are generally not required.Įxample: A fan is selected to deliver 4,000 CFM. Correction for temperatures between 40 deg. Use the Air Density Correction Factors (df) listed below to determine the corrected fan performance. An installations altitude and the air temperature will affect the air density. Operate at 1,100 rpm and will require a 2 hp motor.įan performance tables are based on standard air conditions. The operation point at the new SP the fan will need to The system SP is actually higher than calculated. 75 because fan motor was operating nameplate ampsīHP2 /. 375 SP, 825 rpm and motorĬalculate the required RPM using fan lawsĤ,000 / 3,000 = RPM2 / 825, New RPM = 1100 Once a system is installed if the CFM, SP and BHP are known (by measurement) the CFM, SP and BHP at every other point along the system curve can be calculated.Įxample: Fan is selected at 4,000 CFM. 375 spĪnother rule of thumb that can be used if number of fittings is notįan Laws are equations used to predict a fans performance at other conditions. ft.)ĭuct Velocity 1,400 fpm (air flow / duct area)ĭuct Losses. There are 2 elbows and a grille in the system. Fan is ducted to roof with 16 x 16 duct. Area of room is 20 x 60 or 1,200 sq, ft. 08 per fitting (elbows, register, grille, damper)Įxample: Retrofit a fan for a mechanical room. 40 per 100 feet of duct (based on duct velocity of 1000 to 1800 feet per minute). When the duct layout is not known the following rules of thumb can be used to estimate system static pressure.Retro Fit and Design Build Jobs - the system static pressure can be calculated using the required airflow, the duct size, duct layout and total duct length and duct friction loss chart (Friction Loss Chart needed).Plan and Spec Jobs - required airflow and system static pressure is calculated by the engineer and is listed on the fan schedule.Where more than one method is shown use the method that results in Listed below are ventilation rates for some common applications. ( Note: rule of thumb outdoor / indoor temperature The desired indoor temperature and design outdoor dry bulb temperature.Įxample: 200,000 BTU/hr to be removed, 70 degreeĭesired indoor temperature and 90 outdoor dry bulb temperature.ĬFM = 200,000 (BTU/hr) / (1.08 x 90 - 70) CFM = 9,260 Is needed to calculate CFM is the amount of heat to be removed in BTU/hr, Temperature general ventilation can be used to provide cooling. When the temperature of a space is higher than the ambient outdoor ![]() ft./150 sq, ft p/ personĬFM = 16 people x 20 cfm per person CFM = 320 The occupancy of a general office is one person per 80 to 150 sq. Occupancy Method - Derives the ventilation rate from the number of people that will occupy the space at any given time.Įxample: For an office, the recommended ventilation rate is 20 cfm per person. An auditorium is 80 x 90 with 20 ceiling or 144,000 cu. for a flow rate of 60 cfm.Īir Change Method - Derives the ventilation rate from the volume of the space (in cubic feet) to be ventilated multiplied by the number of total air changes in one hour.Įxample: For an auditorium the suggested air change rate is 4 to15 air changes per hour. inĪrea, HVI recommends an exhaust rate of 1 CFM per square foot. Of the space (in square feet) to ventilated multiplied by ventilation rateĮxample: For residential bathrooms up to 100 sq. Airflow for general ventilation can be calculated by Īrea Method Derives the ventilation rate from the area
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