My current project and a question
 

I am running a Mag-drive pump rated at 500 GPH, but in real world testing through a DD Maze2 and Black Ice Radiator with 3/8 fittings, it is more like 60 GPH with a 1 foot lift(if that), is this about right? I am getting anxious of finishing this project and would like comments,

thanks!

Jim

Ya, that's about normal, and should work well, but you could improve now or later.

Centrifugal pumps lose their flow rate fast with restriction, it has nothing to do with the height unless your computer is a pond. You can make a 20' high loop and as long as it's sealed the pump will only be working against the friction in the tube. It will work even if it's say only rated at a 5' head (because the force required to pump up is equaled by the force regained on the down, the pump just circulates in a sealed system, your not pumping water from a lower level to a higher level, your circulating it).

If you want better flow convert to 1/2" fittings. You may not even need to upgrade the tubing if you can stretch it over the fittings. I found that the 3/8" fittings (with their 1/4" ID) has more to do with the terrible flow than anything else (other than a poor rad, which you don't have, or the waterblock, but here cooling comes before restrictions). 1/2" fittings have 3/8" ID, but bigger tubing does help also. 1/2" can flow over twice as much as 3/8"!!! :)

I also saw an article where flow was increased by 1/3 by drilling the fittings out a bit to increase the ID (without the need for bigger tubing and removing them). But I'd rather not comprimise the strength of the fitting when you can mount 1/2" ones with just a wrench and torch for the rad! (remember the teflon tape!)
:)

Pumps are rated by the flow directly out of the nozzle. So 500GPH is what it pumps with nothing attached. And it has everything to do with height. You loose about 100GPH with each foot of head. Give or take, I don't really remember. Each 90 elbow equals about a foot of head too. You are better to go to a wider tube. If height did not matter then we would not even need pumps would we.

Almost all decent pumps are actually rated at 1 foot head (this is true for the Little Giant, Danner Mag-Drive, Eheim Hobby Pumps, MaxiJets). Just go to one of the web sites to verify this.

Your correct in the fact that the values at each head height display how the pump will handle resistance by friction due to a restriction, but the actual heigh of a closed loop system doesn't matter.

Make a loop 10' in diameter. Run it vertically (with 10' head hight according to your statement), then lay it flat on the ground, run it again (with 0' head hight). You will see there is no difference in flow.

Pump water from one open container to another 10' horizontal, then repeat straight up, chances are the pump won't be able to pump a thing 10' up. This is "Head Hight". In a closed loop resistance to flow (friction) is all that matters.

Also, you cannot give each bend a value in equivalance to feet. They do have a huge influence, the same as any restriction in the loop, but not due to the fact they bend, due to the inside diamater reduction. Water smashes into this abrupt reduction, the fater it's going, the more effect. However if everything was of the same inside diameter (tubing and fittings, like aerospace equipment) then the flow would be much better even with smaller tubing diameter and more bends.

Resistance declines by the speed of the fluid. If your statement where true, a Danner Mag-Drive 500 could only pump threw 10-90 degree elbows. Actually try this and you will see the more elbows you add the less the effect is in a "circulating" system.

I admit I have no experience with closed loop systems. I still don't see why it would make a difference. I didn't understand what you meant about 10-90 degree elbows. GPH is a rate not velocity. If you want more GPH use bigger tubing. Smaller tubing will make the velocity higher. And as a general estimate of a pump size most aquarists go by 90 elbows adding 1 foot of head. I think I stated it wrong last time. But what do I know I don't have a degree in fluid dynamics. I was just stateing what i knew from experience.

In an aquarium the water is pumped straight up to the tank, this is the head height.

In a closed loop system, water returning down exherts a force due to gravity equal to the force required to lift the water (think of syphoning, as long as the outlet is equal to the inlet theres is equal force of gravity, lower or raise the inlet or outlet and you will have flow regardless of the rest of the tubing being higher or lower, as long as it stays in the water). Gravity doesn't exhert an effect on a closed loop, the pump just circulates.

You are correct in that GPH is not velocity, but it is the effect that the velocity of the water in the tubing has against restrictions which causes friction. With one elbow there probably is the equivalance of restriction that a 1' head would create, which in turn lowers the velocity and GPH. The more elbows you add the less the effect becomes as the first one already has lowered the flow rate enough to make these restrictions easier for the water to pass threw. Meaning if one fitting dropped your flow by half, a second would only drop a little more. By the time you get to five or so restrictions of the same size, there is hardly a noticable decrease in flow.

Increasing tubing and fitting size makes all these restrictions lesser. In fact the move from 3/8" to 1/2" more than doubles it's ability to flow from primarily the fact that there is less friction threw the fittings and the tubing, and the water doesn't need to travel at a high velocity to achieve this flow (again restrictions + velocity = friction / restricted flow). But once again there is a large benefit like you said from going to larger tubing.

P.S. I edited my post as I don't believe the last comments where appropriate, sorry, we're all here to learn.
:)

GuyBFF:

Thanks for clearing that up. I wasn't entirely sure what you meant in my earlier thread, but that explained it nicely.