Testing is done! Just a quick recap: My neighbor Craig and I wanted to explore the optimal screen gap. So we built a test collector with three channels with various screen gap sizes. We wanted to find the gap size that produced the most BTU's. Of the few ZP collectors that have been built, the screen gaps range from 1", up to roughly 1.5". I didn't count Seatec's 8 layer ZP since it is really a different beast altogether. We tested the following screen gaps: 1", 1.25", 1.375" and 1.5".
Day 1 of testing: We compared the 1", 1.25", and 1.5" screen gaps. All three channels were outputting 73 CFM’s @ 9.5MPH. Three readings were taken, with the average below.
1" screen gap---------1,796BTU's Produced
1.25" screen gap-----2,341BTU's
1.5" screen gap———2,076BTU's
The 1.25” gap produced 30% more BTU’s than the 1” gap
The 1.5” gap produced 16% more BTU’s than the 1” gap
The 1.25” gap produced 15% more BTU’s than the 1.5” gap
So in this round, the 1” gap was the loser, and the 1.25” gap was the winner!
Day 2 of Testing: Since the 1” gap clearly lost round 1, we eliminated it from competition and changed it to 1.375”, which is right between the two best performers from day one:1.25” and 1.5”.
Again, we matched the CFM output at 65 @ 8.5MPH, tested three times, and averaged the three below. Being that the difference between the screen gap sizes tested below is now only ⅛”, we expected the results to be closer together, and they were.
1.25” screen gap————2,201BTU’s Produced
1.375” screen gap———-2,489BTU’s
1.5” screen gap————-2,099BTU’s
The 1.375” gap produced 19% more BTU’s than the 1.5” gap
The 1.25” gap produced 5% more BTU’s than the 1.5” gap
The 1.375” gap produced 13% more BTU’s than the 1.25” gap
The 1.375” gap produced 39% more BTU’s than the 1” gap.
Conclusion:Clearly you can have a gap that is either too large or too small, and as this test shows, the sweet spot is a 1.375” screen gap. I expected that the 1.5” would be too large as the screens would be too far from the main flow of air. I didn’t expect the 1” gap to perform so poorly. My thought were as long as you had the same amount of air moving through the 1” gap, the closer screens would transfer heat more effectively. Perhaps at a lower CFM the 1” gap might perform better. Testing screen gaps in increments smaller than ⅛” is impractical as it would be difficult to tighten a screen so it has less than a 1/16” amount of sag.
One last note:We compared the airflow entering the intake manifold from the end versus the back of the collector. The CFM results were the same. It was thought that air entering the manifold from the end had an easier entry into the screen gap since it is pointing directly at the gap. A deflector was NOT needed for end entry for our testing. Entry via the end often requires the use of extra elbows, along with straight or corner boots, which add a LOT of EL to the ducting. Entry via the back is far easier and looks better too as it hides the ducts from sight and keeps them out of the sunlight as well. But either way works.
Greg and Craig in MN