Hi,
I get a typical 120fpm out of my thermosyphon collector for an about 60F typical temperature rise.
Its important thermosyphon collectors to make sure the airflow resistance is low. This means good flow paths, good sized upper and lower vent openings, and (if you use a flow through absorber) a low resistance absorber.
About the only way you are going to get the airflow velocity and flow are to measure them. You can calculate the pressure difference driving the flow using the stack equation, but the amount of flow depends both on this pressure difference, AND on how much air resistance you build into the collector -- in other words, two thermosyphon collectors with the same pressure difference driving them can have widely different flow rates depending on how good the flow paths are in each.
The velocity measuring gadgets are not too expensive, and handy for other things, so you might want to just buy one. Here are some:
http://www.builditsolar.com/References/Measurements/measurments.htm
I have the Dwyer vane one and the Kestrel wind meter -- the both work well. I actually like the $25 Dwyer one as long as the airflow you are measuring is flowing in a horizontal direction. It works really well, and its cheap. The Kestrel will only measure down to 80 or 90 fpm -- this should be OK for a good thermosyphon collector.
One common misconception on these collectors, is that a greater temperature difference from inlet to outlet means more heat output. Usually a really large increase in temperature from inlet to outlet means that the airflow is too restricted -- this leads to high temps inside the collector and larger losses and lower efficiency. Its the product of the (temperature rise)(airflow) that determines the heat output -- anything that increases this product is good. "Airflow" above is (Air Velocity)*(Vent Area). Your temperature rise is pretty high -- I think you might find that more airflow (less resistance) and less temperature rise would give you more heat output.
Here is a little blurb on measuring collector efficiency:
http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm
The details on my thermosyphon collector:
http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm
Height about 7 ft,
Depth about 1/15 of the height
Absorber -- flow through, made with two layers of window screen
Vent area about equal to 1/2 the crossectional area of collector (more would be even better).
These groundrules are ones that Steve Baer developed many years ago after quite a bit of experimental work.
Gary
