Backyard Aquaponics
A place to discuss aquaponics

I'm not a person given to liking "multiple" truths. When I see answers or information that are apparently contradictory I assume one of three things:

The contradiction is only apparent it is my understanding that is lacking.
One of the pieces of information is wrong.
Both pieces of information are wrong.

I've posted this after seeing the efficiency figures on a pump calculated two different ways and method one (Franks) found that it was a relatively efficient pump and method two (Mine) found that it was a relatively inefficient pump. This has happened on a number of occaisions.

What I would like to see is a discussion in this thread on the pros and cons of both methods of calculation. Once this discussion has run out of steam or blown up we could then have a poll to decide which way we would talk about pump efficiency in the future to avoid further confusion of primarily me and sedcondarily new people.

We have used this method before when naming and/or renaming hardware components of systems so hopefully we can use it again.

L/Whr Method

For Every head height that you are interested in take the volume (L/hr) pumped at that head height and divide it by the number of watts (W) that the pump is rated to use.

For Example say I have a system where I have calculated that my total effective head is 1.5m so I look at the pumping volumes of a range of pumps at 1.5m.

Pump produces 10000L/hr @ 1.5m head and consumes 125watts to do so.

10000L/hr / 125W @ 1.5m= 80L/Whr@1.5m

You then calculate the figures for other pumps and compare them with each other to find the more efficient pump for your application.

pump energy efficiency calculation expressed in % is done in two stages:

first pump effect is calculated:
pump effect (in kW) = pump head (in bar) * pumped volume (in l/min) @ this head / 600
efficiency is then
power consumed (in kW) / pump effect (in kW)

power consumed = pump rating (unless you measure it)

for ease of calculation, we limit ourselves to water (density = 1) and we stay at the same location
(to be perfectly exact, local gravity acceleration and density of the medium should be taken into account)

so 1 bar can be considered equal to 10 m head
the 600 factor comes from converting bar to m head and minutes to hours

I have assembled a series of calculations which include both efficiency calculations in % as well as in l/w:

you will find there the calculations of the pumps that were mentioned recently on BYAP
this calculator automatically allows you to calculate both expressions for your own pump: in % and in l/w
or for any pump you wish to study

my problem with the l/w expression is that it does not by itself give a reference:
the maximum reachable l/w changes with head as you can see in the spreadsheet
if you don't know what is maximum reachable for each head, you don't know if your pump is performing or not @ this head

The % expression tells you immediately the performance of the pump @ this head

frank

the calculator also allows you to find out for what head your pump was designed:
by inputting the data from the pump chart (or from a data table), you will see @ what head it performs best

just make sure you use data that occur at the same time

do not enter the data given by the manufacturer to describe the pump:
these invariably give maximal flow and maximum head
but these never concur:
maximal flow happens @ zero head
maximal head happens @ zero flow

frank

Frank has put forward the first pro of the % method saying that by referencing it to an unobtainable 100% efficiency you get a clear picutre of whether the pump you are looking at is efficient or not.

I would counter this by saying the only way you know whether the pump is efficient or not forthe purposes of AP is by comparing it to other pumps. Yes the % method shows how much work is being done by the pump in question relative to how much energy is being consumed but by itself this dosn't give me any useful information from a system design point of view.

It is only when you compare the % values of a number of pumps that you can see whether the pump you are considering is efficient relative to other pumps and not to an impossible 100% energy efficiency. Since we are trying to make a descision about pumps it is to pumps that we should compare any result.

I'd rather we looked at why the two methods can produce contradictory results.

they don't, they only seem to
we get the same results
as shown in this table:

0.100 m 6000.0 l/min 360000 l/hr 100 watt 0.010 bar 10 mbar 100 watt 100.00% 3600 l/w
0.200 m 3000.0 l/min 180000 l/hr 100 watt 0.020 bar 20 mbar 100 watt 100.00% 1800 l/w
0.300 m 2000.0 l/min 120000 l/hr 100 watt 0.030 bar 30 mbar 100 watt 100.00% 1200 l/w
0.400 m 1500.0 l/min 90000 l/hr 100 watt 0.040 bar 40 mbar 100 watt 100.00% 900 l/w
0.500 m 1200.0 l/min 72000 l/hr 100 watt 0.050 bar 50 mbar 100 watt 100.00% 720 l/w
0.500 m 1200.0 l/min 72000 l/hr 100 watt 0.050 bar 50 mbar 100 watt 100.00% 720 l/w
1.000 m 600.0 l/min 36000 l/hr 100 watt 0.100 bar 100 mbar 100 watt 100.00% 360 l/w
1.500 m 400.0 l/min 24000 l/hr 100 watt 0.150 bar 150 mbar 100 watt 100.00% 240 l/w
2.000 m 300.0 l/min 18000 l/hr 100 watt 0.200 bar 200 mbar 100 watt 100.00% 180 l/w
2.500 m 240.0 l/min 14400 l/hr 100 watt 0.250 bar 250 mbar 100 watt 100.00% 144 l/w
3.000 m 200.0 l/min 12000 l/hr 100 watt 0.300 bar 300 mbar 100 watt 100.00% 120 l/w
3.500 m 171.4 l/min 10284 l/hr 100 watt 0.350 bar 350 mbar 100 watt 99.98% 103 l/w
4.000 m 150.0 l/min 9000 l/hr 100 watt 0.400 bar 400 mbar 100 watt 100.00% 90 l/w
4.500 m 133.3 l/min 7998 l/hr 100 watt 0.450 bar 450 mbar 100 watt 99.98% 80 l/w
5.000 m 120.0 l/min 7200 l/hr 100 watt 0.500 bar 500 mbar 100 watt 100.00% 72 l/w
5.500 m 109.0 l/min 6540 l/hr 100 watt 0.550 bar 550 mbar 100 watt 99.92% 65 l/w
6.000 m 100.0 l/min 6000 l/hr 100 watt 0.600 bar 600 mbar 100 watt 100.00% 60 l/w
6.500 m 92.0 l/min 5520 l/hr 100 watt 0.650 bar 650 mbar 100 watt 99.67% 55 l/w
7.000 m 85.0 l/min 5100 l/hr 100 watt 0.700 bar 700 mbar 99 watt 99.17% 51 l/w

this table shows that, while best possible result expressed in % stays 100 %,
best possible results expressed in l/w diminishes with head

compare 120 l/w with 1800 l/w:

it looks like 120 l/w is a very poor performance compared to 1800 l/w

you might think pump 1 that does 120 l/w @ 3 m
is doing MUCH worse than pump 2 that does 1800 l/w @ 0.1 m

say 15 times worse

while pump 1 has 100% efficiency and pump 2 only 50 %

so instead of 15 times worse, pump 1 performs twice as good as pump 2
that is because 120 l/w is the best you can expect from any pump @ 3 m head

frank

... and because 1800l/w is only half of what the perfect pump could achieve @ 0.1 m

frank

Well here is an example supplied by yourself from: viewtopic.php?f=8&t=4661&st=0&sk=t&sd=a&hilit=srac&start=135



Well 46L/w @ .038m is pretty poor relative to other pumps you can get but it is not as outstandingly awful as the % value of .49% implies. This would be one of the contradictions that I was talking about.

let's take a closer look at this pump: 0.038 m head and 65 watts
100% would be:
0.038 m 10263.0 l/min 615780 l/hr 65 watt 0.004 bar 4 mbar 65 watt 100.00% 9474 l/w

9474 l/w / 46 l/w = ....

yes: 0.4855%

there is no contradiction, there just seems to be because l/w doesn't give you the reference to what 100% efficiency could be
this pump is absolutely outstandingly awful at this head however you look at it, in % or in l/w
the results are the same

proof lies in mathematics, Stuart (no pun intended, I believe we both support his)

frank

"Once this discussion has run out of steam or blown up we could then have a poll to decide which way we would talk about pump efficiency in the future to avoid further confusion"

This has been very civilised so far. Why don't you two geniuses discuss it out and come to a compromise. I would accept that, and if anyone wants to argue they can with both of you

Ok well I disagree but what about this one: (viewtopic.php?f=8&t=4661&st=0&sk=t&sd=a&hilit=srac&start=135)



249L/Whr @ .038m is one of the best results that has been posted but 2.62% efficiency seems really crappy to me.

I like the first way. Only because I understood it on the first read. And we are only looking at a simple way of working out pumps. We don't do it every other day. And when the time comes to pick a new pump I don't want to have to re-read 50 pages to get my head around it again. I just want something simple to work out. Like how we work out what a pump costs to run per day.

I have a quiz that hopefully will help shed some light on this topic (or add more confusion, probably more likely). I know the answers 'cause I created the quiz. There are two questions in this quiz.

Question 1

I have two pumps; Pump A and Pump B.

Pump A pumps straight up and onto a growbed next to the pump. The total head (including fittings etc.) is 1.0 metres. The flow rate has been measured at 2,100 l/h.

Pump B pumps to another growbed some distance away. The pipework has a number of bends etc. but the total head (including fittings etc.) is 1.5 metres. The flow rate has been measured at 1,700 l/h.

Both pumps require exactly 35 Watts of power to run.

Which pump would you buy?

Question 2

Again I have two pumps to choose from. This time they are both pumping to the same growbed. So both pumps are pumping to the same head height.

Pump A has a large discharge outlet so 40mm PVC pipe is used. The total head (including fittings etc.) is 1.0 metres. The flow rate has been measured at 2,100 l/h. Pump A requires 35 Watts of power to run. [Yes this is the same pump as Pump A in Question 1]

Pump B has an unusually small discharge outlet (because of the type of pump that it is) so 20mm PVC pipe is used. Due to the smaller pipe used, the total head is higher as more friction is created. The total head (including fittings etc.) is 1.2 metres. The flow rate has been measured at 2,100 l/h. Pump B requires 40 Watts of power to run.

Which pump would you buy?

Which one is cheaper?

Someone who is thinking outside of the box! Asking this question is part of what I think the answer should be. +1 to Axl

For Question 1, they both cost the same.
For Question 2 Pump A is 1/2 the price of Pump B, but Pump B is supposed to last twice as long.