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chilled water system controls
7

chilled water system controls

chilled water system controls

(OP)
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.

RE: chilled water system controls

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.

RE: chilled water system controls

(OP)
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?

RE: chilled water system controls

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

RE: chilled water system controls

(OP)
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?

RE: chilled water system controls

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.

RE: chilled water system controls

(OP)
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.

RE: chilled water system controls

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.

RE: chilled water system controls

(OP)
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?.

RE: chilled water system controls

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

RE: chilled water system controls

(OP)
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 ?

RE: chilled water system controls

Quote:

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.

RE: chilled water system controls

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

RE: chilled water system controls

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

RE: chilled water system controls

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.

RE: chilled water system controls

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

RE: chilled water system controls

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: www.taco-hvac.com/track_file.html?file_to_download...

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.

RE: chilled water system controls

(OP)

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

RE: chilled water system controls

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.

RE: chilled water system controls

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.

RE: chilled water system controls

(OP)
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.

RE: chilled water system controls

(OP)
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.

RE: chilled water system controls

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.

RE: chilled water system controls

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

RE: chilled water system controls


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

RE: chilled water system controls

(OP)
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.

RE: chilled water system controls

(OP)
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.

RE: chilled water system controls

(OP)
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!

RE: chilled water system controls

(OP)
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.

RE: chilled water system controls

2
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.

RE: chilled water system controls

I'm with cry22.

Best to you,

Goober Dave

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

RE: chilled water system controls

(OP)

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………..

RE: chilled water system controls

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.

RE: chilled water system controls

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.

RE: chilled water system controls

(OP)
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.

RE: chilled water system controls

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?

RE: chilled water system controls

(OP)
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.

RE: chilled water system controls

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.

RE: chilled water system controls

(OP)
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.

RE: chilled water system controls

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?

RE: chilled water system controls

(OP)
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.

RE: chilled water system controls

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?


RE: chilled water system controls

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.

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