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Primary/Primary Piping System

Primary/Primary Piping System

Primary/Primary Piping System

(OP)
The link below is the flow diagram for a chilled water piping system I am working on for a project.

[url=http://postimg.org/image/dj5s33wc1/full/][/url]
[url=http://postimage.org/]image host[/url]

The project is two separate buildings that are currently served by a combined chilled water plant. The owners want to separate the buildings to two independent plants with a crossover link between them to have one serve as partial backup to the other in the event of a chiller outage.

My design included a heat exchanger between the two systems to prevent any issues with pressure or flow or complicated controls to have two primary systems operating in conjunction with each other.

My boss is trying to convince me that as piped in the linked image the two systems can operate simultaneously and either could provide chilled water to the other in the event of an outages. He said to just tie the supply and return lines of the two systems together and they will operate just fine "because water is stupid and it just goes where its going to go". I didnt find that to be a convincing engineering argument.

This is only an emergency system that operates when one building has lost part of its capacity.

Both systems have variable flow with flow controlled by differential pressure sensors. The pump for both systems would have to operate simultaneously to provide cooling for both buildings in backup mode.

I do not understand how the two systems can operate with each of their pumps running and still have flow in the crossover portion. I thought the flow would essentially deadhead where the pressure for both systems was equal and therefore the flow for each system would continue through their respective supply and return lines but without any crossover flow actually happening.

Also, this system seems far too simplified for there to be any control to allow either building to backup the other.

He did say that he thought there would need to be different pressure setpoints for the pressure sensors to control which building is helping the other. Presumably the pressure would be set higher for the system operating as backup and set lower for the system operating at reduced capacity.

Can someone please help me out and tell me if this system will work or if the heat exchanger between them would be more reasonable?

RE: Primary/Primary Piping System

I know everyone complains about lack of detail, but in this case you need to simplify your diagram so that we (I) can understand it and what your issues are. currently I can't see the wood for the trees (though that maybe because it's Friday afternoon).

What would be good is a simplified sketch showing what you have now and what is proposed. currently I can only see one set of chillers, not two

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Primary/Primary Piping System

(OP)
[url=http://postimg.org/image/9e17durfz/][/url]

[url=http://postimg.org/image/r2sy5h373/][/url]

Here are the two scenarios and my simplified understanding of them.

1400 to 1616, *probably* works

Without any valves, the two pumps are under control of their respective DP sensors. When 1400 is backing up 1616, the flow from 1400 mixes into the tee after the 1616 chiller and at the outlet you get the sum of the two flows. If all AHU at 1616 are calling for maximum flow, the 1616 pump is operating at full flow. Downstream of the tee you get the sum of the flows from the two systems. However, all AHU at 1616 have 2 way valves and are balanced to a sum that is only 250 GPM.

The DP sensor at 1400 is likely to keep its pumps operating at full speed because the flows match. The pump at 1616 would “hopefully” slow down to get the flows matching because the supply side has higher pressure than return side. The flows diverge somewhere in the 1616 return line with each pump pulling its respective total required. The DP sensor at 1616 would need to be placed near the beginning of the system, after the tee. Otherwise when the flows diverge in the 1616 return line it will be picking up a difference between the mixed supply and the 1400 return. Doesn’t work that way.

1616 to 1400, don’t see how this works very well

The air handlers at 1616 are at part load. Say they only need 50% capacity and there is an extra 125 GPM available at the tee. That flow wants to go in the opposite direction at the tee from the scenario above. However, the pump at 1400 is flowing water towards the tee. The air handlers at 1400 are also at part load, say 50%. It is possible that the supply side flows converge somewhere in the supply line and some of the 1400 air handling units are served by 1616 and some are served by the still operating 1400 system. The DP sensor at 1400 slows its pumps to allow the 1616 water to flow into its system.

The best way to control this would be with flow meters I think. Both systems would need supply and return side flow meters as well as a flow meter in the crossover line. The 1616 system determines the flow required to meet its load and determines what spare capacity is available. The 1400 system determines the flow required to meet its load and also determines the available capacity of its system based on maintaining the leaving water temperature. The controls system then slows the 1400 pumps to match the load its system can meet and ramps up the 1616 pumps to provide the capacity it has available. The system would need to make sure 1616 does not starve its own air handlers by providing too much flow to 1400. Fortunately both these plants are operated by the same controls system but they would need to be able to communicate with each other for this to work properly.

If there is a simpler way to make it work well with flow going in this direction that you can see let me know.

RE: Primary/Primary Piping System

I think you're initial impression is correct and that it won't work. Water might be "stupid and goes where it's going to", but won't go there if there is no differential pressure forcing it to. To connect two discharge headers together is ineffectual at best and might end up with reduced flow or reverse flow or no flow at all in the interconnecting line. Far too many variables to work out which one is actually going to happen. Essentially what you are trying to do as drawn is connect two pumps is parallel, but fed from a different source. Even "identical" pumps side by side don't pump evenly so yours have no chance.

Leaving aside for the moment the rather large difference in capacities of the chillers, I don't understand why this wouldn't work (though at lower flow) if the system being backed up simply turns the pumps off and closes the valve on the chiller and allows one system to try and supply all the AHUs?

The only other way this is going to work, though would need some flow control on the bypass, is to plug the end of the discharge line into the suction of the pump and bypass the on working chiller. Return would then need to be via the interconnecting return header line, not to the backed up system pump, essentially a booster pump set up.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

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