Because most hot water/chilled water systems are or should be
utilizing variable speed technologies to reduce energy costs
and improve comfort, control valves play a vital role in success
or failure of an efficient hot water/chilled water system. In
direct return systems, the valves located closest to the pumps
experience the highest differential pressure (DP), those hydronically
furthest the lowest. Typical valves are selected based upon
the equation Q = Cv*DP, where Q is the flow rate,
Cv is a flow coefficient equal to the amount
of flow thru the valve at a pressure drop of 1 PSI, and DP is
the differential pressure across the valve’s control surfaces.
Cv is a constant in all but a very select group of valves that
employ variable Cv technology.
This is all very important because the differential pressure
across the valves’ control surfaces varies constantly as other
system components modulate. The below graph shows a real time
graph of changes in flow rates – for a constant load - for a
typical (constant Cv) control valve and one that uses variable
Cv technology as the differential pressure varies from 5-15
On variable flow systems, control valves experience large changes in differential pressure
across their control surfaces as the flow rates modulate, usually to maintain an established
system differential pressure setpoint established as required to satisfy all coil flow
requirements. This is shown graphically in the chart below.
All of this is important because of the energy scourge known as low system
temperature differentials. The combination of central plant piping, coil
selection criteria and typically used control valve capabilities all
combine – among other things of course – to cause system inefficiencies.
Remember, the most efficient chiller system you have is the one you never