Great Venturi Design

mylesau

hygicell wrote:

if the balance of the increased aeration by a pressurized Venturi is not compensating the loss of volume everybody agrees it causes, (volume that can be used for passive aeration and doubtlessly allows for faster removal of solids which efficiently diminishes DO demands),
then a pressurized Venturi is redundant and contra productive in AP...

This is just such a ridiculous statement. It takes such a small amount of volume/pumping effort to keep up with the removal of waste solids - I really don't think you have a very good understanding of the processes at all Frank!

hygicell wrote:

my reasoning is backed up by explanations of why I think that balance is not reached and TMHO cannot be reached as every conversion of energy has been proven to include energy losses
it is somewhat more than a hunch: it is motivated
so far nobody has tried to explain to me why a pressurized Venturi would swing that balance

Err, yes they have - links were posted to documents that explained exactly this - points you selectively have attempted to ignore were highlighted - you are just unwilling to accept anything but your own, to go back to the soccer story, dribble!!!

hygicell

RupertofOZ wrote:

Glad to hear you're not thinking of actually filling the gutter with media... to do so would have reduced your capacity by about 40-50%... and slowed your flow rate...
As to media in the pots.... given you're essentially "flooding and draining"... or ebb& flow...
And the height of the flood... 70mm.... then I think coco coir fibres are out... they just stay too wet IMO.... more suited to drip feed... although in very hot climates (ie Plachon in Thailand) they might work OK...
Choices then.... Vermiculite holds water really well... but the fine grade wouldn't allow enough air to the roots in that flood height... unless the drain period was fairly long...
Perlite (a pain to work with) on the other hand has much better air holding abilities... but not great water retention.... in your case with that height of flood, in may be suitable...
Again depending on just how long the "ebb/drain" period is... and air temp.... remembering that the channel will also impart a degree of ambient heat transfer...
Expanded clay has great aeration ability, but not great water retention... so again ... depends on the drain period....
You may need to actually determine the drain period to determine the best media....
Flooding that high for extended periods will compromise the ability of the plants to uptake oxygen IMO.... this can be compensated somewhat by flow... if the flow is highly oxygenated...
Question is... will this be the case....
Second possible problem I see is whether the sump receives a large amount of "unfiltered" solids, suspended matter... directly from the overflow, rather than passing through the growbed...
If this was the case, then the chances of solids adhering to the root balls... causing channeling of the water flow.... and/or dry spots or root rot... are increased markedly...
Much of the success or failure of the design... depends on flow rate... and frequency of the flow cycle...
Have you been able able to calculate just how long your timer cycle... sump fill cycle might be?

thanks for a positive approach, Rupe
hope we can finally conclude that we understand each other
thanks for the advice on coco coir fibers
I have experimented with both vermiculite and perlite and am inclined to agree with you when the fine grades are involved
coarse grades might be better
air/water temperatures should be considered too, I agree
the sump will not receive any unfiltered solids as the recirculation will always include the upflow bead filter
so I guess I have provided for that.
frequency and flow rates is where I would ask your support in determining them
in the setup described flood is only taking 10 minutes and the drain is 50 minutes, so I think root aeration will be less of a problem

I know that a lot of this is trial and error
but I do rely on you to help me to avoid the errors as much as possible

Thanks

Frank

RupertofOZ

Quote:

the sump will not receive any unfiltered solids as the recirculation will always include the upflow bead filter

Not sure I understand where the "upflow bead filter" is incorporated in your design...

Had thought perhaps it was integrated into the "solar chimney"... but that appears after the sump.... :dontknow:

Can you explain the "internals"... the working of the solar chimney....

To be honest I've not really spent a great deal of time with "ebb & flow" hydroponics... well not in channels/gutters....

Have done so in shallow trays.... with about those settings... with a 75:25 vermiculite/perlite mix to raise seedlings... but that was in an environmentally controlled nursery....

Dandiman may be able to assist with that one better than I can... although from memory I think that he too concluded that it was hard to manage... at least consistantly...

NFT style channels/gutters... with those settings... maybe... I've always run nft with a continuous flow...

Flood and drain growbeds, with 300mm depth, gravel or expanded clay... no problem....

Again will depend I think on temperature and media...

I still have concerns about oxygen availability... and plant root growth...

A combination of fine media and plant root growth could be problematical... and slow flow considerably... on the other hand a free draining media like expanded clay might dry out too much over the 50 min drain... in a shallow gutter..

I've been running a fine grade vermiculite/perlite mix (75:25) in a half barrel flood & drain... with continuous inflow... but have found it to be a bit too wet... not so wet as to cause root problems, but nowhere near the growth I expected...

Whereas F&F has had good results with the same mix... but flooding for 15 and draining for about 2 hours (from memory)...

I can't adjust the taps to flood slow enough to match that... so had fully intended (this week) to actually replace the growbed with either a 50:50 mix.... or more likely a 75:25 mix of coarse grade vermiculite.... I'll let you know how it goes...

RupertofOZ

P.S. coco fibres might work... if they are the actual loose husk fibres... rather than the husk chips...

But be aware that coco coir runs anything between pH 5.5 - 6.5...

hydrophilia

hydrophilia wrote:

Sleepe wrote:

I am more interested in the effects of humidity on the absorbtion of oxygen into water (I have not found tables showing this). Lets say you place aluminised shadecloth over the FT with some space above the water (a tent if you like) if the air for whatever aeration system you use is mainly drawn from this it would in theory have greater humidity ie less cooling effect. Over here I consider water loss and temp swings to be of greater significance than a few watts here or there on aeration (both fish and beneficial bacteria are effected by these). Unless you are stocking at commercial levels, DO levels are more likely to be effected by other factors ie algae,bacteria which feed on decomposing food etc. OK enough rant.

+1 (as I said in post a couple pages back). If, however, O2 and CO2 diffuse a lot faster than H2O, then having the cover will tend to be like breathing under the blankets: stifling. I would bet, though, that it is the other way around (diffusion of H2O>O2) and that having some cover would be a benefit. If having a cover is good, then having a small amount of air, divided into small bubbles and pushed deep for long contact time would also be better than large amounts of air with less contact time, even if (surface area times contact time) is the same. Any ideas for some simple experiments to test this? It would be nice to know if one is better off with short air-water contact times or long contact times to get the most O2 transfer with the least H2O transfer.

here is a vote for fine bubbles deep in the water:

Thought experiment: put an aquarium upside down in a pool of water. After a few minutes, it will have 100% humidity, but even after many hours there will still be breathable air. Therefore, O2 absorption is far slower than H2O evaporation. This could be wrong, but I don't think so.

I found that some "efficient" air pumps advertised to use 30 watts to pump 1 CF of air per minute 2.5' deep. This comes out to about 5 watts to pump 1g of O2 per minute to 2.5'. This comes out to 12g/(watt.hr). Since absorption (SAE) is about 2lb/hp per hour (or 1 gram/watt per hour) and at my temps (21degC) and O2 (7ppm) the efficiency is .2xSAE, we will actually get more like .2g of O2 dissolving per watt-hr out of a total pumped of 12g, so O2 in the air bubbles will drop from 20% (roughly) to 19.5%(roughly) during its pass through the water. Therefore, if we wish to achieve high O2 while limiting evaporative cooling and water loss it is far far better to have a small volume of very small bubbles in long contact with water than to have a large volume of air in short contact. This is what we achieve with flood/drain (the growbed is taking deep, slow breaths) or with a fine-bubble, deep venturi or a fine (and deep) bubbler. Putting a non-airtight cover over the water to increase surface humidity and decrease evaporation (as Sleepe proposed) should also be a good idea.

hygicell

I second the vote for small bubbles deep in the water
for lots of reasons:
small bubbles expose an almost incredibly higher surface than big bubbles
the surface of a sphere is 4 pi * r² the volume of a sphere is 4/3 pi * r³
one liter of air divided into bubbles of 5 mm diameter exposes 1.2 m² of surface
the same liter of air divided into bubbles of 1 mm diameter exposes 6 m² of surface
the same liter of air divided into bubbles of 0.1 mm diameter exposes 60 m² of surface
small bubbles rise slower than big bubbles, so the much increased surface is exposed for a longer time
deep in the tank the water is more depleted, so DO exchange will be more efficient
deep in the tank the water is cooler so it's DO absorbing capacity is higher
deep in the tank the pressure is higher so DO exchange rate is improving

so if a pressurized Venturi is used it seems to me that pumping the water deep down into the tank would optimize the efficiency

and as the natural tendency of the bubbles is to flow upwards, that would be counter current to the water flow
and contact time would increase even more

if a pressurized Venturi is used, it would be best to place the Venturi in the suction side of the pump rather than in the pressure side
as the impeller will shatter the air into tiny bubbles compared to the big bubbles that a pressurized Venturi on the output flow produces.

I am still not advocating the use of pressurized Venturi's, as I think the loss of flow that can be used for passive aeration and faster removal of solids is not compensated by the air injection

just suggesting ways to make them more efficient.

Frank

hygicell

I would question your conclusions in the submerged aquarium experiment:
it is not because the air in the aquarium is still breathable after many hours that the exchange is faster or slower:
the exchange is limited by the equilibrium, so no matter how short or how long you submerge the aquarium, the air will stay breathable as the water can only hold so much O2
same goes for humidity in air: you cannot go over 100 % humidity saturation in air

as far as I know, the equilibrium is reached as good as instantaneously

this seems to be proven for humidity by the high efficiency of vacuum based dehumidifiers:
creating a vacuum to increase the absorption capacity seems to be more efficient than heating the air with the same energy to increase absorption capacity.

I would guess that the same rules are applicable for O2 exchange

frank

hygicell

in the solar chimney the air is heated by the sun because the exterior is painted black
warm air naturally drifts upwards as it's density is diminished
so fresh cooler air is sucked in to replace it
that is the principle on which all chimneys work
except that in other chimneys the heat source is the furnace (or the barbecue :geek:

)
in a solar chimney the sun replaces the furnace

I have followed a course on ecological building
and visited the house of the teacher

a very small solar chimney served as an airco

and it is absolutely self regulating: more sun, more heat, more ventilation, less sun, less heat, less ventilation
If I remember well, the solar chimney was 3 parallel square galvanized metal pipes of maximally 100 * 100 mm section and 1 m high (I think they were only 50*50 and 500 mm high, but I will not take any risks) enclosed in a transparent housing to eliminate wind chilling.
that was for for the whole house. The day I visited was a very hot day to our norms. it was very efficient
galvanized metal because of the faster heat transfer rate
insulating the shaded side would make that even more performing

frank

Sleepe

Dosn't a solar chimney reverse flow at night? If the material used is a good absorber that would likely make it a good emitter (of IR). There again it would all depend on what is around it also emitting IR.

hygicell

Sleepe wrote:

Dosn't a solar chimney reverse flow at night? If the material used is a good absorber that would likely make it a good emitter (of IR). There again it would all depend on what is around it also emitting IR.

it might, but I'm not sure it would
in the absence of solar radiation there would be no noticeable temperature difference between the in and outside of the chimney
so air flow whether downwards or upwards would not be influenced noticeably
I imagine a throttle or valve on the chimney is advised as in winter the solar chimney would suck warm air out of the house.
that is probably not it's purpose

frank

hygicell

then again, a solar chimney can be combined with an air to air heat exchanger
that would ensure ventilation with limited heat loss

I believe this to be a subject that deserves a new thread
if you start a new thread and give me some time I will make a sketchup drawing of how I can see this realized

frank

mylesau

hygicell wrote:

if a pressurized Venturi is used, it would be best to place the Venturi in the suction side of the pump rather than in the pressure side
as the impeller will shatter the air into tiny bubbles compared to the big bubbles that a pressurized Venturi on the output flow produces.

And I thought you new a lot about pumps Frank? Adding air to the suction side of a pump will kill its ability to pump - huge loss of efficiency... Adding just after the discharge point, however, will increase the contact time of air under pressure.

hygicell

Every Venturi will cause loss of pumping efficiency, Mylesau
that is what I have been explaining all along

the inclusion of a Venturi on the suction side of a pump is a frequently used technique:
lots of protein skimmer pumps are based on it

the pumps I use provide the accessories for adapting them to an inlet Venturi
so do many, many other pumps
some are even marketed especially for this purpose

there are even special impellers and conversion kits for some pump types to produce smaller bubbles

just do a google on protein skimmers and Venturi's and you will see.

a Venturi on the inlet side will produce tiny bubbles
a Venturi on the outlet side will produce big bubbles

tiny bubbles are more efficient at aeration

I do know a little about pumps, Mylesau
I don't pretend to know it all
not by far

frank

Sleepe

Needle wheel skimmers.

Frank
If IR emmitance is greater than IR absorbance temps can go below ambient ie it will reverse flow (I think

)

hygicell

Sleepe wrote:

Needle wheel skimmers.

Frank
If IR emmitance is greater than IR absorbance temps can go below ambient ie it will reverse flow (I think

)

thanks, Sleepe, the term had escaped me :geek:

I presume IR stands for Infra Red
and that you are skipping from Venturi's to solar chimneys
skipping is pure fun but can be confusing :geek:

even then I don't follow your reasoning
My intelligence is really rather limited
much as I try to hide this even for myself

please explain what you mean

Frank

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Great Venturi Design

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