# Water bath system - flow rate problems

## Main Question or Discussion Point

Hi, I hope someone can be of help with this problem I have, as I have tried different forumulas from different places but still havn;t come up with a sensible answer.

I am currently developing a modification to a rotary fatigue rig. The steel tube being tested is required to be placed in synthetic sea water. Therefore a tank is to be fabricated which will allow the steel tube to pass through and be completely submerged. The water must be temp, salinity, and pH controlled. Also the water must be continuously replenished and therefore to house the equipment to control water properties and to create a continuous cycle, a second tank is required. Tank A will be up a height, base of the tank is roughly 1.2m from ground level.

The set up of my system is as follows:

Tank A – pump from tank B will fill this tank with sea water. The sea water must be drained from this tank back in tank B to continually replenish the tube with sea water. This tank is rectangular and holds a volume of 18804500mm3 (18 liters). This will have a sealed lid. Height of the water within the tank will be roughly 0.16m

Tank B - filled with sea water (temp could range from 5-36 degrees) containing pump with flow rate of 300lph. Power of pump does not have to be 300lph but I need a decent pump which can run continuously in sea water. Therefore a pond pump is required. Size of tank is undetermined but both tanks are required to have water in them at all times to allow the continuous cycle of water from one tank to another. This tank must be bigger than tank A, twice the size perhaps.

So tank B will pump water into tank A and tank A will require hoses to drain the water back into tank B. Creating the cycle.

The problem I foresee is that tank A will not be able to drain the water quickly enough, therefore causing the tank to overflow. How do I calculate the flow rate from tank A back into tank B? For example if 1 inch hoses were used to drain the water, how do I calculate the flow rate of this hose to determine how many hoses I need and if I can accomplish draining tank A in a fast enough time from hoses alone? Or do I require secondary pump to pump water back from tank A back into tank B? If so, how to i calcuate the balance required?

Thanks for any help, if there’s any info required I have not given please let me know and I will provide all I have.

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Q_Goest
Homework Helper
Gold Member
Hi Allan,
This is a pretty simple problem if I understand you correctly. The flow from A to B is due to the drop in elevation - tank A is higher than tank B. So you have pressure driving the flow of water from A to B which, assuming neither tank is pressurized, is simply the density times gravity times height (rho*g*h). Countering this flow is the resistance of your hose/pipe/tube. The only thing you need to figure out is the total resistance to flow, then apply the Darcey-Weisbach equation.

This is very simple if you have a program to do it. If you list the various restrictions and height difference it takes less than a minute to set it up and run. Otherwise, use the attached information to determine resistance and do a calculation on it.

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