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# Water velocity

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Member Posts: 28
Hi,
I have two questions.
In closed system with fixed speed circulator :

-Is water velocity would be the same everywhere in the system even if we change resistance, example adding two or three valves?

-Looking at the pump curve, with lower flow rate there is increase in mecanichanical energy (head) added or lost, is that mean that the pump would need more electricity or ( watts ) whenever there is resistance added in the system?

Thanks

• Member Posts: 23,567
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Water is not compressible, so the velocity in a given section of pipe is strictly controlled by the flow rate and the area of the pipe. Therefore if the area of the pipe is the same in a closed system, the velocity will be the same. More area, lower velocity. Valves etc. make no difference at all.

So... if you add resistance to the system, changing the head and the flow with a centrifugal, you may need more electricity -- or you may not.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 22,396
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To add to @Jamie Hall reply, a restriction in the circuit, like a balance valve or partially closed ball valve will show an increase in velocity. Excessive velocity can cause cavitation and excessive wear in the components
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 28
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Water is not compressible, so the velocity in a given section of pipe is strictly controlled by the flow rate and the area of the pipe. Therefore if the area of the pipe is the same in a closed system, the velocity will be the same. More area, lower velocity. Valves etc. make no difference at all.

Let's say we added a partially closed valve to the system, for sure that would result in a lower flow rate, but is that because of a less area available or because of a lower velocity of the water?
Since we have a fixed speed circulator in the system is that mean we should have a fixed velocity of the water ?
• Member Posts: 7,589
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For any given pipe size, the velocity and flow rate will be linked and will rise and fall proportionally.
A fixed speed circ will not have a fixed velocity or flow rate.
A partially closed valve adds resistance to the system.
Maybe this will help.
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_16_na_0.pdf
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 23,567
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chakil said:

Water is not compressible, so the velocity in a given section of pipe is strictly controlled by the flow rate and the area of the pipe. Therefore if the area of the pipe is the same in a closed system, the velocity will be the same. More area, lower velocity. Valves etc. make no difference at all.

Let's say we added a partially closed valve to the system, for sure that would result in a lower flow rate, but is that because of a less area available or because of a lower velocity of the water?
Since we have a fixed speed circulator in the system is that mean we should have a fixed velocity of the water ?
No. A fixed speed "circulator" (a form of centrifugal pump) does NOT have a fixed flow or velocity (note that a fixed speed positive displacement pump does -- but that's not what we have here). Rather the flow varies inversely with the head added -- more head, less flow -- in accordance with the characteristic curve of the pump -- which is fixed by the design of the pump. The low flow rate if you add a partially closed valve comes about from the greater resistance of the valve -- the velocity has nothing to do with it (in fact, the velocity of the water in the valve may well be greater).
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 2,483
edited February 2022
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You are in a car going 50 mph, speedometer reading, that your velocity. In one hour you will have traveled on the highway, 50 miles, that's your flow. Encountering a school zone (resistance) with a velocity of 25 mph will reduce the flow and would take more time to reach that 50 mile mark.

Rule #1--The flow in any closed sys is the same in every part of the sys. However, velocity can change.

A molecule of water leaving the volute of a pump has a change in position and a direction, a velocity. In a 3/4" copper pipe we say that that water molecule moving at 2.48 feet per second ( a change in position and direction), we have a flow of 4gpm.

Any resistance that effects a change in position and direction (velocity) like the wall friction of a pipe or a partially closed ball valve will affect the flow and the flow will be the same in every part of a closed sys.

To answer your question, water velocity can change in a closed sys as a result of valves, etc., you are confusing velocity with flow in terms of gpm.
• Member Posts: 22,396
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Now to confuse it a bit more, the new ECM circulators have sensors and logic built in to maintain certain conditions. Some based on pressure, some based on temperature, or temperature differential. Also some small residential size that you can run from an app
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 23,567
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But, @hot_rod , don't forget that those new circulators are not constant rpm pumps. At any particular rpm it will behave exactly as a standard constant rpm centrifugal.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 28
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According to the circulator curve, increase in the system resistance result in decrease in the flow rate.
Since flow rate is area time velocity,
is that mean each time we add a resistance or valve to the system, water slows down? is that true?
• Member Posts: 3,364
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Hi, That sounds right. I'll say that adding a valve doesn't have to mean that you've added resistance. A full port ball valve won't add meaningful resistance, where a reduced port certainly will. And, you'll move fewer gpm with more resistance, but it will be moving faster through the tighter areas.

Yours, Larry
• Member Posts: 22,396
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Everything in a piping circuit adds some resistance, tees, ells, valves, even a 1” long piece of pipe has resistance, all dependent on the flow rate.  An EL chart shows the values assigned to all components. Reduced port valves much more than full port.

That is the key reason for keeping closely spaced tees on P/S piping as close as possible. Adding even a full port valve between those tees adds some resistance.

Resistance  can be zero, but only when there is no flow🙃
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 28
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hot_rod said:

To add to @Jamie Hall reply, a restriction in the circuit, like a balance valve or partially closed ball valve will show an increase in velocity. Excessive velocity can cause cavitation and excessive wear in the components

I think you are talking about venturi effect which is a local effect
my question concerns the whole system
• Member Posts: 23,567
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I'm a little lost now. What was your exact question again, @chakil ?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 28
edited February 2022
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I'm a little lost now. What was your exact question again, @chakil ?

According to the circulator curve, increase in the system resistance result in decrease in the flow rate.
Since flow rate is area time velocity,
is that mean each time we add a resistance or partially cloded valve to the system, water slows down (water velocity of the system decreases)? is that true?
• Member Posts: 23,567
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Exactly
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 7,589
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@chakil
The way I visualize this is that you have a resistance curve (system curve) that represents the resistance of your system and a pump curve that represents the performance characteristics of the circulator. Where to two meet is your flow rate.
https://www.ksb.com/centrifugal-pump-lexicon/operating-point/191270/
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein