Welcome to the Cleveland Mixer Blog! Here we will post common questions and answers that we get from customers. If you don’t see the answer to a question you are looking for, please feel free to reach out to us on the contact page!

How do I get better mixing at the bottom of my tank?  

The single most important consideration is defining what constitutes better mixing. The more time spent in defining the process goals will save time, cost and heartache after the equipment is installed. The more precisely one defines ‘mixedness’ in their vessel, the more likely they will realize the process goals.  

The second most important consideration is the integration of the tank design and the mixer design. Tanks do not mix fluids. They might have coils, baffles, cones, heads and nozzles, but the tank per se can not impose fluid movement. The best and most efficient results are achieved when tank geometry (Diameter vs Height) and the tank bottom do not militate against the mixer’s operation. While cone vessels facilitate fluid draining very well, they create an equally difficult challenge in solids suspension applications as perhaps an overly obvious example.  

Better mixing performance in the tank bottom or in the lower regions of the tank can be realized in four principle ways:  

  1. The most effective way is to increase the impeller diameter. This is because in the power consumption formula for mixers and blenders, the impeller diameter affects power by the 5th power. Please refer to this formula. The impeller flow rate is affected at the 3rd power as diameter increases. As HP investment increases, so to does impeller thrust and impeller flow rate. The dynamic process response in the fluid will increase proportionately.  
  1. The second way of improving agitation at the bottom of the mixing vessel tank is to change the impeller RPM. RPM increases the HP investment and the impeller flow rate linearly and thus, a linear increase in mixing performance can be expected in the vessel.  
  1. The third way to improve the mixing in the bottom of the vessel is to change the impeller geometry or the impeller type. For example, changing from a hydrofoil impeller to a radial impeller might improve mixing depending on a number of different factors such as fluid type, rotation speed and impeller elevation relative to liquid level. 
     
  2. The fourth principal way of improving mixing in the bottom of the vessel is changing the impeller’s elevation relative to the tank bottom and the liquid level. Sometimes, placing the impeller lower in the vessel will improve mixing near the tank bottom. However, this is not universally true.

What does the “O” in the mixer’s model number mean?

The “O” signifies an open tank, non-sealed mixer shaft. A sealed mixer shaft would have a model name of MTDS or MTDM. In this case, “S” would mean a lip seal or a stuffing box with braided packing material. The “M” would indicate a single mechanical seal.

The open tank mixer configuration would feature a steel base plate that is 2 part epoxy coated that can be bolted to a mixer mounting structure on the vessel. This is in contrast to a closed tank mixer that would have a flange (150# class) that mounts to a tank nozzle.

What barrier fluids should be used with mechanical seals?

Barrier fluids for double mechanical seals are determined by the customer. In general, the barrier media is compatible or non reactive with the process media. The purposes of the barrier media are primarily to provide cooling to the mechanical seal and to capture or block atmosphere from entering the vessel. Moreover, the barrier media prevents process vapors or fluid from the atmosphere in the event of an inboard seal leak. The flow rate of the barrier media through a double mechanical seal system is determined by the cooling amount and rate required by the mechanical seal. The barrier media is typically pressurized at least 1 to 1.50 BAR above the vessel pressure.

When to use a heated stuffing box in side entry applications?

In side-entry applications, there are a variety of seals and stuffing boxes that can be chosen depending on your process goals. When dealing with processes such as asphalt or liquids subjected to freezing temperatures, it is important to provide the mixer with a heated stuffing box to ensure that when the liquid cools or freezes it does not rip the packing out of the seal. We can offer two main types of heated stuffing boxes depending on your application; the first is a stuffing box powered by electricity, the other option is powered by hot oil if working in a hazardous area. The benefits are best summarized by having a constant temperature in the stuffing box to prevent the asphalt from hardening and then adhering to the packing. The packing life and the sealing performance would be improved compared to a stuffing box without heating. This is particularly the case if the mixer is run intermittently. If the mixer runs continuously, the benefits are somewhat muted.

When to use baffles in your tank?

  1. If the impeller Reynolds Number is 1000 and above, the baffles have a width that is normally 1/12 of the tank diameter with an off set from the tank wall of 1/3 of the baffles’ width.
  2. If the Reynolds Number is between 200 and 1000, baffle width typically is 1/24 of the tank diameter. The offset sometimes increases to .5 or 1.0 of the baffles’ width.
  3. Baffles are not used in Impeller Reynolds numbers below 200.
  4. The number of baffles is often 3 or 4. This is based on the Reynolds Number and the desired power transfer into the fluid. Tanks with 5 baffles are rare, but not extinct. 1 baffle should NEVER be used due to the extreme reaction loads it will place on the tank and the agitator’s impeller shaft.
  5. Other factors that determine whether baffles are used or not include
    1. Newtonian or non Newtonian fluids
    2. If a vortex would be helpful in drawing in low density powders or fluids
    3. The process’ sensitivity to aeration.

How do I get the Vortexing to stop in a vertical stand pipe?

  1. The purpose of vertical tank baffles is to convert circumferential flow patterns into an axial flow pattern.
  2. Anti vortex baffles are sized according to the impeller Reynolds Number. In certain cases, we do not have an impeller per se, but, we can roughly estimate the bulk fluid velocity based on incoming flow rate and the tank diameter. Bulk fluid velocity is found by dividing flow rate by the tank’s cross sectional area.
  3. The difficult part in sizing the baffles is the nature of the fluid. It is standard practice to use baffles exclusively in Newtonian fluids. For example, paper pulp stock is a thixotropic fluid – shear thinning, time dependent. Why does this matter? Non-Newtonian fluids will stall at the baffles, especially on the back side of the baffle. As such, the baffle sizing will be non-standard – narrower than typical.
  4. For example; 12,000 GPM (gallons per minute) in an 84” diameter tank roughly calculates to 41 feet per minute in bulk fluid velocity. That fluid is moving at a good rate.
  5. We would recommend an iterative approach for the baffle sizing.
    1. Iteration 1:  2 Vertical baffles, 7” wide with a gap of 7” from the tank wall. The baffle should be supported in three places, equally separated, at the top, middle and bottom.
    2. Iteration 2: If the 2 baffles do not provide enough vortex mitigation, I’d recommend adding a third baffle of equal width and gap as described above.
  6. There are two last things that would need to be calculated. The first is baffle thickness, the second is baffle height. Ordinarily, the baffles’ height is determined by the liquid level and the impeller elevation.