SPRAYED FIBER EROSION IN CEILING PLENUMS Air Return Velocities Are a Key Factor in Determining if Fireproofing Fibers Will be Eroded

By Michael J. Kodaras At first glance it is conceivable return air ceiling plenum are suf- Acoustical Consultant that the passage of high velocity ficiently high to cause substantial return air across the fireproofing fiber erosion. o mineral fibers from sprayed fibers can cause an erosion of these Q: How does a typical office D fireproofing contaminate air fibers. They may then be carried in building return air system conditioning systems? In recent the air stream through the return work? years the question of health air ducts, through air filtration A: The supply air system dis- hazards from various fibers in the systems and then discharged into charges filtered, conditioned air air has raised this type question. occupied spaces serviced by the into occupied office spaces usually With reference to sprayed fire- supply portion of the air condi- from room air diffusers which, in proofing, the question arises from tioning system. turn, are serviced by duct-fan the fact that the space between the It is not within the province of systems of varying complexities. underside of the steel floor deck this writer to offer an opinion as to The supply air reaching the offices and suspended acoustical ceiling in the effect of sprayed mineral fibers is usually a mixture of outside air office buildings is frequently used upon the health, however some and recirculated return air. The as a return air plenum. A large questions can be answered as to supply air, after entering the proportion of these steel decks are the possibility of such fiber erosion. offices, is transferred through protected by sprayed fireproofing Also, whether the airflow veloci- grilles located in the suspended to meet fire codes. ties prevailing in an average acoustical ceiling, then into the plenum space between the top of the ceiling and the underside of the fireproofed steel floor deck. This plenum space is essentially a large return air duct coated on the upper side with sprayed fibers. Return air fans draw this air through the plenum into a much smaller verti- cal return air collection duct located at one side of the building or into a duct shaft, which in turn delivers the return air back into the supply system. Q: What is the velocity of the return air in the plenum? A: Most air conditioning systems in office buildings supply from 1 to 4 cubic feet per minute (cfm) per square foot of floor space. The plenum usually has a free height (from the back of the suspended acoustical ceiling to the bottom of the beams) of at least 1 foot. The return air flow therefore takes place in a simulated duct equal in size to the floor-ceiling area multi- plied by the free height of the plenum. In a 1 foot high plenum, Mineral fiber fireproof- the air velocity would be approxi- ing material is spray applied to steel decks, columns and girders. Continued on page 22

11 KODARAS obtained easily and which shows page 9, a testing procedure that Continued from page 11 the relationship between air veloc- consists of an airtight metal duct ity in the return air plenum as a with a blower and filter on one end mately the same as the cfm of air function of distance from the and an eroded fiber collection supplied per square foot of floor return air collection duct and the system on the other end. The test space; that is, between 1 and 4 feet velocity in the return air duct. The is conducted of a sprayed fiber per minute (fpm). If the plenum is mechanical drawings will provide specimen over a period of 24 hours. very low—say 6 inches instead of information as the the cfm handled The G.S.A. requirements are that one foot, the velocity of return air by the return air duct. Divide cfm the fiber erosion shall not exceed in the plenum may double to 2 to 8 by the size of the duct (in sq. ft.) to 0.025 gram per square foot over fpm. This velocity, 8 fpm, is so low arrive at the air velocity in the this time period with an air as to be imperceptible. For return duct in fpm. It should be velocity of 800 fpm parallel to the example, the design air velocity in noted, of course, that plenum loca- test specimen. offices is usually between 35 fpm tions within a radius of 2 feet from Most well known manufacturers’ and 70 fpm. the return air duct may be exposed products have been able to meet Q: Does the air flow velocity in- to velocities in the order of 400 to this G.S.A. requirement in the crease near the return air duct? 800 fpm. past, however a change in fibers, A: Yes, in the immediate vicinity Q: Is there a recognized test pro- fiber density, binder or overspray of the return air collection duct the cedure for determining wheth- may require retesting. above mentioned very low air er sprayed fiber will erode in a Q: How may the high velocity velocities increase. 400 - 800 fpm air stream? areas of the return air plenum Q: How can I determine what the A: Public Buildings Guide Specifi- near the return air ducts be air velocity may be at a specific cation “Sprayed Fire Protection,” protected to avoid erosion of location? PBS: 4-0920 obtainable from the the fibers? A: An approximation of the air General Services Administration, A: The manufacturer of the spray velocity in the plenum can be Washington, D.C. describes, on materials should be consulted to determine whether its standard product can withstand the maxi- mum velocities of a specific system or whether additional tamping, overspray or other procedures should be followed at locations near the return air ducts. Q: Have any studies been made of the effect on health of fibers in air systems? A: An article entitled, “Fibrous Glass Insulation: Health Hazard Question Resolved” by John M. Barnhart, executive secretary, National Insulation Manufacturers Assn., Inc. published in the August 1971 ASHRAE JOURNAL, quotes the finding of five doctors and hygienists to the effect that small quantities of glass fibers are not harmful. Other fibers, such as rockwool, are not mentioned.

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