variable geometry vav systems: system basics system operation system and duct design considerations
TRANSCRIPT
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