Reason for commissioning sets being in the return leg 1

[dsengine]

This one has stumped me for a while and I can't seem to find an answer.

Why are commissioning sets always specified as being in the return leg of a LTHW system?

Is there a logical reason or just tradition?

Much obliged for your help

 

[cry22]

I think you may get an answer if you put in some US terminology? What is a commissioning set? and what is LTHW? what return leg? is this water? air?

 

[RossABQ]

LTHW = low temp heating water?
Commissioning set = control station?

I was told a long time ago that it was to prevent a coil air bound condition. I've seen it done both ways.

 

[SAK9]

LTHW = low temp heating water

Commissioning set = Balancing valve

British terminology!

Return leg location is to keep the coil flooded as RossABQ mentioned.

 

[mauricestoker]

Being Baltimoron background and not a Brit, I'd say it also depends on whether it's single coil or multiple coil, and 2-way or 3-way valves. Commissioning set would be the flow valve, Pete's plug, and drain. When setting the pump, you will need to know presure and flow. If on 3-ways, the flow is measured and trim set for the pump. If on 2-ways (and not using a recirc including temp control) the commissioning set would include a dP instead of a Pete's plug so the pump can run off VFD. 3-ways can also run off temp control, so commissioning set would include Pete's plug.

 

[cry22]

Yes, Flooded coil indeed is the intention from what I know, but why does the coil need to be flooded you would ask?
With the return location, the coil remains under pressure from the pump at all times and thus insures turbulent flow thru the coil (3 to 5 FPS being ideal). If in supply, the coil gets only the water that is let in by the balancing valve (Oooops soooorry, the commissinoing set) and may result into laminar flow. Laminar flow is also equal to air build-up in the coil as RossABQ says (I suspect him to be a Brit as well with his "airbound" terminology)

Those are my two cents of explanation, there may be other reasons.

PS:
See, even with explanation, you brits confused our Baltimoran mauricestoker thinking about control valves instead of balancing valves.

Smile !!!

 

[KiwiMace]

cry22 - not so sure about the logic of this. The flow through the coil is the flow through the coil regardless of the balancing/commissioning valve/set location. Is it not?

 

[RossABQ]

LOL, the "ABQ" stands for Albuquerque, nowhere near the United Kingdom.

Still I don't see what difference it make w/r/t laminar or turbulent flow; if the valve is closed down, it's closed down regardless and flow thru the coil will be the same value. Only the airbound argument makes any sense to me, although the impact of a 3-way valve needs a little more thought.

 

[cry22]

Kiwi,
Less flow will result into laminar flow, resulting into air build-up within the coil. Most coil MFTR's (and masterspec)recommend a water velocity of 3 to 5 FPS thru the coil.
If you happen to need 0.5 GPM on a coil that can handle 3 GPM, you would run into low velocity within the coil, heat transfer will likely suffer. Imagine pumping 1 GPM thru your car radiator, you wouldn't remove the heat. You need flow AND velocity thru the coil for proper heat transfer in my opinion.

Locating the Balancing valve in the return achieves full flow and proper velocity.

This is a good question for manufacturers, and I will definitely follow up on it with Trane and the likes.

 

[SAK9]

Cry,

You are a star!!!

 

[ChrisConley]

If you don't have enough flow in the coil (under 3 fps), the location of the balancing valve will not increase the velocity of the water through the coil. The balancing valve is nothing but pressure drop, manually adjusted to achieve flow.

Unless, we're talking about three-way control valves? I could see the location being more important in that situation.

I've seen balancing valves on supply and return (I specify on the return). I've seen control valves on the supply, and return to a coil or device.

 

[KiwiMace]

I was trying to be diplomatic. I think this is a crock.

 

[ChasBean1]

Opening an old wound (sorry):

Balancing valve location is usually not critical and only comes into play when the control valve is fully open. Velocity is the same on either side as Chris indicated.

Possible concern however with circuit setter on supply side: if system operates at low pressure and throttling occurs on the supply side of a coil, then the water takes the torturous path at elevated velocities through the coil, gases are more likely to come out of solution. The water will take the same path, at the same velocities, but it will do so at a lower pressure with the circuit setter on the supply. The throttling for these systems (esp. hot systems) is therefore better on the return side.

Control valves should normally be on the return side of hot or cold systems.

Reason for hot systems: 1) also throttling and keeping gases in solution, 2) reduce heat transfer to the actuator. Some actuator mfrs. indicate ~140-160°F temperature limits and require they be installed on returns. Heat can cause damage to electronics or reduce life of rubber parts, etc. in pneumatics.

Reason for cold systems: simply reduce the amount of insulation breaks on the colder supply piping. Also reduce sweat on the actuator (mainly for the electronic variety but can reduce life of pneumatics also).

Sorry to bring this note so late but these are questions that I used to wonder about quite a bit in commissioning systems and found answers I'm pretty comfortable with...

-CB

 

[lowedogg]

control valves should be located at the lowest pressure point in the system. this gives the greatest possible range and finest resolution, in other words better control.

maybe this "commissioning set" is our autoflow device, which typically contains the manual air vent. air vent always goes on the coil outlet, or upper connection. this is to prevent air binding.