Chilled water Pump

[Bashir01219]

We have a closed loop chilled water system with three circulation pump installed in parallel. Capacity of the three pump is not equal, two of them is 55 m3/hr and another one is 45 m3/hr. Did it cause any problem with the smaller one if we run the three pump at a time?

[install monkey]

Depends if each pump outlet has non return valves

[frank]

I would ask if the connected pipework can handle 155m3/hr?

[Bashir01219]

yes we have non return valves for each pump. NRV is good enough?

[Bashir01219]

Connected pipe can handle more then 500 m3/hr. So may be it's not a problem.
But as we know "All parallel installed pumps must have the same head this usually means they must be running at the same speed, with the same diameter impeller". It make me confused.

[Sandro Baptista]

Connected pipe can handle more then 500 m3/hr. So may be it's not a problem.
But as we know "All parallel installed pumps must have the same head this usually means they must be running at the same speed, with the same diameter impeller". It make me confused.

Each pump has a certain flow at a certain head. When all pumps works at the same time it will obligate all the pumps working at the same head. So an equilibrium of each pump will be reached in manner that a specific flow will be obtained at that head. But that it's not a problem if the head is inside the permissible range of the all pumps, so you will not have problems of minimum flow, as example.

It's not mandatory the diameter impeller and the speed be the same.

[Bashir01219]

Thanks.
Is it make any adverse effect on the smaller pump, as smaller pump will take similar load like bigger one?

Each pump has a certain flow at a certain head. When all pumps works at the same time it will obligate all the pumps working at the same head. So an equilibrium of each pump will be reached in manner that a specific flow will be obtained at that head. But that it's not a problem if the head is inside the permissible range of the all pumps, so you will not have problems of minimum flow, as example.

It's not mandatory the diameter impeller and the speed be the same.

[Sandro Baptista]

Bashir,

You find on the market "smaller" pumps with large Head and small flows and "big" pumps with small Head and large flows. That it doesn't matter if the operating point is inside of the operating envelope of the pump.

If you want you can send me the curves of each pump and I will check. Also need to know what's the differential pressure between the inlet and outlet of the pumps. Also what's the density of the fluid (is it water)?

[Bashir01219]

Bashir,

You find on the market "smaller" pumps with large Head and small flows and "big" pumps with small Head and large flows. That it doesn't matter if the operating point is inside of the operating envelope of the pump.

If you want you can send me the curves of each pump and I will check. Also need to know what's the differential pressure between the inlet and outlet of the pumps. Also what's the density of the fluid (is it water)?

I am attaching a simple diagram of our chilled water system. Give me some idea how the system will work? Our total pump flow rate is lesser than chiller required flow rate, so if we add a bigger size pump by replacing on of the three pump, is it will work or we need to do any modification?
In the isometric diagram all AHU are controlled by 3 way valve and FCU are controlled by electrical solenoid valve and sensors. We run Pump and chiller manually when required at morning. Pump model DMT-65-320 & DMT-50-320 brand: MONOFLO.
If you required any further data I can supply.

[Magoo]

an increase in NPSH ( net positive suction head ) will increase pump flow and motor current draw of motor, so allow for power increase on any pump if any other pump stops, comes down to selection of pump and motor Kws. Mono pumps are good selection so long as installed in horizontal arrangement so are self ventilating for any air in system.

[Bashir01219]

an increase in NPSH ( net positive suction head ) will increase pump flow and motor current draw of motor, so allow for power increase on any pump if any other pump stops, comes down to selection of pump and motor Kws. Mono pumps are good selection so long as installed in horizontal arrangement so are self ventilating for any air in system.
Its a closed loop, so may be NPSH is not an big issue only pipe friction is our concern.

[Sandro Baptista]

I am attaching a simple diagram of our chilled water system. Give me some idea how the system will work? Our total pump flow rate is lesser than chiller required flow rate, so if we add a bigger size pump by replacing on of the three pump, is it will work or we need to do any modification?
In the isometric diagram all AHU are controlled by 3 way valve and FCU are controlled by electrical solenoid valve and sensors. We run Pump and chiller manually when required at morning. Pump model DMT-65-320 & DMT-50-320 brand: MONOFLO.
If you required any further data I can supply.

I don't know that mark of pumps. Can you give me the selected curves Q-H.

If you replace one of a pump (for instead the smaller one) by bigger one the flow will increase and the head loss on the pipe, on the aircoolers and on the chiller will be higher due to the increase of the velocity on the pipe (dependence is the square flow, app.). See how much flow is need and what the total head loss you have on the piping. Also have in mind that increasing the head of the pumps each pump will delivery less flow than the present one.

You also could increase the speed (N) of the two bigger pumps using a frequency converter for both the pumps. The H varies with N^2 and the flow with N. Be aware the electrical power will vary app. with the N^3.

[Sandro Baptista]

...Or why not increasing also the speed of the smaller one? All pumps running with the same frequency converter at the same speed.
The system should be pressurized because with the increase of the speed the NPSH of the pump will be higher and they could cavitate mainly when the water is hot, at the cooling period.

[Peter_1]

Each pump has a certain flow at a certain head. When all pumps works at the same time it will obligate all the pumps working at the same head. So an equilibrium of each pump will be reached in manner that a specific flow will be obtained at that head. But that it's not a problem if the head is inside the permissible range of the all pumps, so you will not have problems of minimum flow, as example.

It's not mandatory the diameter impeller and the speed be the same.
Sandro, good reply

[Bashir01219]

...Or why not increasing also the speed of the smaller one? All pumps running with the same frequency converter at the same speed.
The system should be pressurized because with the increase of the speed the NPSH of the pump will be higher and they could cavitate mainly when the water is hot, at the cooling period.
I can measure the required flow rate but How can I measure total head loss on the Pipe (If I understand correctly, there is no head loss in a closed loop system, only friction loss. The chiller is installed in roof top and pipes vertical lift is 60' and horizontal lift is 500' supply+return)? Selected Q-H is not in the installation book, can we prepare it now? If yes then how?
If we increase the pump rpm, will it not affect on pump motor?

[Sandro Baptista]

I can measure the required flow rate but How can I measure total head loss on the Pipe (If I understand correctly, there is no head loss in a closed loop system, only friction loss. The chiller is installed in roof top and pipes vertical lift is 60' and horizontal lift is 500' supply+return)? Selected Q-H is not in the installation book, can we prepare it now? If yes then how?
If we increase the pump rpm, will it not affect on pump motor?

Bashir the head loss is pressure loss, which includes friction loss either. Of course if you have a closed loop with all the pipe path fill with water the geodetic effect doesn't count.

Measure the pressure loss by measuring on the gauges the pressure you have at the discharge of a operating pump (before check valve) minus the pressure loss at the suction inlet.

Without the Q-H curve you can risk damaging the pumps internally. Try to get it from the internet or manufacturer.

If we increase the pump rpm the motor can trip by overloading but also it can happen nothing if presently the shaft power absorbed is far way from the nominal power. You can check that on the the Q-H-P curves and check it with a current meter.

[Bashir01219]

Bashir the head loss is pressure loss, which includes friction loss either. Of course if you have a closed loop with all the pipe path fill with water the geodetic effect doesn't count.

Measure the pressure loss by measuring on the gauges the pressure you have at the discharge of a operating pump (before check valve) minus the pressure loss at the suction inlet.

Without the Q-H curve you can risk damaging the pumps internally. Try to get it from the internet or manufacturer.

If we increase the pump rpm the motor can trip by overloading but also it can happen nothing if presently the shaft power absorbed is far way from the nominal power. You can check that on the the Q-H-P curves and check it with a current meter.
Thanks a lot. It's working.
Do you have any operation guideline for closed loop chilled water system with basic design criteria?

[moideen]

find the link.it might be help you

http://www.cedengineering.com/upload/HVAC%20Pump%20Characteristics.pdf

[Sandro Baptista]

find the link.it might be help you

http://www.cedengineering.com/upload/HVAC%20Pump%20Characteristics.pdf

That's a "hell" good information. Thanks a lot...it's very useful...as much info we can get better...

[Sandro Baptista]

Thanks a lot. It's working.
Do you have any operation guideline for closed loop chilled water system with basic design criteria?

Bashir,

Thanks your feedback.

What you have done exactly?

[Bashir01219]

Thanks.
We have calculated the head loss and find H vs flow curve of the pump. Now we have a clear idea what our problem is. I hope so!
We asked new quotation from the supplier according to our requirement, and also give them details data so that they can suggest us for betterment.


Bashir,

Thanks your feedback.

What you have done exactly?

[Bashir01219]

Thanks Sir.

Already I have collect it. CED engineering have lots of effective reading materials.


find the link.it might be help you

http://www.cedengineering.com/upload/HVAC%20Pump%20Characteristics.pdf

[Sandro Baptista]

Thanks.
We have calculated the head loss and find H vs flow curve of the pump. Now we have a clear idea what our problem is. I hope so!
We asked new quotation from the supplier according to our requirement, and also give them details data so that they can suggest us for betterment.

But said that is working...what changed? I presume you are waiting for the quotation so you didn't install yet the new pump.