Chilled water system design

[mad-dawg]

Help please
I have to design a chilled water system which includes three chillers and three CRAC indoor units.
The pipe and pump sizing is no problem and I know I need a buffer tank of 1000 litres.
The question is; can I use the buffer tank as a feed/ expansion tank (fit a ball valve and mains feed) or do I have to have a seperate expansion vessel and if this is the case do I need a sealed buffer tank and a pressurisation unit which seems excessive as the system is quite small (150 kW). My thinking is that the water volume will expand and contract and this wil be accomodated by the buffer tank, the feed valve will top up if necessary or can the system use a manual fill as and when required?
Thanks for your help

[frank]

Buffer tanks do not contain any facility for expansion, i.e. membrane or air pocket. They are intended to smooth out the return temp to stop the chiller cycling un-necessarily. You need to fit an expansion vessel/pressurisation unit or manual fill

If you go for the expansion vessel/manual fill option, I would install a pressure switch into the control circuit to hold the chiller off on loss of pressure.

[Brian_UK]

I would go for an ordinary feed and expansion tank.

It's simple, cheap and does the job.

Well that's what we did on the last chiller install; it also had a buffer tank.

[Lowrider]

I would place the buffer vessel in the bypass line!

[mad-dawg]

Thanks for the advice.
I was planning to use an open/vented system with the chilled water flow from the chillers into a common primary circuit returning to the open buffer tank pump then back to the chillers, similar to a LPHW heating system. Is this possible of will the pump cavitate because the system is not pressurised?
I have attached a schematic and would be very gratefull if youo can advise whether the proposed design will be ok.
Thanks

[Lowrider]

Here's a layout of a sound hydraulic layout for a chilled water system that works and can be controlled.

In your design you'll have a variable flow through the chillers which is bad practice!

[mad-dawg]

Thanks Lowrider
Your design is the ideal solution but I inherited some of my layout and the pump quantity is fixed. The existing system had no buffer or expansion vessel just a F/E tank.
As usual the client does not want to spend money so I plan to install regulating valves in each chiller return leg to try and equalise the flow.

[Lowrider]

If you stick with that design you'll end up costing the customer more money! In order to get the right temperature from the chillers you'll have to run one at a much setpoint or have two running at very low load!

With a well designed hydraulic layout one can safe in excess of 30% on energy!