Kent Clark | May 18, 2020
So you need a new brewery pump, but you don’t know what size, speed and horsepower. Pick a pump that’s too small, and you’re not going to have the pressure you need (maybe even not enough to overcome the hydraulic resistance - aka your system head). Pick one that’s too big can result in cavitation, damaging your pump, lines, and your beer.
First, gather some numbers.
- Flow Rate – How much do you want to move in how much time? For example, let's say you want to transfer 20 barrels of wort in 20 minutes. At 31 gallons per barrel, that’s 620 gallons in 20 minutes or 31 gallons per minute.
- Discharge head (or “lift”) – Simply put, how high do you need to lift the wort? The height of the fermentation tank plus any additional lift. (i.e. Are your tanks elevated on a platform or on the second floor?)
- Back Pressure (also known as pressure drop) – What is stopping that wort from moving? Valves, heat exchangers, and filters all create backpressure. Determining how much can be found in different ways. The nameplates of your heat exchangers should list it. Filter suppliers will have it for different elements. Add all this together and you know your backpressure.
Next, add together your back pressure and discharge head to get your total head. You may need to convert back pressure from PSI to feet/head (which you can do by multiplying it by 2.3). Finally, on your pump curve locate the point corresponding to your flow rate and total head.
Ok so let's do an example: let’s say you want to transfer 20 barrels of wort in 20 minutes through a heat exchanger to a fermenter.
- Flow rate: (20 barrels x 31 gallons per barrel) = 620 gallons/20 minutes or 31 gallons per minute
- Discharge head: a 20 bbl fermenter is approximately 8 feet high so the discharge head is 8 feet.
- Backpressure: From the heat exchanger nameplate we are told the pressure drop is 7.4 PSI. 1 PSI = 2.3 feet of head, so 7.4 PSI x 2.3 = 17 ft
- Our total head is 8 ft + 17ft = 25 ft
Next we get our pump curve.
We plot the two lines on the pump curve and we see that they meet just below the 5.0 line and just left of the .75 HP line. So for this particular pump, we can determine that you need a pump with a 5” impeller and a ¾ HP motor.
Now if you look at a pump catalog you won't find too many 5" diameter pumps, most of them are 4" (too small) and 6" (too big), this pump curve is from when we only had fixed speed motors and actually trimmed the impeller on a lathe to reduce the diameter of a 6" pump to a smaller size, in this example 5".
Today most of the pumps used are connected to Variable Frequency Drives (VFDs) so the pump curve would have one impeller size and multiple speeds.
This curve shows the same 6" impeller pump at various speeds.
It shows that you can get the same or better performance with a full 6" impeller but at 1350 rpm.
Or you could do the same application with a 4" pump at 2200 rpm
So you could say what is the advantage? I could trim the impeller and be done, I don't need a VFD, but if you trim the impeller you have downgraded your brand new pump. What if you decided you wanted to use the pump for CIP (Clean-in-Place) and need 60 GPM at 40 ft, you can't do it with the 5" impeller, but with the VFD and 6" impeller you can speed the pump back up to 1750 RPM and get 60 GPM @ 40ft for your CIP.
This isn't the only thing VFDs can do for you. As a bonus, they provide soft start/stops and overload protection, the ability to remote control, and many other options.
In my opinion, variable frequency drives are the best thing since sliced bread, especially for pumps. If you want we can generate these multiple speed curves for you and find just the right pump for any application.