Here is a table showing air changes recommended for various buildings.
As an example, if your building is 30,000 square feet with 33 foot high roof (no attic) then the total volume of air is 1,000,000 cubic feet. Based on the table below nearly every space has a recommended change of at least 4 times an hour. Rooftop ventilators come in sizes that are rated in CFM or cubic feet per minute. 1 million cubic feet in 15 minutes is 66,666 CFM.
Below is a comparison of rooftops made by 4 sources, Greenheck, Loren Cook, Edmonds Australia and Invisco Industrial ( the Thorwaldson Tornado). This comparison is based on a 20 inch diameter throat. You can see the leader is the Tornado. For larger buildings Invisco Industrial makes larger fans. An example is the Tornado 48 inch fan. It pulls 20,000 CFM at 975 watts and offers gravity air flow estimated to be over 4000 CFM in a light 7 MPH breeze. . The illustration below is meant to show the value of the new AMCA classification – “Hybrid Rooftop Ventilator”. You can see that the other American made sources have restrictive rain hats. Edmonds and the Tornado use a modern turbine rain hat that allows very good gravity ventilation for free along with motorized air flow when needed. You will also note that the turbine offers air flow when powered at far less electric power than the other US sources.
The conclusion here is that Thorwaldson can make you a fan that not only will cost less to buy and install but will slash your electric power cost. Write Tornado@invisco.com with your building parameters for a free quotation.