Pleated Screen collector

[Skindancer]

Greetings All,

I am working on a couple of screen collectors to help heat the basement of my home and I got to thinking that if I could pleat the screens into the collector so that the screens ran vertically I could effectively double the surface area of the collector without increasing the resistance to the airflow through the system. Another potential advantage I could foresee is that with a 60 deg pleat in the screens you would have more of the face exposed to the sun in the mornings and evenings when you would want the heat the most while in the mid mornings to afternoons you would have a much greater exposed surface. Well, that idea led to another so here I am once again asking what may be a stupid question. That question is, has anyone tried using pleated furnace air filters in place of the window screens? The design of the collector would need to be modified slightly and you would have to spray paint the filters black but as long as you did it it in several light coats that should not clog them.

Am I totally off base with these ideas? Please, do not direct me to Builditsolar.com. I have looked there and while they have lots of good ideas i have not found anything about this. I don't have the time to spend doing hours of research which is why I am asking those of you who have more experience than I.

[Mike90250]

Adding more screen material WILL increase resistance to air flow. So you are on your own here. Sorry.

[Skindancer]

Adding more screen material WILL increase resistance to air flow. So you are on your own here. Sorry.

If I was increasing the screen density or thickness I would agree but I am just increasing the surface area. If anything that should decrease the Delta P across the full screen for a given airflow; that is one of the reasons you use pleated filters in your furnace. it isn't just to give you more dust collection area.

[inetdog]

If I was increasing the screen density or thickness I would agree but I am just increasing the surface area. If anything that should decrease the Delta P across the full screen for a given airflow; that is one of the reasons you use pleated filters in your furnace. it isn't just to give you more dust collection area.

But adding more surface area of collector material in the same space just increases the collection efficiency, which is already probably pretty high, and cannot magically capture more heat than is hitting the area of the collector panel.
Just having the sun hit part of the screen at an angle closer to vertical is not going to improve your collection efficiency at all. The projected area will not change and because of multiple reflections light hitting off perpendicular may be more likely to be absorbed than light hitting straight on.
This is different from the situation with solar PV where capture and conversion efficiency is best at 90 degrees to the cell surface.

Whether it would give better heat transfer to the air going through is another story and if the collector temperature gets much higher than it needs to be while still producing relatively cool output air you might have an advantage for your proposed design.
Measurements of outlet air temp, collector temp and air flow for the two designs would clarify this.

If I understand your proposal correctly, you are talking about feeding the air into the collector from behind the pleated screen and exhausting it from the front of the screen, yes?

[Skindancer]

If I understand your proposal correctly, you are talking about feeding the air into the collector from behind the pleated screen and exhausting it from the front of the screen, yes?

No, the cold air would enter at the front and be drawn through to the back that way you do not have the heated air against your glazing and losing heat to it.

The challenge I am trying to overcome is how to form the pleats and hold them in the screening. I have a few different ideas but they are fairly labor or cost intensive. While labor is not so much an issue cost is and as much as I would like to I can't afford to experiment much.

On a related note I managed to scrounge up two nice sized refrigeration condensing coils that I can repurpose as glycol to air heat exchangers for when I get my glycol solar collector set up latter this year. I even found a 24 volt solanoid 2 way valve so that when I don't need room heating I can automaticly switch the system over to water heating. I think i can also modify a head pressure control system to regulate the fan speed based on the temp. inside the collector.

Go Team ME!
(I have toget my thrills where I can find them)

[russ]

Not too much hope here

[J.P.M.]

Greetings All,

I am working on a couple of screen collectors to help heat the basement of my home and I got to thinking that if I could pleat the screens into the collector so that the screens ran vertically I could effectively double the surface area of the collector without increasing the resistance to the airflow through the system. Another potential advantage I could foresee is that with a 60 deg pleat in the screens you would have more of the face exposed to the sun in the mornings and evenings when you would want the heat the most while in the mid mornings to afternoons you would have a much greater exposed surface. Well, that idea led to another so here I am once again asking what may be a stupid question. That question is, has anyone tried using pleated furnace air filters in place of the window screens? The design of the collector would need to be modified slightly and you would have to spray paint the filters black but as long as you did it it in several light coats that should not clog them.

Am I totally off base with these ideas? Please, do not direct me to Builditsolar.com. I have looked there and while they have lots of good ideas i have not found anything about this. I don't have the time to spend doing hours of research which is why I am asking those of you who have more experience than I.

Part of the reference to biulditsolar is not to use it as a copycat source, but to show what's been done so perhaps not to reinvent the wheel and also as a source for inspiration for new ideas.

If you increase the area of the screens and change the flow path and regimes, it will be difficult to not increase the pressure drop. Depending on how its done the increase may not be measureable or even noticeablebut it will increase. Basically, There ain't no free lunch.

[inetdog]

If you increase the area of the screens and change the flow path and regimes, it will be difficult to not increase the pressure drop..

I do not really see how you come to that conclusion.
You are putting more screen area in parallel with the original screen area, so the total area of the holes that the air passes through increases.
Are you asserting that air passing through the screen at a 45 degree angle has enough additional flow resistance to overcome the additional area? I have my doubts about that.

Consider as a thought experiment a cube with the top face being the air inlet and the bottom face being the air outlet.
Which has more resistance:
1. A horizontal screen across the middle of the cube, or
2. A diagonal screen running from one upper edge to the opposite lower edge?
To me it is a no-brainer that the resistance of #1 is higher for identical screen material. Although for extremely high air velocities and strange screen geometry that might not hold.

[J.P.M.]

I do not really see how you come to that conclusion.
You are putting more screen area in parallel with the original screen area, so the total area of the holes that the air passes through increases.
Are you asserting that air passing through the screen at a 45 degree angle has enough additional flow resistance to overcome the additional area? I have my doubts about that.

Consider as a thought experiment a cube with the top face being the air inlet and the bottom face being the air outlet.
Which has more resistance:
1. A horizontal screen across the middle of the cube, or
2. A diagonal screen running from one upper edge to the opposite lower edge?
To me it is a no-brainer that the resistance of #1 is higher for identical screen material. Although for extremely high air velocities and strange screen geometry that might not hold.

For starters:

Increased screen area = Less flow area = increased fluid velocity

Increased screen area = more surface area

Increased fluid velocity + More surface area = increased pressure drop.

Increasing the surface area can increase the rate of heat transfer within the collector. That's usually good. However, it will come at the expense of an increase in the pressure drop. Finding the best tradeoff(s) is, among other things, is part od the province of what engineering design is all about.

FWIW: Trust me on this one. I spent the first 10 years or so of an engineering career designing large scale heat transfer equipment and heat exchangers, while having solar thermal applications as more than a hobby but less than a profession for the better part of 40 years.

Besides, I've got entropy on my side.

[inetdog]

J.P.M,

Can you address my specific though experiment?
I do not see the increased screen area in that case producing a decreased flow area.
If you are talking about packing a flow section with multiple layers of screen in an attempt to increase heat transfer, than I see you point. But that is not what we are discussing here.

[J.P.M.]

J.P.M,

Can you address my specific though experiment?
I do not see the increased screen area in that case producing a decreased flow area.
If you are talking about packing a flow section with multiple layers of screen in an attempt to increase heat transfer, than I see you point. But that is not what we are discussing here.

I will. But, without trying to duck an answer, it'll take more time than I'd like to devote to it.

Let me say (write) that I was assuming that more mesh is being crammed into the same collector box. That's sure what it sounded like to me. If so, less flow area, greater velocity, more pressure drop. Without having inlet/outlet arrangement that forces the flow through the mesh, in effect creating what I think your thought exp. is getting at, you'll have lower pressure drop but poorer heat transfer.

Give me a bit of time for a response. Dinner's waiting.

Respectfully,

[J.P.M.]

OK. Dinner was good.

Your thought experiment: Assuming air at close to atmospheric pressure and not terribly warm in a square duct in well developed turbulent flow.

A simple 2 dimensional screen will probably act as you might guess: a screen normal to the flow will have a higher face velocity and a pressure drop greater than a similar screen with a "yaw" angle of 45 degrees to the flow.

As for the practical matter of mesh in an air cooled collector:

As the screen increases in thickness by using more layers of screening, or more likely by using some form of mesh like, say, a furnace filter, or st. stl. mesh of the same character, things change.

The pressure drop becomes proportional to the face velocity (note - not the square of the velocity, strange as it may seem), and also proportional to the mesh thickness. In this case, the mesh thickness in practice is usually and actually the thickness as a characteristic dimension that is actually the thickness as measured normal to the flow direction. So, for your thought experiment, if a mesh that has some finite thickness is substituted, the thickness at 45 degree yaw angle is 1.414 times the thickness of the case with the mesh oriented normal to the flow. The face velocity for the 45 degree yaw case is also (1/1.414) times the face velocity for the normal mesh case. Those 2 cancel one another out. Giving "about" the same pressure drop.

However, there is more to the story. The pressure drop is also proportional to something called the filter solidity, also sometimes called the packing factor for such things as packed beds or fluidized bed combustors. My thinking and reading of the posts was such that it looked to me as though the idea was to put more mesh in an already occupied space and thereby increase the mesh solidity, and/or have more than one yaw angle and have the same air cross the mesh more than once. If either or both of those situations were to be the case, then the pressure drop would increase very roughly as a bit in excess of the 1.5 power of the change in the solidity factor brought about by the compression. Or, if not compressed too much and, if in effect using 2 or more of an even number of mesh "faces" in a "V" that divides the mesh face area in 2 and creates 2 flow paths in series where before only one existed, each having half of the area, the face velocity will double and the effective mesh thickness will increase, although I doubt the effective thickness will increase as much as the shallowness of the yaw angle would indicate. Either of those situations will probably increase the pressure drop.

A single mesh that is opaque to sunlight, and a ducting arrangement such that cooler air enters the bottom (lower) corner of the collector above the mesh and flowing in a direction mostly parallel and very slightly into the mesh and/or with a diffuser so as to slow down and spread out the flow, and then eventually passing through the mesh and exiting the collector through a duct below (underneath) the mesh in the top (upper) opposite corner is probably one way to go.

[inetdog]

I can accept that.

The practical application paradigm, which is essentially an extension of my thought experiment, is the pleated air filter for an IC engine.
One reason for the pleats, as mentioned, is to provide a larger number of holes and greater surface area so that it is less affected by dust build up.
But when starting with a perfectly clean filter, and using only a single layer of identical thin filter material (not foam), is the pressure drop greater across a pleated filter than across an equivalent diameter smooth cylinder?

[J.P.M.]

I can accept that.

The practical application paradigm, which is essentially an extension of my thought experiment, is the pleated air filter for an IC engine.
One reason for the pleats, as mentioned, is to provide a larger number of holes and greater surface area so that it is less affected by dust build up.
But when starting with a perfectly clean filter, and using only a single layer of identical thin filter material (not foam), is the pressure drop greater across a pleated filter than across an equivalent diameter smooth cylinder?

Good Morning !

Not ducking an answer, but not being as familiar with the filter material, I'd answer: I don't know for sure.

My guess is that at least part of the reason for the pleats and the increased filtering surface area that it enables is to lower the rate of dirt deposition per square area of filter surface as f(time), and so spread out intervals between servicing. I'd also think (guess ?) that the filter elements are designed more with capture rate and particle size as well as durability in mind than pressure drop as the primary considerations beyond some point. It may very well be that thin(ner) filter elements such as in ICE combustion air inlet filter material have service pressure drops low enough so as to not be a concern if maintained/cleaned per design recommendations, those pressure drops made even lower yet by a lowering of the effective face velocity in some way or measure unknown to me, at least at this time, practical filtration technology not being my strongest suit.

[Skindancer]

Update on air filters

I thought that i would give anyone interested an update on my idea of using furnace air filters in place of screens in a solar air heater. I constructed a series of test chambers each of them 20 inches by 48 inches, this way I could fit all of them onto a single sheet of plywood. These were very basic test chambers, uninsulated except for 1 inch of high density foam in the back and they were all painted flat black. Each chamber had two 4 inch air inlets and a 4 inch air outlet and ran under negative pressure.

The first test chamber was a standard double layer screen absorber using black aluminum screen.
The second test chamber used 2- 1 inch polyester air filters painted black with several very light coatings of krylon fusion spray paint.
The third test chamber used 2- 1 inch pleated air filters painted black with several very light coatings of krylon fusion spray paint.
The fourth test chamber used 2- 2" flat polyester air filters painted with several very light coatings of krylon fusion spray paint.

Right off the bat I noticed that the airflow through the screen absorber was actually the lowest out of the four. Sadly my highly scientific test method for airflow was the "fluttery piece of paper held in the airstream" method but I was careful to balance the airflow so they were as close as possible.

Now, as for the testing. Today was not a good day for such things as there was a high level haze that reduced the available sunlight. The ambient temperature was 46 degrees F. when I started and after 20 minutes exposure to the available sunlight the static air temp inside the collectors were

Chamber 1 98 F.
Chamber 2 98 F.
Chamber 3 137 F
Chamber 4 143 F

When I started the fans the temperatures responded as follows

Chamber 1 83 F.
Chamber 2 83 F.
Chamber 3 87 F
Chamber 4 86 F

The temperature went up and down like a yo-yo as clouds came and went and while the ambient rose to 70 degrees F. by 4:00 PM cloud cover had reduced the the output of the collectors to only a few degrees above ambient.

While these tests were by no means precise and are not conclusive I believe that it does show that a 1 inch flat filter is comparable to a similar area of screen and has better airflow. The pleated filter showed very good Thermal collection and transfer and the 2 inch flat was almost as good.

The nice thing about the air filters is that they are cheap and very easy to work with. You can get them in almost any dimension you want and it greatly simplifies the construction of the collector.

In conclusion I believe I am going to construct a 4X8 collector and use 2 inch polyester filters as the screens.

Oh, one other thing, I really recommend that after painting your filters you let them cure and off gas for several days before installing them.

On a side note, thank God for aluminum duct tape! It is a great product for sealing seams and gaps or if you need to join two pieces of insulation together