Minimum Water Volume 1

[NALUCAS]

Hi

I just wanted to ask a quick question to other people designing chilled water reticulation systems for process cooling and operating during low ambient temps (no less than 5degC).

What minimum water volume are people using for these systems, as we are considering the need for a loop or buffer tank to be installed in the system.

I would appreciate information in SI units, but i'll try and convert imperial if you have them.

Thanks in advance.

Nathan

 

[grim900]

Hi Nathan,

The following is a formula I've used to size a buffer vessel. This gives the total system requirement and sometimes can be used to demonstrate that a buffer vessel is not required (ie if the chiller, pipework and associated equipment already contain more than the calculated volume):

V = (N x 60 x Z) / (4.18 x dt)

V = total water volume (Litres)
N = Capacity of the chiller's first step (kW)
Z = Minimum allowable running time (min 5 minutes)
dt = Temp difference at minimum partload condition (approx 2 deg C)

This will provide sufficient thermal storage within the system to give 5 minutes operation when the machine is not running, therefore ensuring that the equipment will not continuously stop and start at low loads.Of course, you can increase the 5 minutes if you want, this will proportionally increase the volume of water required.

N is in kW
Z is in minutes (but multiplied by 60 to give seconds)
dt is in deg C.
4.18 is approx specific heat capacity in kJ/kg degC

Hope this helps,

Graham