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How Swimming Pools Work

­by Tom Harris

Conceptually, swimming pools are pretty simple — they’re just big basins of water. But on a hot summer day, a swimming pool can seem like the greatest invention known to man. And as it turns out, there really is a lot of cool technology at work in your average pool — much more than you might expect.

In this article, we’ll find out how pools are built, and we’ll take a look at the plumbing system that keeps the water clean and chemically balanced.

Pool Basics

Swimming pools come in all shapes and sizes, but nearly all of them, from the backyard personal pool to the water park wave pool, work in the same basic way. They use a combination of filtration and chemical treatment to continually clean a large volume of water.

An apartment complex pool, mid-construction: It looks like a big hole in the ground, but it’s really much more.

A typical swimming pool needs seven major components:

  • A basin
  • A motorized pump
  • A water filter
  • A chemical feeder
  • Drains
  • Returns
  • PVC plastic plumbing connecting all of these elements

The basic idea is to pump water in a continual cycle, from the pool through the filtering and chemical treatment systems and back to the pool again. In this way, the pumping system keeps the water in the pool relatively free of dirt, debris and bacteria. Some pools also include heaters in the mix, in order to keep the water at a certain temperature.

Gunite pools are the most popular design in much of the United States. To build one of these pools, the construction crew digs a hole, puts the plumbing in place and assembles a framework grid with 3/8-inch steel reinforcing rods (rebar). The rebar rods are spaced about 10 inches apart, and secured together with wire. When the grid is in place, the crew sprays a heavy coating of gunite, a mixture of cement and sand, around the rebar. The sprayer unit combines dry gunite mix with water just before spraying — this produces the wet concrete material. The crew trowels the gunite smooth and lets it sit for a week or so before applying a smooth finish to the rough surface. The most popular finish is called plaster (actually a mixture of cement and marble sand), but a lot of people finish their pools with special concrete paint. Gunite pools can also have tile, exposed aggregate or even fiberglass finishes. Gunite pools (and their cousins, shotcrete pools) are highly durable, and they can be built in any shape or size.

Pool Drain Systems

We’ve already seen that the water in a swimming pool needs to circulate through a filtering system, to remove dirt and debris. During normal operation, water flows to the filtering system through two or more main drains at the bottom of the pool and multiple skimmer drains around the top of the pool.

The main drains are usually located on the lowest point in the pool, so the entire pool surface slants toward them. Most of the dirt and debris that sinks exits the pool through these drains. To keep people from getting their hair or limbs caught in the plumbing, the drains are almost always covered with grates or antivortex covers (a cover that diverts the flow of water to prevent a dangerous vortex from forming).

The skimmers draw water the same way as the main drains, but they suck only from the very top of the pool (the top eighth of an inch, typically). Any debris that floats — leaves, suntan oil, hair — leaves the pool through these drains. The diagram below shows a common system.

In the system described here, the floating weir, the door at the inlet passageway, swings in and out to let a very small volume of water in at a time. To catch debris effectively, the goal is to skim just the surface level. The water flows through the strainer basket, which catches any larger debris, such as twigs and leaves. In addition to the main inlet, the skimmer system has a secondary equalizer line leading to a drain below the surface level. This line keeps the skimmer from drawing air into the pump system if the water level drops below the level of the main inlet.

The water is pumped through the filtering system and back out to returns, inlet valves around the side of the pool. This system involves a lot of suction, but if the pool is built and operated correctly, there is virtually no risk of suction holding somebody against one of the drains. The only way the plumbing system could apply this sort of suction is if there were only one open drain. In a safe pool, there are always multiple main drains as well as several skimmer drains, so if somebody or something blocks one drain, the pumping system will pull water from one of the other drains. This eliminates the suction on the blocked drain.

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