Hydraulics

Filtration, circulation and chemistry are the three basic factors affecting overall water quality in swimming pools. For residential swimming pools, maintaining water quality means that we need to circulate and filter all of the water in the pool two to three times per day. In order to accomplish this, we have to design our pool installation with properly sized equipment. Our design must consider the volume of water in the pool, how frequently we want to achieve water turnover, how much resistance the water will encounter moving through the system and how we will size our pump and filter to overcome this resistance.

Let’s consider:

However, we also have to consider that a number of things create resistance within a closed plumbing system. The vacuum from the suction side of the pump, the length of the pipe, the number of fittings, the type of fittings, the pool filter and the elevation of the pump compared to the pool will all contribute to the amount of resistance within the system.A typical 16 x 32 swimming pool, which holds 17,000 gallons and is plumbed with 1½” pipe, has a maximum turnover rate of approximately 6.5 hours. The same pool plumbed with 2” pipe would turnover in just under 4 hours! If all we were considering was the pipe diameter, the typical pool would have to run almost 20 hours per day, while our pool with 2” plumbing would have to run only 12.As you can see from this table, only so much water will flow through a particular diameter of pipe. We base the flow on the maximum standards for velocity, which is 7 feet per second.

This resistance, measured in Feet of Head, will determine the volume of flow that any particular pump can produce. Notice in the chart above that a more efficient pump, with a smaller motor can actually move more water . By understanding hydraulics, we can actually design systems that not only improve water quality, but also save money!

Consider that a 3/4 HP pump motor consumes approximately 20% less electricity than a 1HP pump motor. But further consider that a pump motor converts electricity into two products; mechanical energy to move the water, and heat. The amount of heat generated depends on the efficiency of the motor, but, it is not unreasonable to expect that we can save an additional 20% with a more efficient pump motor. Therefore, in our typical pool, a 1HP pump consuming 1500 watts will cost approximately $100 per month to turn over our water three times per day. The same pool using a ¾HP high efficiency pump will cost only $60 per month.

Now, adding 2” plumbing lines, our high efficiency pump with 50 feet of head could turn the water over three times in 13 hours, reducing the run time by 46% and the monthly electric bill to only $32.00! By properly designing the system, a five and a half month season will cost only $176 instead of $550. Would you consider taking me out for dinner with the difference?