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    Fresh air make up (2 Posts)

  • David Broome David Broome @ 1:07 PM
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    Fresh air make up

       Had a young person email me a question about economizers and fresh-air make up. Now there is one big difference between the two,They are spelled diffidently lol. He commented that while in tech college the instructor didn't really hit on the subject that much and that is very understandable.
    OK here goes a very simple version of the what's and whys in Kudzu language.

       Fresh air was originally used to bring outside air into the buildings to help cut down on the compressor run time aka electrical consumption in the older cotton mills in the deep south and most folks would agree with that but according to the HVAC scholar's it was first-ed used up north in the larger super markets.The engineers would build a mechanical room for all the refrigeration units and would utilize all the heat from the condensers to help heat the buildings in the winter time.Now fresh air make up has multiple usages. Used in schools,restaurants,hospitals,morgues,chemical plants,sugar cane,chicken houses and several other applications

       Most local and state codes require 10% fresh air in a commercial application and this can and will vary. Operate ting room require in most States 100% fresh air and now we get in to preheat and charcoal filters and it goes on and on.Now most instruct6ors not all ( the last thing I want to do is insult anybody or hurt someones feelings ) just do not have the experience or practical knowledge. Spay paint booths normally require 100% fresh air on the systems to keep the fumes from building up and causing either an explosion or a fire.

       Now there is a difference between a fresh air damper and a economize. Usually but not always fresh-air is a fixed setting on how many cfms of air you bring in to the return air of a unit where an economizer will generally be used to bring in 100% fresh out-side air and generally shut down the compressor to reduce energy cost.Now I have 3 suggestions to help you better understand the two items

    .01 contact the professor Eugene

    .02 contact john Williams Jr of the Carrier corp about carrier college ( on- line )

    .03 order Dooley bible of refrigeration advance hvac issue( not sure if this book   is still available

    Now if you have specific need or a problem with one just email me back

    Remember education is not free but neither should your time be

    PEACE BE WITH YOU

    DAVID C. BROOME
    This post was edited by an admin on October 11, 2013 1:14 PM.
  • pecmsg pecmsg @ 2:55 PM
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    !

    Just to muddy the waters a bit, 10% fresh air is a bad term. The dampers have a 10% minimum closed setting but everything is sized for minimum air changes per hr. At my facility 3-5 CPH for unoccupied areas, store rooms etc and 7-8 CPH for occupied space offices and such. The dampers can never close to a point that reduces these #’s
     
    With 100% OA units there is 1 or more exhaust fans that try and maintain in an operating theatre positive pressure at all times, at this facility neg pressure
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