Calculating Water Flow Using Ohm's Law - Index | JSBachFOA.Org

Page 1
• How to calculate flow using Ohm's Law for Water
• Pressure Drop = GPM squared times Resistance
• HOW TO SIMPLIFY WATER CIRCUITS
• RESISTANCES IN SERIES
• RESISTANCES IN PARALLEL
• PUMPS AND PRESSURE SOURCES

NOTE - Ohm's Law is not as useful working with closed loops which have individual pressure sources

Page 2
• RESISTANCE VALUES FOR 100 FEET OF COMMON HOSE
• RESISTANCE VALUES FOR COMMON NOZZLES
• Formula for Calculating Nozzle Resistance
• Two different ways to think of nozzle pressure
• EXAMPLE 1 Pump at 65 psi, 200 feet of 1-1/2 inch hose, 5/8 inch smoothbore
Page 3
• EXAMPLE 2a Calculate flow in gpm for one 2 inch hose feeding two smaller handlines
• STEP 1 Add the hose resistance and the nozzle resistance together to give an equivlent
• STEP 2 Add the hose resistance and the nozzle resistance together to give an equivalent
• STEP 3 Convert the two equivalent resistances from Step 1 and Step 2 (they are in )
• STEP 4 Add the resistance calculated in Step 3 to the resistance of the 2 inch hose
• STEP 5 Solve for flow
Page 4
• EXAMPLE 2b Calculate the flow in in the individual hoses.
• SOLUTION We have just found the total flow (219.8 gpm) and we know that all FLOW DIVIDES between two parallel resistances based on how the resistances
Page 5
• EXAMPLE 2c Calculate the pressure at each nozzle and at the wye.
• SOLUTION We're going to do this three different ways, just for practice. None of them are hard.
THE EASIEST WAY - Use EK's Friction Loss and Nozzle Flow Calculator. We already know the ...
• Chart Step 1 - Find the pressure at the wye. Draw a line between the 2 inch hose mark and ...
• Chart Step 2 - Find the pressure at the 5/8 smoothbore, which we already know is flowing ...
• Chart Step 3 - Find the pressure at the 7/8 smoothbore, which we already know is flowing ...
• NOW - Let's do it with numbers. We know the flow, so it's easy to work out the pressures.
• Numbers Step 1 - Find the pressure at the wye. The pressure drop in the 2 inch hose equals ...
• Numbers Step 2 - Find the pressure at the 7/8 nozzle. The pressure outside the nozzle, in the ...
• Numbers Step 3 - Find the pressure at the 5/8 nozzle. The pressure outside the nozzle, in the ...
• The nozzle pressures could also be found by calculating the friction losses in the hoses ...
• FINALLY - Let's find the pressure at the wye and at the two nozzles without using the flow.
• Ohm's Law Step 1 - Find the pressure at the wye. We need 3 things: the pump pressure, ...

• How Pressure Divides (General Case)
• In fact, this same thing works for any number of resistances in series - the pressure across ...
• The interesting point is that you don't need to know the flow to see how the pressure divides.
• Next, we'll work out nozzle pressures, using the pressure at the wye as our "total pressure" ...
Page 7
• Talking about Power (Without Numbers)
• POWER - Power is the rate at which energy is generated or used, in terms of Energy per Unit ...
• Three common ways to describe Power are foot-pounds per second, Horsepower and Watts.
• HP = gpm x psi / 1714.55
• This is the basic formula - if you know the gpm and the psi, just go ahead ...
• Let's look at crimps in a hoseline. Do they matter?
• For a given amount of flow, the pump must supply a certain amount ...
• But, and this is important, when the pump pressure goes up 20 percent, then for the same flow, the horsepower also goes up 20 percent.
Page 8
• HORSEPOWER CALCULATIONS USING FLUID WATTS.
• THE BASICS
• For example, a pump which adds 90 psi to a flow of 125 gpm requires 6.56 horsepower to ...
• The HP formula given above is basic, but it's not always the easiest formula to use.
• The trick is to work with fluid watts, where 1 Fluid Watt equals 1 gpm pressurized by 1 psi ...
• 1714.55 fluid watts equals one horsepower
• Other Formulas
• Fluid Watts = Gpm x Psi
Page 9
• THE VIRTUAL NOZZLE and Bernoulli's Equation
• The Frictionless Pump and Hose
• TRASH PUMP Part 1, with warts - something is missing. What is it?
• In our model, the pump is the pressure head, and the virtual nozzle is the size of the hole.
• The value of the virtual nozzle depends only on its diameter, and is the same as the resistance ...
• Virtual Nozzle Resistance Values for 3 Common Hose Sizes
Page 10
• Portable Pond Fill Times with and without a Hose Crimp
• Power Calculations for the Pond Fill Example including at the Virtual Nozzle ...

FUTURE ARTICLES scheduled for the Fall - Winter 2011
• Calculating the Velocity Head using Ohm's Law
• Bernoulli's Equation and related topics
• Centrifugal Pumps
• Nozzles
• Pitot Tubes