Designing for Heat Dissipation with Agitators: Factors to Consider

[amikamil]

hi
i am trying to find the heat that will be created by agitator
what are the parameters? what equ i should use and how good it is?

thanks

 

[russ_watters]

What is an agitator? Is it motor driven? Ultimately, all of the input power of an electric motor is turned into heat.

 

[amikamil]

it is electrical motor driven
if the motor is 500W I will get 500W of heat? this is per the agitator design? what i should look for in the agitator so i can minimized the heat generation? what are the parameters in the agitatore design i should take care for? what eq. i can use to calculate the heat generation?
thanks for the help

 

[russ_watters]

This is simply a matter of conservation of energy. Unless you are converting that 500W of electrical energy into another form such as chemical (splitting water into hydrogen and oxygen, for example) or graviational potential (lifting something), there is only one type of energy being utilized in the end process: mechanical kinetic energy. Mechanical kinetic energy is heat energy. So all of your energy will end up as heat.

Now, that isn't to say that your process really requires 500W. Perhaps your "agitator" requires 300W and a gearbox between the motor and agitator loses 200W (to heat). In that case, improving the gearbox would reduce the needed energy input by reducing the waste.

What are you agitating?

 

[amikamil]

This is simply a matter of conservation of energy. Unless you are converting that 500W of electrical energy into another form such as chemical (splitting water into hydrogen and oxygen, for example) or graviational potential (lifting something), there is only one type of energy being utilized in the end process: mechanical kinetic energy. Mechanical kinetic energy is heat energy. So all of your energy will end up as heat.

Now, that isn't to say that your process really requires 500W. Perhaps your "agitator" requires 300W and a gearbox between the motor and agitator loses 200W (to heat). In that case, improving the gearbox would reduce the needed energy input by reducing the waste.

What are you agitating?

i am agitating water and some dissolved materials
my point is i do not want to heat up the fluid on one hand and need good agitation on the other hand

i wish to design and build aggitatore that will do just this
the way i look at it, if i know what bring the drag up, i will know how to minimized the drag and hance the heat generation
thanks for the help so far

 

[russ_watters]

The problem is that drag is the agitation, so reducing drag reduces the amount of agitation being done. I don't know that there is any way to do what you suggest.

I worked on a project where cocoa for chocolate was ground in huge machines. They generated an enormous amount of heat and the jackets of the grinders had to be water cooled to avoid burning the chocolate due to the excess heat added by the grinders.

 

[stewartcs]

Perhaps some type of ultrasonic tank type agitator would work.

I presume you have some type of tank currently with a paddle that is attached to an electric motor which is essentially stirring the mixture. If so (as Russ indicated) what ever the shaft work is will be equal to the amount energy added to the mixture (assuming the motor is outside of the tank - i.e. not in your control mass).

Hope that helps.

CS

 

[amikamil]

thanks you both
i have a tank where the gear and the motor are on the side so all the heat generated by them is out of the equation
my basic question is what control the heat generated in the tank, i presume it is RPM, shape, size, finish and so of the agitator or in other words the drag created by

you know any equation that describe the heat generated? slitted agitator will help? low RPM?

 

[stewartcs]

thanks you both
i have a tank where the gear and the motor are on the side so all the heat generated by them is out of the equation
my basic question is what control the heat generated in the tank, i presume it is RPM, shape, size, finish and so of the agitator or in other words the drag created by

you know any equation that describe the heat generated? slitted agitator will help? low RPM?

If everything is external to the tank (except the paddle) then it's just the shaft work (the shaft attached to the paddle that is) that is causing the energy of the mixture to increase (as far as the agitator is concerned).

CS

 

[russ_watters]

thanks you both
i have a tank where the gear and the motor are on the side so all the heat generated by them is out of the equation
my basic question is what control the heat generated in the tank, i presume it is RPM, shape, size, finish and so of the agitator or in other words the drag created by

you know any equation that describe the heat generated? slitted agitator will help? low RPM?

Sure, you could lower the rpm or change the characteristics of the agitator, but these things are just other ways of saying you could reduce the agitation. If that's acceptable in your process, you could certainly do it to save energy.

There is no easy equation for the characteristics of the agitator, but the power output of a device in fluid flow is typically a cube function of rpm, so, for example, cutting the rpm in half would drop the power input by a factor of 8.

 

[Dr_Zinj]

I'm a little concerned about your requirement to not heat the mixture by stirring. You're not trying to manufacture explosives by any chance, are you?

 

[Q_Goest]

Hi amikamil,
What Russ and CS are pointing out is that there is a simple, conservation of energy going on. Whatever energy you need to rotate the agitator, that energy is going to go into heating the fluid, simple as that. So whatever energy you need to stir up the fluid… that’s how much energy is also added to the fluid.

There is nothing particularly fancy about agitator design. As far as I know, there is as much black art to designing the agitator as there is science and engineering. The agitator is generally sized and tested in the process to determine if it’s sufficiently powerful to do the job.

Another thing is that when agitators are used, for example to mix chemicals in which there might be a chemical reaction, the agitator vessel is often cooled. Conversely, it may be heated if needed. So if you have a process in which you have to reject 500 watts of heat due to the agitator or even due to some chemical reaction, you can design a jacketed vessel that can cool the fluid. Another possibility is to insert sparger tubes and blow air or some gas into the mix to help cool it. With only 500 watts of heat to dissipate, it won’t take much.