Energy Efficient Buildings
Fan Systems Homework
1. 4,000 cfm of air flows through circular
ducts shown at right. The duct system
includes two 5-gore elbows, a reducer
and 270 ft of elevation rise. The system
draws air from and discharges air to
atmospheric pressure. A) Find the
velocity, elevation, duct, 24-in elbow,
16-in elbow, reducer and total head loss
(in-H20). B) Find the fluid work (hp)
added to the air by the fan. C) If the
ducts are perfectly insulated, find the
temperature rise of the air from inlet to
outlet, neglecting fan inefficiency.
80 ft of 16-in duct
200 ft of 16-in duct
Reduction to 16-in duct
70 ft of 24-in duct
70 ft of 24-in duct
30 ft of 24-in duct
2. Buildings can deliver heating/cooling energy through both air and water. Compare the
transportation energy required for delivering the same amount of heat using air and water. A)
Find the volume flow rate of air (cfm) needed to deliver 12,000 Btu/hr of heat to a house if the
supply temperature is 110 F and the return temperature is 75 F. B) Find the friction head loss (in
H20) if the equivalent length of all ducts and fittings is 300 ft and the ducts are designed at 0.10
in H20 per 100 feet of duct. C) Find the fluid work required by the fan (hp). Now do the same
for a hot water heating system. D) Find the volume flow rate of water (gpm) needed to deliver
12,000 Btu/hr of heat to a house if the supply temperature is 110 F and the return temperature
is 75 F. E) Find the friction head loss (ft H20) if the equivalent length of all pipes and fittings is
300 ft and the pipes are designed at 3.0 ft H20 per 100 feet of pipe. F) Find the fluid work
required by the pump (hp) using the relation Wf (hp) = V(gpm) x h (ft-H20) / 3960. G) Find the
ratio of fluid work required by the fan to fluid work required by the pump. H) Comment on why
this is so, and the implications for energy-efficient design.
3. Consider the duct system shown below. The volume flow rate through each discharge
damper is 1,000 cfm, for a total flow rate at the fan of 12,000 cfm. Each square is 10 feet. Use a
design pressure drop of 0.10 inH2O per 100 feet of duct length.
Use the Equal Friction method in the DuctDesigner software to determine the diameters of each
duct and the total pressure required at the outlet to the fan. Specify 7-gore elbows and low-loss
branches. Each discharge damper has an equivalent length of 20 feet plus the pressure required
for discharge to the atmosphere. To document your solution, paste the data from the
DuctDesigner input file and a screen shot of the DuctDiameter output screen into a Word
document.