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“Free Air” for Virginia’s Factories
September 2011

By: Lance Lewis

Outside air, “Free Air”, can be used to efficiently and economically remove excess heat and contaminants from manufacturing and warehouse buildings. Ambient outside air can be used to cool workers, electrical equipment, motors, and machines so they can operate more efficiently. Using outside air effectively will result in better industrial working conditions, and more efficient employees and machinery.

The amount of airflow for general ventilation requirements of factories and warehouse buildings can be determined using one of two different methods: 1. An analysis of the Heat Gain of the building including the solar gain through the roof and walls, and the heat given off by the process equipment, motors, electrical controls, lights, ovens, dryers, and people; or 2. Using established Air Change Rates with the suggested number of Air Changes per Hour for different types of industries and work related activities.

The procedure for determining the Heat Gain in a building with people, motors, lights, production equipment, and processes is covered in the ASHRAE Fundamentals Handbook, and Manual “N” from the Air Conditioning Contractors of America.
After the amount of Heat Gain is established in BTUH, the ventilation airflow can be determined by using the formula:

Airflow in Cubic Feet per Minute (CFM) = Heat Gain in BTUH
1.08* X Temp. Rise in ° F.

*1.08 is a constant based on the Specific Heat of Air, multiplied by 60 minutes.

Temperature Rise in ° F. is the design temperature rise. For example: using 10° F. allows the incoming air to absorb heat from inside the building and increase in temperature 10 degrees F. before being exhausted.

As an alternate to the above procedure, the general ventilation of factories and warehouses is often designed using Air Changes per Hour guidelines developed by engineers for industrial applications. Air Changes per Hour information is published by fan companies such as Aerovent Company’s “Air Change Rates for Ventilation” from their “Engineering Resource Guide”.

Determining whether to use the higher or lower Air Change Rate depends on the severity of the heat, contaminant conditions, and the end user’s heat relief requirements.

In addition to general building ventilation, the indoor air quality can be significantly improved by exhausting the heat or contaminants at their source of generation. Effective source capture will reduce the total amount of general ventilation airflow needed.

The following items should be considered in the design of an efficient and cost effective ventilation system for controlling specific sources of heat or contaminants: Type of hood(s), and amount of air (expressed in Cubic Feet/Minute, or CFM) that can be used to effectively capture the heat and contaminant(s); Hood location, size of connecting ductwork, number of elbows from the hood to outside, pollution control equipment (if needed), and the required fan; and Source(s) of replacement air: Will fans will be used to supply ambient outside air? Will the air need to be filtered, or heated (air make-up units)?

Specific sources of heat or contaminants are usually captured by using overhead canopy or side draft exhaust hoods specifically designed for the application. The guideline is: The closer and more encompassing the capture hood is to the source of heat or contamination, the lower the amount of air that needs to be exhausted. Please review the “Capture Velocities (or Airflow) For Exhaust Hoods” from Aerovent’s “Engineering Resource Guide”.

In addition, the Industrial Ventilation, A Manual of Recommended Practice published by the American Conference of Governmental Industrial Hygienists is a valuable reference and guide for the hood and ductwork design needed for specific applications.

For both general building ventilation and source capture systems, consideration of the best locations for the exhaust fans and the replacement air intakes will maximize the effectiveness of the system.

With general plant ventilation, roof ventilators located near the center of a building are used to exhaust large quantities of air to prevent the build-up of warm air in a building.

The replacement air can be drawn into the building through open doors, windows, and louvers located as low as practical around the perimeter of the building. The ambient air should flow across the workers and heat producing motors and processes before the warmed air rises up to be exhausted by roof mounted fans.

When using roof mounted fans, a 15° F. temperature rise in the exhausted air may be acceptable. For example, the ambient outside air may be pulled into the building at 75° F. at the level of the workers and exhausted at roof level at 90° F., where there are no workers.

Two-Way Roof Ventilators can be used to exhaust hot air during warm weather conditions. The fans can be reversed to pull in ambient outside air in cooler months. The cold, outside air will mix with warm air near the ceiling and provide tempered replacement air for air that is exhausted year round from ovens, dryers, plating tanks, paint booths, other process systems, and pollution control equipment.

Caution must be used when considering wall fans to exhaust warm air. The outside air is usually drawn in at the workers’ level, and the air warms-up as it crosses the factory before being exhausted at the other side of the building. In this scenario, the area near the wall mounted exhaust fans will have a higher ambient temperature than the air entering side of the building. To reduce the temperature in the area of the wall mounted exhaust fans, more air will need to be moved into, across, and out of the building.

For example, to hold the air temperature rise to 5° F., in the area near the exhaust wall fans, the required airflow will be twice as much using wall exhaust fans, rather then using roof ventilators with a 10° F. temperature rise designed for with the air exhausted well above the normal worker level.

Also, the location of the sources of the replacement air and the wall mounted exhaust fans must be considered to avoid short circuiting. When an exhaust fan is located near an open door or window, it is easy to visualize the flow of the outside air into the building and directly out the closest exhaust fan. This arrangement will be less effective in reducing the heat buildup inside of the building.

In buildings with sources of heat located away from an outside wall, exhaust air systems designed to control the heat with capture hoods and ductwork may be the best choice. A well-designed hood will reduce the amount of air to be exhausted and replaced. This approach increases the efficiency of the heat, or contamination capture system, and reduces the general building heat gain, and the spread of the contaminants.

The design considerations for the replacement air include: 1. The amount of replacement air should meet, or exceed, the amount of air being exhausted by all exhaust fans, including paint spray booths, oven exhaust fans, and natural draft vents on gas combustion heating equipment; 2. The replacement air may need to be filtered year round, and heated in cold weather with air make-up units; and 3. The sources of replacement air should be designed so that the air passes over the areas to be cooled or contaminant controlled, and that uncomfortable drafts or air movement do not disturb the workers or the process.

Outside, ambient air is a free commodity that can be used to help control building air temperature and air quality, cool motors and electrical equipment, and clean, cool, or dry parts and products. The benefits include increased worker productivity, increased efficiency and life of electrical equipment and motors, and lower operating and maintenance costs. Please put “Free Air” to work for a more efficient and healthy plant.


Lance Lewis is president of The Summerell Company, a Roanoke, Virginia-based Manufacturers’ Representative firm. Focused on commercial and industrial HVAC applications, he can be reached via telephone at 540-774-4079, or via e-mail at

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