Why Control Valves on the Return Side?

[scottbalan]

I have been pondering about this for some time now.

Why are the Control Valves - Hot Water Preheat Coil Valve, Hot Water Reheat Coil Valves and Cooling Coil Valves located on the return side of the coil? I thought there would be better controllability if the Control Valves were to be on the Supply side.

SOS, I am preparing for an interview and I am caught with this question stuck to my head.

Scott.

[hands]

Not all control valves are on the return line, some are on the supply, I think it depends on the engineer. I think that most are on the return to keep the coil full of water.

[Shophound]

3-way valves (often found in chilled water apps) can be on the supply side of the coil. Two way valves are typically on the return side, as said to keep the coil full of water when the valve cycles closed.

[ralphtheplumber]

Because the water is cooler on the return. You don't burn your fingers working on them as much.

On chilled water, they sweat less.

[just_opinion]

There are two type of three-way valves that we can set up

a) Mixing valve, which normally connect to the upper pipe or many application as the return pipe.

b) Diverting valve, which is normally connected to the bottom pipe or many application as the suppying pipe.

You cannot mix them two for it will creat a hammering noise.

As for coil has full water. It is not matter. The coil will always has full water regardless.

As for the advantages between the two, I don't think there is as my memory serves becuase it has benn more the 5 years I read on the article.

You are welcom to email deltacp.com or call them 18003772600

[codewriter]

2 types of valves:
Mixing - Exactly as it sounds. It mixes two incoming sources to a single outgoing.

Diverting - Exactly as it sounds. It diverts a single incoming source to either one of two outlets.

Mixing is used when you need to "MIX"
Diverting is used when you need to "DIVERT"

Mixing is modulating
Diverting is two posistion (normally)

I have seen where a diverting valve has been modulating, but its not recommended, the control span is wacky, unless using butterfly type.

Keeping water in the coil is an invalid statement.

The biggest problem I see with inexperianced people designing systems is not properly sizing the CV. I hear too many times "uh, I dunno, I just go with the largest CV available" If I had a dollar for every valve I found sized wrong, I would be rich.

[madhat]

I was told a long time ago by a PE that it was due to the cushion effect of the coil and the valve would last longer. This was a PE that would actually go out on a job and look at the equipment, I don't think they make them anymore.

[yojoey]

Air control. If the valve is on the supply, the pressure will be lower in the coil. If the valve is on the return, the pressure will be higher in the coil. The water with the higher pressure will entrain more air and carry it out of the coil. Same goes for the location of the balancing valve.

[esornivram]

some new fangled enginers idea, you know less filling, tastes great,

its however the PE specs, many all those stated apply, unless its an non functional design then its what ever they think up.
if the air handler was above the water pump then it could be on return to hold water, higher pressure water holds more energy for heat transfer, also is it an steam coil? i hav seen most all thoughts on the idea
is the coil lower than pumps, then possible on supply to allow head on return piping, then there is venting, hi point vents and flow requirements, most all are in the final thoughts applied to the problem, but mostly the budget for the design is biggest constraint.

i was on an hospital start up in nashville, the hot water return pump was only an 1.25 hp pump and was susposed to supply 12 gallons a minute for proper hot water delivery.
test and balance showed pump was only supplying 2.8 gallons, most of building suffered from no hot water.
as tech it was my thought it would be an 7.5 pump, enginer came up with 5. once we got it changed out the hot water return worked and the entire bldg had to be reblanced to shut down flow

[Edited by esornivram on 10-03-2006 at 07:59 PM]

[sammyc]

A couple reasons to install two way control valves on the return side of a coil...first, good engineering practice is to select the control valve CV to produce a pressure drop equal to or greater than the coil itself. This pressure drop will result in air entrainment on the leaving side of the valve. Air bound coils transfer thermal energy ineffectively. Valve seats also last longer at return fluid temperatures. I design systems with control (and balancing) valves on the return side of terminal equipment but don't think it's absolutely critical.