Tuesday, February 14, 2012
How Re$i$tant to Flow is Your Pool?
Total Dynamic Head and Why It’$ Important
Total Dynamic Head is the resistance that your pool water has to overcome to circulate and move within the pool hydraulic system. The flow of the water (hydraulics) is dependent on how easily it can move through the pipes, equipment, and pool fittings.
In our pool system we have a suction side and a return or pressure side. We can take gauge readings to determine what each is on our pool. But to calculate the entire system we must convert the suction and return readings to a common unit of measurement. We use the measurement of “feet of head” for this purpose. The pumps we use have charts supplied to us that show the “pump curve” or what flow the pump is capable of producing given a certain “feet of head” or “total dynamic head in feet” or with the resistance and friction loss.
We measure the suction side friction loss of our system in inches of mercury (Hg). We measure our return side in pressure per square inch (psi). To find common value for our measurements we look at the air pressure that surrounds us. When they have a common value of measurement we can add them together to get a total.
If we take pressure gauge readings 10 feet into a tank of water we will find 4.33 psi (pounds per square inch) is the pressure of the atmosphere pushing down on water. So using math we can determine that for every 1psi there is 2.31 feet of head (10 divided by 4.33 =2.31). When we take a pressure reading on a pool system we multiply that number by 2.31 to give us a measurement in feet of head.
Vacuum pressure is calculated in inches of mercury. At sea level there is 14.7 pounds per square inch pressing down on us by the atmosphere. If we use a vacuum pump you will see that the atmosphere pushing down on mercury will push it up into a tube 30 inches and no higher. If we do the same with a vacuum pump and water in a closed tube it will rise 33.9 feet and no higher. Therefore if we divide the water (which is in feet) by the mercury reading of 30 inches we conclude that we have 1.13 in feet of head. We use the 1.13 feet of head multiplied by the vacuum reading in inches to have a measurement in feet of head.
Now that we have our vacuum and suction readings in feet of head we can add them together to determine the Total Dynamic Head of the system or the total resistance we must overcome.
So, why is this important? This is important because we can determine how much water flow we have if we know what pump we are using. We can determine if we are getting the turnovers we need to keep our water clean and disinfected. We know if we have enough flow of water so that our equipment will work properly. We can also determine if we are producing more flow than what we need. If we are producing more flow than our system will allow than we have more resistance in our system and a higher total dynamic head than what is required.
The lower total dynamic head we have the less our pump and motor need to work. The less our pump and motor need to overcome the resistance the less energy will be required. The less energy that is required the less we pay for energy.
Many states have a new Energy Code that will require calculations to show the minimum pump size needed for your pool system. The days of thinking that the bigger the better are now gone. Energy efficiency is rewarded on some states with rebates on certain qualified equipment. Wattage per gallon is especially helpful to customers who want to have a pool that is cost efficient after installation.