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]

Thank you Drazen.
I will soon be posting a more detailed system schematics. I did not envision that the problem will be as complex as it turned out to be. I had simply plugged in some numbers in the sketch to depict the system, but that obviously is not enough. So please bear with me till the Autocad department favors me with another sketch.

 

[flexiblycool]

Here is the promised more detailed sketch.
1. Each chiller has 2 circuits, in fact two small chillers are packaged as one.
2. The primary pumps are not dedicated for a particular chiller. Any pump can come on to cater a chiller that cuts into service.
3. The bypass line shown on the chillers seems redundant, as it is equipped with a normally closed manually operated butterfly valve. If it is converted to a motorized valve that opened when a chiller cut out of service, the system would at least become a constant volume system, thereby not starving any fan coil units of design GPM. However, another simpler method would be to just let the water run through all the chillers regardless of whether they are ON or OFF.
4. The Secondary pumps are NOT VFD and they are continuous duty. In other words there appears to be no advantage of providing these secondary pumps.
5. The primary pumps rated for 200 GPM are probably downsized taking into account the diversity factor, and this might explain the flow of 280 GPM in the secondary pumps. However, the system might malfunction because secondary pumps are not VFD and therefore unable to give flows lesser than 280 GPM.
6. Due to the absence of the Hydraulic Decoupler, the system can hardly be called Primary-Secondary Pump System. The so called secondary pumps are barely boosting the pressure to take care of the pressure drop in the building they serve. (Would there be any advantage to retrofit a hydraulic decoupler given that the pumps are constant speed, and what would be its location.?).
7. Feedback and reservations about the system would be much appreciated.

 

[SAK9]

My observations are from the sketch are:

1. There is no decoupler pipe in the system which means that primary and secondary pumps are operating in series and the flows handled by pumps are therefore the same. So the pump flows of 200&280 pm need to be investigated on site.
2.The system is essentially constant flow and therefore 3 primary and 3 secondary pumps need to be operated for balanced flow in all circuits.
3.There is a manual bypass valve across the chiller inlet and outlet.I believe this could have been installed for system flushing and cleaning .This will be fully closed during normal operation.So better not fiddle with it.
4.Getting rid of secondary pump and upgrading primary pump head is fine.It is worth checking what is the saving in pump kw before going ahead.
5.As pointed out earlier conversion to a variable secondary flow will make the system more flexible(operating only required number of pumps) and realise better energy savings.But this is serious rework.Therefore setting up an annual energy consumption model will be useful in evaluating savings and deciding on the way forward.

 

[flexiblycool]

SAK9
Thank you for your follow up and feedback.
Best Regards
flexiblycool

 

[317069]

in the existing installation, when a primary pump switch off, does a secondary pump switch off too, did you measure the flow at the inlet and outlet of the set of secondary pumps (I mean the main pipes) in case of one primary is off.
if you load went down, do you need 600 GPM

 

[flexiblycool]

No, the secondary pump does not switch off when the Primary Pump switches off (by it getting the off command from the return water temperature sensor). There is nothing in the system that tells the secondary pump to switch off. Had it been a VFD, its speed could reduce. Had it been a 2 way valve system, the pressure buildup could have been used to switch it off.
I did not measure the flow when one primary is off, but it would be 400 GPM, and the nearest secondary will get the most share of this 400 GPM, then the building next to it and the least share of the 400 will reach the remotest building, but all the flows in the 3 buildings will add up to 400 GPM and no more.
If my load reduces I certainly do not need all the 600 GPM, but I do not know which fan coil units have met their demand due to which the water is bypassed from their coils by the 3 way valves, so I must let full 600 GPM be available so not to starve the fan coil units whose chilled water supply is not diverted by the 3 way valves.

 

[flexiblycool]

I suspect the behavior and performance of secondary pumps is a little irregular due to insufficient available flow. Let me take some pressure and flow readings in the next couple of days and get back to the Forum with my findings. Thanks.

 

[flexiblycool]

SAK9
As suggested the flows of both the primary and secondary have been investigated on site and they are indeed the same. In other words the secondary pumps are also 200 GPM each. Thanks for alerting us to confirm on site.
We still have to get back to you with the field measurements of pressures to see if the secondary pumps could be removed from the system without even having to upgrade the primary pumps because there seems to be a 25% factor of safety built into the primary pump head calculations!

 

[flexiblycool]

A CLARIFICATION
The system described in the previous schematic sketches was simplified hypothetical model in which some buildings and equipment were deleted to fit everything on an A4 sheet. Since the discussion has evolved into a very technical one, I have reinstated the components of the system that I thought could be omitted.
In any case please refer to the additional sketch which together with the last sketch describes the system more closely.
The flow readings could not be taken, but I think, the pressure readings are of substance. In any case the primary pumps are 700 GPM each. The combined GPM of all four primary pumps is 2800 that is divided equally in all the 3 branches.
The pressure readings of building no. 6 are anomalous but we cannot pin down the reason.
The pressure 5 meters upstream of the secondary pump of building 6 is 60 psi and that of building 5 is 57 psi.
Thanks in advance for your anticipated feedback about whether the secondary pumps can be deleted.

 

[cry22]

Flex
The system is badly designed. These fixes will be nothing but lipstick on a pig.
I would get rid of the secondary pumps, get a new set of primary pumps with VFD's for all chillers in a header configuration, install a by-pass with flow meter, replace 3-way valves with 2-way (or close by-pass on existing 3-way if there is one) - Then convert the system into a variable primary flow.

Simple and guaranteed efficient results, with maximum energy savings.

 

[DRWeig]

I'm with cry22.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[flexiblycool]

It looks like the consensus is to make me cry. I can go as far as removing the redundant secondary pumps, but cannot demolish everything to make it perfect. That would be fine for the next project. I hope you agree.
Thanks a lot in any case because the discussion shed much light about the situation. If in the final analysis something else comes up, I will try to reach the Forum team again. Till then………..

 

[Drazen]

i still don't see what is the problem with installing decoupler if major refurbishment is not planned.

it is doubtful whether investment in variable flow would pay off within lifetime of the system. as all fan coil valves are three-way ones, pumps on secondary can do the job, even bms addition is possible.

low-temp syndrome can be an issue, but in this setup that is likely more matter of chiller sizing than matter of hydraulic scheme.

 

[317069]

all your question is about connecting pumps together, just review connecting pumps subject and you will find the easy solution, but your second drawing is little confuse, how do you feed first two branches if the secondary pumps are on the third and it says branch valve is closed.

 

[flexiblycool]

Drazen: To get the benefit of the decoupler, the secondary pumps should be VFD. This retrofit (of Providing VFDs) is a major expense. Furthermore these pumps could never be shut-off because the demand will not have been met in some FCUs.
What can however be done and I have said that previously, is that these pumps can be removed from the system, because the head of the primary pumps appears to be enough to circulate the design GPM in all the fan coil units without boosting the pressure by the secondary pumps. This obviously is a big O&M saving. On top of that, depending on the return water temperature, if the primary pumps are also shut down, this will be another saving, even though, theoretically speaking, the primary pumps could not be shut down to keep the system a Constant Volume System. However, we have noticed that even in the peak summer conditions when one or two primary pumps are turned off, and the flow in the fan coil units must have been reduced, yet there were no complaints from the building occupants.
Conclusion: So if there are no funds available for changes, the minimum that could be done is to remove the secondary pumps and to cycle the primary pumps on and off. I hope you find this trade off acceptable.

 

[SAK9]

The beauty of parallel pumping is that when a single pump operates it can deliver 60~70% more than its design flow.So your 700 gpm pump could be delivering well over 1200 gpm when operating alone and that explains why you have not had too many temperature complaints.But your chilled water temperature could be way off design under such conditions and would degrade the ability to dehumidify.
Interested to know whereabouts are you located?SE Asia?

 

[flexiblycool]

The comment about the pump delivering more than its design flow is not clear. Since the pump can perform along its characteristic curve only, in order to deliver 1200 GPM, the developed head would have to be much lesser than the design head. So with this higher flow, the water would not reach the location of highest pressure drop that could only be reached when the pump develops the design head, or am I misunderstanding the point.
The installation is in Asia.

 

[Drazen]

it is not issue of getting benefit of decoupler, it is that decoupler should enable such flawed design to function at all. that has nothing directly to do with vfd.

disbanding secondary pumps without making sure that fcu's will receive design flow is not a serious practice. making firm decisions without getting grip on all aspects of functioning also does not belong to good engineering practice.

i believe you utterly need support of senior engineer.

 

[flexiblycool]

Your comments are taken very positively and with gratitude. The point I was trying to make in my comment dated 24 August at 11:11 was that the term secondary pumps without the hydraulic decoupler is a misnomer. The continuous duty, constant speed pumps are just boosting the pressure regardless of demand and should be termed booster pumps. It was also said that if the primary pumps can push the design flow through all the fan coil units (and we will verify that during water balancing), then these booster pumps are a redundant accessory.
So in the absence of funds to add, we are endeavoring to save funds by subtracting. I hope this clarifies the intent. The project also does not have funds for a more senior engineer than myself.

 

[317069]

You may need a fundraise to support this owner, removing pumps need some money too, hope he has this amount, or has money to pay your firm.
why the original designer of this project has used booster pumps?