Determining CH Flow

[WAYNE3298]

I always compared the actual shut off head to the pump curve. Good condition accurately sized impellers almost always test within 5 feet of the shut-off head shown on the curve.
This conversation has been fun thanks.

[tridiumtech]

Sounds good. But back to that comment about 'fouled' tubes. How much fouling does it take to really 'wreck' a dP drop across the barrel (ie, >5%). Now I've seen an AHU coiled fouled to about 50% flow restriction - but that unit was put in 1968 (Boeing Plant, WA) and the HVAC crew put in a new coil in 2007 - almost 40 years. What that says it that PM isn't done on AHU coils, but they sure do on Chillers, or they should. In my HON days I'd see the mechanical PM guys do a tube clean pretty regularly. And if you watch the chemistry, the fouling should be minimum concern. So maybe that's a good question to ask the plant staff if you going look at the CH barrel dP - when's the last time you clean yar tubes? Controls PM is good, but mechanical PM is a must - those guys are great. If the plant runs shoddy in "Hand" mode, controls don't mean much.

[tridiumtech]

S

[Ahmed Abdulla]

I wonder when we need to use the secondary pump in the chilled water system. Does it depend on the amount of flow or the pressure of the system. I hope to know the reasons and determinants of using the secondary pump with the primary pump.

[Artrose]

I wonder when we need to use the secondary pump in the chilled water system. Does it depend on the amount of flow or the pressure of the system. I hope to know the reasons and determinants of using the secondary pump with the primary pump.

I just found this discussion, and now I have a small question.......

Is this a constant volume primary/secondary chill water system with a decoupler?

If so, Wayne would you mind clarifying the operating sequence for such a system.

[WAYNE3298]

In our previous discussions the information we were given Artrose indicated the system was primary/secondary. There are more ways than one to control this type of system but the primary flow always has to exceed the secondary flow. With present day electronics I would install a magnetic flow meter in the bridge pipe (de-coupler). That will tell you not only the recirc. (return from the users) flow but the direction of flow. In multiple chiller installations I allowed the supply to exceed the recirc by as much as 15% but did not allow any backward flow. In multiple chiller installations I used the bridge flow meter to start and stop chillers. You can also vary chiller flow as long as you have more supply than secondary return.
I hope I made this clear but if not ask as many questions as you wish. I haven't designed one of these systems in a long while did balance one 3 or 4 years ago.

[Artrose]

I wonder when we need to use the secondary pump in the chilled water system. Does it depend on the amount of flow or the pressure of the system. I hope to know the reasons and determinants of using the secondary pump with the primary pump.

Thank you Wayne. Occasionally , some of us might fall into the assume trap. Today older folks assume that eveyone knows how to read an old clock that has a face and hands. It aint so. Some youngsters only know how to read a digital timepiece.

That being said, from the previous OP's question, I could be wrong, but I suspect the OP is still attempting to grasp the system he's tasked to work with.

Generally speaking, (nothing is cut in stone)..............

There is the primary pump and loop, which circulates water through the chiller at a constant speed. Then there is the secondary loop to the building, which has it's own pump.
They share a common pipe, or decoupler pipe, which basically connects between the supply and return of these two loops. As the secondary loop pump requires cold water, it will pull the cold water out of the primary loop. As it pulls more and more water out of the primary loop, the flow through the decoupler pipe is less and less.

It will reach a point where the building return is equal to the primary pump GPM, and there will be no flow in the decoupler pipe. So in essence, there will be no circulation, but all return water going into the chiller. In fact, the decoupler pipe can actually flow backwards, depending on the GPM of each pump and the mixing of building return with chiller supply.

A constant volume secondary pump never shuts off. This then makes it necessary to use either three way valves at the air handler coils, etc., or some form of a bypass in the secondary loop for when chilled water demand is low in the system.

There are all variations of this basic chill water control scheme. You might use pressure control on the chiller barrels, or volume control on the decoupler based on pressure, or use a constant volume primary with a variable volume secondary, or even a primary variable volume scheme. This is not to mention the varied schemes used with relation to energy recovery, or low temp applications, etc..

Sky's the limit, but an understanding of the basic systems operation sure helps when the knucklehead engineer decides to add another twist. Some of it works very well, some not so much.

Hope this helps a little bit.

[WAYNE3298]

You are correct Artrose after the arrival of VFD pumping and DDC controls the possibilities are almost endless. The probability for an excellent system increased exponentially but unfortunately so did the possibility of systems that performed poorly.
As you alluded to in post #27 my post #26 assumed too much. To be honest I have always thought constant flow primary/secondary was too simple to screw up. The flow is always there the only thing that changes is where it goes.

You made good points THANKS!!