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Pump Calculations, Flow Rate, RPM, Head Pressure, & Impeller Diameter

We're going to be looking at pump calculations and, specifically, we're going to be looking at how to calculate the 

  • Flow Rate 
  • Pump RPM
  • Head Pressure
  • Pump Power
  • Impeller Diameter

If you already have a pump installed in one of your buildings, then you should have a checklist or a sheet that comes which is the datasheet for that pump.

All too often, parts of this design data are missing and that's frustrating. But if you have some data missing then we're going to look at some formulas of how to calculate and fill in these gaps, and also, we'll look to see what would happen if you were to change some of these parameters.

Also Read: How to Calculate Cooling Capacity of a Chiller in HVAC

If you haven't got the pump installed yet, you're in a design phase, and then you can also use these formulas to determine what the performance will be. Remember that these calculations will only give us theoretical values. The actual performance may vary from these. And, you know, certainly, the old a pump gets, the larger the discrepancy becomes between the theoretical and the actual answer. You might need to add on a correction factor to adjust these.

Now we jump into the calculations

Flow Rate (Increase/Decrease RPM)

Here we are going to find what would be the new flow rate if we were to increase or decrease the pump revolutions per minute.

Flow Rate (Change Impeller Diameter)

So the next one we're going to look at is the new flow rate, and that would occur if you were to change the impeller diameter.

Now, I will point out that this is an older way to do, to trim down the impeller to change its diameter. It's not the preferred way anymore, and that's because if you, once you trim it down, you can't, obviously, add that material back on. So you need to replace the entire impeller. A much better way is to actually use a variable speed drive or variable frequency drive to change the speed of the pump to meet your design criteria. But if you are looking to change the impeller or to even trim the impeller down, then you can use these calculations here.

Also Read: How to Calculate Chiller Efficiency in HVAC

Pump RPM (Increase/Decrease Flow Rate)

So that is the speed that the impeller needs to rotate at in order to achieve this new flow rate

Pressure Head (Increase/Decrease RPM)

So the next one we'll look at is how to calculate the head pressure, and this would occur if you were to increase or decrease the revolutions per minute of the impeller.

Head Pressure (Increase/Decrease Flow Rate)

The next one we'll look at is also head pressure, and that will be how to calculate this should you increase or decrease the flow rate.

Pump Power

So the next one is the pump power, how to calculate it? What the new pump power would be? Should you increase or decrease the revolutions per minute of the pump?

Impeller Diameter

And then, last one we're going to look at is the impeller diameter. So if you were to adjust the flow rate and you wanted to achieve this new flow rate, what diameter impeller would you need? And this is usually used in the case where the impeller is being trimmed down to meet a new flow rate.

If you have any questions, please leave them in the comments section below.

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