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Pool Anatomy

Pool circulation is often compared to a living organism’s vascular system. It’s a tempting comparison because they both are mainly composed of a pump, piping network and the function is to the delivery of clean fluid and removal of waste. There are even comparisons with organs in the body such as the heart, kidneys and liver. Without taking the biological metaphor too far, here is a description of pool anatomy:

 Pumps

Several types of pumps exist. Each type has its strengths and weaknesses. Centrifugal pumps are the type used in pool systems as well as most water features. Centrifugal pumps work by accepting water into a centrifuge through an inlet and accelerating water in a circular pattern with a spinning impeller powered by a motor. The impeller then directs the water out through a discharge port connected to the piping system. They also produce high flow rates. Each pump performs based on its design and size. Pump manufacturers provide performance information in the form of pump curves. These curves are a graphic chart that tracks the pumps performance output in GPM for a given TDH. Now you see why TDH is necessary. Pumps are designed to function within certain ranges of TDH. We need to know it so we can size the pump correctly.

Filter

The filter’s job is to remove impurities from the water. It does that by straining the water through one of a number of kinds of media. Any particle larger than the gaps in the straining medium will not pass through and will be removed from the system. 

Heater

The heater’s function is to add heat energy to the water to raise the temperature of the water for the comfort of bathers. There are different types of heaters and each is best employed to different conditions and preferences but the end result is the same. Warm water.

Water Treatment

As with filters and heaters, there are several water treatment technologies to choose from. But the purpose is the same. Filtering the water is not enough to eliminate impurities. Some are microscopic bacteria, dissolved solids, chemicals that upset the chemical balance etc. Water treatment introduces a sanitizer such as Chlorine to eliminate bacteria, and acidic compounds to re balance water chemistry. Many smart systems do this automatically with sensors and controllers. Some work by adding a set dosage to the water regularly. Some pools (probably most) do not have treatment systems. The sanitization and chemistry maintenance is done manually either by the owner or by a maintenance person.

Return / Inlet Fittings

Return fittings are the interfaces between the pipe and the inside of the pool. They are cast into the wall or floor and are designed to safely direct or dissipate fresh clean water into the pool. directional eyeball and adjustable floor returns are the two standard fittings. Directionals may be aimed in oblique directions within a limited angle. They are usually placed in the wall and project water out into the pool. Adjustable floor returns are placed in the floor and discharge water in a near horizontal 360 degree pattern. They have higher flow ratings but discharge water at lower velocities than directionals at the same flow rate.

Skimmer

The Skimmer is a kind of drain. It is installed at water level so that it can draw water from the surface of the pool. It is the main means of cleaning the water surface where up to 80% of impurities are concentrated.  You will require at least one skimmer per 800 square feet of pool surface area. Skimmers are designed to draw water from the surface of the pool where up to 80% of impurities are found. From body oils to dirt and debris.

Vacuum Lock

The Vacuum lock or vac lock fitting is essentially a stopper that plugs a pipe opening in the pool wall below the water line that is connected to the suction side of the piping system. It is used for vacuum cleaning the floor of the pool by unplugging it and connecting it to a hose with a cleaning nozzle at the end. It picks up debris and sends it through the circulation system where it is captured in the filter.

Main Drain / Inlet

The main drain is the primary point of water exit from the pool.  They usually handle higher flow than the skimmers and they remove water from the depths of the pool along with debris that has sunk to the bottom and cooler water that might otherwise sit and suffer from poor circulation. 

Overflow Drain

The overflow drain’s purpose is to direct excess or displaced water – usually from a rain storm or heavy bather displacement – into the site drainage system rather than having it overflow onto the adjacent deck.

Auto Refill System

The pool will lose water to evaporation and heavy use from time to time. The refilling process is automated with a water level sensor and a refill line into the pool, above water level. There is a controller that sends a signal to solenoid to open a valve allowing potable water to enter the pool topping it up until the sensor is satisfied.

Pool

Last but not least, the pool. More than just the sum of the parts mentioned above, the vessel containing clean, sparkling, dancing gurgling water is the reason for all the other bits. The pool’s architectural design has a direct impact on how it’s used and enjoyed. Architecture is beyond the scope of this document but the water contained is still of concern to us. It’s circulation within the vessel of utmost importance. the location and number of fittings is critical for performance and safety.

Piping System

Consisting of pipes, fittings and valves the piping system is what connects all of the above. Most residential swimming pool pipes are made of polyvinyl chloride (PVC). PVC is manufactured in two standards, they are the more common schedule 40 (SCH 40) and schedule 80 (SCH 80). SCH 40 has a slightly thinner wall than SCH 80 and so has a slightly larger interior diameter. That is significant because the larger size affects flow rate and line velocity. Fittings are used to connect pipe segments when it needs to change direction or split into two or more branches, increase or decrease in size. Most pool piping systems will have control valves and sometimes diverter valves or check valves. Control valves regulate the flow of water through the pipes and diverter valves also known as three way valves control the direction of flow through a 3 way branch. Check valves allow water to flow in one direction only and will stop flow if the direction of flow begins to reverse.

Understanding Pool Systems

Designing pool filtration systems is really an engineering task. Engineering is essentially the application of science to solve practical problems. To do that you’ve got to understand the science. Hydraulics is the science of fluids (in our case, water) flowing through pipes and channels. In this book we will focus on flow through pipes. There are a few essential concepts that you should master in order to design effective pool systems. Gaining an intuitive understanding should be your goal. These are: 

  • Volume
  • Pressure
  • Flow Rate
  • Velocity and Pipe Size
  • Friction
  • Head Loss (major and minor)
  • Total Dynamic Head
  • Pumps

Much of the vocabulary of engineering coincides with common speech but has very specific meaning. So my explanations here should be understood in lay terms.

Volume

Volume is simply a measure of quantity by space occupied. Whether still or flowing or falling, for our purposes volume is the amount of space that liquid water occupies (at a given temperature). Nothing more, nothing less. Water is usually measured in gallons in the U.S. (CFMs in large systems) or liters where the metric standard prevails. 

Pressure

Pressure is the force that water exerts against its container. Water in a glass pushes out against the glass. Water in a container, whether a pool, a bath tub,  the Ocean all exert pressure. That pressure is produced by the weight of the water, and the pressure increases with depth. The kind of pressure produced by a static body of water is called static pressure and it is a function of gravity and depth. Now imagine that there is a crack in the glass, now water leaks through. The reason is that crack is a tiny opening where there is no glass to push back against the water. So, the water pushes through. It begins to move. That is how water moves through pipes as well. It flows from the direction of higher pressure toward the point of lower pressure, but more on that later. Water pressure is commonly measured in pounds per square inch (PSI) but there are many other units of measurement more common to specific disciplines or places in the world. 

Flow Rate

Flow rate measured in gallons per minute (GPM) is applied to flowing water. It describes the volume of water traveling past a given point in one minute. Important to note, it is not strictly a measure of velocity but there is usually an element of velocity implicit in any consideration of flow rate as there is a close relationship between flow rate and velocity. Consider this analogy. You are standing by a busy highway observing all the cars zooming by. Now, set your stopwatch for one minute and begin counting each car that passes in that time. Say you count sixty cars, the “flow rate” of cars on that highway is one car per second (average cars/60 seconds). But you don’t know how fast they were going or their “velocity”. You might intuit that they would need to travel at high speed for such a high number. But, maybe the highway had 10 lanes. In that case the cars could have been traveling at a moderate pace and easily achieve the 60 cars / minute. So there is another important variable to consider before we understand the complete picture. That is what I will introduce next.

Velocity & Pipe Size

Velocity is the measure of distance traveled in a given time. In our field the standard unit of measurement is feet per second (FPS). Water’s velocity through pipes is referred to as line velocity. Understanding line velocity is very important in the design of hydraulic systems because it is a pretty reliable shorthand for energy efficiency. Velocity translates to the speed of the cars in our analogy above. All other things being equal, line velocity is determined by the size or the diameter of the pipe. A larger pipe produces a lower velocity for a given flow rate than a smaller pipe will. Think of how a larger highway eg 10 lanes contrasts with a 2 lane road conveying 60 cars in a minute. 6 cars per lane rather than 30 can travel more slowly and therefore conserve more energy. Pool pipes are usually made of polyvinyl chloride (PVC) in two major wall thickness standards. Schedule 40 (SCH 40) or schedule 80 (SCH 80). Schedule 40 is more common except when SCH 80’s added strength is need. The sizes are listed as diameters in inches. So a 2″ SCH 40 PVC pipe has a nominal 2″ diameter. The actual size varies slightly, so we use the actual diameter in calculations. A 2″ SCH 80 PVC pipe has a smaller inside pipe diameter owing to the thicker walls. 

Friction

Friction is the resistance to flow caused by water “sticking” to the inside of the pipe. If the water is not moving there is no problem with the sticky water. But, when it begins to flow, friction steadily increases to rob the system of it’s kinetic (moving) energy. It is possible for friction to become so great that it can bring circulation to a virtual dead stop. Obviously, that would be a very poorly designed system. Friction, then you might now guess is behind the inefficiency of high velocity. Correct. Higher velocity = greater friction = lower efficiency. Friction is the major cause of efficiency loss in systems with long pipe runs. The typical pool system however, loses more from other sources of loss which we will cover later. Friction along with those other sources of energy loss is collectively expressed as head loss measured in feet. Here is how it works:

Head Loss

Head is a kind of shorthand for pressure. The pump in our pool system creates positive head or it creates pressure at its outlet. That pressure is measured in feet of head. That is the equivalent of water pressure under that number of feet deep. Example: A pressure gauge attached to a pump that produces 100 feet of head pressure would read the same if it were applied to the bottom of a 100 foot deep reservoir. So, head loss is the progressive loss of pressure in the system as water travels farther from the pump. A pressure gauge at the pump’s discharge outlet would register a higher reading than one attached 50 feet downstream in a circulating system. As noted before, the reason for this loss of pressure is friction along with other sources. These sources are traditionally divided into “major loss” and “minor loss”. Major loss refers to head loss resulting from friction in the pipes. Turbulence from fittings such as elbows, and valves etc are known as minor loss. Component loss results from flowing through components such as heaters, filters, chlorinators etc. As pressure is what drives flow in a circulating or dynamic system, head loss is a key factor to consider in design.

Total Dynamic Head

This is the head required to circulate water through the system at a given flow rate. In other words, TDH measured in feet is the depth of water that a reservoir would need to provide to drive the system at that given flow rate. Finding the TDH of the system you design is a very important exercise. Armed with this knowledge, you can select the appropriate pump for the job and be certain that you have built the best engineered system possible within the constraints you’re given.

Pumps

Several types of pumps exist. Each type has its strengths and weaknesses. Centrifugal pumps are the type used in pool systems as well as most water features. Centrifugal pumps work by accepting water into a centrifuge through an inlet and accelerating water in a circular pattern with a spinning impeller powered by a motor. The impeller then directs the water out through a discharge port connected to the piping system. An advantage of centrifugal pumps is that they produce a constant steady stream of water rather than pulses as with some other types. They also produce high flow rates. Each pump performs based on its design and size. Pump manufacturers provide performance information in the form of pump curves. These curves are a graphic chart that tracks the pumps performance output in GPM for a given TDH. Now you see why TDH is necessary. Pumps are designed to function within certain ranges of TDH. We need to know it so we can size the pump correctly.

Now that you have the basic engineering concepts down, next I’ll introduce you to the various components that go into a well designed residential pool. Stay tuned!

Why You Need a Pool Consultant

 

Imagine you are working for a client on an high end residential project. If you are an architect you hire the usual lighting, MEP, IT/AV, Interiors consultants. If you are a Landscape architect, you might hire irrigation, planting, landscape lighting specialists to advise you on those details. If you are a builder you sign up your sub contractors in the various trades. Now, imagine your client wants a pool with water features throughout her property. How can you provide the same quality in design and engineering as elsewhere? The usual course is to hire the pool builder with the best portfolio. If you’re lucky that’s enough, but usually there is no way to tell whether the quality extends beyond the photograph. That’s where a pool consultant comes in. As a member of the design team pool consultants are there to answer questions and help integrate water elements into the project through counsel and documentation.

 

 Early coordination avoids mistakes

When questions inevitably arise concerning relationships between pools and other trades, it pays to have someone on the team who can address concerns and questions quickly and reliably. Communication early on can avoid expensive remedial headaches later. One very common conflict between pool design and architecture (or landscape architecture) is the mechanical space location and size. Your consultant will lay out the equipment for best performance and access. If accessibility is difficult, maintenance will suffer – guaranteed. Location is another critical factor. Distance is one of the variables that determines performance.

 

Standard Design Documentation

Consultants are design specialists. Pool builders often tell me how pleased they are to receive a set of thorough, well thought-out documents to build from. It frees them to do what they do best. Proper documentation is essential for efficient communication, coordination and bidding. Consultants speak the language of design and produce documentation that meets professional standards ready for inclusion within architectural documentation packages. The tender process also becomes an apples to apples comparison as all builders bid on the same package. Disparity among bids are often the result of differing equipment and material selections – which usually corresponds to differences in quality. Documentation eliminates these kinds of unknowns.

 

 Stay current

As a design consultant, a big part of my job is to stay abreast of the current developments in the field. Over the last decade or so, residential pools and water features have taken a giant leap forward in design and construction quality, finally catching up to the other design professions. They have integrated more sophisticated automation, lighting and energy efficient technology with new developments taking place all the time. The point is; you want the most appropriate technology for your project, that’s no longer as simple a choice as it once was. Your consultant is there to guide you.

 

There are many technical and design details that go into a successful water design strategy. A consultant can help you address them. After your concept is pinned down, consider a consultant’s assistance in design development. The value from creating a well conceived and executed water design accrues to user and designer. It can mean longer lived equipment, energy savings, better user experience and better developed details and design.