1. Chapter 16B: FANS AND SYSTEM EFFECTS
Agami Reddy (rev- May 2017)
1. Friction loss from airflow in round and rectangular ducts
2. Corrections for duct roughness, altitude and temperature
3. Pressure drop in duct fittings and bends
4. Types of fans and performance characteristics
5. Fan laws
6. Pressure gradient diagram
7. Duct system characteristics
8. Fan and duct system interaction
9. Fan curves
10. Variable air flow control methods
11. Special features and fan installation tips
12. Duct design considerations and sizing methods
13. Flow measurement instruments
HCB 3-Chap 16B: Fans and System Effects

2. HCB 3-Chap 16B: Fans and System Effects

3. Friction Loss due to Airflow in Ducts
Similar charts as for
water piping
Fig (b) Round duct friction chart in SI units based on average roughness of ε = mm and standard air.
HCB 3-Chap 16B: Fans and System Effects

4. HCB 3-Chap 16B: Fans and System Effects
Fig (a) Round duct friction chart in IP units based on average roughness of ε = ft and standard air. Can be used for altitudes up to 1500 ft and temperatures 40 – 100 oF
HCB 3-Chap 16B: Fans and System Effects

5. HCB 3-Chap 16B: Fans and System Effects
Correction for Duct
Roughness
Traditional duct material is G.I.
However, fibrous glass duct liners and fibrous ducts or varying grade are used
Friction factors f ~ 0.02 for GI ducts and 0.03 – 0.06 for fibrous ducts
Charts available for correction such as shown in figure
The Colebrook equation 16.9 is often used
Interior is rough, and joints and fasteners also increase pressure drop
HCB 3-Chap 16B: Fans and System Effects

6. Correction for Altitude and Temperature- due to density change
Friction loss x correction standard air
Adjust the friction loss if:
- Temp. 40 F > T > 10 F or
- Altitude > 1500 ft
HCB 3-Chap 16B: Fans and System Effects

7. HCB 3-Chap 16B: Fans and System Effects
Rectangular ducts of the same cross-sectional area as round ducts have higher frictional loss.
If flow and velocity are specified, do not use the pressure drop chart directly! Calculate equivalent diameter:
Rectangular ducts should be as square as possible.
Aspect ratios of 4:1 or less are best (if space permits), and
ratios greater than 8:1 should be avoided.
HCB 3-Chap 16B: Fans and System Effects

8. HCB 3-Chap 16B: Fans and System Effects

9. for non-circular ducts
Another approach
for non-circular ducts
Determine equivalent
round size duct
D => Dh= (4 x Area)/Perimeter
=(2 W.H)/(W+H)
Similar to hydraulic diameter
First, calculate velocity, and then
use the friction chart
Example: Rectangular duct
0.3 m x 0.5 m (12” x 19.7”)
with 1.5 m3/s-
-From Figure, Dh= 16” or 40.6 cm
Velocity = 1.5/(0.3 x 0.5)=10 m/s
Use velocity and diameter in chart
(dp /L) = 3 Pa/m
HCB 3-Chap 16B: Fans and System Effects

10. HCB 3-Chap 16B: Fans and System Effects
Pressure drop
in Duct Fittings
HCB 3-Chap 16B: Fans and System Effects

11. HCB 3-Chap 16B: Fans and System Effects
Types of Fans
HCB 3-Chap 16B: Fans and System Effects

12. Centrifugal Fans most popular
Types of Impeller blades
HCB 3-Chap 16B: Fans and System Effects

13. HCB 3-Chap 16B: Fans and System Effects
Fan Laws
Similar to pump affinity laws.
Fan laws are groups of relations that predict the effect on performance when such quantities as operating speed, fan size, condition of air are changed
Usefulness of fan laws lies in ability to predict changes from a base condition
The laws apply only to:
- Same fan
- Dynamically similar fans
- Constant duct system
HCB 3-Chap 16B: Fans and System Effects

14. HCB 3-Chap 16B: Fans and System Effects
Table 16.6 Summary of Fan Laws
HCB 3-Chap 16B: Fans and System Effects

15. HCB 3-Chap 16B: Fans and System Effects
Pressure Gradient Diagrams
Pressure gradient diagram- plot of the pressure head versus pipe or duct distance
This is very useful for designing piping and ducting systems and is also used for diagnosing problems associated with improper flow in existing fluid systems
Fig (a) Schematic of a pressure gradient diagram showing how the total and velocity pressures vary along the length of a pipe or duct (b) Example of pressure profile for a small commercial building with a supply fan
HCB 3-Chap 16B: Fans and System Effects

16. HCB 3-Chap 16B: Fans and System Effects
Duct System Characteristics
The relationship between flow rate and pressure drop
of a given duct system with fixed L and D
The head loss is a
quadratic function with flow rate
starting from the origin
Note: We assume friction factor f to be constant
HCB 3-Chap 16B: Fans and System Effects

17. HCB 3-Chap 16B: Fans and System Effects
Fan and Duct System Interaction- Graphical and Analytical Methods
Operating point is the intersection
of both curves
Fig
HCB 3-Chap 16B: Fans and System Effects

18. HCB 3-Chap 16B: Fans and System Effects

19. HCB 3-Chap 16B: Fans and System Effects
Fig Effect of altitude on duct resistance and fan power. The low-density case is an extreme corresponding to the density of process air at 10,000 ft altitude and 250°F. (The bhp is the brake horsepower input to the fan.)
HCB 3-Chap 16B: Fans and System Effects

20. HCB 3-Chap 16B: Fans and System Effects
Fig
Equivalent characteristic curves for piping and ducting networks:
(a)Series flow arrangement
(b)parallel flow
HCB 3-Chap 16B: Fans and System Effects

21. HCB 3-Chap 16B: Fans and System Effects
Fig
Equivalent characteristic curves and operating points for two equal size pumps or fans.
Sketch
System interaction
HCB 3-Chap 16B: Fans and System Effects

22. HCB 3-Chap 16B: Fans and System Effects
Fan Curves
Manufacturers present fan curves, i.e.,
plots of different quantities
versus volume flow rate:
Static pressure
(b) Power consumed
(c) Efficiency
Fig Forward-curved, backward- curved, and radial fan blading arrangements and performance curves. The shaded region is the suggested operating range near peak efficiency, but to the right of the peak pressure point to ensure stability.
HCB 3-Chap 16B: Fans and System Effects

23. HCB 3-Chap 16B: Fans and System Effects
Selection of Fan Size: Manufacturer Table at rated conditions
temperature = 70 F and 1 atm
HCB 3-Chap 16B: Fans and System Effects
Outlet velocity

24. HCB 3-Chap 16B: Fans and System Effects
Note log-log scale
Different fan speeds
can also be
shown
HCB 3-Chap 16B: Fans and System Effects

25. Methods for variable air flow control in HVAC systems
Used at each zone
Empirical curve-fit
equations are used
for modeling such
control methods
Eqs (16.42 – 16.45)
Mounted on fan
motor
Fig Part-load fan characteristics for outlet damper, inlet vane, and variable-speed control methods.
HCB 3-Chap 16B: Fans and System Effects

26. HCB 3-Chap 16B: Fans and System Effects
Fig
Variable-volume fan and system curves for:
(a) outlet damper flow control method and
(b) inlet vane control method.
HCB 3-Chap 16B: Fans and System Effects

27. HCB 3-Chap 16B: Fans and System Effects
Fig Flow control can be achieved either by partial closure of control valves or by variable speed drives.
HCB 3-Chap 16B: Fans and System Effects

28. HCB 3-Chap 16B: Fans and System Effects
Special Features
Ducts need to be insulated when installed
in hot or cold environments (such as attics)
(usually R-6 or R-8)
Ducts need to have aerodynamic transitions
To avoid turbulence and hence increased
pressure drop. Use bends or if sharp turns
are unavoidable, use turning vanes
HCB 3-Chap 16B: Fans and System Effects

29. HCB 3-Chap 16B: Fans and System Effects
Tips for ducting after fan:
Velocity profiles at outlet of fan are not uniform until the air has travelled
“one effective duct length”
It is advisable to provide this length to avoid high pressure drop
Fig
Adjustment of fan outlet velocity profiles with distance from fan outlet connection.
HCB 3-Chap 16B: Fans and System Effects

30. HCB 3-Chap 16B: Fans and System Effects
Fan Installation Tips:
Avoid abrupt changes in velocity and direction
HCB 3-Chap 16B: Fans and System Effects

31. HCB 3-Chap 16B: Fans and System Effects
Duct Design Considerations
The objective of duct design is to deliver the amount of air (at the proper condition) needed to meet the loads in each zone of a building.
Duct design is constrained by many factors:
-available space (often beyond the purview of the HVAC engineer)
- to meet loads in a variety of zones
- to meet economic criteria
- to minimize operating energy subject to previous constraint
- to control noise levels.
HCB 3-Chap 16B: Fans and System Effects

32. HCB 3-Chap 16B: Fans and System Effects
Proper duct design is a specialized skill and involves several years of experience.
The sequence of events in duct design is as follows.
preliminary system is laid out on a set of preliminary drawings including all structural members
ducts are then sized based on needed air quantities in each zone and for each terminal device
pressure drop calculations are made at this point, and a fan is selected.
the next iteration of the design will need to account for potential flow imbalances in the original design, duct runs of excessive pressure drop, and noise problems
after one or more iterations to accommodate these criteria, a set of final design drawings is prepared
at least one cost estimate is necessary as part of the design process, when the design is deemed sufficiently complete
Computer-aided design (CAD) of duct systems is commonplace in large and medium-size design offices; it replaces the formerly tedious, manual iterations
HCB 3-Chap 16B: Fans and System Effects

33. HCB 3-Chap 16B: Fans and System Effects
Duct systems for HVAC installations may be loosely classified into
Low velocity and high velocity groups-
HCB 3-Chap 16B: Fans and System Effects

34. HCB 3-Chap 16B: Fans and System Effects
Fig Recommended operating ranges for low- and high-velocity air systems; pressure drop versus flow rate
HCB 3-Chap 16B: Fans and System Effects

35. Recommended Velocity, ft/min Maximum Velocity, ft/min
TABLE 16.9 Recommended and Maximum Duct Velocities
Recommended Velocity, ft/min
Maximum Velocity, ft/min
Designation
Residences
Schools, Theaters, Public Buildings
Industrial Buildings
Outside air intakes*
500
800
900
1200
Filters*
250
300
350
Heating coils*
450
600
700
Air washers
Suction connections
1000
1400
Fan outlets
1000–1600
1300–2000
1600–2400
1500–2000
1700–2800
Main ducts
700–900
1000–1300
1200–1800
800–1200
1100–1600
1300–2200
Branch ducts
600–900
800–1000
700–1000
800–1300
1000–1800
Branch risers
600–700
650–800
HCB 3-Chap 16B: Fans and System Effects

36. HCB 3-Chap 16B: Fans and System Effects
Table 16.8 Different Types of Duct Sizing Methods
Method
Classification
Basis of Duct Sizing
Equal velocity
Low velocity
For simple systems, equal velocity in duct branches (lot of judgment needed)
Equal friction
Equal friction loss per unit duct length for all branches
Balanced capacity
Equal pressure drops from fan to outlets of all branches
Static regain
High velocity
For large installations, same static pressure along duct length
T- method
Based on life cycle costing (LCC) optimization using dynamic programming
HCB 3-Chap 16B: Fans and System Effects

37. Concept of Static Regain
A loss may not show up as a decrease in static or velocity head
HCB 3-Chap 16B: Fans and System Effects

38. HCB 3-Chap 16B: Fans and System Effects
Static regain method of sizing ducts is most often used for high velocity systems with long duct runs especially in large buildings.
- Initial velocity in the main duct leaving the fan is selected – range of 2500 – fpm
- ducts are sized so that the air velocity in the direction of flow is reduced in such a way that the increase (or “regain”) in static pressure just balances the pressure losses in the downstream section of duct.
- accomplished by progressively increasing the duct cross-section;
Thus system static pressure remain about the same throughout a system. This simplifies terminal box selection and system balancing
Disadvantages are very low velocities and large duct sizes at outlets, and calculation is tedious (several software programs available)
Typical regain factors:
0.75-abrupt changes in area
0.95 very smooth transition
HCB 3-Chap 16B: Fans and System Effects

39. General Recommendations on Duct Design and Layout
Routes should be as direct as possible
Sudden changes in direction and velocity should be avoided
Turning vanes should be used whenever possible
Rectangular ducts should be as square as possible. Aspects ratios greater than 8:1 should be avoided and 4:1 or less should be used whenever space permits
Smooth metal construction is preferred
Since calculations are approximate, a fan with some excess capacity should be selected
Airflow through the fan should be 10% more than the sum of the outlet requirements to allow for leakage
Dampers must be installed in all branches for balancing even if the static regain method is used. Dampers should be installed as close as possible to the main duct in order to reduce noise
Nothing should be put in or through the ducts
For minimum fan power and noise, the air velocity should be a slow as possible
HCB 3-Chap 16B: Fans and System Effects

40. Flow Measurement Instruments
Fig Venturi meter
Fig Orifice plate
Fig Pitot tube
Fig Turbine flow meters
HCB 3-Chap 16B: Fans and System Effects

41. Water and Air as Heat-Conveying Media
Advantages of water vs air
- more heat capacity of water than air
- pipes take up less space than air ducts
- pipes are easier to insulate and so higher temperatures of water for heating is possible
- cheaper first cost and operating costs
Disadvantages
- water leakage has drastic consequences
- better control of comfort with air systems
HCB 3-Chap 16B: Fans and System Effects

42. HCB 3-Chap 16B: Fans and System Effects
Outcomes
Be able to solve problems of friction loss from airflow in ducts using equations and charts- correction for non-circular ducts
Knowledge of how to correct pressure drop for duct roughness, altitude and temperature
Be able to solve problems of pressure drop in duct fittings and bends
Familiarity with the different types of fans and their differences
Knowledge of performance characteristic curves of fans
Familiarity with fan laws
Knowledge of how to analyze interaction of duct and fan interaction
Familiarity with different ways of achieving variable air flow control
Familiarity with good duct and fan installation practice
Understand the concept of the pressure gradient diagram for ducts
Knowledge of duct design considerations and the various sizing methods
Familiarity with various air flow measuring instruments
HCB 3-Chap 16B: Fans and System Effects