Which variables determine the exact output of this condensing device?

[hmmm27]

I was referring to the original design containing no dessicant.

This one ? (larger image from the article)

I added in the purpl'ish arrows in the second-from-top level (missing from the original diagram) to illustrate ambient air movement. The round thing is what I thought was a flow-across turntable, but is apparently (or possibly) a flow-thru (turning) disk.

For the OP's possible edification, the tray with the heat-exchanger on it, the airflows are not actually mixing : inside the hexagonal structure, pipes that carry the airflows carry out the heat exchanging.

 

[cmb]

Here I was in particular commenting on @cmb suggestion that a pressure change would be requiired.

Not something I said. I said "maybe there is some means to do what you want but in a depressurised system?".

Maybe there is some reason I seem to be routinely misread?

But anyway ... it was just a suggestion ...

 

[hutchphd]

You will only get condensate when the working volume you are manipulating drops below its dew point, so worth reviewing that too, maybe there is some means to do what you want but in a depressurised system? You'll have to figure out how to gate a sample charge of gas, lower the pressure, extract condensate, then recover the latent energy used to form the depressurisation.

?This is not a pressure change ?

 

[cmb]

?This is not a pressure change ?

Yes, but not 'required'. I proposed it as one possible means to help achieve necessary dew point.

 

[hutchphd]

Apologies. I did not intend to offend or mischaracterize.

 

[cmb]

No prob. I thought I was adding an alternative train of thought for the OP, didn't want to make an argument of it.

 

[anikad]

I would respectfully disagree with that, and not just because I'm the apparent "accomplice"

I don't see anything wrong with the theory, not qualified to comment on the engineering.

The system transfers water from the atmosphere to a bucket through two sequential fixed media : dessicant and internal air.

Warm air from the outside comes in, interacts with the dessicant and goes out, slightly drier. It has the dual purpose of cooling the dessicant and provides it with some moisture.

In an internal closed loop, very hot air picks up moisture from the dessicant and is moved to a cooler zone where it is cooled and deposits the moisture. It is then heated again, etc.

Note that while it's a continuous process, it might require quite a bit of apparently fruitless circulation before the internal air has picked up enough water to start condensing on the cooler surface.

Thank you .
That's exactly what I've picked up from the original design. I'm working on a design based upon "batch processing" to prevent the highly inefficient water pickup of internal air.

 

[anikad]

No prob. I thought I was adding an alternative train of thought for the OP, didn't want to make an argument of it.

Thank you for your input!
This is still confusing to me.
The condensate forms when there is a temperature difference ( for eg: ambient temperature on outside, & high temperature on inner closed area) .
What does the dew point has anything to do with condensate?
Can you please elaborate what I'm missing?

 

[hmmm27]

What does the dew point has anything to do with condensate?

The Wikipedia article on "dew point" explains it quite clearly in the very first paragraph, IMHO.

 

[anikad]

The Wikipedia article on "dew point" explains it quite clearly in the very first paragraph, IMHO.

I just did a little reading on dew point.
My intuition was: if we trap heat in a closed system, and continue saturating it with moisture- then we let the highly saturated air (that can't hold any more water vapors) come in contact with the wall of the system that is slightly cooler because it's in contact with outside ambient air- the condensate will start to form.

It seems my intuition was correct even without knowing/taking into account the dew point?.

 

[anikad]

I agree (forcefully) that the originally system is not going to work at all, despite the OP entreaty to find an accomplice to defy the laws of known physics.
However this statement is overly broad. There are methods (usually involving changing effective dimensionality) to modify the kinetics for physical changes. For instance a catalytic converter can produce desired reactions on its 2D surfaces. Oxygen concentrators use surface reactions to pull nitrogen from the air in a cycle . The device I referenced in #27 above is similarly cyclic and probably relies on 2D surface kinetics.(?)
So there may be many ways to approach this problem, some may be simple, but none of them are easy.

Hey hutchphd, this is not an attempt to defy the laws of physics.
Perhaps, you didn't understood the mechanism of the device correctly?
If you can prove which law is being defied in the device, I'd take account.

 

[256bits]

Hey hutchphd, this is not an attempt to defy the laws of physics.
Perhaps, you didn't understood the mechanism of the device correctly?
If you can prove which law is being defied in the device, I'd take account.

Well, you just can't say this is how it works and I can get water from air.
There is ( probably ) a limited temperature / humidity levels of the outside air, and heating from the sun, where the system will complete it's function as planned.

The air at the cool surface has to be at 100% humidity for condensate to form.
For example,
If the outside air is at 20 C 30% RH your cooling surface will be at 20 degrees also, since you are not actively cooling that surface.
If you can get the cooler air up to 100% humidity then well you will have condensate.
Then ask yourself the question, how much heat needs to be added to the air at the top section so that the RH of the top hot air is less than 30% for the wheel to transfer moisture from the ambient air to the wheel to the hot air. What is that temperature? If the temperature does not reach that level then the wheel ends up working in reverse and transferring humidity from the hot air to the outside air.
Suppose the temperature rises just 1 degree about that needed just below < 30% humidity.
Then the system does do as planned, but the yield amy be a drop an hour <-- just a simple guess.
You will have to look at your psychometric chart to get answers.

You really have to do an engineering analysis, to determine at what temperatures and RH's where the system can do something useful.

Here's a start.
https://www.odellassoc.com/how-do-desiccant-dehumidifiers-work/
https://www.trane.com/content/dam/T...wsletters/airside-design/admapn016en_0905.pdf

 

[hutchphd]

Hey hutchphd, this is not an attempt to defy the laws of physics.
Perhaps, you didn't understood the mechanism of the device correctly?
If you can prove which law is being defied in the device, I'd take account.

I added in the purpl'ish arrows in the second-from-top level (missing from the original diagram) to illustrate ambient air movement. The round thing is what I thought was a flow-across turntable, but is apparently (or possibly) a flow-thru (turning) disk.

This version can work. Purple arrows.

 

[anikad]

The air at the cool surface has to be at 100% humidity for condensate to form.
For example,
If the outside air is at 20 C 30% RH your cooling surface will be at 20 degrees also, since you are not actively cooling that surface.
If you can get the cooler air up to 100% humidity then well you will have condensate.
Then ask yourself the question, how much heat needs to be added to the air at the top section so that the RH of the top hot air is less than 30% for the wheel to transfer moisture from the ambient air to the wheel to the hot air. What is that temperature? If the temperature does not reach that level then the wheel ends up working in reverse and transferring humidity from the hot air to the outside air.
Suppose the temperature rises just 1 degree about that needed just below < 30% humidity.
Then the system does do as planned, but the yield amy be a drop an hour <-- just a simple guess.

Thank you! Now you really appear to be my accomplice.

So the engineering analysis of this system comes down to this : The greater the temperature difference between hot air & cooler ambient air, the quicker the air picks up enough moisture to reach 100% humidity- to be brought to a cooler surface.Am I thinking correctly?

 

[hutchphd]

Also the choice of dessicant will make a big difference I believe. I found this interesting: a clever guy trying to make a desiccant-based air conditioner:



It didn't work all that well !

 

[anikad]

This version can work. Purple arrows.

Let's say the hot air reached 40°C 70% RH levels (after absorbing lots of moisture from the wheel) . And we bring it in contact with ambient air at 20°C.
Is there a physics equation that can tell whether that hot stream of air when brought to contact with a 20°C ambient air- can reach 100% RH levels ?

Basically what I want to know- the 70% RH air will be increased to higher RH (since cooler surfaces have higher RH) , but what dictates how much it will increase & whether it will increase to 100% RH so we can get the maximum possible condensate?

Any answer to it will be appreciated

 

[hutchphd]

I do not know a simple answer. You really need to know the various rates involved (for a continuous process). I think a far better design (as has been talked about) is to heat the dessicant directly with the solar flux to expel its trapped moisture.

 

[256bits]

Thank you! Now you really appear to be my accomplice.

So the engineering analysis of this system comes down to this : The greater the temperature difference between hot air & cooler ambient air, the quicker the air picks up enough moisture to reach 100% humidity- to be brought to a cooler surface.Am I thinking correctly?

Maybe.
I haven't done any calculations myself so I won't be quick to say yeas on no.
My point is that at some temp/humidity levels the system will never work at all.

Either do calculations, or experiment. ( But I don't not think that you will get rivers of liquid water for a table top size system .)

 

[anikad]

I do not know a simple answer. You really need to know the various rates involved (for a continuous process). I think a far better design (as has been talked about) is to heat the dessicant directly with the solar flux to expel its trapped moisture.

I agree with you on that design!
What my question was essentially: WHAT physics equation relates the following 4 variables.

Initial T ; Initial RH ; Final T ; Final RH

I believe there's ought to be an equation for this simple relation, since we already know that:
Final RH> initial RH & Final T < Initial T

 

[Tom.G]

This screen capture didn't work very well, but here is the approach.
It is about 35% down in the right column at:
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/dew-point

It is an excerpt from the book :

Theoretical Grounds for Humidity​

Dario Camuffo, in Microclimate for Cultural Heritage (Third Edition), 2019

The DP can be easily computed from RH and air temperature, as in the next formulae. Indeed, considering that the DP is reached with an isobaric process, the vapour pressure at the original dry bulb temperature equals the saturation pressure at DP, i.e. e(T) = esat(DP). By substituting this finding in formula (3.38), with the help of the Magnus & Tetens formula, one obtains:
(3.48)u=etesatt=eDPesatt=esat0×10aDP/b+DPesat0×10atb+t=10aDP/b+DP−at/b+t
hence
(3.49)logu=aDPb+DP−atb+t
and
(3.50)DP=b+DPalogu+b+DPaatb+t≈b+talogu+t
where the last approximate finding has been obtained substituting t to DP in the right-hand side of the first identity. Of course, the first term is negative as u < 1 and log u < 0.
Another formula can be derived considering what happens above an evaporating surface. The air temperature lowers, while the increase of MR raises the DP. The air temperature t continues to decrease until the temperature of the evaporating surface, called wet bulb temperature, tw, is reached (see Section 3.9). When the evaporated vapour reaches saturation, t = tw. Starting from the Clapeyron equation and the definition of w and always considering the difference DP − tw, after some steps and approximations, the following formula is obtained:
(3.51)DP≈bblogu+tlogu+atab−blogu−tlogu
where a and b are the Magnus & Tetens coefficients for vapour in equilibrium with the liquid phase. Eq. (3.50) is a better approximation. The formulae can be used once the RH is known, and obviously
(3.52)logu=logRH100=logRH−2
DP ≤ T and DP = T only when RH = 100%. The DP is determined once the air temperature T and the RH are both known, or also when only the MR (or SH) is known. In particular, maxima of MR correspond to minima of DP, and vice versa, so that the DP can be used for diagnostic purposes instead of the MR and may be useful to express the moisture content in °C.

The dew point spread (also called spread), i.e. the difference ΔDP = T − DP basically depends on both the actual air temperature T and the MR. Following the approximation (Eq. 3.50 ), it can be expressed as a function of air temperature and RH
(3.53)ΔDP≈−b+talogu
It physically shows how much the air temperature is close to, or far from, the DP. The zones having smaller ΔDP are more prone to form condensation, to allow microbiological life and more intense weathering. Useful maps of this variable can be easily drawn for diagnostic purposes. However, although the RH is a very different but related variable, in general the areas with RH maximum are the same as those in which the ΔDP is minimum. If one is not interested to know how much the ambient is above the dew point, i.e. how much wall temperature (not air temperature!) should be raised to avoid condensation, maps of RH are sufficient to give a qualitative idea of most critical areas.

Dew has the typical form of droplets and especially forms on leaves during the nocturnal cooling due to the infrared (IR) emission. The formation of dew on leaves is favoured by the local excess of moisture due to stomatal transpiration. The surface tension of water tends to displace the larger droplets to the edges of the leaves and in particular to the points of leaves, especially the lance-shaped ones. The upward IR loss during clear nights is a very effective cooling mechanism. The surfaces on which dew forms are free from any upper shield and in practice are the same that are reached by rainfall. This is the reason why people often believe that dew falls similarly to drizzle. Dew is favoured over vegetated areas, but it occurs on monuments as well, when their surface temperature falls below the DP. When the temperature of a surface falls below the DP, in the viscous layer surrounding the surface RH >100% and condensation occurs.

Hope this helps!

Cheers,
Tom