chilled water system controls 7

[flexiblycool]

The chilled water system consists of 3 chillers and 3 primary pumps (200 gpm capacity). Each building has its set of continuous duty secondary pumps.
According to the existing Sequence of Operation, if there is demand for cooling, the Primary Pump comes on. Upon proof of flow from the flow switch, the chiller associated with this primary pump comes into operation. If the cooling demand is not met with one chiller, the second Primary pump and its chiller comes into operation and likewise upon further increase of cooling load, the third primary pump and its chiller comes into service.
Each building has its own continuous duty secondary pumps that pumps the chilled water through 20 Fan Coil Units each of which is equipped with 3 Way Bypass valves. When the cooling demand in any room is satisfied the 3 way valve routes the chilled water supply back to the chilled water return line, bypassing the Fan Coil Unit.

This control system apparently has the flaw that when one primary pumps is automatically switched off due by the returning chilled water temperature, the flow in the system will be 400 GPM instead of 600 GPM. So all the fan coil units that are still in service because the demand in the space has not been satisfied, will have less than design GPM going through such machines. Likewise, when two primary pumps are automatically switched off due by the returning chilled water temperature, the flow in the system will be 200 GPM instead of 600 GPM. In this case also, the fan coil units that are still in service because the demand in the space has not been satisfied, will have less than design GPM going through such machines.
It would appear that the correct method would be to keep all the 3 primary pumps working regardless of load, and simply bringing (or removing) the chillers in service as per cooling load demand sensed by the chiller from the return water temperature..
The existing control system in which each pump and its chiller cuts in or out of service would be fine for 2 way valves on the FCU but not for the 3 way bypass valves. All the 3 primary pumps must remain in service in order not to starve the fan coil units of spaces where the cooling demand has not been met.
Please confirm if this analysis is correct and if all the primary pumps must be set to work regardless of whether all the chillers are in service or not.

 

[flexiblycool]

317069
To err is human, so I don’t consider the designer infallible, nor do I believe in becoming so reluctant so as not to try to undo the mistake. Evidently the system is flawed as commented by Drazen. This is precisely the reason of my seeking the consensus of this Forum and for trying to come up with a retrofit solution (but not a sweeping overhaul due to financial constraints). The simplest of the cost saving measures that comes to my (junior) engineering sense and acumen is to remove the secondary/booster pumps if it can be done without compromising the design flow of Fan Coil Units.

 

[317069]

I agree they are human and mistake happen anytime, but they could be right too, I also fully agree that retrofit does not mean to swipe everything and start from the zero. but your comments show that the owner doesn't want to spend a penny and want to save energy.
Can you post both primary and booster pumps nominal specification.
nobody here can tell you if you are right or wrong because they don't have enough data of the project and project history, you can run your calculation on paper to see that would be enough if you use primary pumps only , and also check for supply temperature too.
you said existing primary pumps switch off by sensing return temperature,
- do primary pumps have the same set point, because they have the same return pipe.
- when a chiller goes off, does it still have a water flow through it?
- does return temperature control only the primary pumps or chillers too?

 

[tys90]

I think your original problem got lost over the course of this thread.

It's definitely not the most efficient system but it should work the way it's setup if I understand it correctly (which I may not).

You describe your problem as that the fan coil units don't get enough flow when a pump turns off due to return water temperature. I assume this means that it turns off when return water temperature gets too LOW. If return water temperature is low, that means there is a low load on the system since it's essentially a primary only system the way it's setup. Thus, the fan coils should not be starved if the return water temperature is low.

I would make sure the controls are working as intended and that flow is balanced throughout the system. If the bypass between the supply and return off each 3-way valve is not balanced properly you will have a tough time controlling flow through the coil properly.