1. VARIABLE GEOMETRY VAV SYSTEMS: SYSTEM AND DUCT DESIGN CONSIDERATIONS.
SYSTEM BASICS
SYSTEM OPERATION
SYSTEM AND DUCT DESIGN CONSIDERATIONS.

2. VARIABLE TRADITIONAL VAV
(VAV box volume control --- Pressure Independent System
FAN WITH VARIABLE
FREQUENCY DRIVE
FILTER
COIL S
RETURN
AIR
120Pa
VAV BOX
VAV BOX
AHU T T
THERMOSTAT
THERMOSTAT
MULTIPLE FIXED APERTURE DIFFUSER OUTLETS

3. TRADITIONAL VAV – A CLOSER LOOK:
Volume Control Upstream of Diffusers
Volume flow Controlled Upstream of Constant Volume, Constant Geometry Outlet.
Air Diffusion works well at 100% flow rate
After Flow Reduction upstream:
Increased Dumping of Cold Air
Increased Stratification of Hot Air
Reduced Induction (Entrainment) of Room Air – Reduction of Air Change Effectiveness
System Flexibility Compromised – Individual diffuser control costly.
TYPICAL SINGLE DUCT VAV BOX ARRANGEMENT B A
Dumping takes place at reduced Volumes

4. TRADITIONAL SOLUTIONS TO THE PROBLEMS OF HIGH PRESSURE VAV
OPTION PARALLEL FAN POWERED VAV BOX
Fan kicks in at predetermined minimum flow level.
Positive
Fix dumping and air change effectiveness when fan energised.
Negative
Diffusers still dump until flow setting where fan kicks in.
Energy consumption of fan
Noise risk
Maintenance issues
System flexibility compromised.
Individual comfort control compromised.
OPTION SERIES FAN POWERED VAV BOX
Fan running all the time
Fix dumping and air change effectiveness problems
Energy wasted by fan all the time
All of the above under OPTION 1

5. THE IDEAL SOLUTIONS TO THE PROBLEMS OF HIGH PRESSURE VAV ----------
LOW PRESSURE VAV DIFFUSION
FAN WITH VARIABLE
FREQUENCY DRIVE
FILTER
STATIC PRESSURE SENSOR
COIL
RETURN
AIR
500 Pa
60Pa
AHU
MASTER
MASTER
SLAVE T
THERMOSTAT T
THERMOSTATS
MULTIPLE VARIABLE GEOMETRY VAV DIFFUSER OUTLETS

6. VARIABLE GEOMETRY VAV DIFFUSER OPERATION:
CONTROL DISC AT MAXIMUM APERTURE
SUPPLY AIR
SUPPLY AIR
ROOM AIR
INDUCTION

7. A VERY SIMPLE VARIABLE GEOMETRY VAV SYSTEM:
STATIC PRESSURE SENSOR
SUPPLY AIR DUCT
AHU
VARIABLE GEOMETRY VAV DIFFUSERS

8. A MORE COMPLEX VARIABLE GEOMETRY VAV SYSTEM:
MAIN SUPPLY AIR DUCT PRESSURE SENSOR/CONTROLLER LOOP
AHU R
BRANCH DUCTS & SECONDARY PRESSURE SENSORS AND CONTROLLER LOOPS
STATIC PRESSURE SENSORS
MAIN SUPPLY AIR DUCT
PRESSURE CONTROL DAMPERS

9. THE SIMPLEST POSSIBLE MEANS OF CONTROLLING STATIC PRESSURE:
SENSOR
BYPASS STATIC PRESSURE CONTROL DAMPER
PRESSURE CONTROLLER

10. IN EXTREME CASES FACE & BYPASS DAMPERS MAY BE REQUIRED:
STATIC PRESSURE SENSOR POSITIONED
HALF TO TWO THIRDS WAY BETWEEN FIRST AND LAST DIFFUSERS
BYPASS
DAMPER
FACE
DAMPER
AHU
Face damper only required if the static pressure
in the ducting just after the bypass damper is less
than the pressure drop across the bypass damper.

11. BRANCH DUCTS & SECONDARY PRESSURE SENSORS AND CONTROLLER LOOPS
THE MOST ENERGY EFFICIENT DESIGN MAKE USE OF VARIABLE FREQUENCY DRIVES TO CONTROL PRESSURE.
MORE COMPLEX DUCT DESIGNS OR RETROFITS WILL REQUIRE
PRESSURE CONTROL DAMPERS
MAIN SUPPLY AIR DUCT PRESSURE SENSOR/CONTROLLER LOOP TO THE VARIABLE FREQUENCY DRIVE OR VORTEX DAMPER ACTUATOR
AHU
BRANCH DUCTS & SECONDARY PRESSURE SENSORS AND CONTROLLER LOOPS
MAIN SUPPLY AIR DUCT
PRESSURE CONTROL DAMPERS
STATIC PRESSURE SENSORS

12. WHY DOES THE VARIABLE VAV DIFFUSER NOT DUMP COLD AIR OR STRATIFY HOT AIR?
100% Volume flow and Control disc fully open:
Assume the Duct Static Pressure at ‘A’ is controlled at: Pa
Flexible duct from ‘A’ to ‘B’ with length of about 1000 mm
will result in Friction Losses at 100% flow in the order of: Pa
The net static pressure available at ‘B’ in the diffuser neck at 100%: 37.5 Pa
30% Volume flow and Control disc at 30% position:
Assume the Duct Static Pressure at ‘A’ is still controlled at: Pa
Since Volume Flow is restricted at ‘B’ by the control disc,
the velocity on the flexible duct now reduced to 30%.
Using the square law, pressure drop is now (30/100)2 × 100 = Pa
The net static pressure available at ‘B’ in the diffuser neck at 30%: Pa
WHAT IS THE EFFECT ON AIR MOVEMENT?:
The full difference in pressure is converted into velocity pressure.
√48.87/37.5 × 100 = % increased velocity at 30%
Mass flow is reduced to 30% BUT velocity is increased by 14.2%. RESULT: SIMILAR THROW AND AIR CHANGE EFFECTIVENESS A B
SUPPLY AIR
ROOM AIR
INDUCTION

13. COMPARISON BETWEEN THE TWO TYPES OF SYSTEMS
SIMILARITIES BETWEEN THE TWO SYSTEMS:
Both use Similar Air Handling Equipment
Both are Mixed Air Ventilation Systems depending on Fan energy to try and provide good Air Change Effectiveness while maintaining a comfortable, draft free environment for building occupants.
PRESSURE INDEPENDENT VAV BOX SYSTEM ADVANTAGES:
Relatively high duct velocities allow for reduced duct sizes.
First of Cost saving when a single VAV box supply many outlets.
The VAV box will compensate for wide variations in duct pressure
System is tolerant of poor duct design. (Pressure Independent)
BUT WHAT ARE THE NEGATIVES?:
Higher duct velocities result in higher noise risk.
More ducting required since ducting must be routed through VAV Box
Constant Geometry outlets cannot maintain throw at reduced flow rates
The end result is dumping and Bad Air Change Effectiveness
Pressure Independency = Higher FAN ENERGY than necessary.
Reducing cost by using one box for a number of diffusers result in Reduced Flexibility and Losing Individual Comfort Control

14. ENERGY EFFICIENT if Pressure is maintained correctly.
PRESSURE DEPENDENT VAV DIFFUSER SYSTEM ADVANTAGES:
ENERGY EFFICIENT if Pressure is maintained correctly.
LOW NOISE levels if Pressure is maintained correctly.
FLEXIBILITY – Individual or group control achievable at ANY stage in the life of the building.
Because duct Static pressure is controlled at a constant level, VAV and CAV Diffusers can be installed on a Single Duct run.
MAINTAIN THROW at reduced volumes ---- reduced risk of Cold air Dumping or Hot air Stratification. (More detail to follow)
WHAT ARE THE RISKS?
First Cost higher when more than five diffusers per thermostat is planned Using CAV diffusers in areas with constant load can reduce cost!
If pressure is not controlled noise is a risk
but controlling pressure saves energy!!!!
Need focus on efficient duct design!
but improving the efficiency of the duct design saves energy!!!!
Limited risk of cold air dumping or hot air stratification – explanation to follow
First Cost Effective if fewer than 5 Diffusers per thermostat & especially so if Individual Room Control is the Preferred Requirement
Larger duct sizes and more detailed attention to duct design required but one should appreciate that sheet steel increases as the square root of duct area but is of lighter gauge to suit system low pressure aspects
System more expensive where large open plan offices areas required.

15. PRESSURE CONTROL DAMPERS
THE DESIGN BASICS OF A LOW PRESSURE. PRESSURE DEPENDANT VARIABLE GEOMETRY VAV DIFFUSION SYSTEM:
AHU R
MAIN SUPPLY AIR DUCT
BRANCH DUCTS
PRESSURE CONTROL DAMPERS
VAV Diffuser will function quite well with duct pressures at 10% below or 20% above design levels, but there is a need to:
Position pressure sensors correctly /3 down the length of the ducting.
Design ducting with a relative even pressure profile

16. DUCT DESIGN: THE EQUAL FRICTION METHOD:
Used by many engineers by default.
Long duct networks with high friction losses per 30 meter, will have a large pressure differential between the start and the end of the duct network.
Example:
With starting velocity of 7.6 m/s friction loss can be in the order of 50Pa per 30 meters.
In this example a starting pressure of 90Pa will drop to 30Pa at the end of the duct run.
The result is that first diffusers will be noisy supplying too much air, while the last diffusers will not supply enough air even if they are fully open.
Only the diffusers in the middle will operate as required.
This design method can work as a rule of the thumb, when:
Starting velocity is less than 5 m/s in the duct.
The duct run is short enough so that the pressure drop from beginning to end fall within the 10% to 20% range that the VAV diffusers can accommodate.
90 Pa
70 Pa
50 Pa
30 Pa
PRESSURE CONTROL DAMPER
STATIC PRESSURE SENSOR

17. DUCT DESIGN: THE STATIC REGAIN METHOD:(Basic Principles)
A typical fan:
Static Pressure at Discharge = Pa
Velocity Pressure at Discharge = Pa
Total Pressure at Discharge = Pa
A typical fan with Evasé Discharge:
Static Pressure at Discharge = Pa
Velocity Pressure at Discharge = Pa
Total Pressure at Discharge = Pa
This is valid since energy is indestructible.
THE INCREASE IN STATIC PRESSURE IS “STATIC REGAIN”
This example ignores friction losses.
(minimal in this example.)
Maintaining Static Pressure in the Duct, ensures similar Total Pressure in the neck of all diffusers
This principle forms the basis of the
Static Regain Duct design methodology!

18. DUCT DESIGN: THE STATIC REGAIN METHOD:
DESIGN CONSIDERATIONS:
Using the principle discussed on the previous slide, the duct size is reduced after each take off to regain the friction losses in the preceding duct length.
As a result the duct size will reduce to some extent from the beginning to the end.
Due to lower duct velocities, the duct dimensions are typically slightly larger than when using the Equal Friction Duct Design Method. Increased cost is however offset by:
Operating Cost Reduction
Less ducting since more than one zone can be on the same duct run
Savings in System Balancing time.
Manual Static Regain duct design exceedingly tedious --- BUT --- Many Computerized Duct Design Programs are available.
Major advantage is the fact that the Static Pressure Profile along the Full length of the ducting will be very close to that desired for Optimal Performance of the Variable Geometry Diffusers.
50 Pa
50 Pa
50 Pa
50 Pa
PRESSURE CONTROL DAMPER
STATIC PRESSURE SENSOR

19. . NOT NECESSARY! DUCT DESIGN CONSIDERATIONS:
Every VAV diffuser has its own Built-in motorized damper. There is no need to fit a separate balancing or “spin” damper:
Static Pressure is already controlled at constant level in the duct.
If minimum or maximum air is too high or too low, MIN, MAX Control Disc positions can be set electronically.
Installing balancing dampers will more than likely ruin the operation of VAV diffusers:
Setting them for maximum air, will mess up the minimum air setting.
Flexible duct runs of more than 1.5 meters can also ruin the operation of VAV diffusers:
Due to Friction losses at maximum air, minimum air operation can be messed up when friction losses get too high in long flexible duct runs. .
NOT NECESSARY!

20. A VERY GOOD REASON NOT TO USE MANUALLY ADJUSTABLE BALANCING DAMPERS:
100 Pa
100 Pa .
CONTROL DISC AT 30%
CONTROL DISC FULLY OPEN
NECK PRESS 37.5 Pa
NECK PRESS Pa
As am extreme example, let us assume the duct has been correctly designed & the Duct Static Pressure has been set at 100 Pa as the balancing technician feels he would like a safety margin. (Typical VAV box thinking):
AT 100%: Diffuser is typically selected for 50Pa (previous example) so normal friction losses expected in the flex is 12.5Pa. The balancing damper has to be adjusted to loose another 50Pa to get to the 37.5 Pa expected in the neck of the diffuser.
AT 30%: The setting of the balancing damper has a different effect since Friction Losses over the balancing damper follow the square law:
Duct static Pressure = Pa
Friction losses across damper & Flex 100% Flow = Pa
STATIC PRESSURE IN THE NECK AT 100% FLOW: = Pa
Friction losses across damper & Flex combined at 30% Flow = × (30/100)2 = Pa
STATIC PRESSURE IN THE NECK AT 30% FLOW IS THEREFORE = Pa
RESULT – EXCESSIVE NOISE, DRAFTY CONDITIONS AND EXCESS COOLING AT MINIMUM FLOW STATUS

21. A SELECTING THE CORRECT EQUIPMENT FOR THE PROJECT:
VARI-DISC VARIABLE GEOMETRY VAV CEILING DIFFUSER PERFORMANCE – VSD/VSD/VRD
SIZE
READING 20 30 40 50 60 70 80 90
100
150
FLOW l/s
THROW m
NC LEVEL 63
2.0 - 77
2.1 88
2.7 99
3.0
108
3.3 26
117
3.5 28
125
3.7 31
133
4.0 33
140
4.2 35
200 96
118
2.6 27
137
153
3.2 29
169
3.6
184
3.9
195 36
207
4.5 38
218
4.7
250
2.4
171
198
221
242
261
279
296
313
5.1 42
300
176
2.5
216
2.8
280 32
307
332
4.6 37
355
4.8 39
377
5.2 41
398
5.4 43
350
246
301
349
389
4.1
426
461
5.0
492
5.5
525.7
551
5.9 45
SELECTION 1: 56 l/s – VSD NECK PRESSURE Pa
SELECTION 2: 76 l/s – VSD NECK PRESSURE Pa
SELECTION 3: 132 l/s – VSD NECK PRESSURE 40 Pa
SELECTION 4: 170 l/s – VSD NECK PRESSURE 30 Pa
SELECTION 5: 228 l/s – VSD NECK PRESSURE 35 Pa
SELECTION 4A: 170 l/s – VSD NECK PRESSURE 60 Pa
SELECTION 5A: 228 l/s – VSD NECK PRESSURE 50 Pa

22. A SELECTING THE CORRECT DAMPER FOR THE PRESSURE CONTROL STATION:
You have two basis choices when selecting Pressure Control Dampers:
Conventional Opposed blade dampers OR Rickard Airfoil type dampers.
OPPOSED BLADE DAMPER
AIRFOIL BLADE DAMPER
100%
100%
75%
75%
VOLUME
FLOW
50%
FLOW
50%
RATE
RATE
25%
25%
VOLUME
25% % % %
25% % % %
ACTUATOR TRAVEL
ACTUATOR TRAVEL
The Rickard Airfoil design allows for a Linear Relation between Volume Flow rate and Actuator movement. (Similar to the old Pneumavalve)
This result in Improved Control Accuracy resulting in Stable duct pressures.
The Air Foil Damper Prevents Over Compensation of Small Static Pressure Variations.
Rickard PCD dampers are controlled by simple reliable electronics

23. QUESTIONS & ANSWERS