Cooling water pumps

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Cooling water pumps

Postby Chub » Tue Feb 07, 2006 6:44 pm

For those of you who use pumps to reclaim the water in your coil what type seem to work best? I bought a small fish pump and got very little water on the return side. Went to a larger pump and get about 1/4-1/3 gal per min overturn. What should I be shooting for as far as an overturn? Thanks for the help Chub
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Postby agl » Wed Feb 08, 2006 2:54 am

The flow you will require has alot to do with your still design - and the heat input. I have a straight jacketed condenser which requires lots of flow to work properly (probably min flow would be about what your pump is doing). my reflux coil needs only a tiny ammount of water to put the still into total reflux. Mine uses a 1500W element. An offset head design still or one with a jacketed coil condenser would need less flow. My pump is a circulator pump from a hot water heating system it is dead quiet and I can get about 8-10 L/min (2+ gal/min) through the still- way more than I need. These pumps are a bit expensive though ~$80 Canadian new.
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Postby Guest » Wed Feb 08, 2006 11:53 am

Its an offset design It has about 25' of 1/4"copper for the coil I just wanted to make sure my flow is going to be ok. Right now I bench tested a small utility sub pump rated for 1000gph like I said I am getting about 1/4 to 1/3 gal per min I dont know if this restriction will burn the pump up? Chub
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Postby agl » Wed Feb 08, 2006 1:12 pm

Hi, it sounds like you have heaps of surface area so the efficiency of you condenser should be good. I did a quick calculation to determine the rough volume required:


if no heat is lost i.e. it all goes into making steam which must give up it's heat to the condenser- and you want a distillate temperature of 40ºC, and the room temperature is 20ºC.
heat flow(watts)=massflow(grams/sec)* specific heat*(delta T of cooling water)
If we assumed that the distillate and output coolant would be the same temperature- (which they would be if you had the bare minimum flow)
then you end up with a figure of 12 grams / second / KW
so if you used a 1500W heater then the minimum flow would be~ 1L/minute or about 1/4 gal/min. You would want a big overhead beyond this calculation, since you want to be certain that everything is condensed even if your condenser is less effective than 100%. - (there are lots of important factors to consider for that fudge factor- steam velocity turbulence surface area,etc.).

As for the pump and it's ability to withstand that much throttling, if it is submersible it should be o.k. - as long as it is an impeller pump not positive displacement. Maybe run it for a while and see if it gets hot?
agl
 

Postby possum » Wed Feb 08, 2006 1:46 pm

Agl, don't forget the latent heat, energy to convert from vapor to liquid without a change in temp.

Chub, by 1000gph do you mean 1000gallons/hour or grams/hour?
Is your pump a submersable pump or a sump pump?

Too powerfull a pump on too small diamater tubing gets too much resistance,and the pump can cavatate and shake ,eventually burning out.
Hey guys!!! Watch this.... OUCH!
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Postby Chub » Wed Feb 08, 2006 2:14 pm

Thanks guys for your willingness to try to help me on this. It is a submersable pump rated at 1000gal per hour @ a 5 foot head which is about what it is. I had a fish pond pump which just trickeled out that was a magnetic drive 190 gal per hour. This coil was made for another purpose before I even got into this hobby, and on previous coils just like this one I was using a 12volt rv pump rated at 3.4 gal per min and it seemed to work very well ;however the reason I did not want to use this pump was the inconvenience of running an inverter. My coil is not blocked in any way so I guess I am a little shocked at how little is making it thru. I connot be the only person who has a coil that is 25'. Is it that hooking it to a tap in a house is just more common? I will check the pump unrestricted to see if it is working properly. It does not chatter seems to be running smoothly. Thanks guys I hope I was able to give you guys the info you needed. Chub
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Postby agl » Thu Feb 09, 2006 7:44 am

possum, the latent heat is the main trasport of heat in the system- the heater element has to overcome this to generate the steam, and the condeser has to remove the same ammout to convert the steam into liquid. I bypassed the whole mess with the simplified equation by assuming that all the heat you put in with the element must be taken out by the condenser- and neglecting the details.
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Postby agl » Thu Feb 09, 2006 8:04 am

Chub,
The 1000gph - is at 5' head (2psi) so when you force through the 1/4 " tubing it must drop significantly more than 2psi. Different pumps are meant for different purposes and some with that high an output suffer more from a resriction than a low output one like an RV or aquarium pump for instance.
good luck!
agl
 

Postby possum » Thu Feb 09, 2006 9:35 am

Now I see what you are saying agl, heat in = heat out
or, after stuff is up to temp, power in = power out.
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Postby Guest » Thu Feb 09, 2006 3:33 pm

Yep,
heat in = heat out,
crude, but you can figure if you're in the ballpark of correct flow rate.
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Postby agl » Thu Feb 09, 2006 3:37 pm

whoops, forgot to log in- that was me
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Postby possum » Thu Feb 09, 2006 3:51 pm

Chub, here is a thread that discusses a very similar situation to what you have described.
There are several suggestions for rigging a pump that may be too strong for the narrow 1/4 inch coil.

Hope that this eliminates some of your trouble .
You work with compressors and heat exchangers right ?
There are a few others on this board that do similar work.
At least 2 guys that do HVAC.(not me)
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Postby norcal » Fri Feb 10, 2006 6:17 am

There is a formula somewhere that says for a certain size pipe @ x gal per min. add so much head per lineal foot. I can't find it right now. :( I'm sure it is on the web some where. I will look for all my notes later today. I know from experience that for 3/4in tubing @ 600-800gal per min you have to add like 3ft of head for every 10 lineal feet. I learned the hard way but that is an off topic disc.

Also, You can only fit so much water through that tube which I'm guessing is 1/4in O.D. At some point you will reach the maximum flow rate of the tubing.

My point is that 1/4in tubing is a hell of a lot more restrictive than 3/4(I know Duh :roll: stating the obvious) . If you think about it this way, you may be asking your pump to push more like 20' of head. Not to mention, through a little opening. Uphill both ways in the snow with no shoes on...
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Postby Guest » Sun Feb 12, 2006 4:54 pm

The little pump might still be ok. How high up above the pump is the condensor? It might just be a matter of mounting the pump above the condensor to go with gravity instead of against it???
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Postby Grayson_Stewart » Sun Feb 12, 2006 6:38 pm

Th e entire system may need to be raised depending on the pump. Simply raising the pump may not work if the suction head for that pump is not correct.
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Postby crackher » Thu Feb 16, 2006 1:23 pm

I am using a boat bilge pump...unfortunately it is 12V but for the price (14$CDN) you can't go wrong. but that said when i find an AC one for cheap I will make the upgrade
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