prevents pumps from pumping in series with each other
and prevents valves from having a hard time opening or
closing against high head pumps. This piping technique
also allows the valves and injection pumps to be properly
sized to the loads they are intended to control.
What is primary-secondary pumping?
It is a pumping technique that is simple in both theory
and application. It is based upon a simple rule that
states: When two circuits are interconnected, flow in one
will not cause flow in the other if the pressure drop in the
piping common to both is eliminated.
How do you eliminate the pressure drop in the
common pipe?
This is achieved by keeping the supply and return tees to
the secondary circuit very close together! (Maximum four
pipe diameters) This means you can have two circuits
interconnected, (for example, a boiler loop and a radiant
loop, each with its own pump) but the pumps from each
circuit will not cause flow to occur in the other loop.
How do I properly size the mixing device?
The size of the pump or valve is based upon the required
flow rate from the high temperature loop. This flow rate
will then blend with a portion of the cooler return water
to supply the desired “mix” water temperature. This is an
example to calculate the necessary flow rate:
1. Radiant zone load =100,000 BTU/H designed with a
20°F temperature drop.
2. Design radiant zone flow rate =10 gpm
3. Radiant design supply temperature = 120°F
4. Return temperature = 100°F.
5. Boiler loop supply temperature = 180°F
6. The temperature difference between the boiler loop
supply and the radiant return loop is 80°F. To calculate
the flow rate required; divide the BTU/H load of
the radiant zone by the temperature difference
(delta T) x 500. 100,000/ 80 x 500 = 2.5 gpm.
7. The required flow rate is only 2.5 gpm of 180°F
boiler water. This water will mix with 7.5 gpm (10
gpm–2.5 gpm) of 100°F radiant return water to supply
a design water temperature of 120°F of 10 gpm.
Therefore, the control valve or injection pump should
be sized for a flow rate of 2.5 gpm.
If you have any questions or comments, e-mail me at
gcarey@fiainc.com or call me at FIA 1-800-423-7187 or
follow me on Twitter at @Ask_Gcarey. ICM
ICM/September/October 2019 15