Content area
Full Text
Last month, we said that the term drawdown refers to the amount of useable water that can be drawn from a hydro-pneumatic tank from the time the pressure switch cuts out, turning the pump off, until the pressure switch cuts back in, turning on the pump. The amount of drawdown capacity is determined by a formula known as Boyle's Law. Simply put, Boyle's Law states that as the volume of the air cushion in a tank decreases, the pressure of that air cushion increases. Conversely, as the volume of the air cushion increases, the pressure of that air decreases. Therefore, for any given tank, drawdown equals the volume of air at cut-in minus the volume of air at cut-out. Stated as a mathematical formula taking the total volume of a pressure tank into consideration, it looks like this:
Drawdown = P^sub 1^V /P^sub 2^- P^sub 1^V / P^sub 3^ where
P^sub 1^ is the pre-charge pressure.
P^sub 2^ is the cut-in pressure.
P^sub 3^ is the cut-out pressure.
V is the total tank volume.
Remember that all pressures must be stated in terms of absolute pressure. At sea level, add 14.7 psi to the gauge pressure to get absolute pressure. To make the calculations easier, Boyle's Law can be restated as:
Drawdown = (P^sub 1^ / P^sub 2^ - P^sub 1^ / P^sub 3^) × V
Let's try a couple of examples.
Example 1
Consider the following tank conditions, if you will:
A 30/50 pressure switch.
P^sub 1^ the pre-charge pressure is 28 psi.
P^sub 2^ the cut-in pressure is 30 psi.
P^sub 3^ the cut-out pressure is 50 psi.
V the total tank volume is 85 gallons.
Altitude is sea level.
The drawdown formula then looks like this: