HVAC

Question 1

objective

Answer 1

obtain velocity profiles at different locations in a rectangular HVAC duct using pitot-static tubes

from these results the volumetric and mass flowrates through the duct will be calculated

Question 2

equipment: hvac

Answer 2

apparatus consists of a rectangular sheet metal duct attached to residential electric furnace with a centrifugal fan

fan has two speed settings and the electric furnace has a high and low setting

furnace exhausts vertically upward into a 90 degree elbow leading to a differential red oil inclined manometer

damper for flowrate control near duct exit

Question 3

procedure

Answer 3

flowrate damper fully open

turn furnace fan on high with heaters turned off

take pitot-static traverse at both locations in the duct. data points should be obtained at center 3”x3” subareas

turn furnace fan off

Question 4

velocity at each subarea using Bernoulli equation

Answer 4

Vi = square root ( 2 * manometer differential pressure reading for subarea / density)

Question 5

density of the air inside

Answer 5

density = ambient pressure / (gas constant * ambient temp)

Question 6

average velocity

Answer 6

average velocity = (1/24) * sum of all 24 subarea velocities

Question 7

volumetric flow rate

Answer 7

Q= average velocity * duct cross-sectional area

Question 8

mass flow rate

Answer 8

mass flow rate = density * volumetric flow rate

Question 9

Reynold’s number

Answer 9

Re = density * avg vel * hydraulic diameter / abs viscosity

Question 10

hydraulic diameter

Answer 10

Dh = 4A / perimeter of duct