Water velocity

chakil

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

Sorry about my poor English

Jamie Hall

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.

The answer to the second question is a little more complicated. Fundamentally, the answer is... maybe. The energy added by the pump (or lost in the circuit, if it is a closed circuit) is simply the head added times the mass of water moved -- or just as well, the flow times the head. So a lower flow at a greater head may be the same energy as a higher flow at a lower head. The problem is that the efficiency of the pump -- the ability of the pump to translate electrical energy at the motor to fluid energy in the moving water -- is not constant for the pumps we usually work with (centrifugals). At the two extremes -- shutoff head; no flow, high head, and open discharge; high flow, no head -- the efficiency is zero. In between it rises to a maximum at some combination of flow and head depending on the pump.

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.

hot_rod

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 

chakil

Jamie Hall 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 ?

Zman

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

Jamie Hall

chakil said:

Jamie Hall 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).

HomerJSmith

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.

hot_rod

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 

Jamie Hall

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.

chakil

Thanks everyone for the reply

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?

Larry Weingarten

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

hot_rod

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🙃

chakil

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

Jamie Hall

I'm a little lost now. What was your exact question again, @chakil ?

chakil

Jamie Hall said:

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?

Jamie Hall

Exactly

Zman

@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/