Zone pumping strategy 1

[ECab18]

Hi,

Does anyone have any experience on installing a distributed zone pumping arrangement? How do you solve the imbalance on your system when your zone pumps is at part load? I'm thinking balancing valves would be tricky as you need multiple differential pressure on your main. I've attached some sketch to clarify my concerns.




Cheers.

 

[EnergyProfessional]

The only imbalance I see is that your flowrates don't match up. under part load you should have 1000 l/s system flow.
why doyou think you need to balance anything?

what are the branches, cooling coils? Is they are 2-way, all should be good.

 

[ECab18]

Hi,

Thank you for your response. Imbalance will occur when a zone goes on part load. VFD will ramp down the pump therefore there will be a point where two return line meets at different pressure.

 

[EnergyProfessional]

why do you care about the pressure? I assume the pump is controlled to meet an AHU DAT instead of a valve? think of static pressure reset. I think your sketch needs more explanation.

 

[ECab18]

On the point where the return water is at higher pressure, the return water from the other zones will not enter back into the main line.

 

[Lnewqban]

Could you zoom out that schematic to show more?
Are the main pipes pressurized?
Are those boost pumps?
No valves means zero flow cannot be achieved through a pump (if centrifugal type), even when working at minimum practical frequency and that max/min flows may be affected by modulation of other pumps.

"Where the spirit does not work with the hand, there is no art." - Leonardo da Vinci

 

[EnergyProfessional]

What determines pump speed? You really need to tell us more about what this system is supposed to do.
if pumps are controlled to some temperature, they will speed up to get the required flow.

 

[ECab18]

Pump speed will be based upon the diferential pressure setting on each zone. Each zone has multiple AHUs and FCUs with PICV. Load requirement for each zone goes down, pump speed ramp down.
The issue here is the imbalance on each point where the flow/pressure on each zone meets.

 

[LittleInch]

Is that box thing at the LH end a HX?, i.e. you are running a closed loop separate from the district cooling system?

The system will simply balance out. At the point where each branch meets the headers, whether inlet or return, the pressure is the same.

If what you are controlling on is differential pressure from inlet to outlet, then each pump will simply ramp up or down to meet that flow.

If the green loop is a loop then any higher pressure in the return header will either mean increased flow or will raise the pressure in the feed header.

So your problem "therefore there will be a point where two return line meets at different pressure.

So your imagined problem "therefore there will be a point where two return line meets at different pressure" is not physically possible.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ECab18]

I hear you. But saying the system will "simply balance out" can't sink in. These pumps are sized and run at different head. Also note that there is no re-circulating pump on the main which is typical for a secondary/tertiary arrangement.

I've drawn up the pressure diagram and it just doesn't add up to what you are saying.

 

[EnergyProfessional]

the pumps have different head because they "use up" that head in their respective circuit. Unless they are over-sized, they won't impact each other. the key is each pump is controlled based on the actual flow demand, like with static pressure reset etc.

 

[ECab18]

Hi,

Their respective circuit include the main loop, so they are oversized in the sense that zone TDH + main circuit loop.

i.e,

Main loop head = 10 Bar,
Zone 1 head = 15 Bar
Zone 2 head = 20 Bar
Zone 3 head = 25 Bar

Pump Duty
Zone 1 Pump = 25 Bar
Zone 2 Pump = 30 Bar
Zone 3 Pump = 35 Bar

At the poin where the circuit of Zone 1 at Zone 2 meets, Zone 1 could be 7 Bar and Zone 1 could be 5 Bar.
If so, how can we balance it?

Let me have a look at the static pressure reset control. Because in my mind, the static pressure reset would be based on each zone circuit only. It will not see what is happening on the main loop.

 

[Lnewqban]

It seems to me that your approach to the problem is not accurate.
Physically, you cannot have several pressures (big differences) downstream each coil.

If you make all the return pipes big enough, you can see that the Pascal's law will prevail.
Otherwise, you could have flows in different directions within the return pipe.

Each pump is going to naturally give the fluid the amount of energy to overcome the resistance of the coils and control valves and to discharge into a mass of water of relatively similar static pressure.

"Where the spirit does not work with the hand, there is no art." - Leonardo da Vinci