VARIABLE GEOMETRY VAV SYSTEMS:

SYSTEM BASICSSYSTEM OPERATION

SYSTEM AND DUCT DESIGN CONSIDERATIONS.

FILTERCOIL

AHU

T

MULTIPLE FIXED APERTURE DIFFUSER OUTLETS

THERMOSTAT

120Pa

VAV BOX

VARIABLE TRADITIONAL VAV(VAV box volume control --- Pressure Independent System

VAV BOX

RETURNAIR

FAN WITH VARIABLE FREQUENCY DRIVE

S

T

THERMOSTAT

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.

B A

Dumping takes place at reduced Volumes

TYPICAL SINGLE DUCT VAV BOX ARRANGEMENT

TRADITIONAL SOLUTIONS TO THE PROBLEMS OF HIGH PRESSURE VAV

OPTION 1 --- 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 2 --- SERIES FAN POWERED VAV BOX Fan running all the time Positive

Fix dumping and air change effectiveness problems

Negative Energy wasted by fan all the time All of the above under OPTION 1

FILTERCOIL

AHU

T

MULTIPLE VARIABLE GEOMETRY VAV DIFFUSER OUTLETS

THERMOSTATS

500 PaRETURN

AIR

T

FAN WITH VARIABLE FREQUENCY DRIVE

THERMOSTAT

MASTER SLAVEMASTER

STATIC PRESSURE SENSOR

THE IDEAL SOLUTIONS TO THE PROBLEMS OF HIGH PRESSURE VAV

----------LOW PRESSURE VAV DIFFUSION

60Pa

VARIABLE GEOMETRY VAV DIFFUSER OPERATION:

ROOM AIR INDUCTION

SUPPLY AIR

SUPPLY AIR

CONTROL DISC AT MAXIMUM APERTURE

A VERY SIMPLE VARIABLE GEOMETRY VAV SYSTEM:

AHU

STATIC PRESSURE SENSOR

SUPPLY AIR DUCT

VARIABLE GEOMETRY VAV DIFFUSERS

BRANCH DUCTS & SECONDARY PRESSURE

SENSORS AND CONTROLLER LOOPS

R

A MORE COMPLEX VARIABLE GEOMETRY VAV SYSTEM:

AHU

MAIN SUPPLY AIR DUCT

MAIN SUPPLY AIR DUCT PRESSURE SENSOR/CONTROLLER LOOP

PRESSURE CONTROL DAMPERS

STATIC PRESSURE SENSORS

THE SIMPLEST POSSIBLE MEANS OF CONTROLLING STATIC PRESSURE:

PRESSURESENSOR

BYPASS STATIC PRESSURE CONTROL DAMPER

PRESSURE CONTROLLER

IN EXTREME CASES FACE & BYPASS DAMPERS MAY BE REQUIRED:STATIC PRESSURE SENSOR POSITIONED

HALF TO TWO THIRDS WAY BETWEEN FIRST AND LAST DIFFUSERS

BYPASSDAMPER

FACE DAMPER

AHU

Face damper only required if the static pressurein the ducting just after the bypass damper is less than the pressure drop across the bypass damper.

BRANCH DUCTS & SECONDARY PRESSURE

SENSORS AND CONTROLLER LOOPS

THE MOST ENERGY EFFICIENT DESIGN MAKE USE OF VARIABLE

FREQUENCY DRIVES TO CONTROL PRESSURE.

AHU

MAIN SUPPLY AIR DUCT

MAIN SUPPLY AIR DUCT PRESSURE SENSOR/CONTROLLER LOOP TO

THE VARIABLE FREQUENCY DRIVE OR VORTEX DAMPER ACTUATOR

STATIC PRESSURE SENSORSPRESSURE CONTROL DAMPERS

MORE COMPLEX DUCT DESIGNS OR RETROFITS WILL REQUIRE PRESSURE CONTROL DAMPERS

WHY DOES THE VARIABLE VAV DIFFUSER NOT DUMP COLD AIR OR STRATIFY HOT AIR?

WHAT IS THE EFFECT ON AIR MOVEMENT?: The full difference in pressure is converted

into velocity pressure. √48.87/37.5 × 100 = 14.2% increased velocity at 30% Mass flow is reduced to 30% BUT velocity is

increased by 14.2%. RESULT: SIMILAR THROW AND AIR CHANGE EFFECTIVENESS

100% Volume flow and Control disc fully open: Assume the Duct Static Pressure at ‘A’ is controlled at: 50 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: -12.5 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: 50 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 = -1.13Pa The net static pressure available at ‘B’ in the diffuser neck at 30%: 48.87 Pa

SUPPLY AIR

B

ROOM AIR INDUCTION

A

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

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.

THE DESIGN BASICS OF A LOW PRESSURE. PRESSURE DEPENDANT VARIABLE GEOMETRY VAV DIFFUSION SYSTEM:

BRANCH DUCTS

R

AHU

MAIN SUPPLY AIR DUCT

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 ----- 2/3 down the length of the

ducting. Design ducting with a relative even pressure profile

DUCT DESIGN: THE EQUAL FRICTION METHOD:

90 Pa 70 Pa 50 Pa 30 Pa

PRESSURE CONTROL DAMPER STATIC PRESSURE SENSOR

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.

DUCT DESIGN: THE STATIC REGAIN METHOD:(Basic Principles)

A typical fan: Static Pressure at Discharge =

1000 Pa Velocity Pressure at Discharge = 124

Pa Total Pressure at Discharge

= 1124 Pa

A typical fan with Evasé Discharge: Static Pressure at Discharge =

1074 Pa Velocity Pressure at Discharge = 50

Pa Total Pressure at Discharge

= 1124 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!

50 Pa 50 Pa 50 Pa 50 Pa

PRESSURE CONTROL DAMPER STATIC PRESSURE SENSOR

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.

. 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.

.

A VERY GOOD REASON NOT TO USE MANUALLY ADJUSTABLE BALANCING DAMPERS:

100 Pa100 Pa

CONTROL DISC FULLY OPEN

NECK PRESS 37.5 Pa

CONTROL DISC AT 30%

NECK PRESS 94.4 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 = 100 Pa– Friction losses across damper & Flex combined @ 100% Flow = 62.5 Pa

STATIC PRESSURE IN THE NECK AT 100% FLOW: = 37.5 Pa– Friction losses across damper & Flex combined at 30% Flow = 62.5 × (30/100)2

= 5.6 Pa

STATIC PRESSURE IN THE NECK AT 30% FLOW IS THEREFORE = 94.4 Pa

RESULT – EXCESSIVE NOISE, DRAFTY CONDITIONS AND EXCESS COOLING AT MINIMUM FLOW STATUS

A SELECTING THE CORRECT EQUIPMENT FOR THE PROJECT:

VARI-DISC VARIABLE GEOMETRY VAV CEILING DIFFUSER PERFORMANCE – VSD/VSD/VRD

SELECTION 1: 56 l/s – VSD 150 - NECK PRESSURE 20 PaSELECTION 2: 76 l/s – VSD 150 - NECK PRESSURE 30 PaSELECTION 3: 132 l/s – VSD 200 - NECK PRESSURE 40 PaSELECTION 4: 170 l/s – VSD 250 - NECK PRESSURE 30 Pa SELECTION 5: 228 l/s – VSD 300 - NECK PRESSURE 35 PaSELECTION 4A: 170 l/s – VSD 200 - NECK PRESSURE 60 Pa SELECTION 5A: 228 l/s – VSD 250 - NECK PRESSURE 50 Pa

 SIZE

 READING

  

20

  

30

  

40

  

50

  

60

  

70

  

80

  

90

  

100

 150

FLOW l/sTHROW mNC LEVEL

632.0-

772.1-

882.7-

993.0-

1083.326

1173.528

1253.731

1334.033

1404.235

 200

FLOW l/sTHROW mNC LEVEL

962.0-

1182.627

1373.028

1533.229

1693.630

1843.933

1954.236

2074.538

2184.740

 250

FLOW l/sTHROW mNC LEVEL

1402.4-

1712.627

1983.229

2213.531

2423.933

2614.236

2794.538

2964.740

3135.142

 300

FLOW l/sTHROW mNC LEVEL

1762.527

2162.828

2503.330

2803.732

3074.235

3324.637

3554.839

3775.241

3985.443

 350

FLOW l/sTHROW mNC LEVEL

2462.727

3013.228

3493.630

3894.132

4264.535

4615.038

4925.540

525.743

5515.945

A SELECTING THE CORRECT DAMPER FOR THE PRESSURE CONTROL STATION:

100%

75%

50%

25%

025% 50% 75% 100%

VOLUME

FLOWRATE

ACTUATOR TRAVEL

OPPOSED BLADE DAMPER

100%

75%

50%

25%

025% 50% 75% 100%

VOLUMEFLOWRATE

ACTUATOR TRAVEL

AIRFOIL BLADE DAMPER

You have two basis choices when selecting Pressure Control Dampers:Conventional Opposed blade dampers OR Rickard Airfoil type dampers.

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

QUESTIONS & ANSWERS