chilled water diff. press.

[washburn hvac]

I must be missing something simple. I have a building diff press sp of 27 psi and I am now running about 26.85 psi. Water fill station regulator is set to about 22 psi and I have a delta p average of 30 psi across chilled water pump. I am using trane wall mounted fan coils and unit ventilators in this building. Now here is the part I am missing. When the building diff. press. drops to around 16.80 psi I get a whinning sound in the chilled water piping on the Trane units in east side of building, get back towards 27 psi set point and it goes away. Our original set point was 16 psi diff. and a "engineer" advise moving set point to 27 psi and know one bothered to ask why. I am hoping someone here can explain this to me I am sure it is something simple. Thanks Chris

[flange]

The besy way to determine actual setpoint that i have found is to play, but with a purpose. typically there is a lot of math involved in initial design, and I could spend hours on that aspect, but wont because it bores the hell out of me. Initial design is great, IF the pipefitters installed per plans and specs, but if deviations are made, it can throw you off. so, how I approach it is this: pick a number, say seventy five percent. it is the ratio of how many units might be calling at one time, or diversity factor. engineers will tell you that not all units will be at open at a given time due to many factors. So, take your number of units which equals your diversity factor, and manually drive the valves open at each unit. manually close your differential valve, and look at actual system pressure differential. This is your filed setpoint. whatever the number is is your starting point, BUT you need to consider your diversity factor carefully. think about building exposures, interior vs perimeter zones, etc. your factor could be less than 75 percent. anyway, once you have your setpoint, let her rip and put it all back to auto. then go see what your system is doing. if you got lots of water noise, you picked wrong.

[mryan1]

I must be missing something simple. I have a building diff press sp of 27 psi and I am now running about 26.85 psi. Water fill station regulator is set to about 22 psi and I have a delta p average of 30 psi across chilled water pump. I am using trane wall mounted fan coils and unit ventilators in this building. Now here is the part I am missing. When the building diff. press. drops to around 16.80 psi I get a whinning sound in the chilled water piping on the Trane units in east side of building, get back towards 27 psi set point and it goes away. Our original set point was 16 psi diff. and a "engineer" advise moving set point to 27 psi and know one bothered to ask why. I am hoping someone here can explain this to me I am sure it is something simple. Thanks Chris

sounds like your running the pressure to high, just an apinion though. You can always play with it.

[retube]

The answers to the questions you need to have answered are 1) Where is the delta P sensor located, 2) What is the resistance of the worst coil in the system, 3) What is the differential in the actual reading at the Delta P sensor in comparison to the reading of the worst coil? A good guess with a well designed system and sensors located in the correct places will yield a 5 to 7.5psi Delta P under average running conditions.

[Tech Rob]

Take this for what it is; which is an opinion from some guy on the internet who may or may not know a thing or two about this stuff... Almost every garden variety decoupled loop I have seen with vfd secondary pumps controlled to system DP has had a setpoint of somewhere in the range of 12-17 psid. 27 sounds way high to me.

[gbfromsd]

as far as noisy piping - I'd be looking for flow control devices (auto flows, circuit setters, etc. Make sure they're installed and functional. Look over the whole system and keep in mind the minimum system flow rate whatever you do.

[washburn hvac]

Thanks for the input. These pressures also seem very high to me also. Need to do some more research on units and balance reports but I sure do appreciate all opinions and input cause this is a strange set up.

[Russ57]

I have seen certain globes valves make whinning noises when they are near close off with excessive differential pressure.

Personally I keep my DP as low as possible, 5-10 psi max.

[blabath]

I guess the first question should be (How tall of a building are you working with?)
Pressure is a relative term and cannot be determined without knowing the height which your pump has to push the chilled water.
27 psi is plenty of head pressure for a 4 story building but nowhere near enough for a 40 story building. (1psi will push 1 pound of water 27.7 inches. I always use 2 feet cause it's easier to remember)
The water pressure regulator is set at 22 pounds, but remember that that is usually tied to the return side of the pumps and the compression tanks. If the pumps have a 30 psi delta P then the head pressure would be about 52 psi. Thats enough pressure to push that cold water approx. 100 feet in the air, or roughly 10 stories.
There are alot of irregularities in your original question. The big one being why one would have a pump developing 30spi head pressure when only 16psi was originally required per the engineer's specs.

[washburn hvac]

I agree there is a lot of irregular things going on here. the building is 4 stories high and U shaped. I don't understand the 27 psi diff set point myself seems alot high to me. But if that diff presssure drops to about 16.80 its starts whinning in the pipes. That just seem backwards to me but it works, I would just like to understand why. I really appreciate the input and can get any info you need, cause I would really like to understand what they did this way, I am waiting on the balance report and looking for any additions from origal submittals. thanks again

[Russ57]

I would see if the whinning goes away at an even lower DP. It could be that at 18 psi you have forced a disc off a seat on a valve that is supposed to be closed. At 16.8 psi the disc is chattering in the valve seat causing the noise you hear. At 7 psi everything might be happy and act right.

A lot depends on where the DP transducer is located.

[washburn hvac]

I agree 100% thats why I don't understand a engineer saying to increase the differental set point from 16 max to 27 max and it is quite at that set point makes no sense at all and works lol I missed something in school. as for the transducer it is located in the ceiling of the 1 st floor right above the pumps