vav system design

Primary Airflow Capabilities All primary air dampers can shut off the primary airflow completely. However, complete shutoff of primary airflow is not recommended if the VAV terminal unit serves a zone that has a heating load If the zone has a heating load, the primary air damper should be specified to have a minimum airflow position in order for the VAV terminal unit to provide ventilation to the zone during times of low cooling load and during the heating mode of operation.

The inlet velocity sensor requires a minimum air velocity of about 400 fpm through the inlet in order to accurately measure the primary airflow. Therefore, all VAV terminal units have a minimum primary airflow below which the terminal unit cannot accurately deliver primary air. This minimum airflow depends upon the size of the primary air inlet.

The maximum primary airflow that can be delivered by single-duct and fan-powered VAV terminal units is limited by the acceptable radiated and discharge noise criteria (NC)levels for the terminal units. The noise that is generated by a VAV terminal unit increases as the air velocity through the primary air inlet increases and also increases as the differential static pressure between the inlet and outlet of the terminal unit increases.
Most VAV terminal unit manufacturers test their equipment in accordance with the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) Standard 880-98 to obtain the sound power levels for their terminal units at various primary airflows and various differential static pressures between the inlet and outlet of the terminal unit. below Table summarizes the minimum allowable airflows for different-sized VAV terminal unit inlets (based on the minimum airflow required by the inlet velocity sensor) and the maximum recommended airflows (based on the AHRI-rated airflows with a 1.5 in. w.c.
differential static pressure between the inlet and outlet of the terminal unit). These maximum airflows result in radiated and discharge NC levels for the terminal units of about 30, which is an acceptable NC for spaces within commercial buildings.

Capacity Once the heating and cooling loads of the zones within the building have been calculated, the heating and cooling capacity of the VAV terminal units serving these zones can be determined. The cooling capacity (or maximum primary airflow) of a VAV terminal unit must be equal to or greater than the cooling supply airflow calculated for the zone. The zone cooling supply airflow is based on the zone sensible cooling load, the design primary air temperature (which is normally 55°F), and the zone cooling setpoint (which is normally 75°F).

The heating capacity of a VAV terminal unit must be equal to the zone heat losses through the building envelope, plus the heating load of any outdoor air infiltration to the zone, plus the heat required to raise the heating primary airflow from the primary air temperature (which is normally 55°F) to the zone heating setpoint (which is normally 70°F). Thus, the VAV terminal unit heating coil not only needs to meet the heating load of the zone served by the terminal unit, it also needs to heat the heating primary airflow up to room temperature. Once the load on the heating coil is known, the heating supply airflow must be calculated. The heating supply airflow should be calculated based on a supply air temperature that is 15°F warmer than the zone heating setpoint.

which equates to 85°F for a 70°F zone heating setpoint. If the heating supply air temperature is greater than 20°F warmer than the room temperature, the air within the room will stratify. That is, the warmer air will rise to the top of the room and the cooler air will remain near the bottom of the room, resulting in an uncomfortable condition for the occupants. Refer to the Selection sections of the Single-Duct VAV Terminal Units and Fan-Powered VAV Terminal Units sections later for sample calculations of the heating and cooling supply airflows and the capacities of the heating coils for single-duct and fan-powered VAV terminal units.

Standard hot water heating coils for VAV terminal units are one-row and two-row coils. The heating capacity of the hot water heating coil depends upon the number of rows, entering water temperature, water flow, and entering air temperature. Standardsized electric heating coils are available for both single-duct and fan-powered VAV terminal units depending upon the unit size. The heating capacity of the electric heating coil is independent of the entering air temperature.

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