Air-Conditioning-Fans-VAV-System.pdf

Air Conditioning Clinic TRG-TRC014-EN
© American Standard Inc. 1999
Air Conditioning Fans & VAV System
© American Standard Inc. 1999 Air Conditioning Clinic TRG-TRC013-EN
ACES Training
K Y Yow
23 Jan 2015

- Fundamentals
- Fan Types
- Fan Capacity Control
- Application Considerations
Air Conditioning Fans

centrifugal
axial

Test Setup - Model ADA 710

Fan Curve
airflow
static
pressure
surge
region
design system
resistance curve
input
power

Fan Law 1
Static Pressure 2
Static Pressure 1
Fan RPM 2
Fan RPM 1
)
2
=
(
Input Power 2
Input Power 1
Fan RPM 2
Fan RPM 1
3
=
(
Airflow 2
Airflow 1
Fan RPM 2
Fan RPM1
=
(
)
)

Determining Fan Airflow
Velocity Pressure (Pv) = Pt – Ps
Velocity (V) = Constant 
Airflow = Velocity  Fan Outlet Area
 Pv
r

System Resistance Curve
Static Pressure 2
Static Pressure 1
=
Airflow 2
Airflow 1
( )
2

airflow
static
pressure
3,500 cfm
[1.65 m3/s]
0.65 in. H2O
[159 Pa]
2,000 cfm
[0.94 m3/s]
2.0 in. H2O
[491 Pa]
system
resistance
curve
System Resistance Curve

airflow
static
pressure
surge
region
B
C
A
actual system
resistance curve
Higher System Resistance

airflow
static
pressure
D
E
actual system
resistance curve
A
surge
region
Lower System Resistance

Forward Curved Fan

airflow
static
pressure
actual system
resistance curve
surge
line
2 hp
[1.5 kW]
3 hp
[2.2 kW]
1 hp
[0.75 kW]
A
B

Backward Inclined Fan

Airfoil Fan

airflow
static
pressure
actual system
resistance curve
surge
line
7.5 hp
[5.6 kW]
10 hp
[7.5 kW]
5 hp
[3.7 kW]
A
B

Plug (or Plenum) Fan
fan wheel
inlet cone
plenum

Fan Selection
 Forward curved (FC)
 Airflow with low static pressure
 Low Speed
 Not suitable for high static application
 Backward inclined (BI) or airfoil (AF)
 Airflow with higher static pressure
 Higher efficiency
 Twice the speed of FC
 Magnitude of surge is greater

Fan Capacity Control

Fan Capacity Control
 Riding the fan curve
 Discharge dampers
 Inlet vanes
 Fan-speed control

"Riding the Fan Curve"
static pressure
increase
airflow
reduction
B
airflow
static
pressure
surge
line
A
design system
resistance curve
actual system
resistance curve

Fan-Speed Control
variable-speed
drive

Fan-Speed Control
A
C
airflow
static
pressure
fan speed curves
B
VAV system
modulation
curve
sensor
set point
design system
resistance curve

Fan Control Comparison
100
90
80
70
60
50
40
30
20
10
0
10 20 30 40 50 60 70 80 90 100
BI fan with
discharge
dampers
AF fan with
inlet vanes
FC fan with
discharge
dampers
FC fan with
inlet vanes
fan-speed
control
% design airflow
%
design
power
variable-pitch
vaneaxial

Application Considerations

System Static-Pressure Control
VAV
terminal units
supply
fan
static
pressure
sensor
controller

Optimized Static-Pressure Control
static
pressure
sensor
communicating BAS
supply
fan
VAV terminal units

System Effect
Source: Air Movement and Control Association. 2002. Fans and Systems, Publication 201. Arlington Heights, IL: AMCA.
Position D
Position C
Position B
Position A
inlet
Blast Area
Cut off
25%
50%
75%
100% Effective Duct Length
Outlet
Area
Discharge
Duct
Centrifugal Fan
Axial Fan
Effective Duct Length = 2.5 Duct Diameters for 2,500 FPM or less
Add 1 duct diameter for each additional 1,000 FPM
For rectangular ducts, the equivalent duct diameter is
(4 x width x length / 3.14) ^ 0.5

System Effect
airflow
static
pressure
design system
resistance curve
actual system resistance curve
system effect
pressure loss
desired
airflow
C
A
B

System Effect Factors are Real
Fan discharge duct Return air duct that
creates high system
effect
Rectangular duct elbow

Recommendations
 Minimum 2.5 duct diameters on Outlet
 Minimum 5 to 8 duct diameters on Inlet
 Avoid inlet swirl
 If any of these general rules are broken, make
sure to take system effect into account.

Acoustics
radiated
return
airborne
supply
airborne
supply
breakout
1
2
3
4

Sound Transmission Paths
roof
transmission
supply
breakout
return
airborne
supply
airborne
Acoustical analysis: Source–path–receiver model

fan application
Acoustical Guidelines
 Optimize fan and air-handler selection for
lowest overall sound
 Select fan to operate safely away from surge
region
 Minimize system effects
 Use low-pressure-drop duct fittings
(follow SMACNA recommendations)
 Avoid rectangular sound traps, if possible
 Use adequate vibration isolation

Effect of Actual (Nonstandard)
Conditions
1) Air Density Ratio =
2) SPstandard =
3) Use Airflowactual and SPstandard to select fan
4) RPMstandard = RPMactual
5) Poweractual = Air Density Ratio  Powerstandard
Densityactual
Densitystandard
SPactual
Air Density Ratio

Equipment Certification Standards
AMCA – Air Movement and Control Association
 Purpose
 Establish methods for
laboratory testing of air
moving devices

A Trane Air Conditioning Clinic
VAV Systems

VAV Systems
period one
What Is Variable Air Volume?

I’m
freezing!
I’m
fine.
I’m
hot!
It’s stuffy
in here!
single thermostat
constant-volume, variable-temperature system …
Applied To Multiple Spaces

Variable-Air-Volume System
EA
OA
RA
variable-
speed drive
supply
fan
cooling
coil
thermostat
VAV
box
PA
SA

40,000 Btu/hr
1.085 × (75°F – 55°F)
Supply
Airflow
= 1,840 cfm
=
With these design conditions …
space sensible heat gain = 40,000 Btu/hr
space dry-bulb temperature = 75°F
supply air dry-bulb temperature = 55°F
variable-air-volume (VAV) system …
Full Load

20,000 Btu/hr
1.085 × (75°F – 55°F)
Supply
Airflow
= 920 cfm
=
With these part load conditions …
space sensible heat gain = 20,000 Btu/hr
space dry-bulb temperature = 75°F
supply air dry-bulb temperature = 55°F
variable-air-volume (VAV) system …
Part Load

 Reduced fan energy
 Reduced refrigeration energy
why VAV?
Part-Load Energy Savings

why VAV?
Comfort
Ahhh!
I’m
fine.
I’m
quite
cozy.
It’s nice
in here!
 Dedicated terminal units
 Dedicated thermostats

System Comparison
constant-volume,
single zone
 Fan energy
savings
 Refrigeration
energy
savings
 Delivers comfort
to many spaces
efficiently
 Constant fan
energy
 Refrigeration
energy
savings
 Delivers comfort
to only one
thermal zone
VAV

 Variable thermal load profiles
 Multiple, independently-controlled
spaces
 Common return air path
variable-air-volume system …
Building Characteristics

VAV Systems
period two
Components Of A VAV System
© American Standard Inc. 1999 Air Conditioning Clinic 2803-8-1999

supply
ductwork
VAV
box
thermostat
return air path
components of a
VAV System
central
air handler
supply
fan
relief
fan

Near-Linear Performance
0 20 40 60 80 100
100
80
60
40
20
% wide open position
%
nominal
airflow

VAV Terminal Units
primary
air
supply
air
airflow
modulation
device
 terminal heating coil
 filter
 terminal mixing fan

VAV terminal unit types …
Single-Duct, Cooling-Only
primary
air
supply
air

space load
design
cooling load
design
heating load
%
airflow
to
space
100%
minimum
primary
airflow
0%
primary air
Single-Duct, Cooling-Only
maximum
primary
airflow
remote source of heat
heating tempering

series
cool
primary air
warm
plenum air
Fan-Powered
parallel

Parallel, Fan-Powered
primary
air
supply
air
terminal
heating coil
terminal
mixing fan
plenum
air

Series, Fan-Powered
primary
air
supply
air
terminal
mixing fan
plenum
air
terminal
heating coil

 Constant airflow to the space
 Acoustics
 Fan energy consumption
Parallel vs. Series Fan-Powered

Single-Duct VAV Systems
OA
90°F
RA
80°F
PA
55°F
single-duct
VAV terminal units
central air handler
EA
VSD

VAV Terminal Unit Controls

VAV terminal unit controls …
Terminal Unit Control Technologies
 Pneumatic control
 Electronic control
 Direct digital control (DDC)

Direct Digital Control (DDC)
communicating
building automation
system (BAS)
DDC/VAV
terminal units
central air handler

Direct Digital Control (DDC)
 Electronic pressure transducer provides
pressure-independent control
 Digital controller positions modulation device,
controls terminal fan and heat source
 Minimum and maximum airflow settings
adjusted through communication link
 Electric power supply operates modulation
device and electronic controller

Diffusers
VAV
terminal unit
diffuser
flexible
duct
sheet metal
supply duct
primary air

diffusers …
Linear Slot
5 ft
55°F
55°F
air from
space
Coanda effect
linear
slot
diffuser
supply duct
supply air

Supply Duct System
VAV
terminal
unit
diffuser
flexible
duct
sheet metal
supply duct
central
air handler

Duct Design
 Equal friction method
equal static pressure drop per unit length of duct
design can be performed by hand
 Static regain method
relatively constant static pressure throughout system
desirable for VAV system duct design
design often requires a computer program

Duct Design Recommendations
 Keep as simple and symmetrical as possible
 Locate main runs, branch runs, and terminal
units above hallways or unoccupied spaces
 Minimize use of flexible ductwork upstream of
terminal units
 Use duct lining or a duct silencer in first duct
section to attenuate supply fan noise
 Place balancing dampers upstream of diffusers
in all noncritical branches
 Reducing transitions should be several duct
diameters upstream of terminal units

VAV Systems
period four
Fan Modulation

Controlling System Static Pressure
 Fan outlet static pressure control
 Supply duct static pressure control
 Optimized static pressure control

Fan Outlet Static Pressure Control
VAV
terminal units
supply
fan
sensor located
at fan outlet

Supply Duct Static Pressure Control
sensor located approximately
2/3 down supply duct
VAV
terminal units
supply
fan

Optimized Static Pressure Control
static
pressure
sensor
damper
positions
communicating
BAS
fan speed or
inlet vane position
VAV
terminal
units
supply
fan
sensor located
at fan outlet

Air-Conditioning-Fans-VAV-System.pdf

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