While fans are far from the best means to move air for all the reasons mentioned and more, if one is going to use them he had best find high quality ones from such commercial suppliers as W.W. Grainger Co. see:
www.Grainger.com Look there under BROWSE PRODUCTS>HVAC>FANS>AIR CIRCULATING. In their print catalog (obtainable by proving that you are a business)is a section of Reference pages with one labeled: "FAN LAWS"; your question about ratio of air movement may be answered there (can't find my copy or would tell you myself). If that does not help you, notice on Grainger's site the RESOURCES secition with 'Certification Information' link to HVAC organizations, all of which have various sorts of information available. The "Machinist's Handbook" may also include such a table.
There is a chapter in the 'text book' for movie palaces, "American Theatres of Today" by Sexton (1930) {find it through Inter-Library loan at your library, or look for it by title at
www.Amazon.com )about Heating and Ventilation of the theatre which discusses the desired Cubic Feet per Minute of air movement (regardless of heating or cooling) that is desireable for any theatre. In some jurisdictions, the CFM is dictated as a certain minimum according to the building or occupancies codes. Check local authorities.
Fans were used in older theatres since the turn of the century, but were gladly outmoded with the arrival of dependable air cooling which in most theatres was forced by huge blowers in the basement or attic using the "Downflow" system (the Upflow system results in cold feet during air conditioning) as described in American Theatres of Today. With the advent of true air conditioning after the second World War, the dew point of the air could be reduced so far as to make relative humidity comfortable, not just cooler, and fans were completely outmoded, since these freon-based systems were run as Blowers year 'round and were virtually noiseless.
When the freon systems were installed as retrofits in most theatres back then, they always disabeled the original Washed-Air system which had used carbon dioxide or amonia coolers to chill the air without dehumdifying it. Almost always the original blowers were then re-wired to be on only when the compressors or heaters were on, so continual, healthy air movement became a thing of the past. With the building of much smaller Cinemas (as compared to the vast cubic feet of Movie Palaces of thousands of seats) cheaper 'Unit' systems were installed having blowers plus heaters/coolers integrally mounted, and with a minimum of branch ductwork. While the makers advertised these as the successors to the old Washed-Air systems where sprays of water continuously sprayed through the stream of incoming outdoor air and the air was then chilled to remove most of the humidity, then heated if necessary to counteract the cooling, these new Units were no where near the ability to produced the clean, washed air that the Palaces enjoyed. Of course, the Units' cost was also FAR less than the huge Washed-Air systems!
No one advocates trying to reactivate the old Washed-Air systems today, since the upkeep and costs are prohibitive, sad to say, but to rely on fans for most all air movement is not a good idea. Fans do deteriorate much faster than a blower and are not nearly as efficient. They do wobble somewhat depending upon the type of mounting, since some have ball joint mounts that are designed to allow them to wobble somewhat, since that reduces stress on the fan and its mounting. Yes, in all cases, a steel safety cable (chains tend to rattle) should be installed from the superstructure to the fan stem below the mount, since tragic failures of fans as they fell have been recorded. As always, it pays to buy the best fan that your funds allow, if you are convinced that fans are the way to go. Some theatres which want more air movement have made quiet 'Air Risers' by putting a vertical duct on the floor and mounting it to the wall (using suitable cosmetics: paint or panels) where it terminates near the ceiling. They put a blower (surplus blowers are often available from local furnace repair men) in the bottom of the unit behind a grille, and force the air either up or down, as needed by season (some blowers are reversible). Depending upon the size of the room, one or two of these may be cheap and effective with no droped parts hazard and much less noise if the blowers are properly pad mounted, and the ducts curved to avoid Venturi effects.
If you have an old theatre with the ventilators under the seats or in the risers, by all means try to reactivate them, since they were designed to provdide ideal air circulation to every patron. You may have to pull the old Washed-Air spray racks out of the plenum chamber in the basement, but it is a small cost compared to the benefits gained. If the old fans(s) are too difficult to re-motor, if needed, then consider using a framework supporting several blowers wired to work in unison.
Those so-called 'mushroom vents' under the seats also often permitted many items to fall into the plenums under the seats and therefore are often a rich treasure trove of old programmes, play bills, and patrons' items lost over the years. Be sure to take a flash light and crawl down as far as you can, but never alone. If you are re-doing the place and have such vents, you might save yourself some headaches by putting steel hardware cloth over the openings to prevent other things going down there (and vermin from crawling up!) Putting chew-proof steel mesh behind all vent grilles will discourage mice and rats from making a home in your theatre, where the smell of popcorn is irrestible to them.
Finally, do not believe a contractor or salesman who says that the fans will be perfect for your place and will never make noise; great pains were taken in the design of the movie palaces to keep motors and any other such noise makers away from the audience, in fact, many of the large ones had concrete or pyrobar roof decks to keep out outdoor noise, as well. If your ceiling is low, try to avoid fans completely.
Further information may be available from The League of Historic American Theatrs:
www.LHAT.org or from the Theatre Historical Society of America:
www.HistoricTheatres.org The League maintains listings of specialists/contractors in most building categories whom you may consult informally on-line or professionally (for a fee). Best Wishes.
Jim Rankin, member THSA since 1976
P.S. Just found this link for a $90 book on ceiling fan air movement. It is an engineering text, so may be at a local tech school, and you can always ask for it on Inter-library loan through your local library.
http://resourcecenter.ashrae.org/store/ashrae/newstore.cgi?itemid=8568&view=item&categoryid=174&page=1&loginid=243568
[This message has been edited by jimor (edited June 02, 2003).]
[This message has been edited by jimor (edited June 04, 2003).]
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