37 replies to this topic

[douggiestyle]

need help. i wish to make one of these out of an old frig. my refrigeration knowledge sucks.

http://www.marinedep...ne.asp?ast==

ive planed on using a 1/2" x 36" titanium tube for the evaporator. water flow over evaporator/heat exchanger will be apx 500 gph

problems frig runs off cap tube. evaporator is not sized to compressor. will plugging occur due to major conduction from cooling water in direct contact? if evaporator is two small is superheat a large problem? or am i worried over nothing? frig is fairly new and runs off 134a. btu rating is apx 5600 or 1/2 hp

most grateful and humble

doug

[exsearsguy]

douggie, I'm not trained as an engineer, but just from the figures you've given us,it looks like if your compressor runs 24 hrs a day, it'll pull about 11 btus per gallon per hour. If your setup works, that is. And I hope it will.Is that enough cooling for what you want to do?

[douggiestyle]

yeah im no engineer either. so here goes

the average water temperature will be apx 88 degrees f. the required temp is 82 degrees f. the total ammont of water is 220 gal.

 

where did you get your figures? is there some type of table?

[exsearsguy]

Douggie, remember I'm NOT an engineer. The way I figured is btu divided by gph. 5600 divided 500. 11 btu per gallon. Now, gph divided by total amount of water. 500 divided by 220. Roughly. 2 passes per gallon per hour. Apx 24 btu per hour. I would estimate 3 degrees cooling if your rig works. Doesn't sound like enough cooling to me.  Maybe Samurai can help you more than I. I believe he is better trained for this. By the way, why titanium instead of copper?

[douggiestyle]

ti does not corode in salt water. cu does.

dont know if the samurai can help in his pod cast he says he cant remember btu per s/f in room air conditioning, lol.

i never thought of using a formula like that. confuse my self with all those bizzar mollier charts.



the evaporator size would certainly play into it.

mostly i am concerned with plugging or superheating.

if you could, please explain to me plugging or superheating

[exsearsguy]

Let me see if I remember superheat. I believe that is the amount of heat that the evaporated  refrigerant picks up after all the liquid is gone. If you can, shoot for about 6 degrees as I recall.  Not exactly sure what you mean by plugging, but I'm guessing that you mean liquid  refrigerant getting  back to your compressor.NOT a good thing at all. Liquids do not compress!

[douggiestyle]

yeah 6/10 thats what ive read.

I dont think that i have a problem other than efficieny.

my biggest problem would be trying to get a larger ti tube at a reasonable cost.

thanks for the help ex sears guy.

though i would love to try and figure out the actual temp take down.

if you could find anything and post it, it would be greatly appreciated!

thanks again

doug

 

[exsearsguy]

You could use 2 tubes and parallel them could'nt you. I think you need a larger compressor than what you've got. Maybe you could find an old comp out of a window unit? 12, maybe 14000 btu.

[douggiestyle]

that is genius. thanks for thinking outside the box for me.

again i suck when it comes to refrigeration. so i will post a drawing.

maybe you can look at it and tell me what you think.

[Samurai Appliance Repair Man]

[user=437]exsearsguy[/user] wrote:

Let me see if I remember superheat. I believe that is the amount of heat that the evaporated  refrigerant picks up after all the liquid is gone.

That's right, it's represented by the region to the right of the Mollier curve. If the evaporator is sized too small, the risk is that you won't evaporate all the refrigerant and you'll slam in the compressor with liquid refrigerant; this is called "slugging."

An over-size evaporator will evaporate the refrigerant too soon and you'll end with the evaporator mostly filled with superheated vapor, which has zero or even a negative refrigeration effect.

You can calculate the heat transfer from the evaporator in both the forced air and water chilling applications. But I'll be damned if I remember how to do that, that was waaaay back in my undergrad heat transfer class, musta been 1982. I still have all my notes and text books, so if you're really interested in pursuing this, Doug, I'll be glad to blow the dust off of 'em and see what I can remember.

"Plugging," as I've learned about it, is a problem where moisture gets in teh system, or is allowed in through sloppy service practices, and creates a cyclic ice plug at the point in the evaporator where the capillary tube enters it.

[douggiestyle]

so sorry meant slugging.

so an undersized evaporator causes slugging.

not the fact that the evaporator is submerged in water?

[douggiestyle]

could i run two evaporators parallel. splitting the high pressure into two cap tubes. and the low back into one.

[Samurai Appliance Repair Man]

[user=1610]douggiestyle[/user] wrote:

so sorry meant slugging.

so an undersized evaporator causes slugging.

not the fact that the evaporator is submerged in water?

Water, air, the refrigerant and evaporator don't care about the medium, just the heat transfer rate. Heat will transfer between the evaporator and outside medium more efficiently in water that in air because the heat transfer coefficient is higher in water than air due to much lower boundary layer resistance. This means that the same evaporator used in air and then submerged in running water (assuming ambient temps are comparable) will experience a sudden increase in heat transfer rate and refrigerant will boil off sooner in the evaporator when submerged in running water. Slugging would not be a problem here; instead the problem would be excessive superheat.

If the evaporator were too small, from a heat transfer point of view, that would mean that the refrigerant wasn't exposed to the heat source long enough to completely boil off all the refrigerant. Since, in a capillary tube system, the next stop is the compressor, you're looking at refrigerant still in the liquid state slamming into the compressor at its suction port. This will fry a hermetic compressor in a heartbeat. In larger, industrial refrigeration systems, you'll often see an accumulator between the compressor and the evaporator just for this purpose because those big screw compressors (vs. the reciprocating piston compressors used in home refrigeration and air conditioning) can cost thousands of dollars to replace.

[exsearsguy]

I don't see why that wouldn't work.No, the evaporator under water shouldn't cause any problems as long as you have a SEALED SYSTEM.But if you have a leak in that evaporator you're gonna have trouble.  Does titanium braze to copper? I know I've never tried it.

[douggiestyle]

it might with special equipment, ti doesnt like to do much of anything except not corrode.

[douggiestyle]

[user=1610]douggiestyle[/user] wrote:

could i run two evaporators parallel. splitting the high pressure into two cap tubes. and the low back into one.

[exsearsguy]

Douggie, while you are designing your system,if you can, design a way for your cap tube(s) to be in contact with your suction line. Refrigerators don't have them together for no reason(or any other system,either) The cool from the suction line cools the liquid refrigerant from the condenser reducing flashing. You get more cooling from the same refrigerant.This is actually the opposite of superheat, called subcooling. Also, I guess if you're concerned enough about slugging,you could make an accumulator. What are you using for your condenser?

[douggiestyle]

what ever comes stock with the frig or ac will be used for the condenser.

[douggiestyle]

make an accumulator?

is that easily done?

[douggiestyle]

could i just skip the cap and switch to a txv.