Reducing pressure in a chilled water system

[Mamat31]

Hi all,

I am new on this forum, hope you are doing good.
I have a question related to a project I am currently working on.

The system is a 40+ floors mix used building, with a cooling system composed of a couple of centrifugal chillers, and a primary/secondary pumping configuration.

There is a pressure breaker (heat exchanger) on the 20th floors, and another chilled water loop circulating water on the upper floors (20 +), so there are no problems on this side.

Pressure on the lowest floors of the building is about 12 bar (static + dynamic).

What could be the feasible options to reduce the pressure and maintain it at a value below the 10 bar threshold on the lowest floors?

It would be possible to create another "zone" through the usage of another HX and pumping, but if possible, I would like to avoid this solution. Could a pressure reducing device in a certain configuration would work?

Thank you for the help and comments.

 

[Tom.G]

Assuming 10 feet per floor, those 40 stories alone give about 12 bar static pressure. Sure looks like another HX and pumps are needed at the 20th floor!

 

[russ_watters]

What is the height between floors? For 20 floors, static head alone is going to get you near 10bar.

 

[Mamat31]

Correct, static alone brings me to 8.5 bar..

 

[russ_watters]

Correct, static alone brings me to 8.5 bar..

Ok, so what's the pressure drop required across the air conditioning units? 1.5bar is not a lot of pressure to run a chilled water system (secondary, I assume?). You really don't have any easy options beyond reducing the flow rate, adding booster pumps or re-feeding the entire system with 3 secondary loops instead of 2.

 

[Mamat31]

Hi russ, thank you for taking the time to answer.
That is correct, secondary pumps are circulating chilled water through the AHUs. Current pumps are oversized and rated with a total head of around 4 bars, which brings the pressure around 12 bar.

My question is more like this: Is there an option available to maintain the pressure under 10 bar other than making another secondary loop (with HX and additional pumps)? (i.e. reduce my static pressure)

Thank you and have a nice day

 

[russ_watters]

Hi russ, thank you for taking the time to answer.
That is correct, secondary pumps are circulating chilled water through the AHUs. Current pumps are oversized and rated with a total head of around 4 bars, which brings the pressure around 12 bar.

My question is more like this: Is there an option available to maintain the pressure under 10 bar other than making another secondary loop (with HX and additional pumps)? (i.e. reduce my static pressure)

I'm going to need more details and context, but I'm thinking probably not without major changes.

-Why do you want to reduce it below 10 bar? Are you having a problem?
-Was it designed for 10 or 12 bar? Similarly; 4 bar for dynamic pressure is not atypical and 1.5 bar *is* unusually low. Why do you think the pumps are oversized? Are they speed controlled (based on differential pressure?)?
-Is your system dynamically controlled or constant speed and hard balanced (with balancing valves instead of control valves)?

 

[Mamat31]

Hi again russ,

Sorry for late answer!

- There is no problem atm, let's say that by maintaining pressure under 10 bar, I could be able to do a little twist to the system which bring interesting energy savings
- The pumps are indeed controlled with VFD. However there is no DPT neither logic controller, and they are currently set on a constant frequency (which is also far from ideal). With the current situation, I am, at the word point in the piping run, at 11.5 bars.
- The system is, as current, as primary constant/secondary constant. All the loads are equipped with 2 way, motorized valves. Since the secondary pumps are not controlled, you have some pressure unbalances when the valves close...

Let me know if you have some ideas.

We can also continue the discussion in DM if you need more details :)

Have a nice day!