three way control valve balancing 1

[Waramanga]

Hi guys,

According to my CIBSE reference, a three way valve on a coil needs a balancing valve on the bypass line along with one in the return line back to the chiller/boiler. That bypass valve gets set to provide the same dP as the coil when in full bypass mode.

Whats the point? at part load say half the water going through the bypass the hydraulic resistance would be half (two resistances in parallel) so you will still be in overflow. On the other hand if you have a high dP coil on the index run (small dP on the balancing valve) and no bypass bal valve then you will be in even more serious overflow.

Does anyopne have any thoughts on this? Has anyone seen an arrangement without a balancing valve in the bypass??

 

[Drazen]

Well, I would say that you did not consider the fact that at half flow resistance in the main line would be half as well

There is no ideal proportionality but balancing valves would help having it within reasonable limits.

The main reason for having balancing valve on main line is that too often dp on coil is quite small and you cannot find three-way valve with kv small enough to reach adequate hydraulic authority.

Yes, I saw many installation without balancing valve in bypass, in past when that was not mandated, and big trouble in commissioning as well, which often included later addition of balancing valve.

 

[Waramanga]

Thanks Drazen, I agree that there should be a balancing valve in the bypass, its interesting that they havent always been installed! what about a flow controller/limiter? perhaps you wouldnt need the bypass bal valve then....

 

[Wilbur55]

Actually, it is worse than you describe. Pressure drop for turbulent flow is proportional to the square of the flow rate. At one-half flow through the bypass balancing valve, the pressure drop is one-quarter the initial reading.

With no balancing valve in the bypass, the branch in question could divert even more water from reaching other coils on the main when the 3-way valve is in full bypass.

Question: Which has the greater annual energy cost?
[ol][li]The extra pressure drop through a flow limiting valve all year, or[/li]
[li]Running out on the pump curve when all three-way valves are at mid-position and the valve/coil pressure drops are less than half their design values?[/li][/ol]

 

[Drazen]

Wilbur is right, I was more focused on proportionality concept than on actual equations, though I do not full understand question posted.

Waramanga, older installation never had bypass valve, and even now, some "old school" engineering manuals still have their example setups without it. The reasoning is that you need only one branch to balance, to adapt it to other, but, as mentioned, long-term practice showed lot of troubles with it. There is already problem how to measure line without valve to compare it to another.

That would apply to real three-way mixing, modulating. If you use three-way valve only for on-off work with terminal units, than you would not mind having so precise balance, on branch regulating valve would be sufficient, for two reasons:
- you do not care for modulating control preciseness
- effect on eventual change would likely be negligible on hydronic loop level.

 

[mushtash]

why are bypasses are even employed in chiller piping system?...i dont understand their purpose.please be simple in your answers..i am trying to understand HVAC

 

[DRWeig]

Chiller and pump have minimum flows to maintain, no matter the position of your coils.

Too low flow in chiller = freeze.
Too low flow in pump = overheating.

Good on ya,

Goober Dave

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[MTES93]

The balancing valve in the bypass will allow the control valve to operate smoothly. Think of what the water will want to do... When open to the coil the water pressure drop is say 10 feet (as an example) at design.. the bypass pipe pressure drop maybe inconsequential at design flow. As the valve modulates open to the bypass and closed to the coil the water will naturally take the path of least resistance and limit the stroke of the valve.

One can also use a single automatic flow balancing valve (Nexus, Flow Design Inc, Griswold) installed in the return (down stream of the bypass connection) in lieu of two manual valves. The automatic flow device will 'restrain' the water when the valve goes into bypass mode.

 

[MechEngNCPE]

11241 is right, I always specify flowserve autoflow valves, set it and forget it. Balancing is a snap.