speed (rev/s) 

  • D = impeller diameter (m) 

  • µ = dynamic viscosity (Pa•s or kg/(m•s)) 

 

Correctly factoring in SG ensures accurate calculations for both mixing power and impeller performance. 

Learn More on SG Formulas

Industrial Applications Where SG Is Critical ​

  • Mining and Slurries: Heavily loaded slurries with solids like ore or tailings often exceed an SG of 1.5. These high-density mixtures demand high-torque mixers with rugged impeller designs capable of suspending abrasive solids without excessive wear. 
  • Chemical Processing: Multi-phase systems—especially those involving suspended solids, dense reactants, or layered solvents—require careful SG balancing. Effective mixing ensures uniform dispersion and prevents solids from settling or stratifying. 
  • Wastewater Treatment: Solids suspension and separation depend heavily on SG differences between sludge, water, and chemical additives. Proper mixer design ensures efficient agitation without dead zones that lead to sludge buildup—turning the “art” of sludge management into a science. 
  • Food Production: Products like syrups, sauces, and dressings often contain solids in suspension (spices, pulp, seeds) alongside fluids of varying SGs (e.g., oil and vinegar). Gentle, SG-aware mixing preserves texture, prevents separation, and maintains product consistency. And of course; Italian Salad Dressing with separation of Oil and Vinegar is a great example of this.  

Find insights and process strategies at Water Online and The Engineering Toolbox. 

Mixer Customization Based on SG at Dynamix Agitators 

At Dynamix Agitators, every mixer is designed with SG as a core parameter. While viscosity is often the primary input, understanding SG in Solids and Liquid is critical. This ensures each customer receives a system that can handle the weight, flow dynamics, and startup torque associated with their specific application. 

 

Customizations might include: 

  • High-efficiency impeller blades 
  • High flow pumping Impellers 
  • Reinforced shafts and supports 
  • A design process focused on Gearboxes tuned for high-SG startup torque 

 

This level of engineering precision reflects the long-standing trust customers place in Dynamix for performance, reliability, and support. 

Events and Resources to Deepen Knowledge 

 

Keep up with the latest mixing technologies and SG-driven design practices by attending key industry events and leveraging technical platforms: 

 

  • AIChE Annual Meeting 
  • ISA Automation Conference 

 

Conclusion: Specific Gravity Is More Than Just a Number 

If you’re encountering challenges in your mixing process—or if this is the first time SG has been on your radar—give us a call. At Dynamix Agitators, we’re here to review your application, answer your questions, and recommend the right mixer solution for your specific needs. Let’s make sure your system is designed for real-world performance. 

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