Reheat control valve on supply vs. return

[ChasBean1]

Application - standard CAV/VAV supply air terminal box with hot water reheat:

I usually see the reheat coil's hot water control valve at the outlet (hot water return pipe) of the coil.

Occasionally I see a "rebel" design that has the hot water control valve on the supply line.

I've seen a similar post in the past, but coudn't locate it. Is there a reason for having the control valve on the return line, other than convention?

Thanks in advance for any feedback. -CB

(posted this also in valve eng. forum)

 

[quark]

CB!

The thread is thread403-25360.

Regards,

 

[ChasBean1]

Thanks, Quark. (and I had a reply to that post, but nothing written in concrete - possible reasons, not a real definite answer). My assumption was also for a closed loop HW application such that there'd be no flashing on the downstream side of the valve...

 

[haysnick]

If you create cavitation at the valve the collapsing vapor bubbles will not be in the heat exchanger if the control valve is downstream. The Heat Exchanger is (by definition) in the air stream and has a path for the noise to reach the occupant.

 

[tombmech]

quark's response was the correct one in the first thread: you always want the coil "flooded." This is true with hot or chilled water. In the case of steam, a valve on the inlet side will be disastrous.

The only place I can think of where this wouldn't be the best choice is on cooling tower bypasses. There is benefit to switching flow from the fill to the basin. The difference is that the cooling tower is a broken circuit exposed to air anyway, and the return side is designed with a substantial reservoir and static head. Even then, designs must give careful attention to ensuring that transient hydraulics don't empty or overflow the basin "cushion."

 

[KenRad]

Obviously, for any closed loop pressurized system where your pressure and temperature are such that you're not going to see cavitation downstream of the control valve, it shouldn't make any difference where you put the valve on the reheat coil.

Go ahead CB, be a rebel.

---KenRad

 

[tombmech]

Well, yes - assuming you always know the dP leverage for every coil - for now and in the future. Depending on where it is in a system, the back pressure on the return line could be quite small. Most of us like to use every opportunity to make sure the system works.

 

[KenRad]

If you have cavitation on your control valves wherever they are, your system doesn't work properly. I design my closed loop pressurization to always maintain sufficient return system pressure at all points so as to prevent cavitation.

This being said, I also put all of my control valves on the outlet of the coil. But it's been a slow day, and it seemed like a good idea to argue with tombmech for a while.

---KenRad

 

[tombmech]

Well, I've seen air sucked into a coil before. Sometimes systems don't stay the way we want them over a number of years. Other times we inherit others' mistakes. It compounds things if deliberately poor choices are made.

Of course, some people choose PVC for compressed air pipe, too. ;-)

 

[gregha04756]

I think the valve is on the outlet side because the water is cooler, and less stressful to the valve components.

Good Luck,
Greg Hansen