9 replies to this topic

[fredooi]

I need help for project...

i need to know if there is anyway to optimize an under-utilized cooling tower without using variable frequency drives (VFDs).

the cooling towers are 2 500RT counter flow.

Thnx!

[Art Montemayor]

Fred:

I don't believe you can expect to receive an intelligent response on what you furnish as basic data.

First of all, what are the Variable Frequency Drives (VFDs) connected to - the induced (or forced) air fans or the water circulation pumps?

Additionally, what does "2 500RT counter flow" mean? Does this mean two - 500RT (whatever 500RT means)? Or does it mean 2,500 refrigerating tons of cooling duty?

We need to know the full basic data for the cooling towers. If you don't know what constitutes the full basic data, you won't know if the responses you receive are valid or correct.

There are probably many ways to "optimize" a cooling water tower. But you have to clearly state your scope of work, basic data, and explain yourself clearly and accurately.

Supply the above, and I'm sure you will obtain the help you seek.

[Adriaan]

1.) VFR .... frequency controllers (a) use quite a bit of energy themselves ( need cooling (because the energy they use is mostly turned into heat) © cost money

2.) "the cooling towerS are 2 500 RT" so one very obvious answer would be "use just one of the two cooling towers if you are running at a low capacity".

3.) cooling towers are needed but they are never efficient as you use them to dump energy, you might consider ways of utilizing the excess energy somehow (in Rotterdam power plant waste heat is now used to heat houses)

[fredooi]

Thanks Art for the reply,
here are some data, hope it clear things up...

Currently there still isn't any VFD system in place; neither is there any automatic building management system in place. The cooling towers are TWO 500RT (model: NS CTA 500 UN) Couldn't quite get all the specs are there aren't any manuals. couldn't quite find it in the internet either.

The cooling towers (air fans) are currently running on 10hp (415V, 7.5kW) motors with a rated flowrate of 1717 US gpm. Water flows into the cooling tower at a rate of approximately 1600gpm.

Right now the chillers (two 420 ton centrifugal chillers) are running at most 60%-65%of it capacity (around 500RT-550RT)

As for my scope of work, i'm a trainee undertaking a cost saving project in a midsize semi-conductor plant in Malaysia. I've evaluated VFD but it does not seem viable at the moment with its long payback period. Therefore, I would like to know if there are any other options to seek.

[fredooi]

Adriaan,

2.) "the cooling towerS are 2 500 RT" so one very obvious answer would be "use just one of the two cooling towers if you are running at a low capacity".

My country has a warm climate (average max. temp is 31°C) and high humidity (80-90%). So, would it be possible to really do this?

And btw, based on the data from the chillers, condenser water temp. in is around 85°F and condenser water out is around 82°F.

Thanks,

[Art Montemayor]

Fred:

I think we would all love to help you out, but you still haven't given us all the basic data nor the scope of your project. For example:

1) Are the cooling water towers (CWTs) servicing only the chiller units? If so, is one tower dedicated to one chiller unit only?
2) Are the CWTs induced air flow or forced air flow? I presume they are induced.
3) We know the fans are electric motor driven; however, how are they connected to the fans? A reduced gear drive or v-belt pully system may be used. I suspect you have a reduced gear box on top of each CWT.
4) Who made the CWTs? What is the fill material? What is the water distribution method? Are demisters employed?
5) We know Malaysia is a hot & humid country. But what is the design wet bulb temperature where the CWTs are located and what are the existing operating flowrates, temperatures and pressures on the CW circulated.
6) What is the design heat rejection rate per tower? What is the chiller condenser heat duty?
7) You say the CWT are rated at 1717 gpm, but is this per CWT or for both CWTs?
8) What refrigerant are you employing in the chillers and what is the rated temperature and pressure for the refrigerant charge after it exits the condenser? What type and size of condenser do you have on each chiller?

There are more questions, but the above Basic Data serves to give us an idea of what you are confronting. We still also need to know what is your Scope of Work; you told us who you are and that you want to save money. This is not a Scope of Work. You have posted this query in the Student Forum, so I assume you are a student doing a student work problem. In other words, this is an academic exercise. Is that correct? I'll await your reply.

[fredooi]

Art,

here are some basic data. i was unable to obtain all of it but i'll try and get the rest of it tomorrow...

1) Are the cooling water towers (CWTs) servicing only the chiller units? If so, is one tower dedicated to one chiller unit only?
Yes, the cooling towers are only serving the chiller units. There are 3 chillers (2 operrating, 1 redundant) connected in parallel with the CWTs.

2) Are the CWTs induced air flow or forced air flow? I presume they are induced.
Yes. They are induced counter flow CWTs.

3) We know the fans are electric motor driven; however, how are they connected to the fans? A reduced gear drive or v-belt pulley system may be used. I suspect you have a reduced gear box on top of each CWT.
The CWTs utilizes the v-belt pulley system.

4) Who made the CWTs? What is the fill material? What is the water distribution method? Are demisters employed?
I think the cooling towers are locally made. don't really know which company as there wasn't any tags. However, there is Nihon Spindle company name is on the nameplate. There is no demister used.

5) We know Malaysia is a hot & humid country. But what is the design wet bulb temperature where the CWTs are located and what are the existing operating flowrates, temperatures and pressures on the CW circulated.
I could not find the design wet bulb temperature of the CWTs as there are no manuals available. The flow rate based on the CWP is around 1600gpm, temp in is 85 F and temp in 82 F. will try and find out the pressure.

7) You say the CWT are rated at 1717 gpm, but is this per CWT or for both CWTs?
The rating is for a single CWT.

8) What refrigerant are you employing in the chillers and what is the rated temperature and pressure for the refrigerant charge after it exits the condenser? What type and size of condenser do you have on each chiller?
The refrigerant that is used is R134a. Not quite sure what do you mean by “rated” but the readings from the chiller panel is as follow, hope it is what is required.
Evaporator temperature is 41.7°F, Pressure -8.2 psi. Condenser temperature is 86.9°F, pressure 1.0 psi. Will get back to u on the type and size of the condenser.

As for your last question, I am a student but currently I’m working in a company as a pre-requisite for my degree. The company has assigned me to do a study on cost saving focusing particularly on the cooling system.

Any help is truly appreciated.
Regards,
Fred ooi

[Art Montemayor]

Fred:

Thanks for the detailed answer and the additional information. I have a much clearer and more accurate picture of what is confronting you. Your basic data submittal will help to obtain much better and detailed help on your project.

You are indeed a fortunate and lucky student to be assigned this type of hands-on project instead of an academic exercise as your pre-requisite for your degree. I wish all industry would help students (& universities) by offering this type of cooperation and help. This really helps the engineering student get closer to the real truth and facts of engineering life - and faster. I hope other students out there on the forum are reading this and follow this thread. I suspect this is going to be one of the more interesting and profitable threads on the Student forum for a long time. However, we are going to need your cooperation and hard work to make this a profitable experience for all.

First of all, you don't have a cooling water optimization project. Welcome to the real world!
What you have is a heat rejection project that is representative of real-life, engineering as it is carried out and experienced in the real world. You are confronting a real life situation and a lot of the work to resolve the main problem is going to be in your hands to take the initiative and reach a successful conclusion.

The first thing you have to do is sit down and accept the fact that you are being assigned to see to it that the Chillers are kept supplied efficiently with cooling water. In order to do that, you must identify what the actual, true situation is that exists presently. From what you write, the situation is not good - or you don't know if it is good or not. Therefore, you must establish the true heat balance of the system and whether it is satisfactory or not. To do that, you must go back to basics. This is very good for you because it forces you to employ what you were taught (or learned) in university. The basics you must apply are the preparation of an "as-built" PFD, complete with a heat and material balance around the chillers and the CWT. You must calculate the heat and material balance around the refrigeration cycle in order to determine the required heat load to be imposed on the CWTs.

To start off on the calculations, you need thermodynamic data on the refrigerant, taking into consideration the type of refrigerating cycle you have in the chillers. To get the Thermo data, go to: http://webbook.nist....hemistry/fluid/

There, you should obtain all the saturated thermo data for R-134a and download the data into Excel spreadsheets in order to analyze it and have it ready to use in your refrigerating cycle calculations. You should immediately identify what type of cycle the existing chillers have: how many stages of compression there are, the type of compressor employed, the suction temperatures and pressures for each stage as well as the discharge temperatures and pressures for each stage. Find out what is the desired refrigerant evaporator temperature required. With this information (and the wet bulb temperature for the site) you can start your calculations. I will help you set them up and get you started - all done in Excel. For now, work on getting the rest of the Basic Data and I (& others on the Forum) will guide you towards your goal --- if that's OK with you.

Let us know.

P.S.: By the way, your values for the refrigerant temperature and pressure at the evaporator and the condenser are not correct. Check this out with the NIST website and you will find that I'm right.

[fredooi]

Art,

The chillers that are used in the plant are two similar Trane water cooled CenTraVac CHVE 420. They are centrifugal chillers. Based on the manual found on the internet, there are supposed to be 3 stages of compression.
However, I’m not too sure on how to get the pressure and temperature of each of the stage. How can I obtain those data?
The desired refrigerant temperature is 43°F - 45°F in order to maintain a preferred environment temperature.

These are the data which i missed out on the last post,

6) What is the design heat rejection rate per tower? What is the chiller condenser heat duty?
I am unable to obtain the design heat rejection for the cooling tower as the manuals are missing. Is it still possible to conduct the project by making an assumption on this value?
Is the chiller condenser heat duty the heat duty the cooling capacity of the chiller? If it is, then the heat duty is around 500-550 tons.

8) … What type and size of condenser do you have on each chiller?
The condenser is made of copper, tube diameter is 1 inch, and the condenser inside diameter is 2’ and 1 ½” or (3060mm)


Would these data suffice? Or did I missed out on anything?

Anyway, your guidance is really appreciated.

Regards,
Fred ooi

[Art Montemayor]

Fred:

Attached you will find an example of how to calculate your chiller performance and arrive at a cooling water requirement for each of your chillers. I have done this quite hurriedly and I apologize if the calculations are not organized as well as they could be. You will note that I have purposely left the basic design input data in unlocked, Yellow cells. You will also note that from the basic data you’ve furnished and from what I’ve taken from the Trane website, your chillers can barely generate 400 Tons of refrigeration (TR) when you are circulating 1,600 gpm of cooling water. That means if you maintain the same water temperatures around the condenser, you will not be able to obtain the 550 TR that you claim the chillers can put out.

Bear in mind that you stated in your opening post that you want to “optimize an under-utilized cooling tower”. Therefore, I assume you don’t need the 500+ TR and can settle for much less. This will take a bit of load off of your cooling water tower(s). There several reasons I have taken this method to show you the manner of “optimizing” your chiller assignment:

1. I want to use this method to show you – as well as all other Chem E students on this forum - the type and quality of calculations that are going to be expected of you when you undertake your first industrial job. You will note the use of sketch(es), attached references, equations, project identity, dates, Revision Number, and ample definition and explanation of the logic employed and the equations used. This is the type and quality of documentation that is required as a minimum when you are a professional engineer.

2. I want to emphasize the technique of how you can employ detailed, reference data by using Excel workbooks to contain your project. By attaching your reference data, you allow the reader facile access to the data employed and speed up the reader’s process of confirming the results and accepting your logical answer. The easier you make it for your peers to check and confirm your work, the better off you will be – on all fronts.

3. Note that I have not worked out your problem for you. I have shown you the way (by example) to use present engineering techniques to document your solution. If, upon reviewing my calculations, you find it difficult to understand how I came up with one of the answers then you have a serious problem of not being prepared to confront this Thermodynamic process and its industrial application. I chose to respond to your query because I consider you more fortunate than most other engineering students striving to obtain their degrees in Chemical Engineering. This industrial problem you have confronting you is a perfect example of what you must resolve successfully if you are to go on to a rich and rewarding career in engineering. This real-life problem teaches you that you do not become an engineer by going to a university. You become a professional engineer after the university has taught you how to learn to become one. Your industrial experience will teach you how to become an engineer – if you are willing to study and look for the actual experience in real projects and challenges.

4. Look carefully at the presentation of data. Some of the basic data that you gave was erroneous and the Thermodynamic values from NIST confirm this. When you are working on industrial problems you do not have a professor checking your work. If you are not lucky you will have no one checking your work. For that reason, you cannot afford to make mistakes in establishing basic data. Such mistakes will lead to the totally wrong answer and you cannot afford to make such costly errors. Always confirm your basic data. Always seek to obtain peer checks – which is, in reality, what I’m doing for you.

5. Study carefully the basis, the scope, and the nature of the application given you. In this case, you have verified what most of us pros already know: you have a refrigeration cycle application problem and it will involve basic thermodynamics and its application if you are to resolve what is needed to operate efficiently and at low costs of operation. Note that the calculations do not give you a Primer or an orientation on refrigeration thermodynamics. You are expected to already have this knowledge under your belt. If you don’t, then it is time to return to basics and start studying the application of the basic thermodynamics. It is essential that you identify and accurately calculate the required cooling capacity expected from the cooling towers. Without this information, you are locked at a standstill and can go no further in resolving how best to control the cooling water towers.

6. After starting this thread with deficient basic data, you have returned to cooperate and furnish more details and data – plus a description of what basically is being asked of you. This is commendable on your part and I applaud such a positive attitude. It shows you recognized that you were faulty in your communications initially, but have since corrected your method of communicating with others. This type of self-recognition will aid you tremendously if you continue to apply it in the future.

7. Your total scope also involves process analysis and calculations on the cooling tower(s) and that portion will involve simultaneous heat and mass transfer – another challenging technical exercise. I presume you are also ready to tackle that part. But you cannot initiate that portion until you clearly identify the range of cooling loads that will be imposed on the tower(s) by the chiller condenser(s). The identification of the design Wet Bulb temperature will go a long way in helping you arrive at a reasonable estimate on what to expect from you cooling tower(s). A check with your local airport or meteorological government office may furnish you with this information. Try to obtain the local design wet bulb value – not just one or a few random ones.

I hope the workbook contributes to your organizing a work plan and also identifying all the basic parameters that you need to arrive at a recommended method of controlling the operation of the cooling water towers. If you (or any other reader) have any comments or critiques on the calculations and the logic employed, please do not hesitate to make them known. I sincerely hope that that this thread attracts some of our more talented and experienced members and their comments and critiques are graciously welcomed. I am looking forward to some of our members to add valuable comments and recommendations to your request in order to have a thread that is fruitful and profitable to all students participating in this forum.

Remember that I earlier stated that you were welcomed to the real world? Hopefully, you may now be getting the flavor of how real-world engineering problems are resolved. They require you accurately describe and communicate your data and your needs. They also involve you working with others to obtain data and/or information. Note that although I didn’t know the interstage pressures, I assumed some basic data and calculated the required interstage pressures in order to arrive at a complete description of what is generating the total heat load on the cooling water tower(s). Since we are no longer in a class room and there is no text book associated with the problem, we can’t expect others to give us all the basic data we require to resolve the problem. We have to go out and get it ourselves. I presume you will cross-check and confirm my interstage figures with actual field data obtained from the actual machines.

Good Luck!


 R_134a_Chiller.xls   285KB   250 downloads