FLOW/CON FABRIC AIR DIFFUSER
DESIGN GUIDELINES

FOF INC designs and fabricates Flow/Con Fabric Air Diffusers for a wide range of air systems. This includes heating, air conditioning, evaporative cooling, ventilation, dehumidification and makeup air. Flow/Con Air Diffusers are being used in factories, supermarkets, health clubs, swimming pools, broadcasting studios, museums and many other facilities. Each Flow/Con Air Diffuser System is custom designed. The key variables that must be evaluated when designing a fabric air diffuser system include:

1. Air Volume and Static Pressure.
2. Air Temperatures and Humidity.
3. Diffuser Diameter Alternatives.
4. System Static Pressure.
5. Diffuser Length and Height Above Floor.
6. Diffuser Distance to People and Equipment Areas and Air Flow Requirements in These Areas.

1. Most Flow/Con Air Diffusers are designed with round airjet ports in coated fabric. The ports range in size from .125"e; diameter to 6.0"e; diameter and larger. Round ports are very effective at controlling airjet throw and placing air exactly where it is needed. They are highly efficient at entraining and mixing air. The ports also add a great deal of flexibility to diffuser design. Their size and position can be varied along the length of a diffuser, depending on airflow needs and restrictions.

   The smaller ports are called Softflow, and they diffuse quickly. They are used in sensitive areas where drafts could be a problem, particularly with cool air. The larger ports are called Powerflow, and are used to throw air 10 to 50 feet, or to provide cooling ventilation in very hot work environments.

2. FOF INC also uses permeable fabric for air diffusers, but the potential applications are very limited. These diffusers are called Microflow, and they have millions of tiny or micro airjets that diffuse very, very quickly. Normally, we consider permeable fabric for just two situations. One would be when cool air is distributed in a very confined area, and the slightest draft can not be tolerated. But even in these sensitive environments, Softflow designs are generally more effective. The second situation would be an air system that requires a final filter. But for these and other applications, permeable fabric diffusers have limitations.
   1. Even with prefilters, the material plugs up and the diffusers have to be taken down and cleaned regularly.
   2. A plugged or partially plugged diffuser can overload the air handling equipment and will not deliver the air volume required.
   3. If permeable fabric is used to distribute heated air, it will intensify warm air stratification and increase energy costs.

MAJOR DESIGN CONSIDERATIONS

1. Possible condensation must always be considered when designing fabric air diffuser systems. The distribution of cool air into a hot and humid environment is a typical scenario for condensation. Sharp temperature differences at the diffuser surface can cause the humid air to cool and leave water droplets on the diffuser. But all fabric air diffusers with frequently spaced airjet ports have built-in protection from condensation.

   Diffuser airjets quickly entrain and mix with the surrounding hot and humid air. This creates a transitional temperature zone around the diffuser, and eliminates sharp temperature differences at the surface. The frequent spacing of air jet ports along both sides of a diffuser is the key. Two rows of 3.0” diameter ports on 18” centers, twenty rows of .25” diameter ports on 1.5” centers or a million micro airjets discharged through permeable fabric all accomplish the same thing. In a well designed air diffuser, each of the above alternatives will quickly entrain and mix sufficient air to develop and maintain a transitional temperature zone around the diffuser. The result for all three designs is no condensation.

2. When distributing heated air, Powerflow air diffusers are most effective at minimizing the stratification of warm air. Just the right airjet size will deliver warm air to the level that is needed. To destratify warm air and minimize energy costs for heated air systems, coated fabric must also be used for the diffuser. Plastic ports or linear slots can be incorporated in a permeable fabric diffuser, but a large percentage of the warm air is still discharged through the fabric. It mixes quickly and remains at the same height as the diffuser or higher. This compounds the warm air stratification process, and increases energy costs very significantly. Consequently, permeable fabric is not a good choice for heating systems.

3. Round airjet ports in coated fabric is also the most effective way to distribute cooled air. In small spaces or when drafts could be a problem, small Softflow airjets are used. When it is necessary to throw cool air, the larger Powerflow airjets are used. The airjet ports are sized and positioned to deliver cool air exactly where it is needed, and at just the right velocity. To further improve the effectiveness of an air system, the size and position of the ports can change as needed along the length of the diffuser.

   For air systems that distribute both cooled and heated air, round airjet ports in coated fabric is again the most effective design. Just the right port size and position are selected to both cool and heat efficiently and comfortably.

4. Powerflow airjets in coated fabric is also very effective at providing people ventilation. Ambient, air conditioned or evaporative cooled air can be used to ventilate people in hot work stations.

5. Coated fabric air diffusers with round airjet ports can also be used for ventilation systems designed to dilute contaminated air. To minimize the entrainment of upper level contaminates, the diffusers should be installed as close as possible to the people zone, and supply air temperature should be slightly less than room air. Depending on airflow requirements for the system, either Softflow or Powerflow airjets can be used. The clean airjets are directed horizontally just above the people zone. They dilute contaminated air in the people zone, and at the same time, force warmer, contaminated air to rise. Ceiling fans exhaust the contaminated air. This is an efficient system for improving air quality very substantially.

   In some environments, permeable fabric air diffusers can be used to displace rather than dilute contaminated air . The diffuser must be installed as close to the floor as possible. Supply air must be at least 10 degrees cooler than room air and the static pressure in the diffusers must be very low. The micro airjets enter the room at a very low velocity, with minimum air entrainment and mixing. The clean, cooler air flows across the floor and forces warmer, contaminated air to rise. People and machines then contaminate and warm the new air. It in turn, is forced to rise. Ceiling fans exhaust the contaminated air.

6. In environments such as museums or broadcasting studios, noise is a very important design consideration. Generally, fabric air diffuser systems reduce any noise entering the system. But in the very critical areas, it is also important to size the diffuser to minimize inlet velocity, keep air pressure low, use small airjet ports and use softer fabrics such polyester.

© 1999 FOF Inc.