water flow (control) in 3-way valve application

[YuriB]

Water circulates in a closed system either:
pump - supply pipe - diverting valve - heat exchanger - return pipe - pump
or:
pump - supply pipe - diverting valve - return pipe - pump.

Is not flow greater, while the pump total head unconditionally less in the second case, independently on the (reasonable) temperature differences in both cases?

 

[TD2K]

There's no heat exchanger in the second loop, is that a typo? If it's not a typo, why do you need a diverting valve?

Sorry, but it's not clear what you are asking.

 

[YuriB]

Fancoil(s), or a HE of an AHU. 3-way valves regulate water flow through them. BTW there are almost all 3-way valves in my vicinity, no 2-way ones.
If it closes the way to the coil and opens bypass, system's resistance decreases, the flow in the primary circuit increases, the head of the pump supplying the liquid to the machine falls ?

 

[trashcanman]

With a 3-way valve system, if it is balanced right, the pump does not see a change in head when the diverting valve opens or closed. The bypass line is balanced to = the pressure drop through the coil. So the water goes through the coil or bypass. Nice, steady flow.

With a 2-way valve, it is different. As valves close down, the total head on the system increases and the pump operating point will ride the curve up toward shutoff. To avoid this a bypass valve can be added at the end of the run, or use a 3-way valve for the last unit or two (or 3). You have to give the water somewhere to go.

 

[ElSid1]

Heating systems traditionally have 3-way valves and constant volume pumps. How are you pumping (constant or VFD)? If I understand your question, pump - supply pipe - diverting valve (100% bypass of heat exchanger) - return pipe - pump. Then trashcanman's explanation is correct for a constant speed pump! If upgrading a a pumping system, then many other variables come into play.

 

[YuriB]

Constant volume pumps = positive displacement ones ? Centrifugal pumps around here are not those (looking at their H-m diagrams found in internet).
I believe the intrinsic characteristic of the built-in 3-way valves on fancoils units (no balancer on bypass) is equal resistance in all positions ? Or it may be not case depending on a manufacturer ?

 

[ElSid1]

Positive displacement is typically used on higher viscocity fluids or processes. The intrinsic characteristic of a a 3-way valve to to mimic, in head loss, the coil and controls. So if the 3-way vave in a flow through position (2-way + the coil = 20 feet of heat, the 3-way valve in bypass node (nothing going to the coil) should, in theory, be 20 feet of head. I have usually seen between 15 and 22 in actual plants. What exactly are you trying to achieve? What is the system? Is this a replacement? What kind of boiler? I think I've gone off-topic

 

[YuriB]

Thus, the head loss of the coils and the head loss of the valves is in, priniple, a constant - for every DN of connections - the manufacturers should be trying to maintain ?

 

[ElSid1]

It's water. . I doubt it to be possitive displacement pump. Do you have a make and model from the name plate. A schematic, pictures, other information would help.