# When is a buffer tank used and how is its size calculated?

929 views asked Sep 10, 2014

+1 vote

Boilers and chillers in a hydronic system need a minimum water flow at all the times during operation to avoid the buildup of scaling inside them. In some systems which the total water volume in the entire hydronic system such as boilers or chillers, piping, AC units' heating or cooling coils, etc. is lower than the minimum required flow per boiler or chiller manufacturer, then a buffer tank needs to be installed in the system. The buffer tank's capacity should be equal or larger than the difference between required minimum water flow for boilers or chillers and the total water volume in the entire hydronic system. The buffer tank should be installed in the mechanical room, the inlet of the tank should be connected to the main return pipe coming from the building and the outlet of the tank should be connected to the main return pipe going to the boilers or chillers (in fact the tank is part of the main return pipe in system).

A buffer tank is typically used when there is a variable cooling or heating requirement. In such applications the tank is used as storage to cover peak loads or in situations when a surge in demand exceeds the capacity of the cooling or heating system.
When a cooling system starts up, it increases energy consumption and wear on the cooling compressor, in comparison to continuous operation. A buffer tank is well-suited to situations where cooling or heating loads are small because it reduces the number of starts and thus decreases wear and energy consumption.

Buffer tank sizing for boiler:

C = 0.15 x MBH x ƞ x ƒɾ

C = Buffer tank capacity in Gallon

MBH = Boiler firing rate (output) in MBH (BTUH/1000)

Ƞ = Boiler efficiency (from manufacturer)

ƒɾ = Boiler minimum firing rate (from manufacturer)

Buffer tank sizing for chiller:

C = (C1 x g) - C2

C = Buffer tank capacity in Gallon

C1 = Chiller capacity in Ton

C2 = Actual system (piping & users) water volume in Gallon

g = Water flow rate in gallon per ton (usually 3 gpm/Ton)

answered Sep 10, 2014 by
The C2 decription is added to the content. Thanks!