Contractor Will Use Manual D
One U know how much BTUH each room has to remove, our contractor can use manual D to figure our how to design the ductwork to deliver that amount of CFM/BTUH to & from each room.
FIGURING ROOM CFM - DUCT SIZES
Quick method: The heat gain and Btu/hr of cooling is done for each room. At 400-CFM per/ton of cooling, 12,000-BTUH / 400 = 30-Btu/hr for each (CFM) Cubic Foot per Minute of Airflow. At 450-CFM per/ton cooling use 12,000 / 450 = 26.66-BTUH per each CFM.
Then Select Supply duct size by CFM, velocity, & optimal Supply Air *(FR) Friction Rate. Everything has to be figured into the equation including the 90 turns in airflow & the diffusers, the sizing of the free-air area through Return air grilles or filter grille racks, etc.
A Room requires 3000-Btu/hr divided / by 30 equals 100-CFM, or around a 6“ dia. RD metal duct; the Return Air to the blower must be equal to the supply.
Both, the selected blower & the ductwork have to meet those room airflow & BTUH requirements.
Formula for finding CFM Airflow If you can measure the air velocity coming from a duct, here is a rough ballpark formula to get the CFM: CFM = (velocity in (FPM) Feet per Minute times the square footage of the duct area)
I.E., 16” Rd duct 201-sq.ins. X's 0.00694 = 1.39494-sq.ft. X's Velocity of 800-fpm = 1116-CFM
Times 1000-FPM = 1395-CFM. Branch ducts: 7“ Rd duct 38.48-sq. ins. X's 0.00694 = 0.2670512-sq.ft. X's 500-fpm=133.5-CFM
Example, times a velocity of 600-FPM X's 0.00694 = 160-CFM, the velocity is a big CFM & BTUH number changer for rooms.
The contractor needs to follow manual D. If U want to do a lot of reading, Go to my airflow & duct sizing pages, ha. - udarrell __ udarrell