CHW connection into Existing System

[remp]

In a building I have a chilled water header 150dia. Pipe that has branch take off to each floor in the building for supplementary cooling. The branch take offs on each floor connect to fan coil units for meeting rooms etc.…. More floor connections have nothing connect to them as yet; they are just valved and capped off for future use. The fan coil units have 3 port valves on each and the pump is on a VFD and maintains constant pressure (so I have been told anyway). Originally when the building was new the system was balanced and worked fine.

The Question is: Every time I connect a new floor to the system for the first time do I technically have to re balance the entire building (floor take offs at least). I think I do. Colleagues of mine with say no, that the VFD maintains constant pressure…. end of story. Any comments?

 

[quark]

I presume your pipe is sized considering the requirement of the entire building. Generally your colleague is right. Three way valves and variable speed pumps will do the balancing for you. However, the concept of constant pressure(discharge?) troubles me a little bit. Where did you place the pressure sensor? Do you have balancing valves installed for each AHU?

Most of the variable speed chilled water systems use differential pressure across supply and return headers to control the pump speed for optimum use of energy and to take the full benefit of variable speed pumping system.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[MarauderX]

If the other systems are constant pressure, no you should not have to rebalance the entire system.

 

[ChasBean1]

I agree w/Quark specifically with regard to the second paragraph regarding optimum use of energy. The 3-way valve system might require a little MORE pumping power during zero building CHW demand because of less restriction through the bypasses. If you have VFD control of the pumping and appropriate staging ability at the plant (e.g., by primary-secondary loop), why not use 2-way valves throughout the system? With the 3-way valve system, you might also find the central chillers cutting out on low return temperature during shoulder seasons.