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Thread: CHW Bypass valve check
08-17-2008, 07:07 PM #1
CHW Bypass valve check
I need to check some three way bypass valves to see if the bypass is working or not. The original install was a bypass three way valve on many of the cooling coils. A retrofit a few years later was supposed to replace the valves (didn't happen) or block the bypass (maybe?) when they put VFDs on the CHW pumps. I think many of these bypass valves are still open because the CHW delta T is very low at low load conditions (as low as 3 degrees).
I've eyeballed the valves and don't see a blank-off plate installed. I was trying to measure pipe OD temperature at the coil and bypass ports but the bypass line is only about a foot long so I'm not seeing much of a temperature change with the valve in either position.
The best I've come up with is to put the valve in the closed position (full bypass) and wait to see if the chilled water line warms up. If it does, the bypass is blocked 9no CHW flow). If it doesn't, the bypass is still open. It may take a while (half hour to an hour?) for the outside of the pipe to warm up or make sure I didn't give up too soon. I didn't want to shut down cooling for that long. Have you got any other suggestions?If "I have always done it this way" is a good reason to do it again, how many times do I have to do something wrong - before it becomes right?
08-17-2008, 08:16 PM #2Professional Member
- Join Date
- Jan 2005
I'm actually surprised by this one???
Especially from a "Pro" with "Posts: 1,689", but try checking discharge air temp to see if bypass is open or closed.
You must post alot in the General or Food forum.
08-17-2008, 08:42 PM #3
I'll post this one in the "Food" forum next. But until then could you tell me how the discharge air temperature will tell me if the chilled water bypass is open or closed? Obviously I have missed some simple fact you caught. It seems to me that a 2-way chilled water valve will control the discharge air temperature just as well as a 3-way (chilled water bypass) valve. The only difference is the 2-way valve stops the CHW flow it doesn't need and the 3-way valve bypasses the CHW it doesn't need.
When this CHW system was constant flow, the valves had to be 3-way valves to allow constant flow when the cooling coil didn't need cooling. Now that the CHW system is variable flow, if the 3-way valves are bypassing, the CHW delta T gets very low during low load conditions.If "I have always done it this way" is a good reason to do it again, how many times do I have to do something wrong - before it becomes right?
08-17-2008, 08:48 PM #4
can you take pd across chw lines with valve in either position? if they're now 2 way - why are they still possibly controllable? how big is the piping for needing b/o plates. if it's THAT big, shouldn't need a looong time to check t.d's. how many coils(a/h) on a pump? seems if you slow down the pump for t/c, someone's gonna suffer. how does the chiller react? primary/secondary systems? sounds like you have alot to look at
08-17-2008, 08:52 PM #5Professional Member
- Join Date
- Jan 2005
Glad you took that in the spirit it was intended.
Ok, I get it, now.
The valve will close off, supply air temp will go up, but you are not sure if the valve is actually bypassing.
Would checking the temp change in the bypass line help?
Last edited by Carrene #2; 08-17-2008 at 09:15 PM.
08-17-2008, 08:56 PM #6
So put the drive in bypass and try to make sure at least some coils are moveing water
08-17-2008, 09:39 PM #7
I'm not getting a clear picture of exactly what you have, but 3 degrees of delta T at low load seems ok. Why are you expecting it to be higher because they have made the valves work like two way valves?
08-18-2008, 12:40 AM #8
The problem is they want to pull the chillers out of this building completely and go on a district chilled water plant. That way they will buy chilled water for cooling instead of buying electricity to make chilled water. The district chilled water plant wants a 15 degree delta T across their system. If the building is running a 3 degree delta T - they will not let the building connect to the district system.
The chillers have a minimum chilled water flow rate. The pumps have a minimum flow rate. But the cooling coils should be able to operate down to almost zero chilled water flow. If the cooling load is low enough, there shouldn't be any flow thru a cooling coil. When that part of the building is empty, they want to shut it down completely. So the chilled water valve should close and no more chilled water should go thru that coil. If the 3-way valves were blanked off, no chilled water should go thru that bypass. But if the bypass was never blanked off or blocked some other way, the chilled water will bypass the coil and go back to the return side of the chiller. That would reduce the chilled water delta T.
Since the legth of the bypass valve is about a foot long (bypass port to where it joins the CHWR pipe) I'm not measuring much temperature difference between a thermocouple on the bypass line and a thermocouple on the return line. I think there is enough heat conduction along that 1 foot of insulated pipe that I just don't get much of a temperature change.
The ones I've looked at so far are 4" CHW valves. I've been told there are some larger but I haven't seen them yet. There is a lot to look at to check the whole system, but they just want to address the low delta T right now.If "I have always done it this way" is a good reason to do it again, how many times do I have to do something wrong - before it becomes right?
08-18-2008, 07:48 AM #9
Turns out Carren #2 was right - I was being a bone head. I just needed to put it down for a few minutes so I wasn't focused on the bypass valve itself. I should be able to measure the CHWR temperature well down stream of the 3-way valve. If the bypass port is blocked the temperature should rise in a matter of minutes when the valve is fully closed.
My appologies for asking such a dumb question.
08-18-2008, 08:05 AM #10
At least that's the way I see it...
08-18-2008, 04:55 PM #11
You are right - this may turn into a complete redesign and replacement of lots of equipment. But until the decision makers decide that, they want problems identified and corrected.