Manifolded chilled water primary

[mabed]

I have a chilled water system with the following equipment:

-16 chillers 600 gpm each
-6 constant speed primary pumps 1600 gpm @ 85 ft of water each.
-Three sets of variable speed secondary pumps.
* 1st set 4 pumps 1800 gpm @ 130 ft of water. (serving low level)
* 2nd set 2 pumps 700 gpm @ 100 ft of water. (serving intermediate level
* 3rd set 1 pump 1000 gpm @ 115 ft of water. (serving high level)

I have two questions:
1- The primary pumps manifolded to a header and from this header I have branch to each chiller. If for example 14 chillers are running I need 8,400 gpm, so if the 6 primary pumps work, the pumped water into the chillers will be 9,600 gpm. Which mean I have additional 1,200 gpm pumped into the chillers. In this case do I need a bypass (other than the decoupler between primary and secondary loop) between primary pumps suction header and discharge header to circulate any additional flow from primary pump. (The designer didn't show this bypass)

2- I have three sets of secondary pumps each serving a different system with different pump head but the return riser pipe is common for the three systems. My question is do I need a balancing valve at each branch connected to this common return pipe to adjust for pressure difference between the three pumps on return riser. I have automatic balancing valve at each air handling unit and fan coil.

If there is any missing data please let me know, and thanks for your help.

 

[Drazen]

1 - too much of unknown, you could prepare some simple sketch together with data like which kind of balancing valves is used on chiller branches

2 - yes, you would need balancing valve on branches, that should seemingly be automatic balancing valve as well.

 

[cry22]

Drazen
can you elaborate on how these automatic balancing control valves will work? and what will make them modulate?
the secondary systems are on VFD, so your return is whatever is consumed (or pumped) - what would be the relationship between pumping and the automatic balancing valve?
I would put the latest Belimo control valve with flow meter at each zone to maximize delta T and avoid low-delta T syndrome?

Mabed - is this an existing system? or are you designing such a system? cause if you are designing it. I would use one chiller for each pump instead of main header - reduce pipe size and pump size and easier to control (solves your primary GPM problem).
Hey Mabed - how's business in Saudi Arabia? shoot me a line (mtz_2050@yahoo.com) - I am not too far from you.

 

[Drazen]

cry,

you will admit that it would not be easy to elaborate in details when more specifics is known. in general, it looks quite large system to me, and there is likelihood that pipe network is developed.

depending on length of mentioned branches to common return, i would either put bypass valves at the end of branches, or pressure-regulated control valves before connection to common return - or both.

the described system looks pretty overcomplicated as designer felt compelled to make "savings" by all means. i feel that there is much probability that valves will interfere with each other, and it is assumed existing system, as redesign would firstly aim to remove these complications.

 

[EnergyProfessional]

My bet always would be on the Honeywell PICV since they also come with lower pressure drop than the Belimo ones. but this should explain how they work.

Long story short, they are build so that the 10 gpm valve never goes above 10 gpm regardless of pressure (within reason, they are limited to 50 psi etc.). Under that rated capacity the turning valve will linearly limit flow. You want a 200 mesh strainer for them to keep the cartridge clean.
you pay more for the valve, but save on balancing valve, balancer and no one can fool around with it. i have never seen a properly balanced system, and every existing system has some balancing valves turned all the way open. the PICV prevents all these mishaps.

they get controlled the same way like any other control valve. for the controls contractor it is the same. You just never need to worry about balancing and buy them in 1 gpm, 2 gpm, 10 gpm, 20 gpm or whatever you need.

the dT depends on what flowrate you design for. The valve doesn't help you with that. 20 gpm with PICV and 20 gpm with normal balancing valve will give you the same dT

 

[cry22]

Herr
Thanks for the tip about the 200 mesh.
Belimo (from their website) has done retrofit for Harvard with their latest PICV with a built-in Flow meter and temp sensors. The PICV controls the Delta T and flow at the coil or at a branch for a zone.
Has anyone used one of these lately?

DRazen
we do lack some info from Mr. Mabed on the system indeed.
In btw - you mentioned by-pass - Some "schools" tend to go with zero by-pass (whether at decoupler using or at end of main) and rely on head pressure reset, the idea is that if chilled water is manufactured at 44F, then one should use 44F and not mix it. Some other put a check valve on the decoupler to avoid mixing.

 

[Drazen]

most of picv (but not all) manufacturers still like to claim that their valves do everything, and many present schemes where nothing but such valves installed at terminals is needed.

in practice, however, that does not look so in longer and more complex networks.

herr, fixed flow balancing is still indispensable in constant flow systems and price to value ratio has its role as well, not to mention that permanent care for each and every strainer is becoming major maintenance task. that is far from simple "good-no good" decision and there is still large gap between sales rep claims and neutral engineering scrutiny

cry, there are few functions branch bypass does, one of them is to ensure pump minimum flow, the other is to provide timely response in load changes.

 

[DRWeig]

cry22,

I've tuned and commissioned the Belimo PICV line, and I know a product manager for Kele who has evaluated them thoroughly. They work as advertised - as usual for Belimo. However, Belimo's energy saving claims are subject to debate.

Off topic but not completely if you're at design stage:

I would dispute the entire concept of pressure dependent (my preference for avoiding low delta-T, minimizing pumping energy, and easily establishing stable contro) vs pressure-independent operation in a variable-flow system. Also, I'd argue for primary-only variable flow vs primary-secondary. HOWEVER, that's another topic that has been hashed into mincemeat in previous threads. You should look them up. Here are a few, if you're interested.

http://www.eng-tips.com/viewthread.cfm?qid=332582

http://www.eng-tips.com/viewthread.cfm?qid=239619

http://www.eng-tips.com/viewthread.cfm?qid=327234

Best to you,

Goober Dave

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

 

[EnergyProfessional]

sure, PICV are not for every application. but they would work in most systems in 2-way applications. For 3-way you still need a different solution.
all they do is limit the flow to a maximum (their rating), thus eliminating balancing valves and being able to handle different system pressures better (i.e. when you have static pressure reset etc.). they don't do more, not less. there is no magic savings. where you get savings is:
- you know your system is balanced correctly (if you ever worked in the field with balances you know this never is the case) - therefore you know not too much flow, not too little. anything that makes the success less dependent on the contractor is good. the balancer also never talks to the controls contractor and never iterates measurements to set a dP in the BAS. they just set an arbitrary dP, and then just set all balancing valves, thus wasting a lot of energy. If balancers actually woudl do a proper job, we wouldn't really need PICV as much.
- you know some semi-educated mechanics can't just open up some balancing valves to resolve an unrelated problem in a zone. Go to any building after 10 years and you see every single balancing valve has been manipulated by someone who didn't understand the system.
- you save the cost for balancing valve, balance, and space; this is more saving than the added cost to have the valve be PICV as opposed to pressure-dependent.

To control a dT, they don't do more or less than a regular control valves. I'm not familiar with the Belimo solution, but it sounds like a standalone control. this isn't necessarily related to PICV only, someone could offer that for pressure dependent valves as well. I personally prefer control via BAS in either case.

the strainer maintenance isn't a big deal in a system with proper dirt filtration. Once the system is started up, has one of the spriovent dirt separators and water is chemically treated, you shouldn't have much dirt. if you do, you have other problems.

True that sales people promise more than what I mention above, but sales people also promise the same for everything else they sell. If used correctly, PICV are great. but don't use them where they are not good at. Some applications need to be pressure dependent, some applications require 3-way, open systems may be a problem due to dirt,

 

[mabed]

Thank you for your help and I'm sorry I couldn't reply to my own thread before.

I work in a contracting company and this project is at its final design stage and the system can’t be changed (I totally agree with Mr. Cry22 if it’s up to me I would go with one pump for each chiller.)

The building is a 25 floor, first 12 floors are offices and the upper 13 are residential floors. There are 94 air handling units and about 1,000 FCU.

I have attached a rough schematic riser diagram hopefully this will make the system clearer. In the design drawings nothing is mentioned about the excess flow from primary pumps, I raised this issue and waiting for reply, but I wanted to know what you think because there are many good engineers visiting this site, and it’s the 1st time I work on manifold primary chilled water pumps.

For the balancing issue I was mistaken the units don’t have automatic balancing valve, its two way valve and double regulating valve for each AHU and FCU (Shown on attachment). The problem is that in the design drawings only the lower floors risers have balancing valves which I find strange. I think all returns pipes connected to riser need balancing valve.

Do you think connecting different secondary pumps to the same return pipe will create any problems because each pump has different head?

 

[Drazen]

so, it seems i was right in assuming that there is long and developed secondary network - your basic data implied that.

reminding you that this is tip site where conditional opinions are given based on limited data, i daresay the design you presented looks really messy.

it is especially "fatal" to see combination where constant flow primary has dynamic balancing valves on chillers, while variable flow secondary has static balancing valves... that together with common return looks really nasty. so, on secondary system would be "balanced" only at design full flow, and in any other circumstances that would be totally imbalanced. it would be especially interesting to see vfd pumps fighting again each other to put flow through the same return...

again, i would remind you that this is friendly chat only, but would strongly suggest that you hire independent engineer to revise design. if system would not be working properly you as contractor will carry lot of responsibility.

 

[mabed]

I know that this site cant be taken as a reference it is just for tips, and all the problems are already risen to the designer.

the two way valves at each unit will control the required flow for each AHU depend on the room temperature, so the system will be balanced even if the system in not at full load. (two way valve partial close, pressure in system increase, secondary pump VFD will slow down. or the opposite if valve opens)


that is what i am worried about how can this be controlled. then again there are two different flows connected to the same riser at same pressure (pressure is adjusted by the balancing valve at each branch connected to the riser), it is just like having two parallel pumps. but i think the balancing will be very complected.

 

[Drazen]

two way valves will not control flow in your case as static balancing valves will suffocate them at part load. this cannot be described on tips level, but you can google some articles about it.

what do you plan to say to designer? to give up from everything in his concept? i doubt it will work, such cases require third party.

 

[mabed]

I have to disagree with you; the static balancing valves will be used to set the maximum allowed flow in each unit regardless of the two way valve.
If for example a unit need 10 gpm at the maximum load, the balancing valve will be adjusted so that at maximum load this valve will not allow more than 10 gpm ( even if the two way valve is fully opened and can pass more), as the required load by space start to decrease the two way valve will start closing and decreasing the flow to below 10 gpm (the balancing valve still can pass 10 gpm but the pressure drop created by the closing two way valve does not allow these 10 gpm to pass.)

http://www.eng-tips.com/viewthread.cfm?qid=341207

 

[mabed]

sorry about the link i paste it by mistake

 

[Drazen]

it is not an issue of maximum flow limitation, balancing valve will expend too much pressure on part load (if it is set on design load), and vfd pump will additionally reduce flow when excess pressure is sensed, that is why picv valves are introduced.