Hybrid Rooftop Ventilators Made in America Offer More Performance – Here is Why

Kurt Shafer here, founder of the Thorwaldson Tornado brand of hybrid rooftop ventilators. The ventilators here are named hybrid because they combine the best of free air flow caused by gravity with the best of motorized air flow caused by electric power. The man in the banner above is my late father, Al Thorwaldson. More about Al here

Until 2016 the only source for hybrid rooftop ventilators was CSR Edmonds in Australia. Edmonds had been making the best gravity ventilator in history, their Hurricane, for many years. The engineers at Edmonds were brilliant to think of the idea to motorize the Hurricane to make the first hybrid ventilator in history. They made a decision to attach a  motor to the Hurricane turbine rain top to create their first hybrid. They specifically decided against putting a propeller inside the throat. Allan Ramsay, their Export Sales Manager, explained that their goal was to maximize free air flow and they felt a propeller would reduce that free air flow. Their new hybrid was named EcoPower and is offered in 3 throat diameters, 16 inches, 24 inches and 36 inches. They are the EP400, EP 600 and EP900. 

That design decision resulted in a serious deficiency in the air flow when the motor is used. It turned out that the turbine is a poor substitute for a propeller. It cannot move air in a static pressure above about 0.1 inch of water. The result is that their EP series is excellent for buildings with very little air flow restrictious such as 1 story buildings. But when used for 2 stories or more the air flow is no longer effective. As an example, when ZGF Architects in Seattle and the AEI engineers designed the 4 story Molecular Engineering building at the University of Washington, the engineers were forced to derate the EP900 hybrid fans from the rated zero pressure CFM of 5,880 down to 2500 CFM. 

As the founder of Invisco Industrial, interviewed the ZGF and AEI designers and realized that the Edmonds design could be dramatically improved. I  created the Tornado Hybrid Rooftop Ventilator. I specifically used a high performance propeller on a modern EC (Electronically Commutated Brushless DC) Motor – otherwise known as an ECM. 

The use of a propeller offers you the highest performance available because the powerful motor and blade work together to move 1000s of CFM under power and pull air even at high static pressures.

As an example, the Tornado TP900 has a 35 inch propeller inside on a 1 HP motor and pulls 14,000 CFM at zero static pressure. This is compared to the Edmonds EP900 which pulls just 5,880 CFM at zero SP. An even more dramatic comparison is a 0.2 static pressure where the Edmonds fan cannot pull air and the TP900 pulls over 12,000 CFM!

How does the propeller affect the free air flow? An analysis of the area available for air flow gives us a clue. The area of the 36 inch throat is PI times the radius squared or 3.14 times 16 inches squared. That is 803.84 square inches. The 3 propeller blades are 5 inches wide and 12 inches long. If we assume the worst case – that they are flat and block the air flow – they present an area of 5 times 12 inches times 3 blades or 180 square inches of air flow restriction. If we then assume the air cannot flow around them, the total area is 803.84 minus 180 or 623.84 square inches. 

This represents a 23% reduction in free air flow. To offset this the Tornado turbine height is increased by 25%. 

Please visit the case studies – the Studio City study is most important because it offers you a view into a 1 story application where static pressure is not an issue. 

Here is the performance graph for the EP900 – it falls to zero at just over 0.1 :

Here is the Tornado TP900 performance graph – still strong at 0.2 and up 

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