2 replies to this topic

[JEBradley]

Hi all - not been posting for a while due to a password problem.

I just have a quick question to do with a tank design I've been asked to work on.

Im basically designing a tank 3m in diamter and 6m high. The tank will contain a vegetable oil and to keep it fluid (and prevent it from damage) we will need to keep it warm (above 20 degrees all the time - ambient may drop to say -5 overnight in Winter). Im going to achieve this with a trace heating pad on the bottom at 35 degrees C. Now i've been asked to add an agitator to distribute the heat and prevent chemical decomposition (Im gonna check the last bit but I think thats not true).

So my question is - will natural convection be enough to adequately distribute the heat? If so I can save a fortune on the agitator!!!

Thanks for any help

[Art Montemayor]

JE:

I’ve been down this road before and I wouldn’t take the chance of getting by without an agitator and relying fully on just natural convection to spread out your required heat input naturally. Depending on the nature of the vegetable oil (and my experience tells me that it can vary – as well as it’s inherent fluid state: is it absolutely pure or raw?), you can have a marked lag between the actual heat source temperature and the core of the tank contents – and not to mention the side wall effects.

My experience was with oil having a consistency and viscosity similar to Olive Oil and we couldn’t depend merely on natural convection to save the product – and we were not designing on a temperature as low as yours (I assume you mean -5 ^oC).

One spoiled batch of oil and you will have a mess and a loss on your hands that would have paid for an agitator. The advantage with using an agitator is that you can reasonably predict the expected results – and also control the desired temperature, with the appropriate instrumentation.

It is very difficult to predict consistent and reliable natural convection currents inside storage tanks. There is no possible way to practically check them out or to monitor them. All you can do is try it and see if it works. In your case, I would bet that it probably will not work if the oil matches that of my experience. You are the ultimate judge; all I can do is alert you to the fact that you may have more to lose economically than what you initially save in capital investment cost.

Good Luck.

[Ionut]

I've had a similar experience wiht a very small tank (1m3) of hydraulic oil.
It's a tank where amblient temp. oil was pumped for filtreing an heting to about 40 degrees C.
A pt100 sensor wiht a dedicated controller was mounted in order to control the heating.
At jus only 1 meter height the cold oil would be naturally mixed with the existing warm oil in a period of about 10 time longer, than forced mixing.
We had to modify the output of some reservoir filter in order to produce a powerful stream of cold oil in the warm oil.
This way the oil was continously mixed and the temp probe/controller reacted correct.

I don't know for sure if your oil had similar properties with hydraulic oil, but please alow me to post.

Thank you.