By Mike Fowler
Pumps are the heart of a pool circulation system—not only are they the vehicle that moves water through the various systems, but also now the focus of substantial rebates which can drastically reduce the costs of owning andoperating a pool. That said, it is important to get back to basics and understand the significance of hydraulics on residential and commercial pools and why utilities in Ontario, for example, are pushing for consumers and aquatic facilities to install variable-speed pumps (VSPs). To take advantage of utility rebates, pool professionals need to understand how pumps work, what the difference is between single and variable-speed models, and the factors that influence energy savings in the equipment room.
How centrifugal pumps work
All pool pumps are centrifugal. They have a shaft-driven impeller that rotates inside a casing, which allows water into the suction port (inlet) where it is then ‘thrown’ to the outside of the casing before exiting through the discharge port. The impeller is a circular pump part that rotates to provide centrifugal force for propelling water in the circulation system. It spins on the end of the electric motor shaft and is the only moving part of a centrifugal pump.
The velocity (or speed) transferred to the water by the impeller is converted to pressure energy, otherwise known as total dynamic head (TDH). Two things make centrifugal pumps unique:
- They can provide high flow rates.
- Their flow rate varies considerably with changes in the TDH of the particular piping system. This allows the flow rate to be ‘throttled’ considerably with a simple valve placed into the discharge piping, without causing excessive pressure build up or the need for a pressure-relief valve.
However, throttling the flow rate of a centrifugal pump has certain limits, as it should not be operated below the minimum (safe) flow rate that is indicated by the pump manufacturer, or what is required for the given turnover rate. If this happens, it can cause excessive recirculation to occur inside the pump casing, resulting in the pump overheating and increasing wear and tear on internal seals and bearings. The ideal flow rate is the point at which efficiency of the pump maximizes energy use and reduces deterioration of the internal components.
Atmospheric pressure also affects the pump’s ability to operate. A self-priming pump will lose 10 per cent of its ability to lift water for every 305 m (1000 ft) of elevation. For example, at 1534 m (5000 ft) a pump can lift water 1.5 m (5 ft), while at sea level a pump can lift water up to 3 m (10 ft). In other words, the atmospheric pressure at sea level is 6.6 kg (14.7 lbs), while at 1534 m (5000 ft) the pressure is 5.4 kg (12 lbs). This is important to remember because the higher the elevation, the more vacuum required in the system to make the pump operate properly.
Throttling centrifugal pumps with a discharge valve is not as energy-efficient as using a variable-frequency drive (VFD) to slow the pump/motor speed down, but it is much less expensive to install. To understand why many Canadian and U.S. utilities are offering large incentives and rebates to those that switch from single-speed pumps to VSPs, it is important to understand how they differ.
Pump motor differences
Historically, pool pumps with induction motors, which only operate at one or two speeds, draw more energy than is necessary to circulate water and, in some cases, pump more water than necessary. These units must constantly operate at high speed to perform demanding jobs such as running a waterfall or pool cleaner. On the other hand, it takes far less power to filter the pool water—a difference single-speed pumps cannot address.
The programmability of a VSP is where these pumps differ. In fact, some have built-in, constant-flow software, which maximizes the advantages of these pumps, as it will automatically adjust its speed to deliver the required flow rate for each programmed task. For instance, to achieve the maximum laminar jet height of 2.1 m (7 ft) at a distance of 2.4 m (8 ft), a water supply of 37.8 litres per minute (lpm) (10 gallons per minute [gpm]) with 4.8 m (16 ft) of head (measured at the laminar) is required to produce a smooth 1.8-m (6-ft) arc of water. As the filter accumulates dirt, however, the pump will sense resistance in the circulation system and automatically ramp up its speed to continually provide the proper flow rate the water feature requires. With other pump types, the water feature will gradually throw a shorter arc of water as the filter gets dirtier.
VFD pumps can also be programmed to achieve turnover rates of exactly four hours, even if the filter is dirty. This allows for the reduction of motor speed, power, and energy use during periods when filters are clean, instead of sizing the pump to assume worst-case operating conditions.
No matter what type of pump is being used, however, slower speeds save energy, reduce noise levels, as well as wear and tear on other pool equipment the water circulates through.