MECH 8250 – Building Systems Winter 2015 MSYS 4490 - AC Systems MECH 8250 – Building Systems Winter 2015 DUCT DESIGN By Satwinder Singh 10/11/2018 By Satwinder Singh

Duct design Air flow in ducts Major and Minor Losses in Ducts Loss coefficient for some fittings Equivalent length for a fittings 10/11/2018 Satwinder Singh

Air Flow in Ducts

Velocity air pressure, Pv Pv in in water and V in ft/min Pv in Pa and V in m/s Mass Density ρ 62.4 lbm/ft3 and 999 kg/ Pressure changes during flow in ducts. 10/11/2018 Satwinder Singh

Friction Loss Tedious task to solve by equations Pressure Loss Charts have been prepared. 10/11/2018 Satwinder Singh

10/11/2018 Satwinder Singh

10/11/2018 Satwinder Singh

Equivalent of a circular duct The pressure loss due to friction is greater for a rectangular duct than for a circular duct of the same cross-sectional area and capacity. For most practical purposes ducts of aspect ratio not exceeding 8 : 1 will have the same lost head for equal length and mean velocity of flow as a circular duct of the same hydraulic diameter. When the duct sizes are expressed in terms of hydraulic diameter Dh and when the equations for friction loss in round and rectangular ducts are equated for equal length and capacity, an equation for the circular equivalent of a rectangular duct is obtained: Dh = Hydraulic diameter a and b are the dimension of a rectangular duct 10/11/2018 Satwinder Singh

Equivalent of a circular duct 10/11/2018 Satwinder Singh

Elbows are generally efficient fittings in that their losses are small when the turn is gradual. When an abrupt turn is used without turning vanes, the lost pressure will be four or five times larger. When considering the lost pressure in divided flow fittings, the loss in the straight-through as well as the loss through the branch outlet must be considered. 10/11/2018 Satwinder Singh

10/11/2018 Satwinder Singh

Example: Pressure Loss Compute the loss in total pressure for a round 90-degree branch and straight-through section, a tee. The common section is 12 in. in diameter, and the straight-through section has a 10 in. diameter with a flow rate of 1100 cfm. The branch flow rate is 250 cfm through a 6 in. duct. 10/11/2018 Satwinder Singh

Equivalent lengths 10/11/2018 Satwinder Singh

Example: Compute the equivalent lengths for the fittings in the duct system of Fig. 12-24. The fittings are an entrance, a 45-degree wye, the straight-through section of the wye fitting, a 45-degree elbow, and a 90-degree elbow. 10/11/2018 Satwinder Singh

What is the total pressure loss on the critical path? Example: What is the total pressure loss on the critical path? Critical Path: 1-a-3 with Equivalent length of 127.4 ft We pick average pressure (friction) loss for duct and calculate the total pressure loss for the system. Friction loss to be designed for = 0.08”/100ft P = 127.4 ft x 0.08”/100ft = 0.102” Path 1: 1-a-2 Le = Li + 50 + Lwye,S + 50 = 19 + 50 + 4.4 + 50 = 123.4 ft Path 2: 1-a-3 Le = Li + 50 + Lwye,Br + Lelbow-90 + 40 = 19 + 50 + 11 + 7.7 + 40 = 127.4 ft 10/11/2018 Satwinder Singh

Duct Accessories Turning vanes Dampers Fire dampers Linear Airfoil (More efficient) Dampers Parallel blades Opposed blades) Fire dampers Turning vanes prevent turbulence and high loss in total pressure where turns are necessary in rectangular ducts where we don’t have space for large-radius turns. When turning vanes are used, an abrupt 90-degree turn is made by the duct, but the air is turned smoothly by the vanes. Turning vanes are of two basic designs. The airfoil type is more efficient than the single-piece flat vane. 10/11/2018 Satwinder Singh

Turning Vanes 10/11/2018 Satwinder Singh

Dampers 10/11/2018 Satwinder Singh

Duct Design Volume (Q) is a function of cross sectional area (A) and velocity (V) Q=AV however, momentum, friction and turbulence must also be accounted for in the sizing method 10/11/2018 Satwinder Singh

Air Flow in Ducts

Static Pressure Force required to overcome friction and loss of momentum due to turbulence As air encounters friction or turbulence, static pressure is reduced Fans add static pressure Static pressure is measured in Inches-water gauge Positive pressure pushes air Negative pressure draws air Straight ducts have a pressure loss of “w.g./100’ based on diameter and velocity 10/11/2018 Satwinder Singh

Equivalent Length Describes the amount of static pressure lost in a fitting that would be comparable to a length of straight duct 10/11/2018 Satwinder Singh

Duct Construction Round ductwork is the most efficient but requires greater depth Rectangular ductwork is the least efficient but can be reduced in depth to accommodate smaller clearances Avoid aspect ratios greater than 5:1 10/11/2018 Satwinder Singh

Equal Friction Method Presumes that friction in ductwork can be balanced to allow uniform friction loss through all branches 1. Find effective length (EL) of longest run 2. Establish allowed static pressure loss/100’ ΔP=100(SP)/EL 3. Size ducts 4. Repeat for each branch Note: velocity must be higher in each upstream section 10/11/2018 Satwinder Singh

Equal Friction Method - Example The system shown is supplied air by a rooftop unit that develops 0.25 in. wg total pressure external to the unit. The return air system requires 0.10 in. wg. The ducts are to be of round cross section, and the maximum velocity in the main run is 850 ft/min, whereas the branch velocities must not exceed 650 ft/min. Size the ducts using the equal-friction method. Show the location of any required dampers. Compute the total pressure loss for the system. 10/11/2018 Satwinder Singh

Equal Friction Method - Example 10/11/2018 Satwinder Singh

10/11/2018 Satwinder Singh