Primary/secondary pumps
Primary/secondary pumps
(OP)
What would happen if primary pump in a variable primary system is oversized? In other words it can handle primary loop and secondary loop pressure drop by itself but we still have the secondary pump installed.





RE: Primary/secondary pumps
As long as this oversized primary pump is not causing problems at the chiller (assuming this is a chilled water system), like exceeding maximum evaporator flowrate, then it should be no problem at all to the operation of the system.
The additional flowrate in the primary loop will not change anything in the secondary loop; you will still need secondary pumps that are capable of handling the design flow and pressure drop of your distribution system. If you're trying to eliminate the secondary pumps and go to a single loop system, you will have to do a lot of system re-design.
---KenRad
RE: Primary/secondary pumps
RE: Primary/secondary pumps
RE: Primary/secondary pumps
RE: Primary/secondary pumps
RE: Primary/secondary pumps
RE: Primary/secondary pumps
RE: Primary/secondary pumps
RE: Primary/secondary pumps
If you have a loop 6" by 100' long (15 ft head), with a secondary loop off of this one at 4" by 50' long (20 ft head), the pump will only pump water through the 6" pipe because it takes less energy to do so. An improperly sized pump will just follow the pump curve to the actual head loss of the system (higher flow than intended), look at a pump curve, you should be able to figure that one out. A problem occurs with this because now you may end up cavitating your pump (beyond the limits of the pump curves).
The only way to get the primary pump to pump through both primary and secondary systems is to add a block to the primary system after the secondary system branch off. The blockage (valve, coil, whatever) would need to have a head loss that equaled or exceeded 20 ft. head loss to make the pump want to divert down the secondary line.
Run some system head loss calcs using different paths. If you know what you're doing, you should be able to see how the pump will be effected.
PS. If the primary pump is oversized, crank down on the balancing valve.