Questions for all of you HVAC massters, please -)

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Questions for all of you HVAC massters, please -)

Postby vlajky » Sun Feb 19, 2012 6:06 pm

Hi,
I would like to know couple of things ?-)

1. With U shaped manometer attached to the surface of the pipe we are measuring static pressure drop? Pressure exerted by the water on the surface of that pipe? Am i right ?
2. Is that pressure field uniform for some observed cross sectional area of the pipe ? I know that pressure is the function of length. But for some cross section is it distributed equally ( radial ( circular) , in all directions ) among the pipe wall ?
3. in central heating closed circuit installation, what gives the pressure of 6bar in pipes if it is not the centrifugal pump? If it is, how is kinetic energy of the flow transferred in the pressure energy.
Somewhere i read that "common misconception about pumps is the thought that
they create pressure. Pumps alone do not create pressure;
they only displace fluid, causing a flow. Adding resistance
to flow causes pressure " ?
4. I' confused about that, how adding resistance to flow causes pressure, it would be more natural that is cousins pressure to drop ( head loss ).. but than, what generates pressure ?

and for the final and most important for my understanding

5. I know that we have head loss in pipes due to a friction and local resistances like bends, valves, change of pipes diameter ..etc. We all know how to calculate them using Darcy - Weisbach formula. I also know from Brenuli equation that with larger diameter i have less friction losses, lower velocity and gather static pressure and vice verse.

But how is than possible that for same diameter of the pipe , same average velocity, same volume flow ....i still have static pressure drop . I understand the reasons ( listed above) I understand the math.....but i don't understand physics of that phenomena ?
Can somebody enplane me in plane English what is relay going on in there. .. Just some simple Cause and affect story line.

Thank you!-)
vlajky
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- Questions for all of you HVAC massters, please -)

Postby Freon » Sat Feb 25, 2012 10:28 am

1. With U shaped manometer attached to the surface of the pipe we are measuring static pressure drop? Pressure exerted by the water on the surface of that pipe? Am i right ?

Assuming you have the manometer attached at 2 points, one downstream from the other, you are measuring pressure differential between those 2 points. The water column height difference is how you quantify that pressure differential inches of wc. It could also be psig if that's how the manometer was calibrated.

2. Is that pressure field uniform for some observed cross sectional area of the pipe ? I know that pressure is the function of length. But for some cross section is it distributed equally ( radial ( circular) , in all directions ) among the pipe wall ?

If you sliced a pipe perpendicular to it's length then the static pressure around that slice would be constant.

3. in central heating closed circuit installation, what gives the pressure of 6bar in pipes if it is not the centrifugal pump? If it is, how is kinetic energy of the flow transferred in the pressure energy.
Somewhere i read that "common misconception about pumps is the thought that
they create pressure. Pumps alone do not create pressure;
they only displace fluid, causing a flow. Adding resistance
to flow causes pressure " ?

Pumps move fluids and air is a fluid but it's compressible, unlike water. The energy that the pump adds to the air show up as pressure and air flow. For the air to move you need a pressure differential and that's why further down the pipe the pressure is less. The pressure differential is due to the work of moving the air against the effects of friction. If there were no friction you'd have uniform pressure the entire length of the pipe until at its end point where the air would exit at 0 psig pressure into open air. The mixing of the exiting air with the open air would dissipate the kinetic energy the air flow had transferred to it from the pump.


4. I' confused about that, how adding resistance to flow causes pressure, it would be more natural that is cousins pressure to drop ( head loss ).. but than, what generates pressure ?

The pump compressing the air causes the pressure. Making the pipe of air flow requires work to be done... energy supplied by the pump. The mechanism is through compressing the fluid (air). The loss of the initial pump pressure is due to the effects of friction.

and for the final and most important for my understanding

5. I know that we have head loss in pipes due to a friction and local resistances like bends, valves, change of pipes diameter ..etc. We all know how to calculate them using Darcy - Weisbach formula. I also know from Brenuli equation that with larger diameter i have less friction losses, lower velocity and gather static pressure and vice verse.

But how is than possible that for same diameter of the pipe , same average velocity, same volume flow ....i still have static pressure drop . I understand the reasons ( listed above) I understand the math.....but i don't understand physics of that phenomena ?
Can somebody enplane me in plane English what is relay going on in there. .. Just some simple Cause and affect story line.

The pressure drop reflects the work being done to move the fluid against the forces of friction. Friction is a force in the opposite direction of the fluid flow. Force times distance is work so the flow rate and pressure will drop over the distance as the work against the fluid flow continues. This is mostly an energy balancing between the initial energy supplied by the pump and the work against that energy over the length of the pipe due to friction. No friction and you'd have uniform pressure and constant mass flow... no energy loss.

You might like to read a little about fluid mechanics. It's a fascinating field and it is confusing too because so many things are happening at once. Who'd have thought blowing air down a pipe could be so complex?
Thank you!-)
Freon
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