Primary/secondary pumps

[ssn61]

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.

 

[KenRad]

ssn61,

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

 

[walz]

IF the primary pump is variable speed and is oversized also. Why cant you run it at lower speed to provide the design flow. In case of part loads anyway its going to run at reduced speed.

 

[Artisi]

Accepting the previous comments, the only negative of an oversized pump is that is will not be running at its best efficiency which could affect its reliability long term.

 

[imok2]

You should check the pressure drops at the chillers(flo rates)to make sure they are within specs

 

[ssn61]

Thanks for everyone's comments. This is just a hypothetical question. My concern is if we are going to size the primary pump to handle the primary flow and PD, then by some mistake calculate primary pump pressure too high, what would happen to this extra available pressure. Would primary and secondary pumps act as series pumps and provide higher pressure than needed in secondary loop? What happens to primary loop? Will that see extra pressure?

 

[Rabbanee]

It can be used , but oversizing of primary pump without VFD may increase the velocity of chilled water flow thru the evaportaor. As per manufacturer there is limitation in velocity & flow rate. Its better to use constant flow pump in primary circuit & variable frequency drive in secondary circuit.

 

[ChasBean1]

No. If the loop is truly a primary/secondary decoupled loop, over-pumping in the primary would have no adverse affect on the secondary. It would just mean extra unnecessary pumping through your chillers or boilers. However, this depends on how the system is piped. If you could sketch and post a piping diagram it would help.

 

[walkes]

For the primary pumping, loop you would likely have the balancing agent check pressures and flows, if the primary pumps are variable speed the upper flow rate can be set with the correct pressure requirements. If you have overestimated the pressure initially, as long as the actual condition is within the pumps operating envelope (flow and pressure range) you should be fine.

 

[Ziggypump]

Remember, water flows through the path of least resistance. While the pump may end up being sized large enough to push water through the secondary system, it will not do this unless doing so would require less head to accomplish.

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.