Multiple pumps more efficient than a single pump?

[JamesW]

Split off this thread since it's a different topic than the poster is asking about.

JamesW, using two pumps in parallel, while prone to several problems, such as the difficulty priming, is more energy efficient than using a single pump (variable speed or not). It is similar to the principal of running a two speed pump on low for twice as long, except we get the twice as long by having two pumps. There are some ways in which the parallel I just made breaks down, mostly because dynamic head doesn't really follow the same rules with two pumps in parallel instead of one at a lower speed, but the basic idea is the same. Anyway, this is almost never done on residential pools because it adds too much complexity, but you see it on commercial pools sometimes.

I can see how using one 1.5 H.P pump for twice as long as a single 3 H.P pump would be more efficient, but I don't see how using two 1.5 H.P pumps for the same amount of time as a single 3 H.P pump would be more efficient. If you're generating the same flow rate, then the dynamic head should be the same.

How much more efficient do you think it is?

Perhaps they are using the setup as a type of variable speed pump? They could run both pumps when they need full flow and they could run just one when they want to save electricity. It seems like a single variable speed pump would be a better choice.

 

[JasonLion]

mas985 understands this much better than I do. Hopefully the following is correct.

Loosely speaking it has to do with matching the desired flow rate to the optimum efficiency point on the pump curves for the range of available pumps. For example, the largest practical pump in many situations is a 3 HP pump. Taking the WhisperFlo line as an example, the WFE-12 3 HP pump maxes out at around 160 GPM, and that is well outside of it optimum efficiency range. For optimum efficiency you would normally use it at closer to perhaps 100 GPM. Compare that to a WFE-4 1 HP pump which hits optimum efficiency somewhere around 80 GPM. Two WFE-4 pumps run in parallel with properly tuned plumbing can easily achieve 160 GPM and be solidly within their ideal efficiency range. Two 1 HP pumps running in their ideal efficiency range are going to draw less power than a 3 HP pump operating well outside it's optimum efficiency range.

My understanding is that in an ideal world you could specify a pump design optimized for exactly what you are trying to do, and the above analysis would not hold. But the real world is far from ideal and pumps optimized for your specific application are often not available. In practice pump choice is constrained and situations such as the one postulated above do occur.

In addition to limits on available pump designs there are also limits imposed by external constraints. For example some application might be ideally solved with a 6 HP pump, but that pump might require 408 volt 3 phase power, which might not be available on site. The 6 HP pump could be replaced by three 2 HP pumps running on 240 volts single phase. In this application, it might not really be appropriate to call the 2 HP pumps more efficient in the normal sense, but since they are replacing a solution that would not work for other reasons, they will be more efficient than any other alternative that could actually be installed.