Static Pressure Calculations FOR Ventilation Systems - sample problems PDF

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Ashok Kumar...

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EXERCISE 1 FOR THE SYSTEM SKETCHED BELOW, FIND SPh FOR FLANGED TAPERED HOOD.

Solution: Step One: Find target diameter (Dt): Qt = At x Vt Dt = 24 S[Qt / (3.14 x Vt)] = 24S[900 / (3.14 x 2500)] = 8.12" Step Two: Choose an actual diameter: If D < Dt then V > Vt Dt > D then Vt > V Since V must at least 2500 FPM, choose the available diameter closet to, but smaller than 8.12" Therefore, Choose D = 8" Step Three: Find the actual duct velocity (Vd): It is going to be slightly larger than our target velocity of 2500 FPM. V = Q / A = Qt / A = 900 / (3.14 x 8 x 8 / 4 x 12 x 12) Vd = 2578 FPM Now calculate the face velocity (Vf): Vf = Q /A = 900 / (4 x 12 /12 x 12) Vf = 2700 FPM Step Four: Find the SPh SPh = (0-VPf) - Ff (VPf) - Fd (VPf ) + (VPf - VPd) = - Ff (VPf) - (1+ Fd) VPd

= -0.49 (2700/4005)² - (1 + 0.25) (2578/4005)² = -0.22 - 0.52 = -0.74" SPh = -0.74" Note: The air did not significantly change velocity in passing through this hood since its cross-sectional area did not change significantly. It is not a compound hood, but no empirical value of Fh is available for this particular configuration. Therefore, since significant opportunities for entry losses existed at the face entry as well as duct entry, we had to determine both.

EXERCISE 2 FOR THE SYSTEM (SLOT/PLENUM HOOD) SKETCHED BELOW, FIND D AND SPh, GIVEN THAT EACH SLOT IS 3" x 24",TOTAL AREA OF BOTH SLOT IS As = 0.25' x 4', Qt = 2100 CFM and Vt = 3000 FPM.

Solution: Hint: Find Dt, Choose D, find Vd and Vs, find He and Hes, then find SPh 1. Dt = 24S [Qt / (Vt x 3.14)] = 24S [2100 / (3000 x 3.14)] Dt = 11.32" 2. Choose D = 11" 3. Vd = Qt / A = 2100 / (3.14 x 11 x 11 /4 x 12 x 12) = 3182 FPM Vs = Qt / As = 2100 / (0.25 x 4) = 2100 FPM 4. SPh = acceleration - Hes - acceleration - Hed = (0-VPs) - Fs (VPs) + (VPs-VPd) - Fh (VPd) = - Fs (VPs) - (1+Fh) VPd

From Tables: Fh = 0.5, Fs = 1.78 = -1.78 (2100/4005)² - 1.5 (3182/4005)² = - 0.49 – 0.95 = -1.44" SPh = - 1.44"

EXERCISE 3 CALCULATE "Q", "DUCT VELOCITY" AND "HOOD ENTRY LOSS". GIVEN: SPh = 2" WG, Ce = 0.72 AND DUCT DIAMETER = 6".

Solution: Q = V x A = S(4005 x VP*VP x A) = 4005 x Ce x SPh x A =S4005 x 0.72 x0.72* 2 x 0.196 = 800 CFM V = Q / A = 0.196 VP = V² / 4005 = 1.04" WG SPh = He + VP or He = SPh - VP

EXERCISE 4 CALCULATE "Q", HOOD ENTRY LOSS (He), COEFFICIENT OF ENTRY (Ce), AND 'K'. GIVEN: SPh = 2.5" WG, V = 4000 FPM AND DUCT DIAMETER = 18".

Solution: Q = V x A = 4000 x 1.767 = 7068 CFM From Chart: At V = 4000 FPM, VP = 1.0" WG

Hood Entry Loss (He) = (SPh - VP) = 2.5 - 1.0 = 1.5" WG Coefficient of Entry (Ce) = S(VP/SPh) = S(1/2.5) = 0.63 K = He / VP = 1.5 / 1.00 = 1.50...