Using two glycol storage tanks in a cooling circuit ? Please help

Hello
Here is the situation : I have a group of chillers that cool a glycol solution. I have available two glycol storage tanks (about 50 cubic meters each) that are presently connected in parallel. Presently, all glycol streams are mixed in both tanks, ie cooled glycol from the chillers and hot glycol coming from the secondary circuit are mixed in the tanks.
I would like to know if I can use one tank for storing cooled glycol from the chillers, and the other tank for storing hot glycol from the secondary circuit. The primary pump would then pump hot glycol to the chillers and then to the cold storage tank. The secondary pump would then pump cold glycol to the heat exchangers and then to the hot glycol tank.
What would be the pro's and con's ?

Ahmed

hello Ahmed
We need to know what are the temperatures involved, the flow of the mixture, and if the temperature of the chiller and the users are similar.

You must have an overflow connecting both tanks.

There are some general advantages and disadvantages, but there al also specific points to consider (like pressure drop) depending on the design change. The general ones are:

The advantages are:
- Your chillers will receive only hot glycol with a higher temperature difference which will improve heat transfer.
- Your process will receive colder glycol which will improve the heat transfer.
- You have a buffer with low temperature glycol which may be fed to the process for a while even if the compressors are off. This gives some level of capacity control to the system depending on the flow rate to the process.
- You may have different pumping rates in both circuits

Disadvantages are:
- Your compressors may overload if the hot glycol is too hot. This depends also on the working temperature differential of the evaporator.
- You need two pumps (one extra pump) one to circulate hot glycol to the chillers and another for cold glycol to the process.
- Larger temperature differences may cause thermal stress (depending on the magnitude of course).
- If your pumping rate of the process is much larger than the chiller the overflow will work most of the time and you will have a lower advantage in the temperature differential.

I am for what, I call the split buffer system. (needs balancing pipe)

Benefits as described by Aramis.

Plus generally speaking is also more energy efficient.

Thanks for Mark, Aramis and Mad Fridgie for these replies. I am gathering the data (flow rates and temperatures).
As I said, presently, a first pump pumps glycol from the tanks to the chillers. And a second pump pumps glycol from the tanks to a pipe that feeds the heat exchangers. But these tanks cannot act as "decouplers" since all connections are at the bottom of the tanks.
What I dont know is how much energy I can save from the present situation, where all glycol streams are mixed and poured into and from the tanks.
Again, thanks a lot !

Best give you a non real world example

Imagine;
All mixing in the save tank and same flow to process and chiller
you want 5C going to the process
10C is coming back from the process
Therefore
5C must be entering the chiller
0C must be leaving the chiller.
The chiller saturated suction temp (SST) would be -5C.

Let then presume we a have twin tanks .
The 10C from the process effectively goes into the chiller. and 5C leaving the chiller and going to the process.
All things being equal, the new SST would 0C
the chiller .
the increased SST would give approx 20% more net refrigeration, for very little increase in power use.
So your COP could go from say 3.5 upto 4. So around a 6% saving in power.
Of course this will change depending upon refrigerant, compressor type and working conditions (pressures, flows, temps)

Adding to Mad Fridgie's post increasing the differentials lets you decrease the pump rate to absorb the same amount of heat than before, or if you can't change the speed of the pumps you need to cool for less time to reach the same temperatures.

boost you are giving him a good thinking.

I would go for all glycol coming back direct to the chillers, this will improve the exchange in the chiller as boost is telling you.
Use another chiller to keep the buffer on correct temp.
If the chillers can not keep up, then use the buffer tanks.
In the night use two chillers to cool down the buffer for the day time.