However, as pointed out in earlier articles, the pre-charge must be set a minimum of 2 psi below the pump cut-in pressure to avoid the possibility of having the system pressure drop abruptly to zero before the pump turns on. Taking it one step further, if you consider the pre-charge to be the same as the pump cut-in pressure, P1/P2 becomes 1 and the formula then is: acceptance factor = 1 - P1/ P3. If you would rather work with acceptance factors, take the “V” out of the Boyle's Law formula: acceptance factor = P1/ P2 - P1/ P3. In other words, drawdown = acceptance factor x total tank volume. A word of caution: Before expanding the cut-in/cut-out differential beyond 20 psi, check with your tank manufacturer to make sure that by doing so, you will not be over-expanding the water chamber.Īnother term used in determining drawdown is “acceptance factor.” This simply is the factor by which you multiply the total tank volume to get drawdown. This shows how the drawdown changes if we widened the differential between the cut-in and cut-out pressures from 30/50 to 30/60.ĭrawdown = (28 + 14.7) / (30 + 14.7) - (28 + 14.7) / (60 + 14.7) x 85Ĭompare this with the 25.1 gallons from Example 1. Therefore, drawdown = 25.8 x 85 = 21.9 gallons. As we will point out later, it is possible, though not always standard practice, to pre-charge conventional tanks. Use a 30/50 pressure switch, but no pre-charge as could be the case with a conventional tank with no pre-charge.ĭrawdown = (0 + 14.7) / (30 + 14.7) - (0 + 14.7) / (50 + 14.7) x 85ĭrawdown = 0.102 x 85 = 8.7 gallons, about 10 percent of the total tank capacity. Using the same tank conditions, put the job up at Lake Tahoe in California at 6,000 feet above sea level. Therefore, drawdown = 0.295 x 85 = 25.1 gallons. The drawdown formula then looks like this:ĭrawdown = (28 + 14.7) divided by (30 + 14.7) minus (28 + 14.7) divided by (50 + 14.7) times 85. Consider the following tank conditions, if you will:
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