- #1

AbdullahS

- 6

- 1

- TL;DR Summary
- Calculating the pressure set point required for a set of pumps serving a residential building.

Hi Guys,

The problem I am facing at the moment is to calculate the appropriate setpoint for a set of pumps supplying cold water to a building so that electricity and cost savings can be achieved.

Here is the situation:

There are three pumps connected in parallel. They are located on the ground floor of a building which is roughly 21m high. All three pumps are connected to VSDs. The pressure at the water mains is 450 kPa. The current set point is 750 kPa which results in a 3kW pump to operate at 95% speed.

The local standards mention that that minimum pressure at the farthest outlet cannot be less than 50 kPA. This means that in order to sustain the water column and provide 50 kPa of pressure, ignoring pipe loses, the pressure at the outlets of the pumps need to be 256.01 (=50 + 1000*9.81*21/1000) kPa. Surely, that does not mean that we do not need any pumps, right? I cannot get my head around how it works.

Also the nameplate of the pump indicates that its a 3kW pump with pressure head of 33.7m and flowrate of 17 cubic meters per hour. If I do reduce the pressure setpoint let's just say 650 kPa and use use affinity laws in conjunction with the the data from the name plate, I get shaft power as 1.3kW and a flow rate of 13 cubic feet per hour. Is that the correct way to go about it?

Any help on this will be greatly appreciated.

Regards,

The problem I am facing at the moment is to calculate the appropriate setpoint for a set of pumps supplying cold water to a building so that electricity and cost savings can be achieved.

Here is the situation:

There are three pumps connected in parallel. They are located on the ground floor of a building which is roughly 21m high. All three pumps are connected to VSDs. The pressure at the water mains is 450 kPa. The current set point is 750 kPa which results in a 3kW pump to operate at 95% speed.

The local standards mention that that minimum pressure at the farthest outlet cannot be less than 50 kPA. This means that in order to sustain the water column and provide 50 kPa of pressure, ignoring pipe loses, the pressure at the outlets of the pumps need to be 256.01 (=50 + 1000*9.81*21/1000) kPa. Surely, that does not mean that we do not need any pumps, right? I cannot get my head around how it works.

Also the nameplate of the pump indicates that its a 3kW pump with pressure head of 33.7m and flowrate of 17 cubic meters per hour. If I do reduce the pressure setpoint let's just say 650 kPa and use use affinity laws in conjunction with the the data from the name plate, I get shaft power as 1.3kW and a flow rate of 13 cubic feet per hour. Is that the correct way to go about it?

Any help on this will be greatly appreciated.

Regards,