Need refrigerant selection advice for custom chiller project, to freeze water

[twophase]

Hey guys, I'm in the process of fleshing out some design details for a liquid chiller project for a thermal systems design course at school and am wondering if anyone has some advice for what refrigerant would be practical.

We get 7 min to cool a 500 mL bottle of water as much as possible using a bucket of ice water with between 4 and 8 lb of ice. I'm working on determining what sort of system will be able to completely freeze the bottle before 7 min so as to reach a final temp lower than freezing. My preliminary calculations show I'm looking for at least an 800W system with an evaporator temp probably between -20 and -30 degrees C in a glycol liquid bath. The condensor will be a shell and tube HX with the water from the icebucket circulated through it, hence very cold condensor temps.

So my question is, would R-22 be a practical choice in this case? Is there another non exotic refrigerant that would work better? I'm trying to find a solution that does not require a huge compressor (R-134a and R-152a have been ruled out in favor of R-22 so far for this reason, going by the research I've done so far). I'm also trying to limit it to any relatively common refrigerants to keep cost and complexity down (primarily sourcing major components from scrap equipment/ebay, meaning selection is limited to mostly common stuff, plus whatever else that common stuff could be made to work with, like what refrigerants have a chance of working well in an R-22 compressor).

I do have my 608 type 1 certification, so legality is squared away.

Thanks in advance!

[beenthere]

Move to Refrigeration forum.

[craig1]

Are you sure using a mechanical refrigeration system is within he rules of the project? It doesn't seem like it would be. Even if the condenser is cooled by the ice water it still seems like cheating to me.

I would take ice cubes, shred them in a blender with a strong salt mixture and stick your 500 ml bottle in that. It will easily freeze it. And don't use table salt, use calcium chloride ice melter from the hardware store. You can get temps of -32C with ice and CaCl.



If you're into bending the rules, did they say ice must be of the h20 variety? If not, get 5 lbs of dry ice chips, stick your water bottle in that and you'll win for sure.

[VTP99]

LIQUID HELIUM

[twophase]

The only rules are the bottle and the ice must be at least 5 ft apart and everything except the ice must be within 2 degrees C of ambient at the start, meaning no dry ice. The instructor said mechanical refrigeration is within the rules.

I did not realize that ice melt could get the liquid that cold. I may have to do some testing on that. There's about 1500W of cooling power over the course of 7 min in 4 lb of ice assuming all of the ice is melted at the end. The downside to the ice melt system is it won't be as exciting to design and build, but we wouldn't lose in the cost aspect, but that's only worth 5 points out of 100. Final temp is worth 10.

But back to the original question...would R-22 function well at those temps?

[Phase Loss]

Call me when you start chilling beer.

[craig1]

But back to the original question...would R-22 function well at those temps?

Yes, very well.

Since you would be running extremely low condensing pressures due to the ice bath condenser and long term durability is not a concern, you could use r-22 in an r-134a compressor.

You can strip the compressor from a used fridge and the r-22 will give you way more cooling power than the compressor was rated for.

[Poodle Head Mikey]

So if you used 404 you could use a smaller compressor. Just dump out the oil and refill with POE.

Why are you using the S&T condenser? Do you already have it?

What is the metering device?

(ice water condenser water and) "very cold condenser temps" are not going to cut it. How are you going to create the TD across the orifice?

Can the 500ml of water be in a container of any shape? Or is it to be in a fixed size/shape bottle? I ask because a part of your challenge is going to be surface area / heat exchange.

BTW: was the 500ml size chosen because it's one pound? Who came up with the rules of this test?

If you can do whatever you want why not set a bottle of liquid CO2 in the bucket of ice water and the 500ml bottle of water into another bucket - and then blast CO2 into the second bucket? A total-loss refrigeration system.

PHM
------

Hey guys, I'm in the process of fleshing out some design details for a liquid chiller project for a thermal systems design course at school and am wondering if anyone has some advice for what refrigerant would be practical.

We get 7 min to cool a 500 mL bottle of water as much as possible using a bucket of ice water with between 4 and 8 lb of ice. I'm working on determining what sort of system will be able to completely freeze the bottle before 7 min so as to reach a final temp lower than freezing. My preliminary calculations show I'm looking for at least an 800W system with an evaporator temp probably between -20 and -30 degrees C in a glycol liquid bath. The condensor will be a shell and tube HX with the water from the icebucket circulated through it, hence very cold condensor temps.

So my question is, would R-22 be a practical choice in this case? Is there another non exotic refrigerant that would work better? I'm trying to find a solution that does not require a huge compressor (R-134a and R-152a have been ruled out in favor of R-22 so far for this reason, going by the research I've done so far). I'm also trying to limit it to any relatively common refrigerants to keep cost and complexity down (primarily sourcing major components from scrap equipment/ebay, meaning selection is limited to mostly common stuff, plus whatever else that common stuff could be made to work with, like what refrigerants have a chance of working well in an R-22 compressor).

I do have my 608 type 1 certification, so legality is squared away.

Thanks in advance!

[twophase]

Yes, very well.

Since you would be running extremely low condensing pressures due to the ice bath condenser and long term durability is not a concern, you could use r-22 in an r-134a compressor.

You can strip the compressor from a used fridge and the r-22 will give you way more cooling power than the compressor was rated for.

Yeah, I thought about that. I have a recip. compressor from an r-134 fridge, but it's rated for only 600 btu, but that is at -20 deg C evap temps. Not sure if using r-22 or even r404 would make up the difference. I'm also not sure yet if any additional capacity can be gained by running the condensor cold, since pretty much all of these size compressors are rated for air cooled condensor temps. Running r22 would be easier since it uses non hydroscopic oil. I also have a 1100W and a 1300W r22 rotary compressor from dehumidifiers. I know the cooling power will decrease with decreasing evap temps, but how much and how an ice cooled condensor will mitigate that problem remains to be seen.


@PHM

I'm not decided on the metering device yet. I understand the working principles of cap tube/txv but don't have the practical knowledge to know the pitfalls of them in certain applications. I'd like to use a txv, thinking it will give better cooldown performance, better control/easier adjustability for evap temps, and more useful for other projects after this one is done. Problem is finding one cheap on ebay that is small enough and knowing how it will work with my system, since there's probably a snowball's chance in hell of finding a 1/4 ton low temp r22 txv just chilling on ebay. Cap tube is cheap but finding the best size/length might be a bit tricky, and it will only be at optimum performance within a relatively narrow operating range.

I picked shell and tube because it would be easy to fabricate from scratch. We cannot use any pre made hx. It would be a crude setup consisting of a design determined length of 1/4" soft copper formed in flat parallel loops inside of a pvc pipe. A pump will circulate the water through the pipe. The evaporator will be a coiled tube to fit around the bottle of water, submerged in a glycol bath with a circulating pump to increase the convection coefficient. The glycol will be made to swirl in the hopes of imparting random rotating motion to the bottle in order to increase the convection inside as much as possible. The team members must be at least 3ft away during testing, meaning the system must be completely self sufficient. We must use the bottled water as it comes (dasani to be exact) and are not allowed to open it until testing is done. The instructor made the rules. The surface area and internal convection coefficient is definitely the limiting factor. A large enough temperature difference will be required to compensate.

ice water condenser water and) "very cold condenser temps" are not going to cut it. H you going to create the TD across the orifice?

I'm not sure I understand your question. A low condensor pressure would make less work for the compressor in maintaining -20 or -30 deg C evap temps, but the metering restriction would have to be reduced accordingly to work with the lower than typical delta P. Is there something I am missing?

R404 might work pretty well in a r134 compressor, since it would increase its capacity more than r22, and POE is already the oil of choice for r134 IIRC. The biggest problem for me really is obtaining refrigerants in much less than 25-30 lb quantities. I'd only need no more than a pound of chosen refrigerant. I guess swallowing the cost of a full cyl isn't too bad if I can find a deal online/ebay, since I have other personal projects planned this summer. Plus, if I buy a whole cyl of r22 now, I can sell the rest when it gets really expensive.

Realize that this is more about me wanting to learn more about refrigeration system design (and crushing the competition on test day ). There are much easier ways to perform sufficiently on this project to get an A. It is graded as a competition and it's college kids in undergrad engineering, lol...nobody else really cares that much and probably won't be putting a whole lot of effort and advance planning into it.


Thanks everyone for the replies!

[Poodle Head Mikey]

I don't know if you are missing anything but the TXV (or any other metering device) will require a certain minimum TD across it in order to control the refrigerant flow. Typically that is 100 lbs. for a TXV. An ice water cooled condenser just sounds like it might have the head pressure running too low - especially in a crude system like this one.

PHM
------




[QUOTE=twophase;12721621] . . . . I'm not sure I understand your question. A low condensor pressure would make less work for the compressor in maintaining -20 or -30 deg C evap temps, but the metering restriction would have to be reduced accordingly to work with the lower than typical delta P. Is there something I am missing? . . .

[icemeister]

While I commend you on your determination to create a refrigeration system to accomplish this task, I think doing so will over-complicate matters far beyond your knowledge and abilities.

The target here is to transfer as much heat as possible heat from this 500 ml bottle to another location...which should somehow involve 4-8 lbs of ice if I read the rules properly...all within a time span of 7 minutes.

This implies there must be a heat transfer medium of some sort between the water bottle and the ice.

I suggest you initially consider how to maximize your rate of heat transfer from the bottle to the heat transfer medium. Obviously, the greater the temperature difference the greater the heat transfer rate, but you need to also think about heat transfer coefficients, turbulent flow and the like to help maximize the rate.

Personally, I would be looking very closely at the idea of adding salt as previously mentioned. Do you know that a 3:1 mix of ice and NaCl can produce a -20ºF brine temperature? (Think home-made, hand-cranked ice cream)

Do you know what the 500 ml bottle is made of? Glass, plastic or metal maybe? Is there a standard bottle for this competition or is 500 ml the only stipulation? Is the ice in a block form, cubes or perhaps crushed? I see a lot of possibilities to skin this cat.

[Poodle Head Mikey]

Just for some rough perspective: an R-22 air conditioning compressor running at a 25 lb. suction will pump about 40% of it's rated capacity.

You're going to need some fire power to do what you want. Off the top of my head I am thinking 1HP or more to run a -35º F. suction and get all the heat out in seven minutes.

Hell; just the pulldown will take more than a minute - have you considered the the insulating value of the building ice layer? The thermal mass of all the components?

PHM
------

Yeah, I thought about that. I have a recip. compressor from an r-134 fridge, but it's rated for only 600 btu, but that is at -20 deg C evap temps. Not sure if using r-22 or even r404 would make up the difference. I'm also not sure yet if any additional capacity can be gained by running the condensor cold, since pretty much all of these size compressors are rated for air cooled condensor temps. Running r22 would be easier since it uses non hydroscopic oil. I also have a 1100W and a 1300W r22 rotary compressor from dehumidifiers. I know the cooling power will decrease with decreasing evap temps, but how much and how an ice cooled condensor will mitigate that problem remains to be seen.


@PHM

I'm not decided on the metering device yet. I understand the working principles of cap tube/txv but don't have the practical knowledge to know the pitfalls of them in certain applications. I'd like to use a txv, thinking it will give better cooldown performance, better control/easier adjustability for evap temps, and more useful for other projects after this one is done. Problem is finding one cheap on ebay that is small enough and knowing how it will work with my system, since there's probably a snowball's chance in hell of finding a 1/4 ton low temp r22 txv just chilling on ebay. Cap tube is cheap but finding the best size/length might be a bit tricky, and it will only be at optimum performance within a relatively narrow operating range.

I picked shell and tube because it would be easy to fabricate from scratch. We cannot use any pre made hx. It would be a crude setup consisting of a design determined length of 1/4" soft copper formed in flat parallel loops inside of a pvc pipe. A pump will circulate the water through the pipe. The evaporator will be a coiled tube to fit around the bottle of water, submerged in a glycol bath with a circulating pump to increase the convection coefficient. The glycol will be made to swirl in the hopes of imparting random rotating motion to the bottle in order to increase the convection inside as much as possible. The team members must be at least 3ft away during testing, meaning the system must be completely self sufficient. We must use the bottled water as it comes (dasani to be exact) and are not allowed to open it until testing is done. The instructor made the rules. The surface area and internal convection coefficient is definitely the limiting factor. A large enough temperature difference will be required to compensate.


I'm not sure I understand your question. A low condensor pressure would make less work for the compressor in maintaining -20 or -30 deg C evap temps, but the metering restriction would have to be reduced accordingly to work with the lower than typical delta P. Is there something I am missing?

R404 might work pretty well in a r134 compressor, since it would increase its capacity more than r22, and POE is already the oil of choice for r134 IIRC. The biggest problem for me really is obtaining refrigerants in much less than 25-30 lb quantities. I'd only need no more than a pound of chosen refrigerant. I guess swallowing the cost of a full cyl isn't too bad if I can find a deal online/ebay, since I have other personal projects planned this summer. Plus, if I buy a whole cyl of r22 now, I can sell the rest when it gets really expensive.

Realize that this is more about me wanting to learn more about refrigeration system design (and crushing the competition on test day ). There are much easier ways to perform sufficiently on this project to get an A. It is graded as a competition and it's college kids in undergrad engineering, lol...nobody else really cares that much and probably won't be putting a whole lot of effort and advance planning into it.


Thanks everyone for the replies!

[twophase]

I don't know if you are missing anything but the TXV (or any other metering device) will require a certain minimum TD across it in order to control the refrigerant flow. Typically that is 100 lbs. for a TXV. An ice water cooled condenser just sounds like it might have the head pressure running too low - especially in a crude system like this one.

PHM
------
. . .

interesting. I was experimenting with a small r134 system I scrapped from a mini fridge (without losing the charge, total PITA...urethane insulation everywhere!), and it behaved like it was low on charge (frost line shrinks back on evap and temp drops) when the condensor coils were submerged in ice water. I don't have any service ports on it yet, so wasn't able to find out what would happen if gas was added.

Oh sorry, but what does TD stand for?

But yeah, it kind of looks like I'll be pumping salt water for this project if those kind of temps are possible. On the plus side, it will simplify the analysis, won't kill us on cost, and will leave more time for homework. I think the refrigeration system will take a bit too much testing and development to achieve in a month, and if someone else finds out that salt water works, we're screwed unless we build a really killer system.

Really it will now come down to simply minimizing cooled mass and maximizing insulation and liquid flow. The ice is just bagged ice from 7 eleven, so there's lots of surface area.

I do want to mock up a refrigeration circuit anyway just to learn more about it and what would have been possible. I'm a very hands on type and really like building stuff. Moulding a custom chiller vat out of great stuff only goes so far, lol.

[Poodle Head Mikey]

I should have written Pressure Drop.

The refrigerant flow moves through the metering device at a rate based on the size of the orifice and the difference in pressure across it. As the Pressure Drop reduces - so does the flow rate. This reduces the refrigerating effect and is why head pressure must be controlled to maintain a certain minimum Pressure Difference. Typically you would be OK using 100 lbs. as a minimum number.

Sorry about that - I am becoming a dullard apparently.

PHM
------

interesting. I was experimenting with a small r134 system I scrapped from a mini fridge (without losing the charge, total PITA...urethane insulation everywhere!), and it behaved like it was low on charge (frost line shrinks back on evap and temp drops) when the condensor coils were submerged in ice water. I don't have any service ports on it yet, so wasn't able to find out what would happen if gas was added.

Oh sorry, but what does TD stand for?

But yeah, it kind of looks like I'll be pumping salt water for this project if those kind of temps are possible. On the plus side, it will simplify the analysis, won't kill us on cost, and will leave more time for homework. I think the refrigeration system will take a bit too much testing and development to achieve in a month, and if someone else finds out that salt water works, we're screwed unless we build a really killer system.

Really it will now come down to simply minimizing cooled mass and maximizing insulation and liquid flow. The ice is just bagged ice from 7 eleven, so there's lots of surface area.

I do want to mock up a refrigeration circuit anyway just to learn more about it and what would have been possible. I'm a very hands on type and really like building stuff. Moulding a custom chiller vat out of great stuff only goes so far, lol.

[twophase]

I should have written Pressure Drop.

The refrigerant flow moves through the metering device at a rate based on the size of the orifice and the difference in pressure across it. As the Pressure Drop reduces - so does the flow rate. This reduces the refrigerating effect and is why head pressure must be controlled to maintain a certain minimum Pressure Difference. Typically you would be OK using 100 lbs. as a minimum number.

Sorry about that - I am becoming a dullard apparently.

PHM
------

Ok, thanks. The question stirring around in my mind now is why can't you just shorten the cap tube/widen the orifice to restore flow to suit the lower pressure difference? Basicly tune the system for the desired evap temp for a given pressure drop? Does the refrigeration cycle just not work properly without enough pressure difference?

Seems to me that the flow rate would reduce with a reducion in pressure drop if the case was trying to empty out a refrigerant tank passively, but the flow rate, at least in the academic sense of the term, for a pumped fluid is more a function of restriction and the pump (compressor) flow rate versus head pressure curve, right? Reduced restriction should actually increase flow, right? Maybe 50 psi just isn't enough to make the refrigerant change phase correctly?

[craig1]

if someone else finds out that salt water works, we're screwed unless we build a really killer system.

I can guarantee there will be teams using salt. If youre lucky they will be using table salt (NaCl) which had a higher brine temperature than alternatives such as CaCl. You may be able to find somthing with even better ice melting properties. Your biggest challenge is going to be transferring heat through the bottle, especially since Dasani bottles are relatively thick and they are smooth compared to the ridges on other brands that increase surface area. Definitly do something to agitate the bottle to make some circulation inside. Can you remove the label?

[icemeister]

Is fabricating your own 500 ml "bottle" from copper within the rules?

[twophase]

No, the bottle must be exactly as it comes from the store, except maybe the label. I'll have to ask about that.
I'll
Unfortunately, now that it occurs to me, I don't know if i'll be able to find ice melt now that it's so warm out. Probably nobody has it on the shelves anymore. I wonder if glycol automotive antifreeze would work the same way. Maybe epsom salts would work well, too. The circulation system will be designed for as high a flow as practical, and the inlet/outlet will be oriented to create a swirl to increase convection on the outside as well as to impart random rotational motion of the bottle to increase convection inside as well.

[icemeister]

You would benefit from rigging up an agitation system for the bottle...to create internal turbulence like a washing machine...for enhanced heat transfer.

With the bottle immersed in your circulating brine, suspend the bottle by the cap, spin it in one direction for a couple of seconds, then reverse and repeat.

That should be worth some points ...at least for creativity.

[craig1]

Unfortunately, now that it occurs to me, I don't know if i'll be able to find ice melt now that it's so warm out. Probably nobody has it on the shelves anymore. I wonder if glycol automotive antifreeze would work the same way. Maybe epsom salts would work well, too.

I don't think glycol would work well. It has a few properties make it not ideal for this application. The most serious (and unusual) is that its lowest freezing point is at 70% concentration. If you increase or decrease the concentration the freezing point rises quickly. Since melting ice will be continually diluting the solution you will have trouble maintaing a low freezing point. I'm also unsure of whether or not glycol is good at melting ice, or just preventing it from forming.

With salt, mixing the proper concentration is easy. Just put in ton of it. You want to add as much salt as the water will hold in solution, and then add a bunch more so you have undissolved salt circulating with the water. As the ice melts, additional salt will dissolve to keep the brine at the strongest possible level

If you can't find ice melt on at the hardware store, you could try calling up a snow plow company. Im sure they should have some lying around.

alternatively:

http://www.amazon.com/Pellets-CPP50-...1850330&sr=8-3

$60 for 50LBS with free shipping