General Industrial Ventilation Part 1: Powered Forced Supply/Powered Forced Exhaust
An industrial building with a powered forced supply/powered forced exhaust ventilation system uses both motorized supply fans and motorized exhaust fans.
Supply fans are mounted in the perimeter wall of the building about 8 to 10 feet above the floor with a diverter to force the air back down to the floor level. Exhaust fans are mounted in the roof of the building.
This is an extremely effective way of generating airflow toward the center of the building for personnel comfort cooling. Roof exhaust fans are located in strategic areas above high heat generating processes. Fresh air is forced to the areas of highest heat by supply fans and the roof exhaust fans extract the heat. When combined with column-mounted fans, the fresh outside air can be kept moving through critical work areas hundreds of feet away from the wall supply fans. For winter ventilation, direct heated air make-up units can be used (or incorporated into several supply fans) in order to add heat to the work area. The system can be designed to provide either a positive pressure or a negative pressure within the building. Fresh air can be provided through supply fans fitted with filters to maintain interior building cleanliness as required for food processing or where high outside dust conditions exist.
Supply and exhaust fans can be interlocked so that as the summer or winter temperature changes, fans turn on or off depending on the desired inside building temperature.
The primary drawback to a powered forced supply/powered forced exhaust is its up-front cost and operating cost?albeit this is considerably less than air conditioning the entire building.
As for other advantages: a powered forced supply/powered forced exhaust system is not dependent on prevailing winds in order to operate effectively all the times. An overall building air change rate and pressure can be guaranteed. Depending on the air change rate, the temperature throughout the building will be as close to the outside ambient temperature as possible. Workers feel airflow which provides a sensible cooling effect in the summer and with heated air make-up in the winter they are warm and dry for greater year round productivity and health.
Can powered supply fans be mounted on the roof?
Typically, we don?t like to install supply fans on the roof, but sometimes a building is too large or has an unusual process in the interior of the building and we have no other choice. The temperature off a roof in the summer can range from 100-140?F. Therefore, precautions must be taken to reduce the likelihood of bringing that hot air into the building.
In these rare situations, we raise the supply fan inlet off the roof deck so it doesn?t take in the boiling high heat rising up off the roof. We recommend keeping the inlet about 6-10 feet above the roof deck. Inside the building, we drop a duct straight down through the upper stratified layers of heat below the ceiling to put the cooler air past that layer by 10 or more feet.
There are disadvantages to roof-mounted ambient air supply fans. Primarily, because the fans are lifted up off the roof, wind loads must be considered and the fan inlet hoods may need to be secured with guy wires. Most recognizable is the fact that roof supply fans are operating counter to the normal flow of air within an industrial building, given that warm air rises toward the ceiling.
What is the effect of solar load?
Practically speaking, heat load from the sun on an insulated industrial building has a very small impact since the heat load from interior equipment/process or the optimum rapid inside air change rate used?for sizing wall supply fans and roof exhaust fans generally overrules.
Powered forced supply/powered forced exhaust is an excellent way to ventilate an industrial building. Since both supply and exhaust flows are regulated, the airflow in and out is always exactly the desired amount; the ventilation rate is not contingent on exterior conditions like wind, outside ambient temperature, the height of the building, temperature differentials, etc.
The goal of any properly ventilated system is to reduce temperature, prevent the buildup of heat, and create an interior work environment as close to the outside temperature as possible while also keeping an optimal level of air movement for comfort. The equipment, operating costs, and long-term maintenance are all significantly cheaper than refrigerated air conditioning.
The key to building ventilation is to determine a desired air change rate taking into serious consideration work area requirements, and heat-generating processes/equipment.
In the next installation in this series, we will be discussing forced supply with natural exhaust. If you have questions about your own ventilation system, don’t hesitate to contact the ventilation experts at Eldridge for a no-obligation recommendation.