Pumps and flow rate

[reegs210]

If someone can elaborate for me here, I'd be extremely grateful

I bought a couple of the magnetic pumps from KK (~$65 ea), which also seem popular at other HB suppliers, and the flow rate has me confused. Apparently, these should pump ~19-20L/min, however, I just performed a wet run on my set up and was clocking at 20L over 3 and a half minutes.

I noticed this, particularly, as I have only just upgraded from the brown pumps and thought that the flow rate seemed similar whilst performing my trials

I don't get it, and maybe I am a bit clouded with my beer addled brain, but I cannot understand such a big difference between my trial and the max ~19-20L that the pump is purported to handle

Thanks in advance

R

 

[wide eyed and legless]

You have to remember that the flow rate is at the head of the pump, if the pump is pushing liquid through a distance then the flow rate will different to that at the head.

 

[verysupple]

You have to remember that the flow rate is at the head of the pump, if the pump is pushing liquid through a distance then the flow rate will different to that at the head.

Hmm, water is pretty much incompressible and I presume the wet run was done with water (and for practical purposes wort is 95% water so we'll assume it's the same). Because water is incompressible the flow rate should be identical throughout the entire system providing there are no bubbles or pockets of air. I think the reason the OP observed a much lower flow rate is because the specified flow rate will be for a certain head pressure and it's likely the real world scenario head pressure is higher.

 

[wide eyed and legless]

The litres/min is what the op has read on the pump, after his attachments and the distance the liquid is travelling will be the difference in the litres /min at the head to the litres/ min he is actually getting.

 

[neo__04]

Don't know whether this applies here as not how your pumps are powered,
but voltage may cause a difference to this.

Eg. 12v pumps, your power supply is giving it 10-11v?

Just a thought

 

[verysupple]

No, the L/min he gets at the head is the same as he gets anywhere throughout the system, it's just that the L/min at the head is now lower than that specified by the manufacturer because the head pressure is different. Kind of like how your car manufacturer says it can do a certain acceleration but if you fill the boot with cement it will accelerate much slower.

Imagine you have a hose completely filled with marbles. If you push one marble into one end of the pipe, then one marble comes out the other end, not half a marble. Just like marbles, water is incompressible, so if the head is pumping X L/min then X L/min comes out the other end of the hose.

And for the geeky explanation: The length and diameter of the hose affect the head pressure because of the shear forces applied to the liquid by the walls of the hose. The liquid against the walls travels slower due to the friction with the walls - longer walls means more friction. If the hose has a small diameter then a larger fraction of the liquid within is affected by the shear friction. But the flow rate has to remain constant throughout the length of the hose, so to compensate the liquid near the centre has to travel faster. These two effects make it harder for the pump to pump (increases the head pressure). So, going back to the hose and marble analogy, the longer the hose is the harder it is to push a marble in. But for every marble you do push in, a marble has to come out the other end.

 

[reegs210]

I'm working with roughly a metre of tubing to the pump and the same to the receiving vessel. Surely, this can't be restricting flow rate?

If I were to connect intake of pump directly to the water out vessel, and a little bit more for receiving, then I would get the max flow rate?

Edit: It makes more sense now. Thanks

 

[verysupple]

Well, Neo brought up a good point. What's your power supply? Also, the height it has to pump the liquid plays a big role. So pumping 1 m horizontal isn't the same as pumping 1 m vertically up.

Do the specs say it's a maximum flow rate? If so, you're never going to get that in the real world because of all the stuff mentioned above. In any case, the specs should state what head pressure that flow rate was measured at. You can make a rough estimate of your system head pressure by following this example. If yours is significantly higher than what the manufacture measured it at then you'll get a lower flow rate.

 

[wide eyed and legless]

Very Supple that is what I am saying, the litres/ min has diminished through resistance I am not saying he is still getting what he has read on the pump at the at the head the 20 litres/ min, of course he is getting less through the resistance which would come from the fittings and distance travelled.

 

[verysupple]

Very Supple that is what I am saying, the litres/ min has diminished through resistance I am not saying he is still getting what he has read on the pump at the at the head the 20 litres/ min, of course he is getting less through the resistance which would come from the fittings and distance travelled.

Goodo, I must have misunderstood. I thought you meant the head was still pumping at whatever the rating was but it was coming out the other end slower. Anyways, the OP seems to know what's going on now.

 

[Burt de Ernie]

Hmm, water is pretty much incompressible and I presume the wet run was done with water (and for practical purposes wort is 95% water so we'll assume it's the same). Because water is incompressible the flow rate should be identical throughout the entire system providing there are no bubbles or pockets of air. I think the reason the OP observed a much lower flow rate is because the specified flow rate will be for a certain head pressure and it's likely the real world scenario head pressure is higher.

This theory is incorrect.

Pipe loss and head loss all affect flow rate.

 

[Burt de Ernie]

No, the L/min he gets at the head is the same as he gets anywhere throughout the system, it's just that the L/min at the head is now lower than that specified by the manufacturer because the head pressure is different. Kind of like how your car manufacturer says it can do a certain acceleration but if you fill the boot with cement it will accelerate much slower.

Imagine you have a hose completely filled with marbles. If you push one marble into one end of the pipe, then one marble comes out the other end, not half a marble. Just like marbles, water is incompressible, so if the head is pumping X L/min then X L/min comes out the other end of the hose.

And for the geeky explanation: The length and diameter of the hose affect the head pressure because of the shear forces applied to the liquid by the walls of the hose. The liquid against the walls travels slower due to the friction with the walls - longer walls means more friction. If the hose has a small diameter then a larger fraction of the liquid within is affected by the shear friction. But the flow rate has to remain constant throughout the length of the hose, so to compensate the liquid near the centre has to travel faster. These two effects make it harder for the pump to pump (increases the head pressure). So, going back to the hose and marble analogy, the longer the hose is the harder it is to push a marble in. But for every marble you do push in, a marble has to come out the other end.

Again...this is totally incorrect.

Anyone who has correctly specced any pump has undertaken a pipe loss calculation and factor the losses into the final spec.

 

[Burt de Ernie]

If someone can elaborate for me here, I'd be extremely grateful

I bought a couple of the magnetic pumps from KK (~$65 ea), which also seem popular at other HB suppliers, and the flow rate has me confused. Apparently, these should pump ~19-20L/min, however, I just performed a wet run on my set up and was clocking at 20L over 3 and a half minutes.

I noticed this, particularly, as I have only just upgraded from the brown pumps and thought that the flow rate seemed similar whilst performing my trials

I don't get it, and maybe I am a bit clouded with my beer addled brain, but I cannot understand such a big difference between my trial and the max ~19-20L that the pump is purported to handle

Thanks in advance

R

What is the size and length of pipe you are pumping though on the outlet side of the pump?

What is the size of the pipe on the inlet size?

 

[pk.sax]

On your inlet side:
is your ball valve full bore or reduced bore?
Is the nipple a full half inch one (hard to slip hose on) or the 3/8" one (easier to slip hose on).

Besides that, any other obstructions on the I let side reducing the pressure available to the pump?

I'd suggest setting the pump physically lower than the vessel to get a little head pressure to compensate for the fittings etc causing flow restriction. As little height rise from pump outlet to end if hose also.

See how close you get.

 

[reegs210]

On your inlet side:
is your ball valve full bore or reduced bore?
Is the nipple a full half inch one (hard to slip hose on) or the 3/8" one (easier to slip hose on).

Besides that, any other obstructions on the I let side reducing the pressure available to the pump?

I'd suggest setting the pump physically lower than the vessel to get a little head pressure to compensate for the fittings etc causing flow restriction. As little height rise from pump outlet to end if hose also.

See how close you get.

The ball valve is fully open and tubing fitted to a 3/8" barb

I have two pumps and both are sitting ~1mtr below my HLT/MT/BK vessels. Both pumps have identical lengths ~1 mtr tubing on inlet and outlet of pumps. All tubing that I am currently using has an inner diameter of 0.315"

Some tweeking to do still it would seem then?

Edit: got inner diameter of tubing wrong

 

[shaunous]

You have confused the 'law of pressure' with the 'law of flow' verysupple. In a given closed system the pressure will be the same at all sizes and volumes, flow will not be.

As others have mentioned, distance from outlet to pump, height of outlet to pump, hose size, head pressure, valves, joiners, etc.... will all play a role in affecting the rate of flow.

You say your using .315" i.d. hose, thats about 8mm, the pumps outlet is bigger then this if its the standard mag pump size, I think there 13mm / half inch. I cannot remember if thats the i.d. of the outlet or the thread size of the fitting at the pump head.

Just check the i.d. of the pump outlets, and try match your valves, hoses and fittings to that, and as long of your heights and distance of hose isnt crazy long (1m is OK) you should get pretty close to that 19L/min.

 

[pk.sax]

Yes, a bit restricted on the inlet isn't helping. Try to remove as much restriction as possible. Use 1/2" I'd hose, the hosetail is a restriction but I won't go changing that in a hurry, do the hose first.

 

[verysupple]

Again...this is totally incorrect.

Anyone who has correctly specced any pump has undertaken a pipe loss calculation and factor the losses into the final spec.

It's a simplification, but it's not incorrect. Yes, you can calculate what the real word flow rate will be for your system, but it's unlikely to be the same as the pump manufacturer spec precisely because of the pipe losses etc. When you "correctly spec any pump" you look at what it can do in ideal circumstances and then calculate what it will do in your circumstances.

You have confused the 'law of pressure' with the 'law of flow' verysupple. In a given closed system the pressure will be the same at all sizes and volumes, flow will not be.

I think you're making up your own laws there. I never came across the "law of pressure" or "law of flow" when I studied fluid mechanics or fluid dynamics. I did learn about Bernoulli's principle though. The force remains constant, not the pressure.

In brewing applications we rarely have large changes in pipe or hose diameters (we wouldn't usually go from, say, an 8 mm hose to a 20 mm hose; we'd try to keep everything about the same diameter), so we can ignore the effects of the Hagen-Poiseuille law as the difference in pressure drop in the two hoses would be quite small.

By the way, all this only applies because the pump in question is not a positive displacement pump (constant flow), it's a centrifugal pump which is a type of velocity pump. The pressure output of a velocity pump is governed by Bernoulli's principle, and is therefore not constant for all situations.

There are many things I don't know much about, but first year undergraduate physics isn't one of them.

 

[verysupple]

Pipe loss and head loss all affect flow rate.

This is of course correct. The bit you're missing is that the total pipe loss and head loss are constant for a given system (here 'system' means a set of physical circumstances. Obviously it'll change if you change the length of a pipe or the elevation for instance). And so the volumetric flow rate at any point within the system remains constant.

 

[shaunous]

I studied to be a busted arse diesel fitter and I can tell you there is a law of pressure.

Pascal's law is a principle in fluid mechanic's that states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure variations (initial differences) remain the same.


I clearly made the 'law of flow' part up.



Your getting to technical for what is as you said, a 'Non-positive Displacement Pump', best example being the water pump on our cars. As long as he is within the spec's of mounting the pump, and his inlet and outlet hoses/fittings/valves are not a smaller i.d. than the inlet and outlet of his pump, then he should get within the spec of 19L/min.