Variable Speed Primary Loop 2

[KentRo]

Howdy,

We've got an opportunity to affect the re-design of a chilled water plant. Currently the plant has five 900-ton chillers, each with its own constant volume CHW pump. The plant has had to run an "extra" chiller to ensure both secondary loops get "all" chilled water. The plan is to replace the chillers, one or two per year, until all are "new" again. With the suggestion from one of the chiller reps, we're considering changing the primary loop from constant volume to variable volume (using VFDs on pumps) so as to better match the secondary loops. By keeping a slightly positive flow in the bridge (primary water flow greater than secondary flow) we should be able to keep the chillers loaded and save pumping energy as well.

Question: Anyone out there with experience using a variable volume primary loop? What are the pluses and minuses?

Thanks!

 

[PacificSteve]

Chillers need to see a certain minimum GPM, regardless of load. Be careful that any control system would be fairly failsafe, and not tend to starve the chiller under any foreseable circumstances, including failure of other pumps or equipment, or normal off cycles of other equipment.

Don't run water through a chiller that may be off. Close off the flow path using a motorized valve when the chiller is off.

The primary loop head tends to be pretty small, so potential savings are proportionally smaller than on secondary side. It is helpful to have knowledge of the load profile so you can run some fairly accurate economics.

Sometimes pumps that have run at constant speed, have operational problems when a VFD is applied. If you go ahead, new pumps would be a good bet. While picking new pumps, you can reassess the original design head at rated load, and very likely pick an inherently more efficient pump. You have a great advantage over the original deisgners....the hydraulics are now measurable, allowing you to select a pump to operate closer to the envelope.

Suggest you spend an hour on the "back of an envelope" to rough out the costs and savings. If you want more help, email me some spreadsheets, and Ill have a look.

pacificenergy@hawaii.rr.com

PacificSteve

 

[KentRo]

Thanks, PacificSteve.

The alternative design for the plant is adding a 400 ton chiller to allow for a "half-step". This may be the easiest method of making the plant match the load.

Whaddya think?

KentRo

 

[PacificSteve]

Don't go with the "half-step". Modern chillers unload well, and actually get more efficient at part loads, due to increasing heat transfer area per ton load, and also due to expected cooler condenser water.

Depending on your electric rates (what are they by the way?) and your load variability, you may even wish to put a VFD on the chiller itself. One chiller with VFD can track the load, while the other chillers are controlled via EMS to operate in their "sweet spot" of efficiency.

There are another 20 tricks to optimizing chiller plants, but heading in the right basic direction is a good start. Most well intentioned people know a "piece of the pie" but can't focus on enough of the technologies and interactions to create an optimized system, while also operating in their low-bid services arena that us engineers seem to let ourselves get trapped into all the time.

What location of the country are you in?

PacificSteve

 

[ChasBean1]

(Tried to star PacificSteve's first post but system not responding! - will try again later). Kent, I wouldn't try to configure the primary plant flow to match the secondary, especially if there's the hydraulic decoupler (bridge) that you mention. That's the purpose of the bridge - so the secondary plant can draw the cooling that it needs without having to vary the primary flow and potentially mess up any one chiller's performance. I don't know if you're more familiar with AC units, but a likewise comparison could be reducing or increasing airflow across a DX coil. Usually a simple DX system (or likewise, a simple chiller system) enjoys a pretty steady flow across the evaporator.

Use the KISS method for this replacement. Isolate one, replace one. Re-commission the whole plant after replacement to determine how to optimize, but keep the primary flow simple. Also, I agree with Steve that you shouldn't have a need to half size a unit for better control when you have five chillers. That would give you ten stages of cooling... generally not necessary - half-staging is better for plants with two or three chillers. Good luck, -CB

 

[ChillerGuy]

We're adding a chiller to an existing chiller plant that is primary/secondary CHW flow. The new chiller will have variable flow to help match secondary flow better.The chiller manufacturer must tell you how low of flow you can go on the chiller you're going to use(usually 3ft./sec velocity)and the time/amount of flow changes you can make. Rapid changes in flow can make chillers hunt and flood which causes the tonnage and efficiency to not be good.

 

[235zzzo]

Hello Forum

Let me add some points:
1. As said before, you better not to complicate the things with the primary-loop, keep the primary flowrates constant!
2. Matching the chiller power with variable secondary-loop needs (loads), is something you have to focus, at the secondary loops, not in chillers primary-loop circuit.
3. You can do it with by-pass circuits and three gates valves with fine modulating control, parameter is the the going-temperature of each secondary circuit, keeping all the pumps you have, don't need to repace none of them, unless misfunction.
4. Besides that, for that power every chiller has at least 2 or 4 independent circuits, each equipped with at least, 2 compressors.
5. So you have already parcial, "modulating" production capacity, don't need any more.
6. Try to get contact with someone good, expert in chiller control, (not some guy from the chillers company, believe me, they want to sell you another chiller or two. The question is: do you really need it??)
7. Before that: try to optimize your existing production system, concerning the distribuition needs you have, starting to minimize all the significants losses in the your distributions systems;
8. Evalute carefully the real maximal need; (have you thought about recovery energy devices?);
9. Have present the suggested changes in the secondary circuits.

And only then, consider finally to buy the new chiller and those "very nice variable things", which are very expensive. So, you have plenty of money "to clean the house first", that means to try such a kind of approach. Unless, am I out of context???
Good Luck
zzzo

 

[235zzzo]

Hello Again

I forgot one more important aspect:

10. Have you thought to implement cooling storage capacity in a such significative installation. You can couple the maximal needs with that cooling capacity you stored by nigth at low rates, having very interesting pay-back. Otherwise, it will be cheaper than a chiller, and it represents a recovery energy device, paying some attention and respect to the Kyoto Protocol....
zzzo

 

[KentRo]

Thanks for all the input!

The reason we are considering either the variable volume primary or a half-size chiller is to better match the secondary flow. Right now, due to the piping configuration (two separate secondary taps), the "most critical" of the two loops gets mixed (supply and return) water when the primary loop flow is less than the secondary loop. To maintain the proper temperature in the critical secondary, the chiller output is kept at 40 degrees or less. (This is not the design temperature for the chillers). The alternative is to run an "extra" chiller to increase the primary flow so that both loops get full chilled water.

A fellow engineer found some more information (Trane Engineering Newsletter Vol 28, No. 3), that stated basically there is little or no power savings in the chiller due to variable flow. This is because the heat transfer coefficient decreases with the decrease in flow, which "decreases the overall heat transfer effectiveness...the net effect is that the power consumption for a given chiller is virtually the same whether the system's primary flow is variable or constant."

With this little nugget in mind, and with what PacificSteve noted about the proportionally smaller savings from the primary pumps, I believe the KISS method of replacing like-for-like will prevail.

Variable primary flow systems are most effective when the secondary pumps can be eliminated, and all the pumping is done by the primary pumps.

Thanks again for all the responses!

KentRo

 

[PacificSteve]

Kentro,

A few parting comments to this active group. You are probably paying $1.5 to $3M a year in electric alone to operate this chiller plant now. A standard "re-design" should knock 20% off of that just due to better modern equipment. An optimized design should be able to take an extra 20% savings above and beyond the standard re-design.

Driving your chill water to 40F is painfully expensive in terms of efficiency. Even the expense of some major re-design on the large piping system seems justifiable to address your unusual primary/double secondary system. Bottom line---it is worth it to put some serious energy engineering into your project on the very front side.

Thermal storage systems (usually ice) will increase your total kWH consumption, although perhaps decrease your operating cost, dependent on utility rate structure. Sizing of such a system is usually done wrong due to innappropriate guesswork. However, prior to my first coffee today, it seems that some strategic tie-in of ice to your first secondary system could be the kicker to bring this return water back to a decently low temperature, with many spin off benefits. Plus it is cool.

Good luck!

PacificSteve

 

[235zzzo]

To PacificSteve

Maybe you're rigth about spending more kWh's, shall we say more 10 ~15 %, but I can buy those for th~e less of its half-price, along the night?, so I got really a very important bonus!

Besides, thermal storage can be obtained in several ways, even hybrid ones, as you know.

A plant like this one must have large water reserves, namely against-fire reserves, we can estimate, something like 500 m3 (or ~ 132100 gallons).

If that is allowed, suppose to use this water in a "closed-circuit". I mean to cool this water reserves from the initial temperature, 57 ~ 58 to 40 ~ 41 ºF, by nigth-electrical rates, (that is, when 1 kWh is half-priced).
(By the way: why not consider a cogeneration facility-gas combined-cycle?).

Back to the subject: Then, that quantity of water absorves a 305 Tons chiller, (with a COP>3!) working by nigth during six hours and it stores a quantity of availability (effective energy) about 5.527.000 kcal (or 21.933.120 Btu).

There is an extra-condidition to ask our concern: You have to isolate those water tanks to prevent the moisture processes, even you can admit partially that phenomenon, because of some difficulty to isolate some parts...

Please try to think Kyoto Protocol...
Kind Regards
zzzo

 

[quark]

ZZZO!

Once I planned to install thermal storage system to our 1500TR central air conditioning system. Upon discussion we came to know that they are useful in two conditions.

1. you should have dual tariff facility.
2. the chilled medium temperature should be less than
0 deg.C

Note: One hidden advantage is that in a green field project your maximum demand (MD electrical) can be less because you are not going to start all the equipment at a time.

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.

 

[235zzzo]

To QuarK:

Your mentioned two conditions, which are quite possible to deal with:

- the first one is just a matter of changing the contract, and the electrical supply conditions (contract), which is not difficulty and if you have that necessity;

- the second, why impose strictly 0ºC? (the system can't work at this ice temperature, unless you use glycol solution, that is impossible in the present case, as you know!) Why not + 4 ~ + 5ºC ? Still you have plenty of sensive storage capacity, as i mentioned above. You can be sure that is possible to get very short pay-backs! You can save money, not to buy that extra-chiller;

- As i said, the unadvantage is to obtain those perfect isolation conditions and this solution demands you a very good design at the in and out hydraulic conditions in the tank;

Please, give more information about your conclusion/ position. Thank You!
zzzo

 

[PacificSteve]

Gentlemen, I think that thermal storage, and in particular, ice storage is a very interesting project. Perhaps we should switch discussion to the Energy Storage forum?

I don't mean to sharp shoot anyone's comments, but would like to add:

1) The phase change involved with going to ice storage allows much greater BTU storage per given volume. Storage tanks cost money, and take up space. Proper economic evaluation requires this consideration to be thrown into the mix. AT 144 btu/lb captured during phase change, compared to 1 btu/lb for each degree. Rule of thumb states that chillwater storage required 5 times the space compared to ice. Perhaps this is why Quark said "must be less than 0deg C.

2) At least in the US, utilities hold alot of political power. If an advantageouse rate schedule exists, great, but if it doesn't exist already "changing the contract" could be a multi year endeavour with no guarantee of sucess.

3) I cant imagine many thermal storage facilities making sense (reasonable payback), for any individual company, unless both of these apply:
a) Advantageous rate schedule.
b) Tax credits and / or utility rebates.
If both these factors don't exist, it would very unlikely for thermal storage to make sense, in other words, companies pursuing thermal storage would be put at a competitive disadvantage compared to other companies pursuing more economically efficient decisions.

The advantage of thermal storage for the "community" is that peak kW demand can be controlled, reducing the need for more and larger power plants. Nobody wants a power plant in their backyard. Thus the community benefits, and in order for thermal storage to make sense for any one business/facility, legislation must be put in place to support that decision and reward the business for performing an expensive and complex project that benefits the community.

I would say that 90% of all thermal storage systems are not "optimized" for the client. 2 main reasons: lack of understanding of real life cooling load profiles, lack of understanding of real life implmentation and maintenance costs.

PacificSteve

 

[KentRo]

PacificSteve is right on the money.

Ice storage takes up much less room than water storage, but without incentives or a decent rate structure, it's a real tough sell.

Something to keep in mind: to keep from running glycol throughout the chilled water distribution and worrying about leaks and decreased AHU capacities, run the ice-loop through a plate-and-frame heat exchanger. Requires an extra pump and some safety controls, but the overall cost (not to mention the environmental mess should there be a leak!) makes it worthwhile.

Kent Ro

 

[Energia]

The key to evaluation on this issue is if the plant is or can go to a indepth redesign. The big advantage of variable primary flow is the ability to lower consumption in the pumps and mantain the chiller Kw/ton constant. When partial loads arrive you will still have a very efficient system.

The secret is that it should not be design as a decouple system ( primary and secondary loops). This type design was created for the inability of older chillers to deal with flow variations safetly.

Now a days with computer control panels and very reliable flow (DPS)and Temp. sensors manufacturers feel safer to deal with these variations.

As some body said the flow trhough a certain size of chiller has a minimum. This min. is determined basically by the reynolds num., you always need turbulent flow inside the tubes to ensure heat tranfer.

That's why there is the call for the half step chiller. But there is more, you said you have five 900 t units. This is a total of 3,600 tons peak ( assuming one is backup)then; why not use (1) 1,200 t, (2) 900 t and (2) 450 tons?

Well, probably because it is more difficult to visualize. May be it is not the most cost-effective first cost option. There is no easy answer. The system shall be evaluated for different options using computer emulation programms (Trace 700 or DOE).

Remember the real savings of a variable primary flow are in the elimination of the secondary loop pumping and directly pass the the return water ( by passing it one way or the other in the decouple is a loss of energy) trhough the chillers. Also remember that you can modulate the condenser water gpms and fans rpm's when operating at patial loads and still have the same kw/ton on the chillers.

When using lower gpms the head lowers reducing the kw's for pumping. So if you design with lower temperature it is right that the chiller itself will be less efficient but the system in total might be more efficient.

Again, all alternatives shall be evaluated with a life cost cycle of the investment vs. savings. Include the reduced maintenance for the elimination of secondary pumps. Include the advantages of the required Chiller Plan Manager software and go for it. You will be the hero in the next year for reducing the utilities by 40%!!!