There is a single major reason for the enhanced energy efficiency of variable-speed pumps. It’s a concept known as the Pump Affinity Law. In hydraulics, this law governs the relationships between the variables (head, flow rate, shaft speed, power) involved in pump performance.
There are three relationships described by the law that are relevant to the discussion of pumps. The first is that the shaft speed in revolutions per minute is proportional to the flow rate. The second is that the pressure or head is proportional to the square of the shaft speed. The third is that the power is proportional to the cube of the shaft speed.
Speed (rpm) ≈ Flow (gpm) Speed (rpm) ≈ Pressure2 (feet of head) Speed (rpm) ≈ Power3 (kW) So, if we reduce the speed by half, we reduce the power by an eighth, or ½ speed ≈ ½ x ½ x ½, so power = 1/8. Similarly, if we reduce the speed by a third, we reduce the power by a 27th.
Because the shaft speed is proportional to the flow rate, power can be significantly conserved by lowering the flow rate and running the pump longer.
The concept is not intuitive, but the fact is, operating pumps for normal filtration at high flow rates is an unnecessary waste of energy.
This is the primary benefit of a variable-speed pump. With a single speed pump, it must be sized to perform the most demanding task, which may include running the filter or pool cleaner, operating jets or fountains, or running a solar heater.
And because single-speed pumps cannot change their flow rates, for normal filtration purposes — the primary purpose of the pump — they are providing far greater circulation than the filtration system needs.
Switching to a variable-speed pump, and operating it at the lowest flow rate that provides adequate filtration will significantly lower the energy usage, thereby saving money.
To convince pool owners of the merit of switching pumps, consider the potential financial savings possible for an average 15,000-gallon pool.
A standard, traditional singlespeed pool pump is typically 1½ to 2 horsepower and operates using a single-speed induction motor to generate motor speeds of 3,450 revolutions per minute, resulting in a flow rate of 87 gallons per minute. The power necessary to accomplish that amount of flow is approximately 2 kilowatts.
Using this single-speed pump for 6 hours a day will consume 12 kilowatt hours. The price of electricity varies by region, but using a value of 0.16 cents per kWh, a single-speed pump costs about $700 per year.
With a variable speed pump, however, we can set the pump to 30 gpm, which draws 300 watts of power, and run it for 8 hours per day. Now, one years’ use of a variablespeed pump will cost $140.
That’s a savings of $560 per year! The math shows that the financial savings is real.
A pool flow meter is a great tool for any swimming pool operator. By monitoring it, operators can make themselves aware of any issues that might be affecting a swimming pool’s circulation.
The circulation of water in a swimming pool is a crucial part of keeping the pool clean. Pools rely on manual circulation of water within the shell to prevent stagnation and through the filter to collect debris for the overall health of the water. Lack of circulation promotes algae, cloudiness, and can allow bacteria and disease-causing pathogens to flourish. It’s important to maintain a minimum flow (in gallons per minute) within swimming pools for proper circulation. This is defined as the turnover rate, which is the amount of time it should take the entire volume of water within the pool to be pushed through the filtration equipment one time.
For commercial applications, health departments mandate the amount of time they want that entire volume to be filtered. Most health departments have set a requirement that your pool's turnover occurs every six hours. For residential pools, while there is no such requirement, the APSP/ANSI standards for residential pools recommends the pool water be turned over once every 12 hours.
A swimming pool flow meter is a device that measures the speed of the water moving through the system such that operators can know if the pump is turning the water over at the proper rate.
The operator must determine the minimum flow that is acceptable for the pool in question. This means the slowest that the water can move before it falls out of the six-hour turnover requirement of the health departments, or for residential pools, the desired turnover of the operator.
The way to calculate the minimum flow in a pool is as follows:
Minimum Flow = Pool Volume ÷ 6 ÷ 60
In this equation, the number 6 is the number of hours it should take for the water to complete turnover. For residential pools, longer turnovers may be substituted.
Once this calculation is made, it is a good idea to write it down and display it somewhere in the equipment room or pad so that everyone can see if the flow at the pool is where it needs to be. They can do this by reading the flow meter. Note: Although the pool flow rate can be manually calculated, using a flow meter provides a much more accurate measurement.
But in addition to filtration schedules, an extremely important reason to verify actual flow rates is to make sure they’re within equipment parameters.
For example, all filters have a maximum designed flow rate, which is the maximum flow volume that the filter can handle before risking damaging the filter or filtration media.
This used to be much less of an issue, but newer pumps are more powerful than ever, and many of the larger variable-speed pumps are capable of exceeding maximum designed flow rates for some filters.
Heaters also have minimum flow rates as well as optimal flow rates, and it is necessary to have a flow meter to verify that this flow is optimal.
Running a variable-speed pump at either too low or too high rpm speed could potentially cause the pool water to be under-filtered or potentially exceed the maximum designed flow rates for the other pieces of equipment.
There are several types of flow meters available on the market today. These are: 1. Analog 2. Digital 3. In-Line
Analog Flow Meters
Analog flow meters are the old school flow meter. They are mechanically driven, which means they don’t use electricity and work using the flow of water alone.
They are made of a clear plastic housing that stands on one of the pipes usually directly before or after the filter (after filter is recommended.) Inside the housing is a metal float. When water flows through the main pipe, water shoots up into the flow meter and the floating pointer indicates the flow rate. It is necessary to drill a small hole on top of a pipe and the meter fastens over the hole with a clamp.
Analog flow meters are relatively inexpensive, usually costing well under a hundred dollars. Analog flow meters are manufactured for specific sized pipe.
Digital Flow Meters
Digital flow meters are often found inside an automatic chemical controller, and generally show the flow on the main screen of the controller box. These flow meters are accurate, easy to read, and they aren’t that much more expensive than analog.
Inline Flow Meters
Inline or integrated flow meters are a section of pipe with a flow meter installed inside it. Typical models have an inlet and an outlet with a bulb shaped housing in between where the flow rate can be read. Digital versions of inline flow meters are also common. For these types of flow meters, it is necessary to chop out a section of pipe and install the flow meter in the gap using PVC glue.
Where to install a flow meter
When installing a pool flow meter, there are two things to consider: One — it should be someplace easily accessible. Put it somewhere on the top of a pipe that is easily seen.
Two — try to ensure that the meter is located someplace after the filter. A flow meter installed anywhere prior to the filter can become clogged with debris, making it inoperable. How to troubleshoot a pool flow meter
The most common cause of a problem with a flow meter is that it is clogged with debris or stuck in one position. To remove a clog, you need to access the flow meter.
Inline models have a series of screws that hold the lid on. Simply remove the screws, and inspect the float, or flapper, and housing for any debris.
Ensure that the float is moving freely.
For new installations, be sure to install the meter in the right direction. There is usually an arrow to indicate which way the water needs to move through the flow meter.
Analog and digital flow meters will need to be unscrewed or twisted out of the pipe for inspection and cleaning.
Remove the clamp, remove the meter from the pipe, and check for debris clogging the float.
Digital flow meters sometimes have a filter screen. Ensure that this screen is clean.
Scale and mineral deposits from the pool water can also affect the function of flow meters.
This is yet one more reason to make sure pool water is always well balanced.
The bottom line is that a flow meter is a great tool for determining the overall health of the circulation system.
For instance, operators thinking the water seems to be moving a little slower than normal might assume the filter is dirty. So, a service tech may take the time to backwash the sand, pull out the cartridge, or clean the grids and hope the problem was solved.
Flow meters can show operators whether cleaning the filter media fixed the flow issue.
And if the flow rate didn’t return to normal, they can look for other areas that might still have an issue, such as a skimmer obstruction, a dirty pump hair and lint basket, or misaligned valve positions.
And if the pool uses a variablespeed pump, a flow meter can also help to fine-tune the operation of the system to save energy and save your customers money.