Closed loop head loss again!?!

[dedfish]

Does anyone have any links with specific facts about head loss, or the lack of, in a closed loop. I tried to explain why I thought that a closed loop had little head loss on another forum and the person I was responding to acted like I personally attacked his suggestion. I like to be able to back up my position.

 

[NateHanson]

I don't have any links, but try this explanation:

Head loss is the energy the pump uses just to get water to not flow backwards through the pipe. On your sump return pump, when you turn it off water flows backwards through the plumbing and into the sump. That's because there's a gravitational potential up there. The water 3 feet above, in the display tank, has energy it's waiting to spend on making the trip back down to your sump. If you could vary the power of your pump, then imagine that instead of turning it off completely, you just turned it down a bit. The flow in the tank would decrease. Keep turning it down, and eventually you'll reach a point where the pump is still running, still using energy, and still pushing on the water in the return pipe. But the water won't be going anywhere. It will be suspended in the return plumbing by the small pumping power you're exerting with the turned-down pump. The pump has to do work to counteract the potential energy of the water in the plumbing, and it's tendency to obey gravity. It has to do some work to fight the head pressure of the water above it.

Now on to a closed loop. Let's say your closed loop pump is three floors below you in the basement, with plumbing connecting it to a suction bulkhead and a couple output bulkheads in your display tank on the third floor. What happens when you unplug your closed loop pump. Does water suck backwards into the outputs, rushing down into the basement, through the pump, back up the plumbing and spray back into the tank through the suction bulkhead? No. Of course not. It just stops moving. The water in the pipes just stops moving when the pump turns off. The bulkheads are all connected to the same body of water, so there is no potential energy difference between water at two different points in the closed loop plumbing. You don't need to have your pump running even a little bit to keep the water from flowing backwards. If you used that hypothetical adjustable pump, and turned it on just a tiny bit, water would start moving through the closed loop immediately (albeit very slowly). So 100% of the pumps energy is going into moving water through the closed loop (the only flow losses and pressures are those due to frictional losses related to the size of the plumbing, number of elbows, etc.).

Is that any better?

 

[ReeferMedic]

Can't argue with that!

 

[RichConley]

THere is a LITTLE bit of head loss in a closed loop, simply because of any sort of elbows, etc, pipe restrictions, algae in the pipes, etc, but its really negligible. You're looking maybe 1-5%. Its essentially the same as having a powerhead in the tank.

 

[nitrofish]

kind of the same way a canister filter works.

 

[Scuba_Dave]

Some people may say gravity is working on the water, pushing down. But if you think about it, the water that needs to go down should be the same as the water that is going up - so it equals out.
Any force down helps = any resistance up hurts

I see very little head loss in my regular return due to (2) 45's & a 90 elbow

When you compare head loss from a squid (30%) then the loss in a closed loop is very small

 

[NateHanson]

THere is a LITTLE bit of head loss in a closed loop, simply because of any sort of elbows, etc, pipe restrictions, algae in the pipes, etc, but its really negligible. You're looking maybe 1-5%. Its essentially the same as having a powerhead in the tank.

You're certainly right. Although I think that might not be called head, but rather frictional loss. Maybe someone like Matt L. can correct/affirm that for me. Head loss is only dependant on the difference of height of the two bodies of water, and frictional loss is only dependent on pipe geometry and rate of flow.

 

[NateHanson]

You can calculate the theoretical frictional loss of your closed loop using the ReefCentral Head loss calculator, just put zero for the vertical distance, and report all your length of pipe (horizontal and vertical) as horizontal distance.

http://www.reefcentral.com/calc/hlc2.php

 

[Jim Tansey]

Nate,

You are correct about the frictional loss, a point that may be confusing however is that frictional loss is often stated in terms of the EQUIVALENT head loss with the word equivalent often dropped. This is a quick way of stating the total losses in a plumbing system regardless of the cause. Clear as Mud?

Jim

 

[NateHanson]

Clear as mud Jim! So would the original units for frictional loss be a pressure (psi)? And that's translated into an equivalent height unit, like feet of water?

Thanks for lending some actual knowledge to my layman's understanding.

Nate

 

[Jim Tansey]

Correct Nate, and most often expressed in feet of water column, or just feet of water.

Jim

 

[dedfish]

I think some of the misconceptions arround head loss and closed loops stems from a misunderstanding of the terms used like head loss, frictional head loss, equivilant head loss. I think a lot of reefers just figure any loss of water exiting the output of their pump as head loss.

 

[Jim Tansey]

I should have been more clear on this point the reason for converting frictional and other losses into equivalent head loss is that it gets all the losses into the same units, for example if the loss due to a 90 degree fitting is "X" feet of water it can be simply added with the other losses for a total.

Jim

 

[NateHanson]

That makes sense Jim. Thanks for the professional perspective.