Sciencemadness Discussion Board

Quick Pump Question - Promise to do a Write-Up for the Board afterwards.

oxybate - 29-5-2007 at 16:24

After endless reading, it's become clear (or perhaps I'm still in the dark - that is for you Rhodes scholars to decide) that for a typical vacuum distillation, two pumps are required (assuming one isn't so environmentally amoral as to squander a gajillion gallons of potable water):

1. One pump to circulate (cold) water through the condensor.

2. One pump to either (1) act as a direct vacuum upon the system, or (2) act as a means of circulating water through an aspirator whereby the Venturi effect provides the vacuum.

Although there are several posts that talk about torr, inches pulled, feet of head, this, that, and everything else - I can't for the life of me figure out the specs that will enable me to walk down to my local parts store and buy the pumps that are necessarry. Is there anyone here willing to simply state what is needed for each?

Considering that the condensor only requires a constant flow of cold water through what is, at most, 2-3 feet of distance, I imagine the pump required isn't a "strong" one (I say strong because, as said before, I lack the understanding/vocabulary to clarify that further!). So, without further ado, here are a set of questions that, if answered, will hopefully give me the information I need to make the purchases I need to make, and provide you the least amount of input necessary - for your convenience, of course.

1. For the condensor circulation: Will a submersible pond pump that pushes about 250 gallons per hour, suffice? Are there other variables besides a GPH spec that needs consideration?

2. For the aspirator setup (and I've chosen to construct a self-made "aspirator station" with recirculated water, unless one of you provides some compelling reasons - like a particularly great pump suggestion -- for a pump that may be used directly for a decent price), what are the minimum specs I should be looking at? Is it correcct of me to think that another Pond pump, with greater GPH specs can be used in such a setup?

Any and all information is greatly appreciated, and as the title suggests, I *will* do a nice write-up so the question never need be asked again. Ever.

Fleaker - 29-5-2007 at 19:47

I don't have time to answer all of your questions in detail, but I can tell you this straightaway: a pond pump is overkill for a condenser. We used a decent (30 gallon) aquarium pump for our condenser and it works just fine. As for making a venturi pump, it's not so much quantity of water as it is the velocity that it goes through the orifice. You could use your garden hose or tap and hook it to another hose and then to your aspirator. No need for a big fancy pump. A good venturi-based vacuum will boil water without a problem.

alancj - 29-5-2007 at 22:49

Quote:
Originally posted by oxybate
2. For the aspirator setup (and I've chosen to construct a self-made "aspirator station" with recirculated water, unless one of you provides some compelling reasons - like a particularly great pump suggestion -- for a pump that may be used directly for a decent price), what are the minimum specs I should be looking at? Is it correcct of me to think that another Pond pump, with greater GPH specs can be used in such a setup?


For powering an aspirator, you would need a pump to provide a good amount of pressure. 40-60 psi I would think. That's about the pressure household water is at. I don't think pond pumps have near that kind of pressure, they wouldn’t need it... they are more for high volume, low pressure work.

-Alan

markgollum - 30-5-2007 at 19:51

As far as the condenser pump is concerned very little water really needs to be flowing, I have had great success with the pump/hoses/ reservoir of a windshield washer from a wrecked truck, (I got it free) I power the pump with a 6V battery charger, while the pump is designed for 12-18V it is too noisy and the flow rate is far higher than needed.
I have only attempted one vacuum distillation using the sink and not recycling the water, I found this to be very unsatisfactory.
The joint size of my glassware is 24/40.

bio2 - 30-5-2007 at 20:06

For an aspirator the manufacturers specification should
be consulted. The given max vacuum is determined at
a particular flow and pressure.

This information is then transposed onto the pumps
characteristic operating curve.

Aspirators construction varies widely so quess work
is not recommended.

There are many old threads on this topic so read them
for quidance on building the aspirator and
pump selection.

oxybate - 30-5-2007 at 20:12

I have a Nalgene aspirator. I guess I'll try and look up the specs of it.