inlet size part 2

[wannabedivin]

After all the lessons learned, I am getting ready to build a better one.  I noticed on retired2's project that for 5 inch pipe (19.63 square inches)  the inlet opening was 3 7/8W X 5 7/8 tall (22.76 square inches).  For a 4 inch pipe (12.5 square inches), what would be the right measurement?  Building a top hat side inlet

[retired2]

After all the lessons learned, I am getting ready to build a better one.  I noticed on retired2's project that for 5 inch pipe (19.63 square inches)  the inlet opening was 3 7/8W X 5 7/8 tall (22.76 square inches).  For a 4 inch pipe (12.5 square inches), what would be the right measurement?  Building a top hat side inlet

It probably doesn't mean much for an inlet pipe, but the HVAC Industry says rectangular duct is less efficient than round and should be upsized 10% to maintain similar performance. 

So, just choose a rectangular dimension that yields 10% more area than the area of the 4" round.

[wannabedivin]

Thanks much.  An increase of 10% would put it at 15.  And everything I have read so far is that rectangle is better than square.  A 3 X5 gets 15 square inches.  Two questions:  what is a good ratio of height to width, and if I go above a 10% increase at what point would I be doing more harm than good?  I am using a 1500CFM 2 HP collector. 

[retired2]

Thanks much.  An increase of 10% would put it at 15.  And everything I have read so far is that rectangle is better than square.  A 3 X5 gets 15 square inches.  Two questions:  what is a good ratio of height to width, and if I go above a 10% increase at what point would I be doing more harm than good?  I am using a 1500CFM 2 HP collector.

It probably does not matter if you go over the 10% recommendation simply because you are so close to entering the large separation chamber.

With regard to the best rectangular ratio, I'd simply advise you not to take it to extremes.  Your idea of 3" x 5" is probably ideal.  It might also depend a little on whether you are building a 2H unit or just a 1H.  The rectangular inlet is probably more advantageous on a 1H unit, so maybe you could stretch the ratio just a bit further if that is what you are building.