Hydraulics and energy efficiency

April 1, 2010


By Connie Gibson Centrella

For decades, pool professionals believed over-sizing pumps and filters would give them a competitive edge in their pool presentations. Unfortunately, this supposed advantage has backfired significantly. The pool industry has been promoting excessive energy usage with little understanding of the cost to the customer and the planet. In the interest of environmental responsibility, professionals must take new approaches to equipment selection to create the most energy-efficient pool operations possible. Accurate hydraulics are vital to achieving this goal.

The objective of today’s pool professional should be to effectively marry pool volume, pipe size, velocity, pumps and filters. Proper sizing of pool equipment will not only facilitate an energy-efficient operation, it will also allow for proper circulation and chemical distribution. Pool energy workgroups across North America are convening to determine the appropriate hydraulic formulas to meet new energy standards and initiatives. The target is to achieve the desired circulation using the least amount of energy.

Understanding the science of hydraulics is the first step toward elevating products to meet demands of the energy efficiency movement. Getting back to the basics of pool calculations is a must. To size the new pumps, pool litres (gallons), turnover rate and litres per minute (lpm) (gallons per minute [gpm]) must be known, while pool piping and fittings must be taken into consideration to determine the entire circulation system’s total dynamic head (TDH).

Efficiency relates to flow and resistance

In lay terms, TDH is defined as resistance to flow. The entire loop around a pool comprises the total length of piping; friction losses generated by each fitting, value, filter, heater and chemical feeder are combined to compute TDH. The term ‘head’ is further modified by two factors:

In new pool construction, engineering of TDH can be determined by reviewing the pool plans. Velocity, pipe size, length of pipe and fittings each have a specific resistance, which can be determined using a series of calculations and then totalled to assess TDH. In many municipalities, building code officials are demanding these calculations be verified and approved prior to installation of a new pool. The bottom line is the TDH will determine the size of the pump, whether it is variable-speed, two-speed or single-speed.

Retrofitting an existing pool is much easier. Obviously, pool professionals cannot simply look under decks to see piping and fittings. However, TDH can be estimated with some degree of accuracy by using two instruments—a vacuum gauge and pressure gauge. The vacuum gauge, located on the suction side of the system, measures friction loss on the suction side and is expressed in inches of mercury (Hg). The pressure gauge measures friction loss on the discharge side and reads in pounds per square inch (psi).

Do the math

To calculate TDH, both gauge readings must be converted to a common factor, such as feet of head. In order to achieve this, one must multiply the vacuum gauge reading by 1.23 and pressure gauge reading by 2.31. Once both readings are converted, the sum is the TDH of the pool system.

(Pressure gauge reading x 2.31) + (Vacuum gauge reading x 1.23) = TDH

In residential pools, a vacuum gauge may not be present. In this case, it can be screwed into the drain plug of the hair and lint strainer, while the pressure gauge is installed on the pipe between the pump and filter. Once the system is turned on, with a clean filter in place, readings can be taken. Main drains can also be retrofitted to meet current entrapment regulations—these hydraulic calculations can be used to find the maximum flow rate based on the existing pool’s pump curve.

How resistance relates to flow

When TDH increases, flow decreases; conversely, as resistance decreases, flow increases. The goal is to increase flow with less resistance; this will require less horsepower to achieve the same circulation. The lower the horsepower, the less energy is consumed; thus, a more energy-efficient pump.

As the velocity of water travelling through a pipe increases, so does resistance, creating higher TDH and lower efficiency. In new pool construction, it is therefore prudent to consider increasing the pipe size to slow water movement. Swimming pool pumps are centrifugal, meaning they are designed to push water and challenged by pulling water. Easing the pump’s workload by increasing its size on the suction side will result in lower energy consumption.

Various energy groups are currently reviewing velocity and putting together recommendations on lower speeds. As an example, a 38-mm (1.5-in.) pipe at 2.1 metres per second (mps) (7 feet per second [fps]) can handle 163 lpm (43 gpm); by upgrading to a 50-mm (2-in.) pipe, 273 lpm (72 gpm) can be achieved at the same velocity. In a larger pipe, more water can be circulated with the same velocity, thereby decreasing resistance and reducing energy consumption. Furthermore, increasing the main drain suction from a 38-mm (1.5-in.) to a 50-mm (2-in.) main drain, and increasing skimmer(s) to 50 mm, will dramatically reduce resistance and friction loss.

Why does this matter?

It is important to balance all aspects of hydraulics to achieve the greatest energy efficiency and lower energy costs. Overall, it makes good business sense to downsize the pump and upsize the piping. Everyone wins—not only will users conserve energy and lower operating costs, they will also help prevent future damage to the plumbing and wear and tear on pump motors and filters. Of course, consumers want to know how long it will take to recoup their investment when they purchase energy-efficient pool equipment. Most pump manufacturers provide energy-saving calculations, including details on calculating the pay-back period.

The more knowledge industry professionals accumulate on hydraulics, the better prepared they will be to achieve energy efficiency and communicate benefits with customers. While this article is a good first step, enrolling in a hydraulics class can help professionals better understand the issues and calculations at hand.

Readers are also invited to submit experiences and opinions as to how we can best promote energy efficiency. E-mail suggestions to ccentrella@keiseruniversity.edu[2]. Comments will be shared with the other participants in the threaded discussion forum.


Centrella_Headshot_edited-1Connie Gibson Centrella is professor and program director for the online Aquatic Engineering Program at Keiser University eCampus. She is an industry veteran with more than 40 years experience and is a former pool builder with extensive knowledge in pool construction, equipment installation and manufacturing.

  1. [Image]: http://psm.poolspas.ca/wp-content/uploads/2010/04/Waterfall_4471646_edited-1.jpg
  2. ccentrella@keiseruniversity.edu: mailto:ccentrella@keiseruniversity.edu

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