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  1. #14
    Join Date
    Sep 2002
    Posts
    12,715

    I'm sorry - I didn't write what I meant

    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
    ------




    Quote Originally Posted by twophase View Post
    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.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  2. #15
    Join Date
    Aug 2011
    Location
    Northern VA
    Posts
    38
    Quote Originally Posted by Poodle Head Mikey View Post
    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?

  3. #16
    Join Date
    Jun 2003
    Location
    Chicago, IL
    Posts
    4,490
    Quote Originally Posted by twophase View Post
    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?

  4. #17
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,428
    Is fabricating your own 500 ml "bottle" from copper within the rules?

  5. #18
    Join Date
    Aug 2011
    Location
    Northern VA
    Posts
    38
    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.

  6. #19
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,428
    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.

  7. #20
    Join Date
    Jun 2003
    Location
    Chicago, IL
    Posts
    4,490
    Quote Originally Posted by twophase View Post
    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

  8. #21
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,066
    It may sound crazy.

    But are you allowed to boil the bottle of water?

    It has something to do with the boiled water displacing oxygen molecules. the displaced oxygen will allow the water to freeze faster than room temperature water.

    It's called the Mpemba effect.

    The agitated bottled water experiencing the Mpemba effect with the swirl of brine around it should drop temp nicely.

  9. #22
    Join Date
    Mar 2001
    Posts
    1,499
    Calcium chloride available through concrete companies
    One way to outthink people is to make them think you think. They'll think you're not really thinking what you're trying to get them to think you think...........

  10. #23
    Join Date
    Feb 2012
    Location
    Toronto Canada
    Posts
    1,090
    What type of schooling is this project for? This is to get a refridgeration lic or cert ??

  11. #24
    Join Date
    Aug 2011
    Location
    Northern VA
    Posts
    38
    No, it's junior year undergrad engineering.

    The official test bottles are provided on testing day, so no chance of boiling. I'll have to see what kind of salt I can get my hands on and test to see what works the best.
    Thanks for the ideas.

  12. #25
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,428
    Quote Originally Posted by twophase View Post
    No, it's junior year undergrad engineering.

    The official test bottles are provided on testing day, so no chance of boiling. I'll have to see what kind of salt I can get my hands on and test to see what works the best.
    Thanks for the ideas.
    http://www.walmart.com/ip/Morton-Ice...-4-lb/10318347

  13. #26
    Join Date
    Aug 2011
    Location
    Northern VA
    Posts
    38
    Thanks for that. I found some at the grocery store. Didn't know there was such a thing. It doesn't say what kind of salt it is, though.

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