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.

 

[EnergyProfessional]

The problem seems the use of 3-way valves.

If you convert many to 2-way, then the flow will be less at less demand. Especially in chilled water the pump losses are bad since they add heat to the water.

I'm also curious that your system flow of 600 gpm mathes the 600 gpm chiller, this way you don't seem to have spare capacity in case of chiller failure. I would think the reason to use 3 chillers was to only need 2. Normally you would shut off primary pumps of the respective chiller. If you run water through the chiller all the time anyway, then you would not need the hydraulic separation int he first place.

 

[flexiblycool]

Thanks for response.
1) There is no chance to convert the 3 way valves to 2 way valves.
2) The standby pump/chiller is not shown in the sketch. The sketch is just to depict the system in a general way. In any case there are 3 duty pumps and chillers in the system.
3) The question remains: Should all 3 primary pumps be kept running even when only 1 chiller is in service, to ensure that the fan coil units get the design flow rate?

 

[SAK9]

Your analysis is right.Your system is basically constant flow and as such you can not turn off primary pumps.This would mean at low loads you have 200 gpm going through the active chiller and 400 gpm going through the idle chillers with the result that chilled water supply temperature will be floating up.This may cause humidity swings in the conditioned area but that is inherent flaw of such a system

 

[flexiblycool]

Thanks SAK9. Furthermore, the Secondary Pumps are fixed speed constant duty pumps, they appear to be a redundant accessory. The Primary pumps could have been selected for a little higher head and there would be no need of secondary pumps at a cost saving of O&M of these unnecessary pumps. Would you agree?

 

[SAK9]

Exactly(you took the words out of my mouth!)That is how we used to design CHW systems a few decades ago.If the secondary pump has enough head, replace primary pumps with secondary pumps and the system would work just fine.You will save a lot on energy and maintenance.

 

[flexiblycool]

SAK9: Thanks for agreeing with me. Actually, the Primary pumps cannot be cancelled, because they are taking care of all the losses from the chillers and piping all the way to the secondary pumps. The secondary pumps could be cancelled out if the head of the primary pumps could be increased to account for the pressure drop through the pipes and FCU coils in the remotest building.
Any other ideas would be welcome.

 

[Noway2]

With constant flow / three way valves, you are undoubtedly going to have to make some form of trade off: efficiency, chiller cycles, or temperature and humidity control. In the worst case (and I've seen this happen in your situation is that you get a serious mold problem). About the only thing I can think of in your situation, besides eliminating the secondary pumps would be to create a recirculation loop around the building FCUs. For example, you could use a temperature control valve that restricts the flow to and from your chilled water loop (bypass) that is designed to keep the return temperatures above a certain set point. As long as the coils are working, this should not post a problem with regards to your cooling capacity and the coils could all have maximum flow.

 

[flexiblycool]

Noway2
Vow electrical has quite an insight about mechanical!
Another question:
Since the secondary pumps are constant duty pumps, their cancellation and replacement with an untrimmed pump impeller (to give a higher head) is being contemplated. However, there is another reservation. The actual GPM being handled by each secondary pump is 300 GPM instead of the 200 GPM shown in the sketch. This 300 GPM each means 900 GPM for all three secondary pumps whereas the total GPM of all three primary pumps combined is 600 GPM. Is this 300 GPM due to diversity factor in which the Primary pumps are sized for diversity whereas the secondary pumps are sized for the maximum possible demand?.

 

[dbill74]

If the primary pumps are cycling on/off with system demand and the secondary are always on, there is bound to be problems. Might want to double check your plans and verify that there isn't a by-pass around the chillers.

As I see you have a couple options.
1) Effectively make it a variable flow sytem by:
a. Replace the 3-way valves with 2-way valves
b. Add VFDs to secondary, building pumps (or replace).
c. Add a bypass in each building.

2) Add a bypass around the primary/chiller pumps. Control Chillers based on CHW temperature downstream of the bypass.
This will allow all 3 secondary/building pumps to run a continuous 200 GPM each. Meanwhile the primary pumps and chillers can cycle on/off as needed to maintain your CHWS temperature.

This second option will be better suited to handle up to 900 GPM while the chillers are only cooling 600 gpm at a time. You might want to take a closer look at your building cooling loads and compare with your chiller capacities.

Bill

 

[flexiblycool]

dbill74
If all the secondary pumps are running, each demanding 300 GPM or a total of 900 GPM from the network, while there is only 600 GPM coming from the primary, would this situation cause pump cavitation in secondary pumps since only 600 is all that is available ?

 

[Noway2]

If all the secondary pumps are running, each demanding 300 GPM or a total of 900 GPM from the network ....

Is your piping really designed as shown in the diagram in the initial post? This doesn't look like a primary secondary loop. It looks more like a variable primary (lack of bypass on the chiller output) without the VFDs on the buildings.

 

[flexiblycool]

Noway2
Thanks again. Let me reassemble the schematic in a couple of days for a closer look.

 

[Drazen]

Is this not a typical example where to apply hydraulic decoupler?

 

[SAK9]

Have you verified the flows of primary and secondary pumps on site?Unless you have a decoupler,there is no way secondary pumps can deliver 900 when primary is delivering 600.If a decoupler exists,the flow imbalance is serious enough to degrade your chw supply temperature.

I am assuming there are only FCUs in the system and the FCU 3 way valves are on-off type type(not modulating).If my assumption is right,you can blank off(either use a blind or close a valve if there is one) the bypass port and convert this into a variable flow system working on a single pump.
You need to do it on around 40 FCUS and keep the remaining 20 as they are.This will give you the flexibility of running only one pump and chiller at light loads.

 

[flexiblycool]

SAK9: Good Idea
Drazen: Do you a hydraulic decoupler in the system. If not, where would you apply it? Thanks.

 

[Drazen]

sak, what do you mean by flow imbalance? decoupler makes actual separation of primary and secondary flows and they become independent of each other. the concern would be to have too high flow on primary, that could pose threat of low-temp syndrome, but this does not look like such case.

flexiblycool, decoupler is installed between primary and secondary loop. if you are not familiar with the sole concept, you could study some of manufacturer's literature.

this is good result from ordinary googling:

http://www.taco-hvac.com/track_file.html?file_to_download_id=17396‎

you have to select per maximum flow.

i believe there is little sense in attempting to make variable-flow secondary if all control valves are on-off three way type - the least you should do is to change their actuators, shotoff bypasses, and eventually change valve orifices if manufacturer gives such option. that is far from small retrofit.

 

[flexiblycool]

Drazen: Very useful website. What would be the most convenient location to install the decoupler in the given situation? Thanks

 

[SAK9]

Drazen,

If you have a decoupler,primary flow should be greater than secondary flow by a small margin.If the secondary flow is significantly greater than primary flow as in the above case,return water will flow through the decoupler into the supply side raising supply chw temperature well above design value.

 

[Drazen]

sak,

you are right, i assumed by default that fan coils have higher design temperature than primary circuit, but as we are unclear about the whole picture, that does not have to be the case.

flexiblycool,

at your circuit, decoupler connection points should be downstream of chillers and downstream of last secondary circuit. you should verify, though, design temp. of fan coils as per sak's comment.