Help for a university refrigeration project

I was wondering if any of you would be able to help me with a problem.

For my university course I have to complete a project for an undisclosable company, where I need to design or specify a secondary refrigeration system.

The primary system which needs to be cooled has a working fluid of demineralised water at 45 deg C, this demineralised water then needs to be cooled to 30 deg C.

I was wondering if there was an off the shelf self contained refrigeration solution to this problem, instead of having to design and specify each individual part of the new system myself.

Any help would be much appreciated.

I suspect the whole point of the exercise is for you to learn how to "design and specify each individual part of the new system" yourself. Have fun. :)

For the design temperatures you have given, I would go as far as saying that you wouldn't require a refrigeration system.
Here in UK a drycooler (heat exchanger with ambient air blown across it) would do the job 340 days a year.
For the remaining 25 days, wet sprayers could be added to the design, spraying a fine mist of water across the drycooler on hot days.

To design these drycoolers you will need to know the total heat-load on the system and the fluid flow as well as the temps.

Here you will find more information (and also people able to assist you with the design)
P.S. Personally, I never dealt with these people, so I have no experience of what they or their products are like.

Hi, Gary :)

Are you sure:confused:

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Originally Posted by Gary

I suspect the whole point of the exercise is for you to learn how to "design and specify each individual part of the new system" yourself. Have fun. :)

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nowadays "engineers" are only looking how to buy (cheap) and sell (expensive) ;) that is complete exercise....forget about individual training like learning....

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Originally Posted by uni_student

I was wondering if there was an off the shelf self contained refrigeration solution to this problem, instead of having to design and specify each individual part of the new system myself.

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I am in doubt...not sounds like a student:p

....but maybe I am wrong:rolleyes:

Best regards, Josip:)

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Originally Posted by Josip

Hi, Gary :)

Are you sure:confused:



nowadays "engineers" are only looking how to buy (cheap) and sell (expensive) ;) that is complete exercise....forget about individual training like learning....



I am in doubt...not sounds like a student:p

....but maybe I am wrong:rolleyes:

Best regards, Josip:)

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I don't want you all to think I'm cheating by doing this at all, we're supposed to be acting as design consultants for the company so ideally they would like to know an off the shelf solution, instead of having to have a unique system custom built.

The nature of the project is to experience a real life industry project whereby the results can be passed on to our contact company's clients meaning referring them to a ready built system would be more useful for them.

The design must be installed in a fully enclosed room, so after doing the primary analysis of the problem we have concluded a refrigeration system would probably be best to avoid large amounts of air being moved around in the rooms.

I was hoping to find a single self contained refrigeration unit where we can attach the pipes of the demineralised water system to the front, and the heat is removed from the water and rejected from the fridge into the room, with the cooled demineralised water then flowing back out of the fridge and back to the source. Do these exist or would I need to implement a collection of components such as a pump and evaporator into a self designed system?

Hi, uni_student :)

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Originally Posted by uni_student

I don't want you all to think I'm cheating by doing this at all, we're supposed to be acting as design consultants for the company so ideally they would like to know an off the shelf solution, instead of having to have a unique system custom built.

The nature of the project is to experience a real life industry project whereby the results can be passed on to our contact company's clients meaning referring them to a ready built system would be more useful for them.

The design must be installed in a fully enclosed room, so after doing the primary analysis of the problem we have concluded a refrigeration system would probably be best to avoid large amounts of air being moved around in the rooms.

I was hoping to find a single self contained refrigeration unit where we can attach the pipes of the demineralised water system to the front, and the heat is removed from the water and rejected from the fridge into the room, with the cooled demineralised water then flowing back out of the fridge and back to the source. Do these exist or would I need to implement a collection of components such as a pump and evaporator into a self designed system?

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I'm sorry was not my intention to be rude to you, hope this will be of some help to your project;)

http://www.sabroe.dk/products/standa...-chillers.html

Best regards, Josip :)

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Originally Posted by uni_student

The design must be installed in a fully enclosed room, so after doing the primary analysis of the problem we have concluded a refrigeration system would probably be best to avoid large amounts of air being moved around in the rooms.

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As The Viking pointed out, the best solution would be a dry cooler. The air could be ducted in and out.

If you are using a refrigeration system, then the condenser would need to be outdoors OR ducted to the outdoors OR use a water cooled condenser.

If the water cooled condenser is hooked up to a tower, then you might just as well have the dry cooler.

Which pretty much narrows it down to a water cooled condenser using domestic water and dumping the discharge down the drain.

But then, why not eliminate the refrigeration system and just use a heat exchanger with domestic water and dump the discharge down the drain?

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Originally Posted by uni_student

I was hoping to find a single self contained refrigeration unit where we can attach the pipes of the demineralised water system to the front, and the heat is removed from the water and rejected from the fridge into the room, with the cooled demineralised water then flowing back out of the fridge and back to the source.

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You need to rethink this. The heat is being rejected into the room. The room is going to get very hot and the system is going to get a hernia and stop working... perhaps permanently.

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Originally Posted by Gary

You need to rethink this. The heat is being rejected into the room. The room is going to get very hot and the system is going to get a hernia and stop working... perhaps permanently.

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Hernia, nice description Gary!:D

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Originally Posted by Gary

You need to rethink this. The heat is being rejected into the room. The room is going to get very hot and the system is going to get a hernia and stop working... perhaps permanently.

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That's exactly what we told our contact in the company we're doing the project for.

He said it would be best if we could try and find a solution where we transfer the heat away from the demineralised water and leave it inside the room, however if this is not possible (which it clearly isn't) then a small external outlet may possibly be installed if sufficient evidence for it is given.

From this information and other poorly worded answers from our contact, we have deduced they are only happy to have an extraction fan installed into the walls of the cooling rooms meaning large inlet and outlet ducts are not possible.

Does this mean our original idea of a refrigeration system would still be incompatable?

Hi, Gary :)

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Originally Posted by Gary

You need to rethink this. The heat is being rejected into the room. The room is going to get very hot and the system is going to get a hernia and stop working... perhaps permanently.

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good point...due to my "excellent" English :o, I miss that....

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Originally Posted by uni_student

attach the pipes of the demineralised water system to the front, and the heat is removed from the water and rejected from the fridge into the room, with the cooled demineralised water

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how big is that room....maybe like limbo:confused:....something is not ok with your design....the only one good sized room is mother nature...to reject heat

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Originally Posted by uni_student

The design must be installed in a fully enclosed room, so after doing the primary analysis of the problem we have concluded a refrigeration system would probably be best to avoid large amounts of air being moved around in the rooms.

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why you need to avoid movement of air around:confused:

....please, uni_student can you explain a little more your idea

Best regards, Josip :)

I'm not sure I understand this. Is he wanting to heat the room with this system... presumably during the winter? Is his company located at the north pole? Do reindeer drink demineralized water?

Going back to the air cooler, you would only need to provide holes in the walls for the pipework as the cooler would be positioned outside.

No heat added to the room and no airflow within the room.

The provision of refrigeration plant would appear to be a waste of energy which must be taken into account when designing systems nowadays. Include the sums to demonstrate the financial gains of such a scheme to show that you have thought about the matter.

If you insist on using a packaged machine then a simple search via Google will bring plenty of results such as...

http://www.industrialcooling.co.uk/chillers.htm

Mr Student,

Can you give us some more info.

From what you said about rejecting the heat in to the room, the picture I got in my head of this system has somewhat changed.

First, we do need to know the total heat-load.

Then, what it's intended to cool.
From your second post I get the feeling that this is to cool some piece of machinery, like a plastic moulding machine standing in the middle of a production area......

Hi, uni_student :)

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Originally Posted by uni_student

That's exactly what we told our contact in the company we're doing the project for.

He said it would be best if we could try and find a solution where we transfer the heat away from the demineralised water and leave it inside the room, however if this is not possible (which it clearly isn't) then a small external outlet may possibly be installed if sufficient evidence for it is given.

From this information and other poorly worded answers from our contact, we have deduced they are only happy to have an extraction fan installed into the walls of the cooling rooms meaning large inlet and outlet ducts are not possible.

Does this mean our original idea of a refrigeration system would still be incompatable?

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sorry even with this explanation I cannot get a picture of cooling demand/solution:confused:


maybe I am too old for new cooling technologies;), but here we have some younger members which maybe understood all this...

Best regards, Josip :)

I think, as Viking said, without knowing the total heat load, this project, from our point of view (advice given etc) cannot proceed any further.

From the basic info we have so far, it appears to be a straight forward chiller application, unless of course, Uni_Student want to incorporate some sort of heat recovery into the system.

Dry air coolers have their use, but to attempt to control a secondary water loop constant load with one is surely a difficult task, given that their duty can swing considerably with ambient temperature.

These are all of the numbers we've been given for the project, about the current system and the heat exchangers which we need to replace:

DEMINERALISED WATER SYSTEM

Flowrate 12 kg/sec
Temp inlet / outlet 45 / 30 deg C
Pressure drop 25.12 kPa



SEA WATER SYSTEM (To be replaced with our new system)

Flowrate 25 kg/sec
Temp inlet / outlet 25 / 32 deg C
Pressure drop 97.64 kPa


These two systems currently interact via 4 counterflow heat exchangers where the heat is transferred from the demineralised water to the sea water.

Our task is to replace the sea water system and the heat exchangers which are currently in place, with a new system of our own design.

The heat exchangers are enclosed inside a building (which we have since been lead to believe is underground) so minimal outlets through the walls of the buildings are required.

Hmmm... an underground building by the sea. I can "sea" why they don't want to poke holes in the walls.

:cool: It would be nice of them to give you some idea of the original system, Probably defeats the object a bit but it could be upgraded, or totally re designed. It also makes me feel they are asking you to place your coil,(Outside Coil) underground using this to transfer heat, Any thoughts people?

Sparrow

Possibly it would help to know why the sea water system is being replaced.

The system itself is easy, but here's the problem: Where do we put the heat?

You object to using a dry cooler in the room because it would require lots of airflow... but an air cooled chiller would be shedding both the process heat and the heat from the chiller and would require at least as much airflow as the dry cooler.

Either way, the need for airflow would increase as the temperature in the room rises.

So where do we put the heat?

Possibly it would not be objectionable to simply transfer the heat to the Earth using a large underground loop.

A modulating bypass loop could be used to regulate the water temperature and a fan/coil unit (dry cooler) could be placed in that loop to be used if/when the room actually needs heat.

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Originally Posted by Gary

Possibly it would help to know why the sea water system is being replaced.

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The reason we've been told as to why the sea water system is being replaced is it doesn't allow enough control over the magnitude of cooling imposed on the demineralised water system.

This means when the demineralised water is not at its peak temperature, the sea water system can currently over-cool it to below the desired temperature range.

They need us to design a replacement system with a method of controlling the magnitude of cooling acting on the demin. water system.

From everyone's posts I take it a dry cooler could be the best solution for the problem. Can these be found in self contained units where pipes for the air and the working fluid from the primary system can be simply attached?

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Originally Posted by uni_student

The reason we've been told as to why the sea water system is being replaced is it doesn't allow enough control over the magnitude of cooling imposed on the demineralised water system.

This means when the demineralised water is not at its peak temperature, the sea water system can currently over-cool it to below the desired temperature range.

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This can be fixed by installing a simple temperature controlled bypass loop, bypassing part (or all) of the warm water around the heat exchangers.

Hmmm,

So we have a perfectly operating free cooling system that's coping well with the demands. In fact, I would go as far as saying that the current system has the best COP you will ever find, whatever you/they decide to replace it with will have higher running costs as well as higher maintenance costs.

As Gary said, the only thing they really need is better controls.
There are many options on how to achieve the control improvements needed:Bypass loop/buffer tank in secondary circuit+pump on-off on seawater side/speed controlled pump on the seawater side, to name but a few.

If they really insist on a refrigeration solution....
You are looking at a chiller with a capacity of 750kW, that's a really big radiator. Approximate 1100kW of heat to get rid of!!!
(In comparison, my house need ~9kW of heat on cold days and the largest warehouse heater I ever worked on had a capacity of about 150kW )
So the option of rejecting this heat in to the room......
What you would have to do is place the chiller, or at least it's condensor outside at ground level.
BUT (BIIIG BUT), you are at the sea. Plant installed at the sea side has a really short lifespan, a 750kW chiller would have a lot of air going through the condensor and all that air would be laden with salt, corrosion would be a major issue.

Exelent summarize by The Viking.

I have got the understanding that this is a sort of a learning project. If so the answer on what to learn is when a customer should be advised against investing in a refrigeration system. From what you have explained, the only advice i can give is for you to offer a better control system for the existing sea water cooling system.

Thanks to all of you for all of your help so far, I'll have a look into all the ideas you've suggested and see what I can come up with.

I would be grateful if you could carry on suggesting ideas because obviously the more ideas we have and the better we analyse the situation, the better mark we'll get for the module.


EDIT: I've just been reminded by one of my group members that the sea water system has to be replaced due to erosion in the piping. Hence the reason of wanting a complete new system with a new working fluid to avoid the current erosion problem.

In which case, replace the the whole of the seawater loop with "plastic" (polyethylene or similar) pipe.
(A long length of pipe coiled around a structure of treated wood would make an excellent heat rejector)
Replace the seawater circulation pump with a pump designed for seawater (Stainless?).
Replace the heat-exchanger(s) with new plate ones designed for seawater (Stainless?).EXAMPLE
Control this new seawater pump with an inverter to allow for varied flow depending on cooling demand (temperature of the secondary loop).

The above system would last almost forever, be almost free to run and require almost no maintenance.

EDIT:
Sorry, forget what I have said.
They do need a chiller installed above ground with the secondary loop extended to it.
(I just realised, system like the one I described would render me without a job)

I'm kinda fond of the underground loop idea. :)

Sorry guys,
I must have had a blond moment during my previous post.
(What can I say, my Absolut time has begun)

The system I described above would of course have a suitable heat transfer fluid in the primary loop (or even suitably treated fresh water).
Therefore there wouldn't be any need for the pump and heat exchanger to be able to operate on seawater!

Again, Sorry.
(Maybe I should stop posting on Saturday evenings)

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Originally Posted by The Viking

Sorry guys,
I must have had a blond moment during my previous post.
(What can I say, my Absolut time has begun)

The system I described above would of course have a suitable heat transfer fluid in the primary loop (or even suitably treated fresh water).
Therefore there wouldn't be any need for the pump and heat exchanger to be able to operate on seawater!

Again, Sorry.
(Maybe I should stop posting on Saturday evenings)

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I'm sorry but I'm not a thermodynamics expert so cannot really understand what you're trying to get at here.

Would you possibly be able to explain your new idea in greater detail please?

Thanks.

The first question is where to put the heat? We can transfer it to the ground (underground loop) we can transfer it to the air (dry cooler) or we can transfer it to the water (underwater loop).

Transferring it to the room air is a very bad idea. Apparently your customer thinks the heat can then be ejected from the room by simply exhausting air to the outdoors. If you exhaust air from a room you must replace it with fresh air or you have a vacuum. Its like sucking air out of a bottle. The air very quickly ceases to move. Air in must equal air out. So why not simply have a dry cooler in the room, ducting the air in and out?

Next, why use a secondary system when the primary fluid is suitable for transferring heat to the earth, air or water? Secondary systems are less efficient because the heat is transferred twice. Why add this unnecessary layer of heat transfer?

If for some reason unknown to us there must be a secondary system, we still need to ask, "Where do we put the heat?".

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Originally Posted by Gary

Next, why use a secondary system when the primary fluid is suitable for transferring heat to the earth, air or water? Secondary systems are less efficient because the heat is transferred twice. Why add this unnecessary layer of heat transfer?

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Thats a very good point. So do you suggest not bothering with a refrigeration system coupled with an air flow system, and just go straight for a dry cooler system?

Would a dry cooler system be able to cope with the cooling requirements of the problem, bearing in mind the flow rate of the demineralised water to be cooled is approximately 12 kg / sec and it the temperature needs to be reduced by 15 deg C?

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Originally Posted by uni_student

Thats a very good point. So do you suggest not bothering with a refrigeration system coupled with an air flow system, and just go straight for a dry cooler system?

Would a dry cooler system be able to cope with the cooling requirements of the problem, bearing in mind the flow rate of the demineralised water to be cooled is approximately 12 kg / sec and it the temperature needs to be reduced by 15 deg C?

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The answer is... it depends.

If the outdoor air at this unknown location rises above 30C/86F, then it isn't going to do the job on a hot day no matter how large the coil. A misting system would help overcome this problem IF the wet bulb temperature is sufficiently below 30C/86F.

Then there are the corrosion problems. Salt laden sea air is going to be going through the drycooler coil. Possibly there are suitable materials/coatings to overcome this problem. There are others on this site who are far more knowledgeable about such things.

The underground coil would seem the least susceptible to corrosion, but again there are others who are more knowledgeable in the area of underground coils and I would welcome their input.

What would probably make the most sense is to completely re-design the customer's current system with stainless pumps, plastic pipes, a control system that actually works and whatever else it takes to overcome the problems he is currently experiencing.

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Originally Posted by Gary

What would probably make the most sense is to completely re-design the customer's current system with stainless pumps, plastic pipes, a control system that actually works and whatever else it takes to overcome the problems he is currently experiencing.

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Yes, I fully agree with that.

My argument for keeping a primary loop in the sea is based on the fact that the secondary loop is filled with de-mineralised water, hence I assumed that it is feeding equipment that can't take seawater. Should something happen with the primary loop in the sea, there wouldn't be any cross contamination.

My personal preference would be a primary loop in the sea, as this could be cheapish to install (there already is something there, just to replace), no moving parts that could break, the sea will have limited temperature variations, and, It would be the cheapest to run.

U-S,
The only thing my post about heat transfer fluid clarified, is that if you utilise a sealed primary loop sunk in the sea, you wouldn't fill it with seawater....therefore there wouldn't be any requirements for the equipment to handle salt water.

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Originally Posted by uni_student

the flow rate of the demineralised water to be cooled is approximately 12 kg / sec and it the temperature needs to be reduced by 15 deg C?

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12kg/s with a delta t of 15 = 754.2kW - that needs to be a large dry cooler!

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Originally Posted by frank

12kg/s with a delta t of 15 = 754.2kW - that needs to be a large dry cooler!

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Just the fans would probably consume at least 30 kW at full speed. For comparison the 12 kg/s pump probably consumes some 2-3,5 kW at a reasonably head.

damn! I was going to suggest a groundloop

Thinking outside the square
Is there an oppertunity where the heat could be xfered to another process nearby? ie where any sort of heating is required a heatpump could be utilised. Scroll compressors perform well. Then options of saltwater or saltair - both are very corrosive, titanium heat exchangers are used at one seawater cooling plant I maintainted. To reject air through a small hole needs high temp and water/moisture content to reject vast heat. At Xmas I bought a portable a/c unit which rejected heat via 4" hose to window, I removed the covers and they pumped the condensate water over the condenser to increase efficeincy (hence size then cost).

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Originally Posted by frank

12kg/s with a delta t of 15 = 754.2kW - that needs to be a large dry cooler!

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For those on this side of the pond, 754 kW is somewhere in the neighborhood of 2.5 Million BTU. Perhaps there is a small city nearby that needs heating.

But wait... the customer wants all that heat dumped into an underground room... :eek: