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All 18 posts   Subject: Aspirator station help   Please login to post   Down

 
    MrJ
07-06-04 22:43
      Aspirator station help
(Rated as: UTFSE!)
    
 
 
 
    abolt
(Hive Addict)
07-07-04 05:37
No 517894
User Picture 
      I've used it to try distillation of sass, and...     

I've used it to try distillation of sass, and nothing comes over until about 185 C, which makes for a really shitty vaccuum.

Are you using large blocks of ice in the reservoir?

Is the nipple of the aspirator slightly lower than the vacuum adaptor nipple?

Are you removing the vacuum tube before you turn off the water pump?

Of all Michael Moore's accusations, only 97% are true


 
 
 
 
    MrJ
(Stranger)
07-07-04 06:18
No 517898
      RE:aspirator     

Are you using large blocks of ice in the reservoir?
-does the reservoir temp really matter that much?

Is the nipple of the aspirator slightly lower than the vacuum adaptor nipple?
-i'd say the vaccuum adaptor nipple is maybe about 1 foot higher than the aspirator and it is connected with about 2 ft of hose

Are you removing the vacuum tube before you turn off the water pump?
-i usually turn off the vaccuum before disconnecting the tube

MrJ
 
 
 
 
    ApprenticeCook
(Hive Bee)
07-07-04 06:45
No 517904
      does the reservoir temp really matter that...     

does the reservoir temp really matter that much?
- Yes. The pump will heat the water as it pumps it (esp after prolonged use) and the hotter water will lower the aspirator vacuum, ie as you keep running your water temp goes up and your vacuum goes down.
Ice the water, and put something over the intake to stop the ice from being sucked in.

i'd say the vaccuum adaptor nipple is maybe about 1 foot higher than the aspirator and it is connected with about 2 ft of hose
- Your prob is more than likely here.... the hose? what type? vaccuum hose? check when its running the tube is not crushing down under the vacuum... go buy vacuum hose if your not using it...

And you need to release the vacuum before turning off the pump, so in swims setup there is a tap which is opened to release the vacuum, then you turn off the water pump, otherwise unless you have a nrv (non return valve) the water at the output of the aspirator will be sucked in to the vacuum area.

Hope this helps...
-AC

Its just my opinion, but no-one listens to me anyway, and rightly so...
 
 
 
 
    abolt
(Hive Addict)
07-07-04 07:14
No 517914
User Picture 
      Your prob is more than likely here....     

Your prob is more than likely here.... the hose? what type? vaccuum hose? check when its running the tube is not crushing down under the vacuum... go buy vacuum hose if your not using it...

......also, lessen the length of your hose if possible by placing the unit closer to the vac nipple. Standard Rubber hoses do lessen the vacuum at the vacuum nipple. Therefore the longer the hose, the more vacuum is decreased.

UTFSE, and look for some excellent info posted by Osmium in relation to vacuum properties.

Of all Michael Moore's accusations, only 97% are true


 
 
 
 
    Osmium
(Stoni's sexual toy)
07-07-04 09:01
No 517933
User Picture 
      > And you need to release the vacuum before     

> And you need to release the vacuum before turning off the pump, so in swims
> setup there is a tap which is opened to release the vacuum, then you turn
> off the water pump, otherwise unless you have a nrv (non return valve) the
> water at the output of the aspirator will be sucked in to the vacuum area.

I once flooded the whole setup of a finished distillation containing 100g of P2P by doing something stupid like that. Always let air in before turning off the water. Don't trust those valves.

BUSH/CHENEY 2004! After all, it ain't my country!
www.american-buddha.com/addict.war.1.htm
 
 
 
 
    MrJ
(Stranger)
07-07-04 11:06
No 517942
      damn asp station     

thx for the help guys.. but i think that stupid aspirator station is more trouble than it's worth!!!

friggin pump cost me a bundle... i think i'm gonna just get a friggin yellow jacket vaccuum....

i think i grew some white hair over this shit!!!

MrJ
 
 
 
 
    hypo
(Hive Addict)
07-07-04 11:55
No 517945
      could be worse     

> I once flooded the whole setup of a finished distillation containing
> 100g of P2P by doing something stupid like that.

with an aspirator? consider yourself lucky - separating the p2p and drying
it is a nuisance, but not more.
when my oil pump broke, she spilled her guts (nasty, dirty oil) all over
the collected product. that was a tragedy. frown

He always lied while on the earth and now he's lying in it
 
 
 
 
    Sredni_Vashtar
(Hive Bee)
07-07-04 22:13
No 518022
User Picture 
      These take some tinkering
(Rated as: good read)
    

Not fair to UTFSE on this one - he has the nous to set the thing up and is asking for more advice.

Water temperature is important. The limiting vacuum of practical water-jet pumps (aspirators to Yanks) is the vapour pressure of water. (Because the pumping chamber is always contaminated with water splashes for Hypo etc.). If you use ice-cold water, you can achieve close to 5 mbar Hg with a good setup.

Your water reservoir is too large (if you are filling it to capacity). A bucket (2 gallons/ 10 L) is sufficient and easier to cool with ice.

Water pump horsepower is secondary - what matters is pressure and capacity. Most water-jet pumps require around 1.5 bar (45 foot head / 15 m head / 150 KPa) but some low flowrate lab. jobs take a higher pressure of above 3 bar (90 foot head / 30 m head / 300 KPa). Your pump has easily enough capacity, but has it the correct pressure? You could be over-pressuring your water-jet. If you greatly overpressure a water-jet pump, it will flood and your vacuum will die. You say that water is coming out of the vacuum inlet - maybe this is what is happening? Try restricting the water inlet to the water-jet pump with a valve or such and see what happens.

Ditch the bathtub and get a large bucket.

Vapour Pressure of water

Celsius  Pressure (mm Hg)
-10        2.15
0           4.58
5           6.54
10         9.21
11         9.84
12         10.52
13         11.23
14         11.99
15         12.79
20         17.54
25         23.76
30         31.8
37         47.07
40         55.3

Edit: easiest thing is for you to get a vacuum gauge and see what is really happening. Cheapest are automotive gauges but you can get cheap ones on auction sites that we know and love etc.

Sredni Vashtar went forth,
His thoughts were red thoughts and his teeth were white.  
 
 
 
 
    abolt
(Hive Addict)
07-08-04 04:58
No 518098
User Picture 
      Your water reservoir is too large (if you are...     

Your water reservoir is too large (if you are filling it to capacity). A bucket (2 gallons/ 10 L) is sufficient and easier to cool with ice.

I would say that ISwE, a larger reservoir volume with a larger volume of ice allows the returned water to cool down better and therefore provides a better, (and more importantly)constant vacuum.

Free Tip: Go to the outdoors/sporting store and buy some of those 20 litre water drums that people use for camping. They are usually very cheap. Look for the ones that have a rubber "O" ring that seals the screw on lid in order to make them "water tight". Fill them up with water and place them in the freezer for a couple of days and voila!, you have a great chunk of ice for your aspirator reservoir (~40-50 litre volume), which can maintain a pretty constant reservoir temp that lasts for several hours.wink

Of course, bigger reservoirs require more containers of ice.

Of all Michael Moore's accusations, only 97% are true


 
 
 
 
    biotechdude
(Hive Bee)
07-09-04 02:03
No 518304
      need it tight when hot     

If using an aspirator, ensure that the hose connections are really secure (use clamps etc).  Otherwise after a few hours they will be be hot, expand and pop off and spray shit everywhere.  Its not cool to be woken at 2am and lying in bed thinking "what's that hissing sound??.."
 
 
 
 
    WizardX
(Wizard Master)
07-09-04 13:41
No 518391
User Picture 
      anti-freeze     

Add anti-freeze to the water so you are able to cool the water close to 0 oC to get better vacuum.

The vapour pressure of water @ 5 oC is 6.54 mmHg. If you go 2 x 6.54 mmHg = 13.08 mmHg of vacuum (approx) at the vaccuum adaptor nipple.

The vapour pressure of water @ 0 oC is 4.58 mmHg. If you go 2 x 4.58 mmHg = 9.16 mmHg of vacuum (approx) at the vaccuum adaptor nipple.
 
 
 
 
    goiterjoe
(Title on BackOrder)
07-10-04 16:07
No 518557
User Picture 
      large coolers work well     

Not only can you hold several gallons of water in a cooler, but the insulation will also help prevent the ice from melting as fast.  I always used coolers to hold the water and ice for my condensers as well.

skeed pills
 
 
 
 
    abolt
(Hive Addict)
07-12-04 01:57
No 518791
User Picture 
      Toxic     

Add anti-freeze to the water so you are able to cool the water close to 0 oC to get better vacuum.

If you are going to utilise anti freeze, I suggest you use an enclosed system that minimises the inhalation and contact with any "misting" that may occur.

Pharmacology:

Ethylene glycol is rapidly absorbed once it is ingested and is then widely distributed into body tissues. Peak blood levels are generally seen in 1-4 hours. Exposure to the skin and lungs may cause irritation but does not cause the systemic toxicity in the way that methanol does. Lethal quantities in adults are considered to be 100 ml, but in children much less may cause serious cardiac, renal, and CNS toxicity.

Ethylene glycol itself is relatively nontoxic. After absorption the unchanged compound undergoes glomerular filtration and passive reabsorption. It is then broken down into metabolites that are highly toxic and cause the associated findings of ethylene glycol toxicity. Ethylene glycol is converted to glycoaldehyde by alcohol dehydrogenase. This is the rate limiting step of a reaction in the liver that continues to breakdown the glycoaldehyde into glycolate, glyoxylate, and oxylate (see below).

Inhalation:
 
Vapor inhalation is generally not a problem unless heated or misted. Exposure to vapors over an extended time period has caused throat irritation and headache. May cause nausea, vomiting, dizziness and drowsiness. Pulmonary edema and central nervous system depression may also develop. When heated or misted, has produced rapid, involuntary eye movement and coma.



http://www.embbs.com/cr/alc/alc7.html

Of all Michael Moore's accusations, only 97% are true


 
 
 
 
    Sredni_Vashtar
(Hive Bee)
07-12-04 23:41
No 519000
User Picture 
      Viscous too     

The viscosity of water increases significantly close to zero, add ethylene glycol and it will increase further. A powerful pump will handle this but will maybe double the heating effect on the water from work done.

That's a good point about ethylene glycol, it is acutely and chronically toxic. Propylene glycol is harmless by comparison but more viscous.

I think pure water is best:

You can use antifreeze and refrigeration to get subzero coolant temperatures, but for what? Say you wanted to cool down to -5 C: you'd need around 15% ethylene glycol by mass. This would bring the vapour pressure down to around 3 mbar Hg, but the viscosity would be 3 times as much as water at 0 C. Is it worth it for 2 mbar extra vacuum? Plus, you'll need a refrigeration method (old chest freezer etc.). 5 mbar is good enough for most needs. Better to concentrate on getting a very reliable 5 mbar.

Converting an old fridge to cool the pump water might be a good idea though. No more ice. I used to have a 1HP garden pump and that melted a lot of ice.

Sredni Vashtar went forth,
His thoughts were red thoughts and his teeth were white.  
 
 
 
 
    WizardX
(Wizard Master)
07-14-04 03:51
No 519295
User Picture 
      Antifreeze     

Antifreeze solutions for automotive (automotive radiator fluid) contains about 30 to 50% w/v of ethylene glycol in water with some corrosion inhibitors.

A concentration of 15-20% w/v is all we need, because at 5 oC water is still liquid.

If viscosity of water is an issue, you can minimize the viscosity by making a 10% w/v ethylene glycol solution and test the freezing point by placing it in your home freeze. Take periodic temperature measurments to see what temperature it freezes at? Increase or decrease the ethylene glycol concentration as desired.
 
 
 
 
    Sredni_Vashtar
(Hive Bee)
07-14-04 23:45
No 519456
User Picture 
      Antifreeze conc.     

Wizard, the point I was trying to make is that I don't think sub-zero cooling is worth the hassle. You need to add antifreeze and the combined effect of adding anitfreeze and the low water temperature increases the viscosity of the water so much that you need several times more work pumping: this heats the water, so you need much more cooling. All this to get from 5 mbar Hg to 3 mbar Hg.

If you just float loads of ice in circulating water, you can cool it to almost freezing point and enjoy 5 mbar vapour pressure.

If you look at commercial recirculating aspirator stations, they are often like this:

10 L water capacity
150 W water pump
2 aspirators
Optional refrigerated cooling

commercial_aspirator.gif

This goes with what I have been saying: you want as low a power water pump as possible to do the job - anything more wastefully heats the water. If you calculate from flow and pressure requirements, a couple of aspirators need less than 150 W. High pressure, low flowrate pumps are hard to find, so everyone uses massive 0.75 H.P. jobs for a single aspirator and consequently melts loads of ice. Insulating the system helps too.

Refrigerated cooling is a complicated: difficult to break up an old fridge/freezer and suspend the cooling element in the water. I was thinking: use a separate cooling circuit. First, get your recirculating aspirator station working well, then if you want to avoid ice, add a cooling circuit.

Get a fridge/freezer and place a bucket of antifreeze solution inside and use an aquarium pump to circulate the antifreeze through a cooling worm in the aspirator water. Haven't tried this, but a domestic freezer would have have enough power to cool the water, maybe a fridge would too (with fridges, constant workload is the issue - overheating the compressor).

His enemies called for peace, but he brought them death.  
Sredni Vashtar the Beautiful.
 
 
 
 
    Sredni_Vashtar
(Hive Bee)
11-03-04 23:49
No 539525
User Picture 
      Homebrew Water-Jet Pump (Aspirator) Part 1
(Rated as: excellent)
    

Part 1: The Theory
Part 2: Making One From Plumbing Fittings
Part 3: Constructing an Efficient Recirculating Water-Jet Pump

Water-Jet Vacuum Pump

A cheap source of rough vacuum for the amateur scientist is the water-jet pump, known as the 'aspirator' in the USA. Laboratory sized water-jet pumps typically require 12 litres of water per minute at 1.5 bar pressure to operate. The pumping speed is 4 litres per minute and the limiting pressure is the vapour pressure of water - 20 mbar at room temperature. Reducing the temperature of the water supply lowers the limiting pressure - down to 5 mbar at 0° C.

Water-jet pumps are constructed in 2 forms: the venturi and the jet-ejector. The jet-ejector is the most efficient: it has the highest pumping speed for a given water flow rate and is the type dealt with here. Both types rely on the Bernoulli effect, whereby the dynamic pressure of a fluid falls with increasing velocity. Jet pumps using a liquid as a fluid cannot in general pump below the vapour pressure of the liquid due to outgassing from the liquid itself. In the jet-ejector type, a high velocity water jet is projected through a pumping chamber into a narrow throat. In addition to creating a low pressure area at the mouth of the throat, the jet entrains fluid molecules as it passes through the pumping chamber, so increasing the flow towards the mouth. Dimensions are critical for an efficient pump. Fundamental, is the velocity of the water jet - this must be high enough for the dynamic pressure to reach the vapour pressure of water, which in turn governs the nozzle diameter required for a particular water supply flowrate. Throat diameter is also critical: too narrow and pumping speed is reduced; too wide and air can return past the water to the pumping chamber. Throat length is also very important to ensure a good seal against the atmosphere without impeding water flow too much, thereby creating back-pressure. Other important dimensions are the conical angles of the nozzle and the mouth, and the displacement of the nozzle from the throat. Optimally chosen dimensions in a simple homemade pump yield a pump performance that equals or exceeds commercial models. Schematic 1. shows the cross section and critical dimensions for an ejector water-jet pump.

Schematic 1. Water-Jet Ejector Pump
waterjetschematic.jpg

The optimum size ratios between different components of the pump has been determined empirically in various papers (not uploaded for the sake of space)

Given D, the pumping chamber diameter, and Dn, the nozzle diameter, the other key dimensions of the pump are determined by ratios:

Dimension Value
α 20°
β 10°
θ 43.9°
Dt 1.69 x Dn
Lt 2.1 x D


From Bernoulli's equation, the required water velocity to give a drop in dynamic pressure of 100 kPa can be calculated:

ρV2/2 = 105 Pa Where density of water, ρ = 1000 Kg/m3 Therefore: velocity, V = √(105/500) = 14.14 ms-1

The water velocity and flowrate determine the nozzle cross-sectional area: velocity x area = flowrate Given flowrate = 12 litres/minute = 2 x 10-4 m3s-1 and velocity = 14.14 ms-1,

area, A = 2 x 10-4 / 14.14 ms-1 = 1.4 x 10-5 m2 = 14 mm2,

which corresponds to a nozzle bore of 4.2 mm.

The jet will decelerate due to frictional losses in the pumping chamber and throat. To allow for this, the nozzle diamater is reduced to 3.5 mm, increasing exit velocity by 44 % to 20 ms-1. This yields the following optimal dimensions for a pump of 25 mm diameter pumping chamber (typical of plumbing fittings):

Dimension Value
D 25 mm
Dn 3.5 mm
α 20°
β 10°
θ 43.9°
Dt 5.9 mm
Lt 52.5 mm


His enemies called for peace, but he brought them death.  
Sredni Vashtar the Beautiful.
 
 

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