Conserving wasted energy by replacing an overpowered cold-water booster pump station is becoming the norm for many high-rise buildings.
A question that gets asked very frequently is… “Why are you recommending a replacement booster pump system that has much smaller motors than the pumping system that is currently servicing the building?”
Why are my current booster pumps over-sized?
Most buildings in Ontario are designed with multiple, single stage centrifugal pumps that can operate together to meet water pressure demands of the building.
What is a single stage centrifugal pump?
A single-stage centrifugal pump has one, single impeller – like a propeller on a boat – that is driven by an electric motor. So long as the impeller is in contact with water, it will add pressure energy to that water. How much water and how high a single-stage pump system can push water will be determined by how many pumps are running, the size of a pumps impeller, and how quickly each impeller is spinning. A lot of time and effort goes into making sure a single-stage pumping system will safely meet a buildings water demands.
Best Efficiency Point
A single-stage pump will have a Best Efficiency Point (BEP) were it runs most economically. This would mean that you are matching the horse power (HP) created by each motor with the output of water being discharged. Due to system pressure drops and friction loss, it is very hard to achieve a BEP without varying motor speed and torque.
Therefore, most high-rise buildings are designed with large, overpowered single-stage pumping systems that use control valves to regulate the high pressure being created.
Why are you recommending a pumping system with smaller motors? Can it meet my buildings demand?
As stated earlier, pumping systems will have a BEP. If you match up the correct size of motor, control the motor speed and torque with Variable Frequency Drives (VFD) – and most importantly – if you use the correct type of centrifugal pump(s) this result is achievable. The right pump in this case would be a “multi-stage” centrifugal pump.
What the heck is a multi-stage centrifugal pump?
A multi-stage centrifugal pump adds energy to water using two or more smaller impellers, housed in a cylinder that are called “stages”. These closely grouped stages spin together on a central axis being driven by an electric motor. How high you can push the water is now more dependent on how many stages a pump has, not how big a motor is. There is a very good reason for this…
Kinetic energy is generated as water passes from one stage (impeller) to the next. This action continually increases water pressure through the passing of each stage. All things being equal, the motors driving a multi-stage pump can be significantly smaller, achieve the same water distribution result, and conserve more energy than a single-stage system when pumping clean water.
Multi-stage pumps can be sized by flow and pressure simultaneously (over a wide range of flows and pressures) and match BEP with average consumption. As a result, peak efficiency is obtained with smaller motors that optimally drive multi-stage pumps.
A tremendous amount of energy savings comes from the ability to use a smaller motor size alone.
So how do you achieve the “overall” best efficiency point?
The best way to get the most efficiency out of your booster pumps system is to have it regulate motor energy used by water demand changes, or rather, pressure drops in the system. VFD’s can regulate the motor speed (RPM’s) to match pressure variances in real time as water being used by residents change in a building (to learn more about variable frequency drives, see my article on controlling motor speed).
Combining VFD’s with multi-stage pumps – your best choice
In addition to the energy saved by allowing smaller motors to be used, a multi-stage pump is 37% more efficient than a single-stage pump when conserving electricity by varying motor speed. It is this reason moreover that companies recommend a complete system replacement as apposed to simply adding VFD’s to your buildings current single-stage pumping station.
I hope this helps to answer the question of how and why booster pumps systems can be down-sized (use smaller motors) to maximize energy efficiency. Please bear in mind, it is a complicated process to properly assess consumption data and build an efficient pumping system to match a buildings water demands. Always use a company that has experience with proper pump/motor sizing when considering replacing a booster pump system to conserve energy. You will always be guaranteed a successful result when you do.