Most health codes require a minimum frequency for filtering the water (turnover) in a commercial swimming pool and many of these codes also mandate the engineering solution for resistance (total dynamic head - TDH) in these systems. The TDH assumptions are often conservative resulting in the selected pump being larger than required to move the required amount of water through the system. As a result, the system is hydraulically throttled down to the required flow rate. Hydraulically throttled systems result in wasted energy and the wasted energy can be eliminated by incorporating a Variable Frequency Drive. The payback for a facility that is operating the pump 24 hours a day, 365 days a year, can be measured in months with the resulting savings dropping to the bottom line. Today, VFD’s are becoming common place with specific adaptation for the aquatic industry that occurred in the last couple of years.
How Does A VFD Work?
Simplistically a variable frequency drive (VFD) adjusts the amount of power sent to the motor to adjust how fast it spins the impeller resulting in the water being pumped. As the impeller spins faster or slower a corresponding amount of energy is consumed. By controlling the speed of the motor based on the actual flow rate of water the VFD is able to maintain optimal water flow as the hydraulic resistance of the system changes based on the loading of the filter.
A more technical description of the VFD interface with electric motors is that a motor-driven system is controlled by the frequency of the supply voltage and rotates on a fixed speed. An alternating current that is applied produces a magnetic field that rotates at synchronous speed. The only way to alternate synchronous speed is through a VFD, which converts the power in three stages. In the rectifier stage, the power is converted to a higher adjustable DC voltage. In the inverter stage, the power transistors in the rectified DC are switched off and on. This produces a voltage waveform at the frequency desired matching the pump load requirements with the energy consumed. From an energy perspective, the power consumed by a pool pump varies by the cube of the change in speed at which it runs – the third affinity law. Therefore a small reduction in pump motor speed can have a significant impact on the energy consumed. For example, reducing the pump’s speed by 15% will reduce the energy used by 39%. To apply this in real dollars, let’s assume a 15 hp motor (typical for an eight lane, 25 yard pool) operating year around in an area of the country with energy costs in the 9.5 cent kilowatt per hour range. An average 15% reduction in pump operating speed would result in an annual energy savings in the range of $2,700 – $3,100.
There are other potential applications for VFD technology if the industry can get health department cooperation in maximizing public health in a cost efficient manner. For example, today’s health codes do not take into account if the pool is occupied or unoccupied with respect turnover requirements. What if we had the speed of filtration was tied to the water quality? As water gets dirtier, the filtration rate automatically increases and as the water becomes cleaner, the filtration rate slows thus saving money. This aligns the filtration solution and operating costs with the actual water quality and does not use an inefficient one size fits all solution. This in my opinion is where the industry needs to go to provide the best result for the patron, owner and public health.