An Introduction to Air Pollution Chapter 8. Auxiliary Equipment: Hoods, Ducts, and Fans 薛人瑋 Ph.D 16th October 2012 Role of hoods in an industrial process Performance assessment • Static pressure – Potential energy put into the system by the fan • Velocity pressure – Pressure arising from air flowing through the duct. This is used to calculate velocity • Total pressure – Static pressure + velocity pressure – Total pressure remains constant unlike static and velocity pressure 3 Hood Operating Principles Enclosing Hood Canopy (Receptor) Hoods Use and misuse of Canopy Hoods Basic Air Flow Equations • Q=V*A • TP = SP + VP • V = 4005(VP/d)0.5 Hood Flow Rate Determination For an enclosure capture velocity at the enclosed opening is the exhaust flow rate divided by opening area The capture velocity at a given point in front of the exterior hood will be established by the hood air flow through the geometric surface which contains the point For a theoretical unbounded point suction source Q = v * a = v * 4 * π * x2 = 12.57 * v * x2 Where Q = air flow into suction point, cfm V = velocity at distance X, fpm A = 4 * Π * X2 = area of sphere, ft2 X = radius of sphere, ft 9 Hood Flow Rate Determination For an unbounded line source Q = v *2 * π * x * l = 6.28 * v * x * l Where L = length of line source, ft In general the equation used is Q = v * (10 * x2 + a) Where Q = air flow, cfm V = center line velocity at X distance from hood, fpm X = distance outward along axis of flow in ft A = area of hood opening, ft2 D = diameter of round hoods or side of essentially square hoods, ft 10 Hood Losses Entry losses occur due to formation of venacontracta at the entrance of duct. The hood entry loss represents the energy necessary to overcome the loss as the air enters the duct. The losses increase with increase in flow area. Hoods with two or more points of loss are compound hoods. The basic equations used are (for simple hood) SPh = hed + VPd Where SPh = hood static pressure, ―wg hed = entry loss transition (Fh * VPd ) VPd = duct velocity pressure 11 Hood Losses For compound hoods: SPh = (FS) (VPS) + (FD) (VPD) + VPD This is when duct velocity is greater than slot velocity. Where: SPh = hood static pressure, ―wg FS = entry loss factor for slot VPS = slot velocity pressure, ―wg FD = entry loss factor for duct VPD = duct velocity pressure, ―wg total hood loss = hood entry loss plus air acceleration loss P (1.0 Fh )VPduct 12 Designing a Canopy Hood over a Hot Source • For a low, circular canopy hoods Qt = 4.7 ( Dh)2.33 (ΔT)0.42 Qt = total hood air flow, cfm Dh = diameter of hood, ft ΔT = difference between temperature of the hot source, and the ambient, F • For a low, rectangular canopy hoods Qt = 6.2L ( W)1.33 (ΔT)0.42 L = length of the hood, ft W = width of hood, ft Capturing Hoods Hood has to generate sufficient airflow at and around the source to ‘capture’ and draw in the contaminant-laden air Ducts Bernoulli’s equation P1 2 2 v1 P2 v2 w hf 2g c 2g c • • • • • • • • ρ = fluid density (ft3/lbm) p = static pressure—gauge (lbf/ft2) z = height of fluid above some reference point (ft) v = fluid velocity through duct, hood, etc. (ft/sec) gc = gravitational constant (32.2 ([lbm-ft/sec2]/lbf) η = fan efficiency w = work added by fan, etc. (ft-lbf/lbm) hf = head loss due to friction (ft-lbf/lbm) Ducts Bernoulli’s equation • ρg/gc = γ 2 P1 2 v P v 1 2 2 Hf 2g 2g • ρ = specific weight of the fluid, lbf/ft3 • g = gravitational acceleration, 32.2 ft/sec2 • Hf = head loss due to friction, ft of fluid • Hf fLv 2 Dc 2 g Hf Kf Hv v v 2 2g 2 2g • Hv = velocity head of air, ft of air Kf = fitting loss factor The Pitot-static tube Ducts VPact VPstd act std f 0.0001575QP w 4A Dh P act Pact Pstd std Interchangeability of static and velocity pressures in air systems TP1 TP2 SP1 VP1 SP2 VP2 pressure loss Fans Primary air moving devices used in industrial applications Basic groups of fans are: Axial fans Centrifugal fans 20 Fan Types Air exits with centrifugal force Rotation Pressure side Axial Fan - Air is moved through the fan in an axial direction. High flow very low pressure development. Suction side Centrifugal Fan Operation Fan Types Air exits with centrifugal force Rotation Pressure side Backward Curved Centrifugal - Air enters the eye of the impeller drawn in by the suction side of the blade. Less flow than axial but more pressure development. Suction side Fan Curve, Backward-Curved Centrifugal Blade TP=total pressure SP=static pressure TE=total efficiency =QΔPtotal/Wshaft SE=static efficiency HP=horsepower Fan Types Forward Curved Centrifugal - Generates more air volume and higher static pressure than backward curved of the same size but at the expense of lower efficiencies. The scooping action of the forward curved generates a high air velocity which in turn give a high dynamic pressure. The fan scroll converts this into static pressure. Fan Types Mixed Flow Fan - A compromise between an axial and backward curved centrifugal. Axial throughput but with the performance of a backward curved. Tangential Fan - These move air in a strange manner. Air exits in a similar way to a centrifugal fan but air enters the impeller through the blades. Comparison of each fan type Fan Selection Various factors effecting fan selection are: Volume required (cfm) Fan static pressure Type of material handled Explosive or inflammable material Direct driven vs belt driven Space limitations Noise Operating temperature Efficiency Corrosive applications Fans 28 System Pressure Effects • Fan curves are typically given in terms of total pressure vs. volumetric flow rate • A typical fan running at a fixed speed can provide a greater volumetric flow rate for systems with smaller total pressure drops (if we’re to the right of the peak in the fan curve). • Total pressure loss=static pressure loss + dynamic pressure loss 1 2 Ptotal Pin,static Pout ,static Vin2 Vout 2 • If exit and inlet area of a duct are about the same, the dynamic pressure loss (or gain) may be minimal. Total pressure of fan The total work a fan must do to move a specified volume of air against the static pressure plus the velocity pressure is defined as total pressure of the system Rating Tables FSP TPoutlet TP inlet VPoutlet FSP ( SPoutlet VPoutlet ) ( SP inlet VPinlet ) VPoutlet FSP SPoutlet SPinlet VPinlet FSP Psuction side Pdischarge side VPinlet Fans 31 Fan static pressure rise System Resistance • • Sum of static pressure losses in system Increases with square of flow rate Actual with system resistance calculated 33 Fan Laws Fan data for geometrically similar fans can be collapsed onto a single curve using dimensionless numbers FSP N 2 D 2 w N 3 D5 Q=volumetric flow rate D=fan diameter N=fan rotational speed W=fan power =fluid density P=fan pressure rise Fan Laws Example Fan Operating Efficiency Evaluation The parameters to be studied in detailed are: Air /gas rates of fans / main ducts Static pressure and dynamic pressure and total pressure Power consumption of fan (for estimating the operating efficiency of the fans) Monitor present flow control system and frequency of control valve operation if any (for application of variable speed drives) 37 38 39 Fan efficiency k (P)Q w FSP1 1Q1 2 Fan Operating Efficiency Evaluation Fan static kW Q in m3/ s x static pr. developed by fan in mmwc 102 = = Fan static efficiency % Fan mechanical Efficiency % Fan static kW x 100 Input kW to motor x m = Fan total kW x 100 Input kW to motor x m Parameter Details Unit Q Air flow rate m3/ s Static pressure Difference between discharge & suction pressure mmwc Fan static/ total kW Static / total power consumption of the fan kW Input kW to motor Measured power consumption of the motor kW m Efficiency of the motor at operating load Total pressure Difference between discharge & suction pressure mmwc 41