AHU – Mixing Plenum

If the equipment is closely coupled to outdoor louvers in a wall,
the minimum outdoor air damper should be located as close as possible to the return damper connection. An outside air damper sized for 7.5 m/s gives good control. Low-leakage outdoor air dampers minimize leakage when closed during shutdown. The pressure difference between the relief plenum and outdoor intake plenum must be measured through the return damper section. A higher velocity through the return air damper—high enough to cause this loss at its full open position—facilitates air balance and creates good mixing. To create maximum turbulence and mixing, return air dampers should be set so that any deflection of air is toward the outside air. Parallel blade dampers may aid mixing. Mixing dampers should be placed across the full width of the unit, even though the location of the return duct makes it more convenient to return air through the side. When return dampers are placed at one side, return air passes through one side of a double-inlet fan, and cold outdoor air passes through the other. If the air return must enter the side, some form of air blender should be used.

Although opposed blade dampers offer better control, properly proportioned parallel blade dampers are more effective for mixing airstreams of different temperatures. If parallel blades are used, each damper should be mounted so that its partially opened blades direct the airstreams toward the other damper for maximum mixing. Baffles that direct the two airstreams to impinge on each other at right angles and in multiple jets create the turbulence required to mix the air thoroughly. In some instances, unit heaters or propeller fans have been used for mixing, regardless of the final type and configuration of dampers. Otherwise, the preheat coil will waste heat, or the cooling coil may freeze.

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