# Pressure Required to Circulate Liquid in a Closed System?

Steven Bolgiano
TL;DR Summary
Example: 3/4 pvc in closed circuit with pump. Linear distance of pipe=30ft / head 8ft ...Force?
Example: 3/4 pvc in closed circuit with pump. Linear distance of pipe=30ft / head 8ft .... so not counting friction, what general description of force can describe what's required to make the liquid circulate?

## Answers and Replies

Mentor
More correctly, it's pressure, not force. And it's pressure difference across the pump. In a closed system full of liquid, any microscopic pressure difference will cause a flow. More pressure difference will cause more flow.

We can give you much better help if you make a simple pencil sketch of your system, and tell us more about it - what is the fluid, what are you trying to accomplish, what is your level of understanding of fluid dynamics and physics, etc.

berkeman and hutchphd
Example: 3/4 pvc in closed circuit with pump. Linear distance of pipe=30ft / head 8ft .... so not counting friction, what general description of force can describe what's required to make the liquid circulate?
Is there any gas in the system?
If the system is filled with liquid, and there is no gas present where fluid might partly fill the tube and flow downhill, then the head is not important since all hydrostatic pressures will cancel.

Lnewqban
Gold Member
TL;DR Summary: Example: 3/4 pvc in closed circuit with pump. Linear distance of pipe=30ft / head 8ft ...Force?

Example: 3/4 pvc in closed circuit with pump. Linear distance of pipe=30ft / head 8ft .... so not counting friction, what general description of force can describe what's required to make the liquid circulate?
Not counting friction in a closed loop, you can get any flow at zero pump head. No real closed system will have this characteristic though.

So if you are trying to actually specify a pump for your 3/4 inch pvc loop…you must consider friction…a.k.a. viscous dissipation.

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russ_watters and Chestermiller
We can give you much better help if you make a simple pencil sketch of your system, and tell us more about it - what is the fluid, what are you trying to accomplish
@Steven Bolgiano : this ^^

russ_watters
Steven Bolgiano
Thanks!

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Gold Member
Are you talking about a real system, or is this a theoretical exploration? In a theoretical system you describe there is no "force" i.e. pressure added by the pump - pump head. However, If you are actually trying to design that system, there certainly is a necessary pump head for any desired flow rate.

Steven Bolgiano
More correctly, it's pressure, not force. And it's pressure difference across the pump. In a closed system full of liquid, any microscopic pressure difference will cause a flow. More pressure difference will cause more flow.

We can give you much better help if you make a simple pencil sketch of your system, and tell us more about it - what is the fluid, what are you trying to accomplish, what is your level of understanding of fluid dynamics and physics, etc.
Thanks! I wanted to make sure I wasn't overlooking factors beyond friction.
I have only a passing knowledge of hydrodynamics, and an Associate degree in design engineering.
I wanted to check on my choice of pumps, as I don't need more than a slow flow rate, and I need the minimum parasitic load of the pump's energy foot print.

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Gold Member
Thanks! I wanted to make sure I wasn't overlooking factors beyond friction.
I have only a passing knowledge of hydrodynamics, and an Associate degree in design engineering.
I wanted to check on my choice of pumps, as I don't need more than a slow flow rate, and I need the minimum parasitic load of the pump's energy foot print.
So you are trying to design a real system.

What is the desired flow rate, pipe sizes, length etc. That info will lead you to calculating your system curve. After generating the system curve you will attempt to find a pump which satisfies the systems head requirements at the design flowrate. This is generally achieved by plotting the system curve with the manufacturers pump curve. The intersection of the two curves gives the theoretical operating point.

Steven Bolgiano
Are you talking about a real system, or is this a theoretical exploration? In a theoretical system you describe there is no "force" i.e. pressure added by the pump - pump head. However, If you are actually trying to design that system, there certainly is a necessary pump head for any desired flow rate.
Real. "Solar Assisted Anaerobic Digester"
.... it utilizes solar heated water and solar heated forced air to maintain biologically generated methane gas.

Gold Member
Ok, well if you want some help specifying a pump, you'll have to be specific about the design characteristics of the closed loop system. In the video it looks like it might be a heat exchanger for the process?

You will also need a header tank that will allow for thermal expansion, and the escape of gas from the system.

erobz
Steven Bolgiano
Yes. There are several heat gathering systems that deliver heat to the anaerobic digester tank.... which has copper radiator pipes underneath tank, and forced air into the tank's surrounding "cabinet"
• Traditional solar hot water "serpentine" pipe collectors.
• Single solar hot water, large tube collector in peak of greenhouse
• Solar electric powered forced air from greenhouse peak.
• Solar electric powered forced air "mini fans" to utilize excess heat in solar collector containment.
I do have a pressure bleed valve in the system, but I've never see it relieve pressure.
I always assumed the circulation reduced the water temp to the extent that the pressure was not enough to create damage.
But if you have a "poor boy" solution for constructing a small header tank, ... I'm interested.
Thanks!

Steven Bolgiano
Apologies. I'm not as much interested in the pump size, in that I've got one on there now.....
And I just put a flow rate monitor on, because I couldn't observe the actual flow.
I did want to check on what I was pretty sure of for closed circulation systems, and see if anything indicated a larger pump was needed (its a tiny system so that would have been unusual).

Gold Member
But if you have a "poor boy" solution for constructing a small header tank, ... I'm interested.
Thanks!
"poor boy" solution...How much did it cost to make this system? It doesn't look cheap. I would imagine a hydronic expansion tank is chump change in comparison. But perhaps the change in temperatures in the system are not so great that you've experienced a problem.

Gold Member
Apologies. I'm not as much interested in the pump size, in that I've got one on there now.....
And I just put a flow rate monitor on, because I couldn't observe the actual flow.
I did want to check on what I was pretty sure of for closed circulation systems, and see if anything indicated a larger pump was needed (its a tiny system so that would have been unusual).
Ok.

But if you have a "poor boy" solution for constructing a small header tank, ... I'm interested.
A simple 'T' fitting at the high point in the system, would allow air to escape, while heated water would not flow through the small exposed reservoir tank.

Air or water vapour in the system can block the flow by preventing the hydrostatic pressure in the rising and falling legs from cancelling. There should be some hydrostatic difference, because you have actually built a thermal siphon. Maybe, if the thermal siphon can drive the circulation, you would not need to buy, fit or power a pump.

If you do have a pump, make sure you save power by pumping it in the same direction as the thermal siphon.

There seems to be no advantage gained by running the system at positive pressure.

Steven Bolgiano
You nailed what I have been wondering ... how much thermal siphon exchange can take place.
Even though there are two sources for heated water, it is still a single series connection.
I tried putting check valves in strategically, but removed them thinking they were either ineffective or an inhibition. I do have several blue tooth temperature monitors placed throughout the system, that provide a graphed data record

You nailed what I have been wondering ... how much thermal siphon exchange can take place.
You can work that out by knowing the temperature of the water around the system and computing the density. Compute a path around the loop, accumulating hydrostatic pressure and head.

Thermal siphons work best when the flow velocity is low, and the pipes are larger diameters.

I tried putting check valves in strategically, but removed them thinking they were either ineffective or an inhibition.
Unless they are simple flaps, check valves will take more pressure to operate than is available from a thermal siphon. Check valves work well in pumped systems where backflow must be prevented.

Steven Bolgiano
"poor boy" solution...How much did it cost to make this system? It doesn't look cheap. I would imagine a hydronic expansion tank is chump change in comparison. But perhaps the change in temperatures in the system are not so great that you've experienced a problem.
It didn't really cost that much to build the system, but it's because this project is an exercise in designing alternative methods to construct and fabricate. So a cheap expansion tank design would be nice. Inside the peak of the greenhouse at the highest point of the system is a 4" schedule 80 PVC, that could act as the "tank", and I can mount a pressure relief valve there (do I set the threshold pressure just below what the pip's psi rating is?).

I built this contraption entirely by myself, which reduced costs significantly.

Here is the original 800 cubic system, after traveling to and working with I designed and built after working with (with the help of structural, chemical, and thermodynamic engineers.) ....
And it was not cheap!

Steven Bolgiano
You can work that out by knowing the temperature of the water around the system and computing the density. Compute a path around the loop, accumulating hydrostatic pressure and head.

Thermal siphons work best when the flow velocity is low, and the pipes are larger diameters.

Unless they are simple flaps, check valves will take more pressure to operate than is available from a thermal siphon. Check valves work well in pumped systems where backflow must be prevented.
exactly what I suspected I observed

Inside the peak of the greenhouse at the highest point of the system is a 4" schedule 80 PVC, that could act as the "tank", and I can mount a pressure relief valve there (do I set the threshold pressure just below what the pip's psi rating is?).
Why must it operate at great pressure, can you not just let it breathe ?

Look at the radiator cap in a car. If the pressure rises by half or one bar, it will blow excess gas or coolant out to the reservoir tank. When it cools, it draws water in from the reservoir tank without a pressure step.

Steven Bolgiano
Great info. Can you send a description or diagram of one?

Steven Bolgiano
Nice! OK .... so how does one measure the thermal siphon's transfer of heat through a system? Is it by monitoring the temperature from different locations, calculating the difference (Delta?) and maybe the rate of change? (And) ..... I've got a low gpm sump pump .... does that have to be taken out in order to evaluate a thermal siphon?
Thanks!

Nice! OK .... so how does one measure the thermal siphon's transfer of heat through a system?
You study for a couple of years, so you can solve any general case.

Or you might provide an accurate description of the example you want to evaluate, and then see how to solve that one case.

Steven Bolgiano
Ha ha! Isn't that the truth. thanks again.

Gold Member
You study for a couple of years, so you can solve any general case.

Or you might provide an accurate description of the example you want to evaluate, and then see how to solve that one case.
Can you demonstrate the calculations for the simplest closed loop you can think of - a "text book problem" ? The calculations are completely novel to me.

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Can you demonstrate the calculations for the simplest closed loop you can think of - a "text book problem" ? The calculations are completely novel to me.
The analyses I have been involved in studied the natural circulation flow in a system with a heat source down low and a heat sink up high. A closed loop with pumps installed but not running.

We did this numerically with a computer program that divides the system up into ~50 "nodes" and solves mass, energy, and momentum conservation. This allows for temperature & pressure-varying physical properties (eg, density), and flow-dependent resistance, etc. as well as varying the heat input and removal rates.

It can be done by hand but you have to make simplifying assumptions.

Steven Bolgiano
Thanks so much! I guess a better way to ask my question is:
In a closed loop system, what information or monitoring tells you a thermal siphon is active or in active?

Gold Member
The analyses I have been involved in studied the natural circulation flow in a system with a heat source down low and a heat sink up high. A closed loop with pumps installed but not running.

We did this numerically with a computer program that divides the system up into ~50 "nodes" and solves mass, energy, and momentum conservation. This allows for temperature & pressure-varying physical properties (eg, density), and flow-dependent resistance, etc. as well as varying the heat input and removal rates.

It can be done by hand but you have to make simplifying assumptions.
I was just thinking there must be some base model, where the heat input establishes the thermal gradient in the water column, and a free convection within a column is creating the flow. Warm fluid is less dense and rises, cold fluid sinks. I can see this happening vertically, but it's harder to swallow that a bulk flow is established in a loop as if there were a pump circulating the flow. What is the base model for this is what I'm asking...its probably going to be complicated even with simplifying assumptions; whatever they may be.

Steven Bolgiano
Ah! So while the pump may need very little force to circulate water in the circuit ..... I see what you mean, ... without a pump, a thermal siphon generating heat transference on the downward link of the circuit is difficult to envision..... How could that happen?

Gold Member
Ah! So while the pump may need very little force to circulate water in the circuit ..... I see what you mean, ... without a pump, a thermal siphon generating heat transference on the downward link of the circuit is difficult to envision..... How could that happen?
I'm imagining a single column joining a warm reservoir on bottom to a cold reservoir above. I can imagine a natural circulation occurring where heat is brought to the top cold reservoir by free convection in the column and is released to the environment in the top cold reservoir, that seems like it can have a steady state circulation. I can't see it happening with the hot reservoir on top, nor can I see it with two vertical columns establishing some kind of net clockwise\counterclockwise flow like a pump would provide around a loop.

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