Ventilation 5 Fans Vladimír Zmrhal (room no. 814) http://users.fs.cvut.cz/~zmrhavla/index.htm Dpt. Of Environmental Engineering 1 Introduction Fans air pump that creates a pressure difference and causes airflow generally for gas transport rotary blade machine not only for HVAC applications Fan parameters total pressure difference p volume airflow rate V power input P 1

Types of fans According to the direction of airflow centrifugal fan axial fan mixed-flow fan cross-flow fan Drive arrangements direct drive coupling drive belt drive 3 Types of fans According to pressure rise across the fan Centrifugal fans low pressure p < 1000 Pa medium pressure p = 1000 to 3000 Pa high pressure p > 3000 Pa Axial fans propeller tubeaxial vaneaxial 4

Types of fans According to using residential fans ducted fans jet fans roof fans smoke-ventilating fans hot-gas fans corrosion-resistant fans etc. 5 Centrifugal fans impeller wheel (1), inlet (), discharge (3), scroll (4), motor (5), frames (6) scroll (diffuser) - kinetic energy pressure energy Centrifugal fans blow air at right angles to the intake of the fan; the impeller rotates, causing air to enter the fan near the shaft and move perpendicularly from the shaft to the opening in the scroll-shaped fan casing 4 1 3 1 5 4 6 6 3

Centrifugal fans impeller design a) forward-curved blades b) backward-curved blades c) radial blades d) aerofoil blades low pressure fans with forward-curved blades most widely used in ventilation and air-conditioning lots of blades (40 to 50), η m = 0,55 to 0,65 medium pressure and high pressure fans with backward-curved blades small number of blades (6 to 15), η m = 0,8 to 0,85 7 Centrifugal fans a) b) c) d) 8 4

Axial fans the axial-flow fans have blades that force air to move parallel to the shaft about which the blades rotate because the discharge opening is inline with its entrance, and axial flow fan offers the advantage of simplified duct arrangement wide variety of applications, ranging from small cooling fans for electronics to the giant fans used in wind tunnels standard axial flow fans have diameters from 300 400 mm or 1800 to 000 mm and work under pressures up to 800 Pa 9 Axial fans Propeller fans low pressure fans Tubeaxial fans generally considered to be heavy-duty propeller fans Vaneaxial fans basically tube axial fans plus vanes. behind the fan blades are vanes which straighten the spiral flow of air, thus increasing the static efficiency 10 5

Axial fans Jet fans 11 Mixed-flow fans in a mixed flow fan the air flows in both axial and radial direction relative to the shaft. mixed flow fans develops higher pressures than axial fans the angle of leaving air < 90 example: roof fans 1 6

Cross-flow fans also tangential fan or tubular fan main flow moves transversely across the impeller, passing the blading twice forward-curved blades, the fan is usually long in relation to the diameter L = (1 to 5)D example: fan-coil units, η m = 0,45 to 0,55 13 Fan parameters Volume airflow rate V [m 3 /s] volume of air, which is transferred by fan performance data are based on dry air at standard conditions 101,35 kpa and 0 C ρ = 1, kg/m 3 Total pressure difference p [Pa] the fan have to pass the system pressure losses (static pressure) Electrical power P [W] V p P = η tot 14 7

Fan parameters Specific fan power SFP [W/(m 3 /s)] P p SFP = = V η tot Energy consumption [MWh/a] Volume airflow rate V [m 3 /h] 15 Energy consumption E Pdτ P tot τ 0 n = = 0 [kwh/year] E tot V p = τ = SFP. V. τ 1000 η tot V p V p 1 V p Etot = τ + τ + = τ i 1000 1000 1000 n 1 1 i i 1... ηtot,1 ηtot, 1 ηtot, i τ working time of the fan [hours] 16 8

Fan performance curves p = f (V) P = f (V) η tot = f (V) n = const. 17 Fan laws n = var.; ρ = const. ρ = var.; n = const V n = V1 1 n 1 V = V n p = p1 n1 p = p ρ 1 ρ 1 n P P n = 1 1 3 P = P ρ 1 ρ 1 18 9

Temperature rise across fans P = M + Q P pv Q = M = pv = V ρc t η η tot p SFP ttot = = ρcη ρc c tot p tfan = (1 ηc ) ρcη c 19 Temperature rise across fans Temparature rise across the fan t fan Total temparature rise t tot Total pressure difference p [Pa] Total pressure difference p [Pa] 0 10

Fan and system pressure relationship 1 Fan and system pressure relationship dynamic pressure p d w = ρ total pressure pt = ps + pd total pressure difference of the fan p = p p = p + p = p + p + p t t1 t1 t l1 l d 11

Fan and system pressure relationship p l = 0 3 Duct system charakteristics Pressure losses of ductwork friction local losses l w w l w p λ loss KV d ρ ζ ρ λ d ζ = + = + ρ = 1443 1443 friction local losses K λ l = + d 8 ρ ζ π d 4 4 1

System and fan charakteristics System curve Fan charakteristics Working point 5 Parallel fan operation System curve Fan charakteristics fan operation 6 13

Serial fan operation System curve Fan charakteristics fan operation 7 Control of the fans System characteristic change dampers reducing of airflow by increasing the system pressure required incresing power consumption P = V ( p p ) ' 8 14

Control of the fans Fan characteristic change economical speed control n 9 Control of the fans a) variable speed motors variable speed motors (twin or three speed motors) number of motor poles p (s slide of the motor) b) frequency control optimal control control in wide range od 0 do 100 % for bigger systems 10f n = 1 p ( s) c) voltage control 30 15

Control of the fans d) EC motors electronically commutated (EC) motors brushless DC motors where the direction of the electric current is switched using electronic controllers variable speed control, without the drawbacks of brushes DCV (demand control ventilation systems) 31 Control of the fans Adjustable pitch-blades a) Controllable-pitch in suction of centrifugal fan for backward-curve impellers only b) Controllable-pitch of impeller wheel for axial fans high operation efficiency complicated mechanism a expensive 3 16

Thank you for your attention 33 17