No Flow thru FCU???

[dblyle]

I recently replaced a FCU for a customer with a like-for-like unit. To my surprise, when we opened the valves, we experienced no water flow through the unit. The customer then informed me the system hasn't worked for 8 years or so, and I should just hot tap a different supply line (I thought that was sort of strange). After tapping the new line and running new pipe, the system still didn't work.

I have removed the chill water control valve from the system and am recording 120 PSI on both sides of the coil. Of course, without a differential pressure, water won't flow, so I have concluded the customer has a water flow issue (aka water pressure issue). The customer, however quickly stated he has no issue and I just need to upsize the pipe.

The size pipe I am currently using requires a pressure drop of 6.9 PSI; upsizing will require only 5.3 PSI. Should that little of a difference (1.6 PSI) really matter?

 

[quark]

I think your post requires a little more explanation. When you say no water flow through the unit, how did you check it?

Is it by coil inlet outlet pressure difference or by a flow meter or bucket fill?

What is that 120psi pressure? Is it in running condition? How is your return line and where is this FCU (at the end point or starting point?). First, I will doubt the 120psi pressure in a running return line.

I would do the following checks.

1. Isolate the return from the header, open it and exactly check the water flow, if there is a problem
2. Isolate the FCU and check inlet flow, if there is a problem
3. Check for presence of air in the system.
4. Check for blockage
5. If there is no problem with Point No. 1 then try to connect the return line at a different point of the return header (you may have some flow problems if this FCU is nearer to a much bigger FCU)
6. Check pressure gauges for calibration and zero errors.
7. What is the flowrate and what is the pipe size?

1.6 psi doesn't really matter that much

PS: Water flow issue is a correct term and water pressure is not.

 

[dblyle]

Thanks for your response, Quark. I'll scan in a sketch of the system tomorrow morning.

As for your suggestions:
1. The unit sits in the basement of a 15 story building. My guess is that most of what I am seeing is standing pressure on the system.
2. The building is part of a larger campus loop.
3. We have isolated the supply and return headers. Both flow nicely when not connected.
4. Pressure is measured on both sides of the coil with accurate pressure gauges.
5. Flowrate is 11.2 gpm. Pipe size is 3/4" (I know - a little small).
6. All AHUs / FCUs are piped in parallel.

Our conclusion of "no water flow" comes from the lack of differential pressure and the simple observation that the unit is just not cooling.

 

[quark]

OK. Then two more questions.

What was the need for replacing FCU?

Secondly, what are the physical changes you made while hooking up the new one? For ex. hooking it at different locations, inlet/outlet configuration (with respect to top and bottom nozzles of the coil)

3/4" for 11.2 gpm is not bad at all. If by disconnecting the outlet from header (inlet being connected) and running the FCU works well, then you may have to relocate the return connection closer to the pump.