If by buffer vessel you mean an expansion tank. Then it would be to pick up the thermal expansion of the chilled water from ambient temp. to operating temp. and thermal expansion during operation.
A minimum chilled water volume is required to minimise cycling of compressors and prevent excessive rates of temperature change. Carrier recommend a minimum of 3.5L/kWr chilled water in the system.
If the pipework is minimal, then a tank may be required to achieve minimum system chilled water volumes.
The buffer vessel is there to even out some fluctution in return temperature to the chiller, since Carrier sugests to put the buffer vessel in the return system.
There is a requirement for a minimimum of water content when you have got a small system. With a primery and secondary system, you only look at the content of the primery system.
And last it is to reduce the amount of starts and stop to 6 times per hour or other wise indicated by supplier.
And is it to (buffer) or hold an amount of (chilled) cold water at say 7 degr. C when you are using a 7-12 degr. C system.
Well it was a question. But looking at the last three helps is what I have mentioned as its four purposes for a buffer vessel. Some info I did get from other web sites.
