Many processors rely on mixing equipment that is more than 25 years old. Even if the equipment isn’t worn out, it almost certainly was designed for different conditions and often different processes or products. The design may not suit the current application or product. In the last 25 years, many products have become more viscous or non-Newtonian, making mixing more difficult.
Over that same period, new types of impellers and ways of applying them have improved mixing with the same power and speed. These developments often open up opportunities to enhance performance without replacing the equipment or resorting to expensive upgrades.
While not a recent improvement, hydrofoil impellers have replaced many pitched-blade turbines in medium- to low-viscosity blending and solids suspension applications. A typical pitched-blade turbine (Figure 1) has four blades made of flat plates. The most common hydrofoil impellers have three blades with a curved cross-section (Figure 2). The curved cross-section is called camber; it furnishes a degree of lift and a smooth change of flow direction. Just like the wings on an airplane, the hydrofoil blades provide a downward force with an efficient transfer of power. The advantage of the hydrofoil impellers is greater efficiency in creating axial flow. The axial flow pattern generated in a stirred tank produces good vertical circulation, which helps to carry surface additions throughout the vessel. In solids suspension, the axial flow directed toward the bottom of the vessel lifts solids off the bottom. Delivering the same power at the same speed with a slightly larger diameter hydrofoil impeller may improve mixer performance at a modest cost for a new impeller.
Some mixers have under-loaded motors. A unit with a 25-hp motor actually only may deliver 10 hp to the mixed liquid. This points up the difference between two important and often confused characteristics: motor power and impeller power. The motor power establishes the maximum power the impeller can deliver to the fluid. The impeller power reflects the rotational force (torque) the impeller applies to the fluid and the rotational speed or rate of energy dissipation. Impeller conditions, power, torque, tip speed and pumping rate are the key factors that create the mixing intensity and process performance of a mixer. Increasing the impeller power may offer an opportunity for improvement.
The first step in assessing a unit is to establish the current design. Older equipment may lack good drawings. Even when drawings are available, they may not include modifications made after the original equipment was installed. Many equipment modifications can enable cost-effective improvements to process performance without completely replacing an old mixer. Beyond basic tank dimensions and impeller type, the two most critical characteristics of mixing equipment are impeller diameter and rotational speed. For low viscosity turbulent liquid mixing, impeller power is proportional to the rotational speed cubed and the impeller diameter to the fifth power. For instance, a 10% boost in the rotational speed will raise the impeller power by 33%. Increasing the impeller diameter by only 5% will push up the impeller power by 28%.
When analyzing mixing equipment, you must ensure accurate measurement of the speed and impeller diameter. Changing the speed of an existing mixer may be difficult and expensive. Modifying or replacing an impeller may be easier and less costly. However, an impeller modification, especially if it adds weight, may cause mechanical problems that require evaluation before changes are made.
All mixers are 100% efficient with respect to impeller power input. All the power delivered to the fluid eventually becomes molecular motion, which is heat. In most situations involving low viscosity blending, the amount of power input to the fluid doesn’t noticeably affect the process temperature. In cases of intense or high viscosity mixing, the power input can cause a measurable and sometimes undesirable increase in the process temperature.
The Impact Of Viscosity
In a variety of mixing processes, viscosity plays a major role in determining success or failure. Viscosity basically is the internal resistance to fluid motion. However, that resistance takes many different forms. Newtonian viscosity, which is the simplest type, follows Newton’s definition of viscosity, i.e., a constant describes the relationship between shear stress and shear rate. However, that doesn’t mean that viscosity is always the same. Temperature affects viscosity. Viscosity usually is lower at higher temperatures, which means that blending becomes easier. Merely adjusting the process temperature can solve some mixing problems.