Ammonia chiller with VSD

[MedicineEng]

Dear All:
Some of you might remember in the past that I ask your oppinion about some lousy ammonia chillers that we had here in our site. By that time I learnt a lot regarding this issue from great experts like Montemayor (among others).
Well, after two years of tremendous headaches with those chillers, finally we manage to convince the top management that we should change them. Now we are finalizing the procurement for a new chiller.
One of the suppliers (York), proposed a chiller with VSD in order to control the load of the chiller (Model Rotatune SAB 163H). According with them, this chiller in partial load can achieve significative cost reduction in terms of electricity and this is quite appealing for us since the electricity here is extremely expensive.
I would like to know if any of you already had experience with those chillers since here in Asia they just sold a few of them and in Portugal, (where we have our HQ)they only sold one. Is there any hidden problem with these chillers?

Thanks a lot for your help.

PR

 

[quark]

I don't have experience with ammonia centrifugals but I commissioned and run York Millenium 3x500TR R123 chillers. Initially we had VSD for only one chiller to observe the savings assured by York based on a theoretical load pattern. It did give us results and subsequently we went for VFDs on the other two chillers.

Having said this, you have to first check your load pattern and particularly the low loads and their duration. A chiller with a VFD is not as easy to operate as one with simple IGV control. But a centrifugal chiller with both IGV control and VFDs give you better control over compressor surge.

The VFD drivers are more prone to damage(due to external noise, is what the manufacturer said when it happened)and take care that York has a good service setup and spare part management system at your place.

Further, this article may be of interest to you.

http://www.automatedbuildings.com/news/mar02/art/hrtmn/hrtmn.htm

Regards,

 

[MedicineEng]

Quark:
Thanks a lot. Just one thing that I forgot. The chiller that we are considering is screw type.

PR

 

[HVAC68]

Screw chiller with VSD ? I doubt if anybody has come out with such a product as of now, though Carrier had plans to bring one out. Maybe somebody is already offering and I am ill-informed.

My experience with Centrifugal chillers with VSD has been good. Of course, as quark said, you need to look at your load pattern as well as your ambient conditions before you take a call. The VSD will more than pay back its investment only if you have lot of hours of low load conditions.

HVAC68

 

[MedicineEng]

Yes, York has the Rotatune model that is a screw type compressor that has VSD. In fact I am expecting a lot of partial load situations since our production is not constant in cooling needs, it works a lot with peak loads. That is why this is so appealing in terms of power savings.

 

[Montemayor]

When it comes to comparing the performance between centrifugal and positive-displacement compressor controls, I believe the gap is too wide to allow comparing the results of one to another. A centrifugal compressor is a formidable challenge to control under normal conditions - and really starts getting demanding when subjected to varying speed(s). The normal nature of the beast is steady, unvarying process conditions - especially with respect to capacity.

I would expect that any supplier capable of putting together a dependable centrifugal chiller control system based on variable speed for capacity turn-down should have no trouble doing the same with a screw-type compressor. Screws, by nature, have the inherent advantage of going through capacity variance with little or no change in their P-V characteristic performance as do centrifugals. In other words, their performance curve is essentially linear within a given range. This capacity linearity is accentuated when the screw is oil-flooded (as I suspect it is, in Paulo's case). I have put together screw compressor chiller packages for a company bought out by York, so I'm familiar with what the expected performance should be like. I do give a lot of importance on the exact type of Variable Frequency Design that is ultimately applied on the machine AND the rest of the process. The savings being talked about, I presume, are the TOTAL savings: cooling water recirculation, oil recirculation, net horsepower demand, power factor, etc..

The application of VSD to a screw compressor is, I believe, straight-forward; what I would stress is the quality of the instrumentation employed and its reliability - as well as its maintenance requirements. Not all VFD systems are the same.

 

[HEC]

I may be a bit old fashioned, but PauloRibeiro did mention a variable load, I have had much experience in Dairy industry where there are wide swings in cooling load. In these cases we have used smaller chilling plant and an ice build burn system to provide the required cooling. This system has advantages for a more constant load on the compressor, smaller compressors, and the ability to take advantage of off peak electricity usage if there is such a tarif where you are.
Regards
Mark Hutton
hutton4eng@picknowl.com.au

 

[MedicineEng]

Dear all:
Thanks a lot for your replies.
Montemayor:
You are right. The screw is oil flooded type. Your point on the control and instrumentation is very well done.

HEC:
I believe that you are refering the ice bank concept. This chiller will operate in the -25C to -20C range, so I don't think that it is possible to do so.

I still have some second toughts about this option because if it is as good as the supplier says, why don't they sell more? In fact in a reality where the oil prices are going skyhigh this would boost the sale of this type of equipment, dont'you think?
Tat is why I am so worry. After the last three years with a constant headache because of the actual chillers, the less that I wanted to happen is to replace one headache by another.

PR.

 

[quark]

Paulo,

As Mr Montemayor rightly said, the capacity control of a screw chiller is linear with its speed and so power. The power consumption of a PD device is a function of the mass flowrate of the fluid and the pressure ratio. As pressure ratio is constant here, the power consumption completely depends upon the mass flow rate of fluid or no. of rotations of screw linearly.

Generally, screw chillers come with slider valves for capacity control, which were misinterpreted as stepless controls in old days, which can control at 75%, 50% and 25% loads. If you require an accuracy of 0.5⁰C with a designed delta T of 5⁰C, your process becomes a 10 step control system(5/0.5). Air conditioning loads, unlike process loads, do not vary drastically due to themal inertia of control spaces and other material. If your system operates at full load for 8 hours in a day then your power savings calculations with vfd should not base on 16 hours. I would prefer them to be for 16*(9-3)/9 = 11hours approximately (You have 9 variable load capacities with vfd and 3 with slider valve mechanism). This basis can be used to evaluate the power savings correctly to some extent. I would prefer a 2 year payback period.

If you have plans to implement all variable systems now or in future, you can definitely go for a vfd driven screw.

Don't overlook the hidden advantage of increased life of chiller with vfd.

Good luck,

 

[MedicineEng]

Thanks quark:
Just one remark, the chiller is not for aircon is to cool down reactors in production process.
I am also not shure about your claim of extended work life, since with the VSD the chiller will work at higher speeds so the wear is usually bigger.

PR

 

[quark]

Why should it work at higher speeds during low loads?

 

[Montemayor]

Paulo:

I believe quark has a valid point. This item may be important to your management and strengthen your position with them. You said you have peak refrigeration loads and also are expecting a lot of partial load situations. This, to me, is the crux of the application and I would expect that the design, specified basis for your screw machines would be the peak load amount. Any lesser (partial) loads would be attended to by a combination of the conventional slider valve (as mentioned by quark) and the subject VFD. Quark has done an admirable job in describing the variable capacity mechanical operation of the screw and I can add little except that the “turn-down” capacity control would be dependent on employing the slider, the VFD, or both. We don’t know how both systems would inter-act as designed by York – and this is an area of great concern (with good reason) as the operation applies to your load distribution. But quark rightfully expects to experience less “wear and tear” on the machines when they are working with a turn-down capacity using the VFD.

Your load distribution, I also believe, is the key to your projection of future cost savings – and justification to management for an investment in the VFD. Your ability to accurately describe and document it to management and to the proposed VFD supplier will not only lend confidence to management of your control of this project, it will also strengthen your position with the supplier since you have given him the added tools to accurately design and propose the correct and justified capacity control mechanism for your future machines. In the end, a successful project is one where the application works and BOTH the supplier and the user profit from it.

I agree with you totally that the most important role in managing and overseeing this future critical installation is that the choices and decisions you make now are the CORRECT ones for your application. This is very critical because, as you correctly state, you could wind up in a worse situation than before. It is constructive, in this type of project, for a young engineer to be cautious and study well his application before making an important decision that will have a large impact later on. The more information and data that you can supply your proposed or selected chiller designer, the better off you are with respect to negotiating a performance warranty or an understanding on what to expect. The refrigeration load distribution (as it relates to the expected capacity) is an important factor to lay on the table at the very outset. It should be in the interests of your supplier that you succeed in every goal that you have set forth in this application. And when he is given the opportunity to share in all your basic data, it strengthens his ability to carry out his best design.

I would also recommend that you insist on personally visiting similar, screw compressor refrigeration applications in the field that are operating with similar capacity variances and using VFD. I realize that management may frown on these costs, but I believe that you can rightfully point to this proposal as a direct means of obtaining captive information valuable towards a correct decision and contributing to company know-how in predicting and controlling future critical energy costs. Additionally, when you are reinforced with actual, field experience on the proposed application, a lot of the final decision to purchase and apply the VFD is shared with management – instead of being shouldered directly by you. This is always a good thing for you and your career: if the application fails, you don’t take 100% of the blame; if it succeeds, you make heroes of the management that worked directly with you. It’s almost a win-win situation. In my experience, you will find that companies like York will gladly cooperate with you in identifying similar, existing installations and work with you in visiting these. This type of experience can be a big boost in your learning curve and gives you the opportunity to learn first-hand the pros and cons of an actual VFD installation without the usual sales talk of supplier representatives.

I wish you luck in your application and congratulate you on achieving this big step forward in improving your present chiller application.

Respeitosamente

 

[EdStainless]

This is a non-compressor comment, it is a VFD one.
The quality of VDFs is critical. The better the output waveform and filtering the less wear there will be from harmonic problems.
I have seen units on pumps that when you turned them down you could hear the cogging due to the steps in the wave form.
This is also a function of the motors. If you are running sync you can use medium quality motors and be fine. For VFD use you need the very best motors. There are ones rated for this service.
And don't cut corners on the surge/lightening protection for your VFDs. I went to an outdoor instalation once and we had trouble finding the pad where the VFD unit had been mounted, there was nothing left, just black concrete.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[sterl]

There's a whole lot of interest in Big Machine VFD's and there's some special units currently available, though the suitability and feasability has to be pursued on a case by case basis.
Frick and York have focused the development of the VFD Screw Compressor (This nearly-new stuff...) on bottling plants, the basic rationalization being the alternative costs of adressing their load swings. With conventional carbo coolers: The heat transfer drops from 100% to something like 5% every time the bottle filler has a disruption, in a period of maybe 20 seconds, sometimes less; and in worst cases it will resume just as fast.
Current methodologies for such plants include: Very Large glycol tanks to provide a thermal flywheel (similar in thought to the Ice Builder, but without the benefit of latent storage); running multiple recip compressors down to lowest cylinder based turndown, then hot gas bypassing to prevent the machines from shutting off thus avoiding the inclemencies of restart when the load resumes; and so on....A variety of different machine limitations and maintenance scenarios are being addressed by the breadth of such control schemes. And, of course, the system has to be able to tolerate and react to the rapid upswings and downsings as well.
Slide valve screw compressors will modulate capacity in a continuous manner (not stepwise) and will also continually adjust their internal volume ratio to optimize COP at low loads or low condensing pressure, but every slide valve machine has a decaying COP at part capacity....the actual shape of the curve is available from Frick and others. Conventional Slide Valve screw compressors take more like 90 seconds to load or unload through a breadth of 20% to 100%. With the VFD, the rate of load and unload can be as fast as 10% to 100% in 8-seconds.
Frick will not consider Overdriving a screw compressor, and the packages are different than their catalog units.

That said: The motors are special; the power side electronics are more sophisticated than a typical VFD for driving say, a fan; and the Drive is itself, fluid cooled.

Does it fit? Hard to tell without knowing a lot about the plant and its operating cycle, and the cost of power, but if this is going to be built up as a chiller package, there is inherently some heat capacitance to the brine side, and the turndown rate may be a moot point. Based solely on power savings, the Plant Duty Cycle and the COP curves become even more important.