Ductwork

Question 1

What is the conversion between Absolute and Gauge Pressure?

Answer 1

Absolute pressure (psia) - 14.7 = Gauge Pressure (psig)

Question 2

What are two additional units relevant to HVAC systems (related to pressure)?

Answer 2

Inches of Water Column (wc) or Water Gauge (wg)

Question 3

What is Static Pressure?

Answer 3

The force per unit area exerted by a fluid at rest

Question 4

Static pressure acts in _____ direction(s)

Answer 4

All

Question 5

Static Pressure represents _____ energy

Answer 5

Potential

Question 6

What is Velocity Pressure?

Answer 6

the force per area resulting from the motion of fluid

Question 7

Velocity Pressure act in _____ direction(s)

Answer 7

one

Question 8

Velocity pressure represents ____ energy

Answer 8

kinetic

Question 9

When looking at a flow ring in a VAV box, the low side tube is measuring …

Answer 9

Static Pressure

Question 10

When looking at a flow ring in a VAV box, the high side tube is measuring …

Answer 10

Total Pressure

Question 11

Bernoulli Equation follows what principle?

Answer 11

Conservation of Energy

Question 12

When the cross-sectional area of the duct decreases the velocity pressure ______ and the static pressure _____.

Answer 12

Velocity Pressure: Increases
Static Pressure: Decreases

Question 13

What are Friction Losses?

Answer 13

Pressure drop in a straight duct due to surface roughness

Question 14

What are Dynamic losses?

Answer 14

Pressure drop through fittings (elbows or junctions) due to flow disturbance

Question 15

Both Friction and Dynamic Losses result in a drop in ______ _______

Answer 15

Total Pressure

Question 16

Why do we try and limit the use of Flex Duct?

Answer 16

There is a large pressure drop

Question 17

To get Pressure Drop, what two things do you multiply together?

Answer 17

Velocity Pressure and the Local Loss Coefficient

Question 18

What do the subscript for the loss coefficient stand for (C, B, & S)?

Answer 18

C: Common
B: Branch
S: Straight

Question 19

What assumption do all loss coefficients have?

Answer 19

That there is a well-developed profile at the inlet

Question 20

What is the Fan System Effect?

Answer 20

The decrease in fan performance resulting from poor fan inlet and/or outlet connections

Question 21

What is the Duct System Effect?

Answer 21

The increase in system resistance due to close-coupled (consecutive) fittings; such losses cannot be accurately estimated since each fitting combination is unique

Question 22

Two consecutive elbows can have a _________ % higher pressure drop or more than two elbows separated by long straight sections at the inlets and outlets.

Answer 22

40-50%

Question 23

What are the (4) duct design methods for commercial buildings?

Answer 23

• T-Method
• Equal Friction
• Static Regain
• Friction Rate Reduction

Question 24

What duct design method do we use?

Answer 24

Static Regain

Question 25

What is the goal of static regain?

Answer 25

pressure loss in each segment is offset by static pressure regain from a reduction of velocity pressure at the beginning of the next downstream segment

Question 26

What is the claim for the Static Regain Method?

Answer 26

Self-balancing at design and remains in relative balance during modulated flow

Question 27

What offering does Trane have that revolves around the Static Regain Method?

Answer 27

VariTrane Duct Designer Software, which will help us land BOD!

Question 28

What are the (4) major duct shapes?

Answer 28

- Spiral Round - Flat Oval - Rectangular - Flex

Question 29

Why is spiral round ductwork preferred? (3)

Answer 29

- Weight is less than rectangular of flat oval - Perimeter (surface area) is less - Quieter: excellent resistance to low frequency breakout noise (rumble)

Question 30

What are the (8) general guidelines for ductwork?

Answer 30

1) Go Straight! 2) Minimize the number of transitions and joints 3) Use round spiral Ductwork 4) If you must use square duct, avoid square elbows 5) Use conical or 45-degree taps to VAV boxes from medium-pressure duct mains 6) Avoid close-coupled (consecutive) fittings 7) Use flex duct between VAV box and the diffuser 8) Do NOT use flex duct from main duct tap to VAV box

Question 31

Flex duct should be limited to _____ feet in length

Answer 31

6