Variable Volume

Variable Volume

Variable Volume regarding water distribution systems is a very loose term that describes many combination of pumps, valves and pipe work layouts. However, the same rules apply. Change the flow, and the pressure in the system will change accordingly.

There are so many different combinations of variable volume systems, from simple switching pumps on and off, to invert drives, to inclusion various types of valves. Equipment such as Chillers do not work well with variable volume as the flow and differential temperature are linked. Lower the flow, the cooling capacity stays the same, the delta changes and the Chiller starts to ice up.

Simply switching of the pump off to a Chiller of boiler makes the system variable volume. Many systems use this method with more than one chiller and pump to manage demand. Other methods of splitting the system in to Primary, Secondary and tertiary allows different systems to work to distribute heating or cooling throughout a building.

  • Variable volume systems
    Can have problems with
  • Noise
  • Over pressure on valves, too much flow
  • Under pressure on valves, not enough flow
  • Cavitation (large pressure drops that destroys the seat)
  • Solution Differential Pressure Control valves

Balancing Valves
Balancing Valves at Branches should be replaced for Venturi’s
Flow Measurement is all that is required.
Design flow achieved using second seat in PICV valve
By-pass to keep water circulating
 

PICV valves
Variable volume Systems have been simplified by
the development of the PICV valve.
One valve replace three

  • Two Port Control valve
  • Differential Pressure Control valve
  • Balancing Valve

DPCV and PICV
When the differential pressure is out of range
of the PICV valve. Typical 5 bar
The Differential Pressure Control valves
can be used to reduce the pressure locally.
The start pressure of the PICV should be maintained

Primary & Secondary
One set of pumps serves

  • Terminal Units
  • chillers

Difficult to Stage pumps to meet load
Problems with the Delta T

Individual Pumps for Primary & Secondary
Constant Primary (could use Buffer Vessel)
Constant Secondary Low Delta T
Very inefficient waste Pump energy
Difficult to Control

Constant Primary
Constant Speed pump used on a variable volume secondary system
As two close, System curve climbs pump curve
Reverse Acting Differential Pressure Control valve required
to avoid over pressure in system
pump over heating

Variable volume Primary and Secondary
Inverts used to control pump.
Differentail Pressure Sensor to control pump
Some Constant volume systems to give minimum turndown
on pump.

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