Fresh Air Systems

Fresh air systems is the process of “changing” or replacing air in any space to control temperature or remove moisture, odors, smoke, heat, dust and airborne bacteria. Ventilation includes both the exchange of air to the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in the buildings. Methods for ventilating a building may be divided into mechanical/forced and natural types. Ventilation is used to remove unpleasant smells and excessive moisture, introduce outside air, and to keep interior building air circulating, to prevent stagnation of the interior air.

Mechanical or forced ventilation “Mechanical” or “forced” ventilation is used to control indoor air quality Excess humidity, odors, and contaminants can often be controlled via dilution or replacement with outside air. However, in humid climates much energy is required to remove excess moisture from ventilation air. Kitchens and bathrooms typically have mechanical exhaust to control odors and sometimes humidity. Factors in the design of such systems with Mechanical or forced ventilation through an air handling unit or direct injection to a space by a centrifugal fan.

Fresh Air Systems Overview

Ducting System

  • The equipment of the exhaust system should be located to permit, as far as possible, a symmetrical layout of pipes about the central fan, to minimize inequality in airflow resistance in the branches.
  • The shortest lengths of straight duct work should be used; long runs of small diameter duct, and all unnecessary elbows, tees or entries should be avoided.
  • Exhaust duct takeoff should, wherever possible, be located in the line of normal contaminant travel.
  • Duct work should be located so that it is readily accessible for inspection, cleaning and repairs; ductwork should be protected against external damage.
  • For large and shallow hoods, multiple takeoffs could be used to attain the desired distributions of exhaust airflow. Interior baffles or filter banks could also be used to attain satisfactory air distribution.

Air Cleaning Equipment

  • Air cleaning equipment should be compatible with all the components of the local exhaust ventilation system.
  • Fire safety and explosion control must be considered when designing or selecting an air-cleaning device.
  • Emission rate, capacity, and resistance of the air cleaner should remain as constant as possible throughout its daily operating cycle and be nearly independent of entering dust, fume, or vapor concentration.

Metal Thickness And Classification

Classification of ducts for Local Exhaust Ventilation systems for non corrosive applications.

Class 1 Light duty for nonabrasive applications such as replacement air and general ventilation.
Class 2 Medium duty for applications with moderately abrasive particulates in light concentrations such as woodworking and grain dust.
Class 3 Heavy duty for applications with highly abrasive particulates in low concentrations such as abrasive cleaning and sand handling.
Class 4 Extra heavy duty for applications with highly abrasive particles in high concentrations such as canopying systems in heavy industrial plants.




Diameter of Straight Duct Class 1 Class 2 Class 3 Class 4
100 mm to 200 mm 22-20 22-18 16 14
>200 mm to 450mm 22-12 22-12 16-11 14-11
> 450 mm to 760 mm 18-7 16-7 16-6 14-6
> 760 mm 14-2 14-2 12-2 12-2

Continuous Control

Some experts recommend continuous ventilation to simplify controls and to avoid unhealthy indoor air for the several hours it may take a system to flush out pollutants after having been off for an extended time.  If pollutants continuously released from furnishings or building materials are the principal concern, continuous ventilation is most effective.

Programmable Timer

Rather than operate a whole-house ventilation system continuously, a programmable timer can be employed to operate the system intermittently for a selected period of time each day.  To provide equivalent ventilation, such a system must have a higher capacity than one that operates continuously.


The most efficient ventilation ducts are smooth, short, straight, and properly sized. Smooth sheet metal ducts offer low airflow resistance. Because corrugated ducts have greater flow resistance, it is important to keep them as short as possible-stretch the corrugated material to its full length and cut off the excess. Minimize the number of elbows. Provide adequate support. Use mechanical fasteners and sealants (preferably duct mastic) at all joints. Ducts located outside the conditioned space should be insulated.

Fresh Air Systems Application Areas

Fresh Air Systems are widely used in Chemical reactor environment Fresh air ventilation, Automobile assembly area Fresh air ventilation, Painting booth Fresh air ventilation, Metal polishing ventilation, Foundries Fresh air ventilation, plastic injection molding area Fresh air ventilation .


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