Avoiding Dead Zones by Selecting the Right Pump
By Mike Fowler
Proper pump selection and optimal flow rates are extremely important to ensure proper flow, avoiding "dead spots" in pools, especially in today's pools that often have multiple curves, inlets, grottos and vanishing edges. Pool professionals can also increase the energy savings for their customers with properly sized pumps for the pool. This article focuses on how to choose the right size and speed pump for different pools. With a few tips and rules of thumb, proper pump selection will result in better water flow while minimizing energy consumption.
Affinity laws indicate the power demanded by a pump is proportional to the cube of the flow rate. For example, if the pump's flow rate is doubled, then its power demand is increased by a factor of eight. Therefore, it is important to utilize the smallest pump that is capable of completely turning over the pool water in an acceptable amount of time.
Furthermore, during the pump selection phase, the facility's auxiliary features (e.g., spray pads, fountains and waterfalls) should also be considered, as it is common for them to use the pool's main pump. Some building codes, however, require the use of a multi-speed pump, or in some cases, a separate pump for each auxiliary pool load. Pumps on many pools are oversized by design, sometimes more than 20 to 40 percent bigger than they need to be; this happens because many architects and engineers look at what is required, then pick the next size up to be sure the pump can handle the job.
Pump selection tips:
- Determine flow rate in gallons per minute.
- Calculate total dynamic head (TDH--the pressure head difference between the inlet and outlet of the pump) to account for friction loss. Adding 20 feet of head for a dirty filter is optional.
- Refer to the pump's performance curve to select the preferred unit.
- Locate the required horsepower of the pump by plotting GPM vs. TDH. (If plotted point falls between two pump sizes, select the next larger pump size in terms of horsepower.)
- Do not oversize the pump. Choose the best pump available for the facility's flow rate requirements (i.e., do not install a 20-hp pump where a 10-hp pump will suffice just because that is all that is available). If the preferred pump does not provide a proper fit, consider a different pump model.
- Verify the selected filter can handle the system's flow rate, and be sure the minimum backwash flow rates can be achieved.
Avoiding Dead Zones
There are two solid ways to make sure a swimming pool is designed and circulated to eliminate "dead zones" of circulation in today's swimming pools, which are often designed with multiple insets, curves, grottos, water fountains and slides:
- Properly balanced plumbing: One of the most important parts of a well-balanced circulated pool is having a hydraulically sound plumbing layout throughout both the suction and return sides of the system. Taking larger pipe sizes to the pool and then breaking them down to smaller sizes in a balanced way around the pool will ensure that you have proper circulation throughout the entire body of water and don't leave any dead spots in the pool.
- Proper placement and directions of return lines: When bringing water back to the pool after it passes through the filtration system, the whole key is to make sure it goes back to the pool completely and as evenly distributed as possible. Once again, the importance of balancing your piping around the pool and location of the returns will help to make sure that all areas of the pool are circulating water. Many contractors today will align the return lines throughout the pool so the water is returned in a circular motion around the pool and designed in a way that as the circulation around the pool is complete the water is also passed by skimmers in a strategic fashion to aid in cleaning the surface, as well as distributing the water. One suggestion is to cap the end of the return line plumbing to increase the performance of those returns that are farthest away in your plumbing layout. This will ensure that all areas are getting proper circulation around all spots of the pool.
In larger pools, it often makes sense to divide the plumbing and pumps into three sections, each with its own pump system. In this way, each section of the pool is its own "zone," and the equipment and plumbing is set up for each zone. For example, in a 20x40 pool with a grotto with cascading waterfall and a slide with its own water system in the deep end and a sundeck with fountain and cuddle cove/play area/reverse radius section in the shallow end, you would divide the shallow end area of the first 10 feet into one zone, the mid-section of 15-plus feet into a second zone, and the deep-end area, or last 15 feet, into a third zone with pumps and plumbing sized accordingly.
Historically, pool pumps with induction motors, which operate at only one or two speeds, have drawn more energy than is required to circulate pool water. These units must constantly operate at high speed to perform their most demanding jobs, such as running a waterfall or pool cleaner. However, it takes far less power to simply keep the pool water filtered—a difference single-speed pumps cannot address.
Variable speed pumps, on the other hand, are able to be programmed to operate at set speeds to deliver the correct flow rate for each task they perform. This allows the pool's pump to reduce energy consumption and ultimately reduce operating costs. Variable speed pumps can also be programmed to achieve turnover times of exactly six hours, even if the filter is dirty. This allows motor speed, power and energy to be reduced during times when the filters are clean, instead of sizing the pump to assume worst-case operating conditions.
Some VSPs have built-in constant-flow software, which maximizes the advantages these pumps have, as it will automatically adjust its speed to deliver the required flow rate for each programmed task. For instance, if an arcing laminar water feature requires 40 gpm to produce a smooth 6-foot arc of water, the pump will automatically ramp up its speed when it senses resistance in the circulation system (e.g., as the filter accumulates dirt) to continuously provide the proper flow rate. With other pump types, the water feature will gradually throw a shorter arc of water as the filter gets dirtier.
No matter what type of pump is being used, however, slower pump speeds save energy. Slower speeds also dramatically reduce noise levels and wear and tear on the other pool equipment the water flows through.
© Copyright 2021 Recreation Management. All rights reserved.