Hydronics Primary/Secondary loops - secondary loop pump placement question 2

[USAeng]

I read it is not desirable to install a centrifugal secondary loop pump in the return line of hydronic system as it decreases static pressure in the loop upon pump startup. (see attachment "rule #2")

The engineering firm that designed a system at our plant placed the pump in the return line - the system is already constructed but nothing ran yet.

Can someone give me an idea how we could know if the pump placement is an issue here? Is the concern of decrease in static pressure about cavitation in the secondary pump?

 

[bimr]

If the separate circulator is not in the secondary circuit, you will not obtain consistent flow in the secondary circuit.

Why don't you post a sketch of the system?

 

[USAeng]

See attachment. This is what we would like to understand - why is it shown as incorrect and what is concern?

 

[USAeng]

Bimr,

Thanks for reply. The secondary circulator is in the secondary circuit. A simplified sketch is provided in the last post.

Perhaps the normal placement in hydronics is in the supply to provide some desired pressure to secondary loop? We only require a given flow so perhaps our placement does not matter per the sketch?

 

[USAeng]

Anyone?

 

[bimr]

Potential problems with recirculator on the discharge end:

1. With the pump on the discharge end, you would have to pressurize the common loop to force water into the secondary loop.
2. Difficult to fill the loop on startup.
3. Low suction pressure at the pump.
4. Pump suction problem if pump is installed on the multistory downleg.
5. If the common piping loop was long, you may not have much pressure to push liquid into the secondary loop where you tee off.
6. Centrifugal pumps are designed to pump, not to pull liquid.
7. In general, it is advisable to minimize the length of the pump suction lines.
8. Potential for problems with installations in multistory structures, but may not be a problem on single story structures.

Agree that the pump at the end of the secondary loop is incorrect.

 

[Compositepro]

It always has amazed me how in the field of HVAC everything is made to seem so complicated, and special jargon is used which does more to confuse than to illuminate. To answer your question about placement of the pump in a flow loop, what you show in the sketch could work with no problem. But, it is usually poor practice to place the pump in a return line rather than a supply line. Placing restrictions in the suction of any pump may cause cavitation if the suction pressure drops too low. So, if the primary loop pressure is 20 psia the pump will never be able to create more than 15 or 20 psi of pressure across your your secondary loop, regardless of the pumps capacity. If the primary loop pressure is 200 psia then the pump can create up to 200 psi, if it is capable of it.

Part of the problem is that the terms "supply" and "return" and "primary" and "secondary" can have whatever meaning you want them to have.

 

[USAeng]

Thanks guys. This confirms my thoughts. Much appreciated.

 

[LittleInch]

I agree with compositepro - it could easily be OK depending on the actual conditions (flow/pressure) being seen by the loop. It might be a desire to reduce temperature going into the pump (if this water is hot) or just that the pressure drop in the secondary system is much less than the primary system and hence issues of low pump suction don't happen or if the secondary system pressure is somehow limited and can't be increased above the primary pressure to generate flow in the secondary system.

Hence, yes, often not a good idea, but can be done depending on the particular issues of your system.



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