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Fan, AHU calculation

Selecting the correct duty(VOLUME) of any fan whether for ventilation, extract, air conditioning system, fume exhaust, dust removal, paint etc.  The designer must take into consideration the design of the Ductwork,the application and the Friction loss of the designed Ducted system, Filters,Grilles,Louvres etc. First to establish the cubic area to be ventilated, according to the activities of the occupants. Either air changes per hour(ACH) or to the local health and safety recommendations as to the volume of fresh air per person.
If calculating to ACH:
Calculate the total cubic area to be ventilated Height x Width x Length = CUBIC AREA, multiply the AREA by the amount of ACH divide this by 60 = VOLUME required either in cubic Meters per Minute(M3/pm) or Cubic Feet per Minute(CFM). For example: An area of 12 x 12 x 3 = 432 cubic feet or meters, Multiply 432 x ACH (4 air changes per hour) = 1728, Divide 1728 / 60(Minutes = 1 Hour)  = 28.8 = Fan Duty(VOLUME) of 28.8 cubic meters/28.8 cubic Feet per Minute.
If calculating to recommended fresh air to occupants:
According to the activity of the occupants local rulings, guidelines or regulations the VOLUME of fresh air required will be described as either cubic Feet per minute(CFM),cubic Meters per Hour(C3/h), cubic Meters per Minute(M3/m) or cubic Meters per Second(M3/s). If the required VOLUME fresh air required per person = 20 CFM/ 0.566 M3/m then simply multiply the VOLUME x number of inhabitants.For example: 14 inhabitants require 20  CFM/0.566 M3/m  =
14 x 20 =  280 CFM or 14 x 0.566 = 7.924 M3/m.  

FAN DUTY SELECTION (ventilation only) Free to download and use!  Importantly select either Metric or Imperial units, 
 Enter dimensions of area, amount of air    changes required, select "Enter" the tool will   return the necessary VOLUME of air required. 
If the user requires VOLUME of air  per person,  simply enter required Volume M3/m / CFM  then number of occupants, select "Enter" the tool will return necessary VOLUME.
Also included the conversion of the most used units to cubic Meters per Hour.  

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 Having determined the VOLUME the calculation of the Duct size requires the VELOCITY, The Velocity is obtained with either experience or recommendations by HVAC organisations. Depending on the application the VELOCITY can vary between as little as 200 FPM/60 MPM (low pressure) to above 5,800CFM/ 1800 MPM (high pressure). For the same VOLUME of air the smaller the Duct will result in higher VELOCITY creating more FRICTION loss through the Duct.