As discussed last month, the primary function of a pressure tank in a pumped water system is to store water under pressure so the pump does not have to come on every time there is a small, intermittent demand for water. The amount of usable water stored in a pressure tank is the drawdown, and is the amount of water that can be drawn from the tank between the time the pressure switch cuts out, turning off the pump, and cuts back in, turning the pump on. Tanks are sized with enough drawdown to allow the pump to run a minimum of one minute between cycles, letting the motor cool, as recommended by the motor manufacturers.
There are two types of pressure tanks, captive-air tanks (also called pre-charged, diaphragm, or bladder tanks), and conventional tanks, (also known as galvanized, ASME, and epoxy-lined tanks). In this article, we will use the terms "captive air" and "conventional" to differentiate the two types of tanks.
Drawdown factor - The drawdown of a pressure tank, whether captive-air or conventional, can be calculated using a formula known as Boyle's Law. Boyle's Law takes into account amount of pre-charge and cut-in and cut-out settings of the pump pressure switch to come up with a ratio of total tank capacity to drawdown. A pressure tank with a 30/50 pressure switch setting and 28 PSI pre-charge will have a drawdown ratio of .3. This means 30% of the tanks total volume is available as drawdown. An important concept to visualize and understand is that 70% of the tank's total volume compressed air which is available to push out the 30% of water.
For a conventional tank to have the same drawdown ratio as a captive air tank, it is necessary to pre-charge the conventional tank and to maintain same air/water ratio as you would have in a captive air tank. Logic states if you have the same amount of air available to push the water out, you will be able to deliver the same amount of water between pump cycles regardless whether the tank has a diaphragm separating the air from the water. In a captive air tank, the diaphragm assembly is virtually out of water when the pump turns on. To get the same performance from a conventional tank, the water level can be set such that the pump turns on just before the tank runs out of water.
The easiest method of optimizing performance of a conventional tank is to use a small air compressor in conjunction with a water level probe. The advantage of this set-up is the probe level can be set anywhere in the tank, to optimize the drawdown. For low pressure settings like 20/40, the probe will be set higher and for high pressure switch settings, the probe will be lower. There are two such units commercially available today, one by Maass Midwest (800 323-6259) and one by Whitewater (800 966-3110).
Using a compressor/ probe unit, the probe controls drawdown and the compressor maintains the right amount of pre-charge. This is how to set the probe level.
1. Using Boyle's law, calculate drawdown in gallons for the pressure switch settings you will be using. For this calculation, figure the tank pre-charge and the pressure switch cut-in pressures are the same.
2. Calculate the water level for this much water for the tank diameter you are working with.
3. Set the probe 2 inches above the top of the outlet fitting. The 2 inches is a safety margin to assure the tank does not run out of water before the pump turns on.
4. Before filling the tank with water, turn on the compressor and short out the probe wire going into the water level electronic module. This will make the module think the water is on the probe and it will allow the compressor to turn on. Run the pressure up to the pump turn on pressure.
Example - 30/50 pressure switch, 30" diameter 220-gallon tank, top of water outlet 4" above bottom of tank.
Boyle's law calculation
Drawdown
= 1 - (Pump Cut-in + 14.7 / Pump Cut-out + 14.7) x gallons
= 1 - (30 + 14.7/ 50 + 14.7)
= 1 - 44.7/64.7 = 1 - .691
= .31 x 220 gallons
= 68.2 gallons
Water level calculation
Volume of drawdown - 68.2 gallons / 7.5 gallons per cubic ft. = 9.1 cubic ft.
Volume of tank per foot
= Pi x radius squared x height
= 3.14 x (1.25 feet) squared
= 4.9 sq. ft. per foot
Height of drawdown
= 9.1 / 4.9 = 1.86 feet x 12 = 22.3 inches
Therefore the probe should be set 22.3 + 4 = 26.3 inches above the bottom of the tank.
Compressor/probe air charging systems are typically used on larger conventional tanks for economic reasons but you will also see them on smaller conventional tanks where optimizing drawdown is necessary due to space limitations, Next month we will talk about some of the more commonly used air charging techniques for smaller conventional tanks. Till then....
