National Oilheat Research Alliance 15 To find the duct system’s total equivalent length, add the trunk duct measured length, the branch duct length and the equivalent length of the fittings used from the furnace to the register. Figure 14. Duct Design Goals There are three basic goals when designing a duct system. In order of importance they are: 1. Allowing the system to circulate enough air to operate properly. All heating and cooling equipment has a minimum requirement for airflow. A duct system that will not allow the unit to deliver this minimum airflow will cause inefficient operation and potentially shortened equipment life. 2. Distributing the air proportionally to where the load is. To design a duct system that will move an adequate amount of air, the designer needs to know: System CFM Minimum available system static Duct friction rate 3. Keeping airflow noise below objectionable levels. System Airflow Airflow is measured in cubic feet per minute or CFM. Systems can operate correctly over a wide range of airflows. The proper airflow requirement can be found in the manufacturer’s design and specification literature. If you do not have the manufacturer’s specification sheet to work from, use 400cfm per ton for air conditioning or use the following formula for an oil furnace: CFM = output Btuh 1.08 x TD All duct systems should be designed to the higher airflow requirement, be it heating or cooling. The cooling air requirement is commonly used because the cooling airflow requirement is usually higher than the heating requirement. Air conditioning or cooling is designed to both lower the temperature of the home and reduce humidity. The airflow over the coil has an impact on the amount of sensible cooling (temperature reduction) versus the amount of latent cooling (humidity removal). Reducing the airflow increases the amount of humidity removal and decreases the sensible cooling. The amount of sensible and latent loads can be determined by using the Air Conditioning Contractors Association’s Manual J. It is important to remember that duct sizing is calculated on the basis of sensible cooling, not total cooling. With the sensible load known, the following formula may be used to find the airflow requirements: Cooling Factor (CF) = building cooling CFM sensible heat gain This cooling factor would be used to multiply the room Btu requirement to determine the airflow requirements for the area. On the heating side, we can use the same type of formula to determine our airflow requirements for heating: Heating Factor (HF) = building heat CFM sensible heat loss The heating factor would be used to determine the CFM requirements for heating each area by multiplying the room Btu requirements. Maximum Available System Static Pressure The maximum static pressure the equipment can operate against is specified by the equipment manufacturer. Everything the air must travel through or over will create some static pressure. Static pressure drops will be caused by evaporator coils, humidifiers, air filters, grilles and registers. The amount of pressure drop across any of these devices is directly related to the amount of air going through that device. Increased airflow creates an increase in static pressure loss. The available static pressure is determined by subtracting all the losses
NORA Advanced Air Flow
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