Better mix: raise high the outdoor air inletEditor's note: "The Right Mix" (Oct. 2001, p. 69) author Gene DeJoannis, P.E., supplies an addendum to his story. In a recent article in CSE, I wrote about the difficulty of insuring good mixing in large factory packaged air-handling units (AHUs), and proposed a compact mixing device to physically "tunnel" cold air into the warm return-air str...


Better mix: raise high the outdoor air inlet

In a recent article in CSE , I wrote about the difficulty of insuring good mixing in large factory packaged air-handling units (AHUs), and proposed a compact mixing device to physically "tunnel" cold air into the warm return-air stream—the "Injecti-mixer." Although small air handlers permit outdoor air to be connected to the return duct before the AHU—thereby achieving good mixing—I commented that this is not practical for large AHUs because the ducts are so large they do not permit enough room. But there is a way: In cases where the AHU is backed up against an outdoor-air plenum, with the return or exhaust fan suspended in-line in an overhead return duct, the outdoor air plenum can be moved from the floor to ceiling of the mechanical room. This way, the return-air damper is installed in the room side of the outdoor-air plenum and the return duct connected to it. Note that the outdoor- and return-air streams enter nearly opposite each other and both must turn 90° downward and then 90° horizontally to enter the AHU. Although it is risky to predict air-mixing behavior, it seems reasonable that this arrangement will mix air better than traditional practices. Additionally, this reduces the likelihood of pulling in dirt as the inlet is elevated above the outside roof or surface.

GENE DeJOANNIS, P.E., van Zelm Heywood & Shadford

Integrity of fire cabling questioned

Mineral insulated (MI) cable manufacturer, Pyrotenax, has a videotape demonstrating a danger of circuit-integrity cable. When the insulation "ceramifies," it creates a flammable smoke which can spread fire through conduit wired with the U.L. 2126-approved cable. "Circuit Security," (Oct. 2001, p. 55) is silent on this aspect.


Author Dean Wilson's response: I am not familiar with the video and feel that it would be inappropriate for me to comment on these allegations until I view it myself. With regard to the general topic of smoke migration through conduit: This subject has been discussed in various forums over the years, and I am not aware of any notable fire occurring where propagation of smoke through the conduit played a significant ro le in physical damage or the threat to life.

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