Supply fan warm-up control for systems having a return fan must prevent the supply fan from delivering more airflow than the return fan maximum capacity during warm-up mode Figure 7.. Re
Trang 2Design and Application of Controls 45.3
the sensor malfunctions or is placed in a location that is not
repre-sentative, operating problems will result
An alternative approach to supply fan control in a VAV system
uses flow readings from the direct digital control (DDC) zone
terminal boxes to integrate zone VAV requirements with supply
fan operation Englander and Norford (1992) suggest that duct
static pressure and fan energy can be reduced without sacrificing
occupant comfort or adequate ventilation They compared
modi-fied PI and heuristic control algorithms via simulation and
dem-onstrated that either static pressure or fan speed can be regulated
directly using a flow error signal from one or more zones They
noted that component modeling limitations constrain their results
primarily to a comparison of the control algorithms The results
show that both PI and heuristic control schemes work, but the
authors suggest that a hybrid of the two might be ideal
Supply fan warm-up control for systems having a return fan must
prevent the supply fan from delivering more airflow than the return
fan maximum capacity during warm-up mode (Figure 7)
Return fan static control from returns having local (zoned)
flow control is identical to supply fan static control (Figure 5)
Return fan control for VAV systems provides proper building
pres-surization and minimum outdoor air Duct static control of the
sup-ply fan is forwarded to the return fan (Figure 8) This open loop (no
feedback) control requires similar supply and return fan airflow
modulation characteristics The return fan airflow is adjusted at
minimum and maximum airflow conditions The airflow turndown
should not be excessive, typically no more than 50% Provisions forwarm-up and exhaust fan switching are impractical
Airflow tracking uses duct airflow measurements to control the
return air fans (Figure 9) Typical sensors, called flow stations, aremultiple-point, pitot tube, and averaging Provisions must be madefor exhaust fan switching to maintain pressurization of the building.Warm-up is accomplished by setting the return airflow equal to thesupply fan airflow, usually with exhaust fans turned off and limitingsupply fan volume to return fan capability During night cool-down,the return fan operates in the normal mode
VAV systems that use return or relief fans require control of flow through the return or relief air duct systems Return fans arecommonly used in VAV systems to help ensure adequate air distri-bution and acceptable zone pressurization In a return fan VAVsystem, there is significant potential for control system instabilitydue to the interaction of control variables (Avery 1986) In a typi-cal system, these variables might include supply fan speed, supplyduct static pressure, return fan speed, mixed air temperature, out-side and return air damper flow characteristics, and wind pressureeffect on the relief louver The interaction of these variables andthe selection of control schemes to minimize or eliminate interac-tion must be considered carefully Mixed air damper sizing andselection are particularly important Zone pressurization, buildingconstruction, and outdoor wind velocity must be considered Theresultant design helps ensure proper air distribution, especiallythrough the return air duct Using the technique described by Dick-son, the designer may be able to eliminate the return fan altogether
air-Sequencing fans for VAV systems reduces airflow more than
other methods and results in greater operating economy and morestable fan operation if airflow reductions are significant Alternation
of fans usually provides greater reliability Centrifugal fans are trolled to keep system disturbances to a minimum when additionalfans are started The added fan is started and slowly brought tocapacity while the capacity of the operating fans is simultaneouslyreduced The combined output of all fans then equals the outputbefore fan addition
con-Vaneaxial fans usually cannot be sequenced in the same manner
as centrifugal fans To avoid stall, the operating fans must bereduced to some minimum level of airflow Then, additional fansmay be started and all fans modulated to achieve equilibrium
Unstable fan operation in VAV systems can usually be avoided
by proper fan sizing However, if airflow reduction is large cally over 60%), fan sequencing is usually required to maintain air-flow in the fan’s stable range
(typi-Supply air temperature reset can be used to avoid fan instability
by resetting the cooling coil discharge temperature higher (Figure10), so that the building cooling loads require greater airflow
Fig 7 Supply Fan Warm-Up Control
Fig 8 Duct Static Control of Return Fan
Fig 9 Airflow Tracking Control
Trang 3Design and Application of Controls 45.5
thermostat would control a hot water or steam valve to keep water
temperature above freezing
Economizer Cycle
Economizer cycle control reduces cooling costs when outside
conditions are suitable, that is, when the outdoor air is cool enough
to be used as a cooling medium If the outdoor air is below a
high-temperature limit, typically 18°C, the return, exhaust, and outdoor
air dampers modulate to maintain a ventilation cooling set point,
typically 13 to 16°C (Figure 16) The relief dampers are interlocked
to close, and the return air dampers to open, when the supply fan is
not operating When the outdoor air temperature exceeds the
high-temperature limit set point, the outdoor air damper is closed to a
fixed minimum and the exhaust and return air dampers close and
open, respectively
In enthalpy economizer control, the high-temperature limit
inter-lock system of the economizer cycle is replaced in order to further
reduce energy costs when latent loads are significant The interlock
function (Figure 16) can be based instead on (1) a fixed enthalpy
upper limit, (2) a comparison with return air so as not to exceed
return air enthalpy, or (3) a combination of enthalpy and
high-tem-perature limits
VAV warm-up control during unoccupied periods requires no
outdoor air; typically, outdoor and exhaust dampers remain closed
However, in systems with a return fan (Figure 17), the outdoor air
damper should be positioned at its minimum position, and supply
airflow (volume) should be limited to return air airflow (volume) to
minimize positive or negative duct pressurization
Night cool-down control (night purge) provides 100% outdoor
air for cooling during unoccupied periods (Figure 18) The space is
cooled to the space set point, typically 5 K above outdoor air
tem-perature Limit controls prevent operation if outdoor air is above
space dry-bulb temperature, if outdoor air dew-point temperature is
excessive, or if outdoor air dry-bulb temperature is too cold,
typi-cally 10°C or below The night cool-down cycle is initiated before
sunrise, when overnight outside temperatures are usually the
coolest When outside air conditions are acceptable and the space
requires cooling, the cool-down cycle is the first phase of the
opti-mum start sequence
Heating Coil
Heating coils that are not subject to freezing can be controlled by
simple two-way or three-way modulating valves (Figure 11) Steam
distributing coils are required to ensure proper steam coil control
The valve is controlled by coil discharge air temperature or by space
temperature, depending on the HVAC system Valves are set to open
to allow heating if control power fails In many systems, the outdoor
air temperature resets the heating discharge controller
To provide unoccupied heating or preoccupancy warm-up, a
heating coil can be added to the central fan system During warm-up
or unoccupied periods, a constant supply duct heating temperature
is maintained and the cooling coil valve is kept closed Once the
facility has attained the minimum required space temperature, thecentral air handler will revert back to the occupied mode
Heating coils in central air-handling units preheat, reheat, orheat, depending on the climate and the amount of minimum outdoorair needed
Preheating coils using steam or hot water must have protection
against freezing, unless (1) the minimum outdoor air quantity issmall enough to keep the mixed air temperature above freezing and(2) enough mixing occurs to prevent stratification That is, evenwhen the average mixed air temperature is above freezing, inade-quate mixing may allow freezing air to impinge on the coil.Steam preheat coils should have two-position valves and vacuumbreakers to prevent a buildup of condensate in the coil The valveshould be fully open when outdoor air (or mixed air) temperature isbelow freezing This causes unacceptably high coil discharge tem-peratures at times, necessitating face and bypass dampers for finaltemperature control (Figure 19) The bypass damper should be sized
to provide the same pressure drop at full bypass airflow as the bination of face damper and coil does at full airflow
com-Hot water preheat coils must maintain a minimum water velocity
in the tubes of 0.9 m/s to prevent freezing A two-position valvecombined with face and bypass dampers can usually be used to con-trol the water velocity More commonly, a secondary pump control
in one of two configurations (Figure 20 and Figure 21) is used Thecontrol valve modulates to maintain the desired coil air dischargetemperature, while the pump operates to maintain the minimumtube water velocity when outdoor air is below freezing The system
in Figure 21 uses less pump power, allows variable flow in the hotwater supply main, and is preferred for energy conservation Thesystem in Figure 20 may be required on small systems with only one
or two air handlers, or where constant main water flow is needed
Fig 16 Economizer Cycle Control
Fig 17 Warm-Up Control
Fig 18 Night Cool-Down Control
Trang 4Design and Application of Controls 45.7
A desiccant-based dehumidifier can lower space humidity
below that possible with cooling/dehumidifying coils This device
adsorbs moisture using silica gel or a similar material For
continu-ous operation, heat is added to regenerate the material The
adsorp-tion process also generates heat (Figure 26) Figure 27 shows a
typical control
Humidification can be achieved by adding moisture to the
sup-ply air Evaporative pans (usually heated), steam jets, and atomizing
spray tubes are all used for space humidification A space or returnair humidity sensor provides the necessary signal for the controller
A humidity sensor in the duct should be used to minimize moisturecarryover or condensation in the duct (Figure 28) With proper useand control, humidifiers can achieve high space humidity, althoughthey more often maintain design minimum humidity during theheating season
Outdoor Air Control
Fixed, minimum outdoor air control provides ventilation air,space pressurization (exfiltration), and makeup air for exhaust fans.For systems without return fans, the outdoor air damper is inter-locked to remain open only when the supply fan operates (Figure29) The outdoor air damper should open quickly when the fan turns
on to prevent excessive negative duct pressurization In some cations, the fan on-off switch opens the outdoor air damper before
appli-Fig 23 Cooling and Dehumidifying—Practical Low Limit
Fig 24 Cooling and Dehumidifying with Reheat
Fig 25 Sprayed Coil Dehumidifier
Fig 26 Psychrometric Chart: Chemical Dehumidification
Fig 27 Chemical Dehumidifier
Fig 28 Steam Jet Humidifier
Trang 5Design and Application of Controls 45.9
For spaces requiring heating, a reheat coil can be installed in the
dis-charge As the temperature in the space drops below the set point,
the damper begins to close and reduce the flow of air to the space
When the airflow reaches the minimum limit, the valve on the
reheat coil begins to open
Single-duct VAV systems, which supply warm air to all zones
when heating is required and cool air to all zones when cooling is
required, have limited application and are used where heating is
required only for morning warm-up They should not be used if
some zones require heating at the same time that others require
cool-ing These systems, like single-duct cooling-only systems, are
gen-erally controlled during occupancy
An induction terminal controls the space temperature by
reduc-ing the supply airflow to the space and by inducreduc-ing return air from
the plenum space into the airstream for the space (Figure 35) Both
dampers are controlled simultaneously, so as the primary air
open-ing decreases, the return air openopen-ing increases When the space
tem-perature drops below the set point, the supply air damper begins to
close and the return air damper begins to open
A bypass terminal has a damper that diverts part of the supply
air into the return plenum (Figure 36) Control of the diverting
damper is based on the output of the space temperature sensor
When the temperature in the space drops below the set point, the
bypass damper begins to open, routing some of the supply air to the
plenum, which reduces the amount of supply air entering the space
When the bypass is fully open, the control valve for the reheat coil
opens as required to maintain the space temperature A manual
bal-ancing damper in the bypass is adjusted to match the resistance in
the discharge duct In this way, the supply of air from the primary
system remains at a constant volume The maximum airflow
through the bypass must be restricted in order to maintain the
min-imum airflow into the space Although the airflow to the space is
reduced, the total airflow of the fan remains constant, so the fan
power and associated energy cost are not reduced These terminals
can be added to a single-zone constant volume system to provide
zoning without the energy penalty of a conventional reheat system
A fan-powered terminal unit has an integral fan that supplies a
constant volume of air to the space (Figure 37) In addition to
enhancing air distribution in the space, a reheat coil can be added to
maintain a minimum temperature in the space when the primary
system is off When the space is occupied, the fan runs constantly to
provide a constant volume of air to the space The fan can draw air
from the return plenum to compensate for the reduced supply air As
the temperature in the space decreases below the set point, the
sup-ply air damper begins to close and the fan draws more air from the
return plenum Units serving the perimeter area of a building caninclude a reheat coil Then, when the supply air reaches its mini-mum level, the valve to the reheat coil begins to open
A plenum fan terminal has a fan that pulls air from the return
plenum and mixes it with the supply air (Figure 38) A reheat coilmay be placed in the discharge to the space or in the return plenum
Fig 33 Constant Volume Single-Duct Zone Reheat
Fig 34 Throttling VAV Terminal Unit
Fig 35 Induction VAV Terminal Unit
Fig 36 Bypass VAV Terminal Unit
Fig 37 Fan-Powered VAV Terminal Unit
Trang 6Design and Application of Controls 45.11
closed or mix cold supply air with bypass air when the hot deck
damper is closed
A single-zone system (Figure 44) uses a constant volume
air-handling unit (usually factory-packaged) No fan speed control is
required because fan volume and duct static pressure are set by the
design and selection of components Single-zone systems do not
require terminal boxes because the zone temperature can be
main-tained by varying the temperatures of the heating and cooling coils
During warm-up, as determined by a time clock or manual
switch, a constant heating supply air temperature is maintained
Because the terminal unit may be fully open, uncontrolled
overheat-ing can occur It is preferable to allow unit thermostats to maintain
complete control of their terminal units by reversing their action to
the unit During warm-up and unoccupied cycles, outdoor air
damp-ers should be closed
A unit ventilator is designed to heat, ventilate, and cool a space
by introducing up to 100% outdoor air Optionally, it can cool and
dehumidify with a cooling coil (either chilled water or direct
expan-sion) Heating can be by hot water, steam, or electric resistance The
control of these coils can be by valves or face and bypass dampers
Consequently, controls applied to unit ventilators are many and
var-ied The three most commonly used control schemes are Cycle I,
Cycle II, Cycle III, and Cycle W
Cycle I Control Except during the warm-up stage, Cycle I
(Fig-ure 45), supplies 100% outdoor air at all times During warm-up, theheating valve is open, the OA damper is closed, and the RA damper
is open As temperature rises into the operating range of the spacethermostat, the OA damper opens fully, and the RA damper closes.The heating valve is positioned to maintain space temperature Theairstream thermostat can override space thermostat action on theheating valve to prevent discharge air from dropping below a min-imum temperature Figure 47 shows the positions of the heatingvalve and ventilation dampers in relation to space temperature
Cycle II Control During the heating stage, Cycle II (Figure 45)
supplies a set minimum quantity of outdoor air Outdoor air is ually increased as required for cooling During warm-up, the heat-ing valve is open, the OA damper is closed, and the RA damper isopen As the space temperature rises into the operating range of thespace thermostat, ventilation dampers move to their set minimumventilation positions The heating valve and ventilation dampers areoperated in sequence as required to maintain space temperature.The airstream thermostat can override space thermostat action onthe heating valve and ventilation dampers to prevent discharge airfrom dropping below a minimum temperature Figure 49 shows therelative positions of the heating valve and ventilation dampers withrespect to space temperature
grad-Cycle III Control During the heating, ventilating, and cooling
stages, Cycle III (Figure 46) supplies a variable amount of outdoorair as required to maintain the air entering the heating coil at fixedtemperature (typically 13°C) When heat is not required, this air is
Fig 42 Variable, Constant Volume (ZEB) Dual-Duct
Terminal Unit
Fig 43 Zone Mixing Dampers—Three-Deck
Multizone System
Fig 44 Single-Zone Fan System
Fig 45 Cycles I, II, and W Control Arrangements
Trang 745.14 1999 ASHRAE Applications Handbook (SI)
needed in each supply duct A controller allows the sensor sensing
the lowest pressure to control the fan output, thus ensuring that there
is adequate static pressure to supply the necessary air for all zones
Control of a return air fan is similar to that described previously
in the section on Fans in the paragraph on Return Fan Static Control
Flow stations are usually located in each supply duct, and a signal
corresponding to the sum of the two airflows is transmitted to the
RA fan volume controller to establish the set point of the return fan
controller
The hot deck has its own heating coil, and the cold deck has its
own cooling coil Each coil is controlled by its own discharge air
temperature controller The controller set point may be reset from
the greatest representative demand zone: based on zone
tempera-ture, the hot deck may be reset from the zone with the greatest
heat-ing demand, and the cold deck from the zone with the greatest
cooling demand
Control based on the zone requiring the most heating or cooling
increases operation economy because it reduces the energy
deliv-ered at less-than-maximum load conditions However, the expected
economy is lost if air quantity to a zone is too low, temperature in a
space is set to an extreme value, a zone sensor is placed so that it
senses spot loads (due to coffee pots, the sun, copiers, etc.), a sensor
is located in an unoccupied zone, or a zone sensor malfunctions In
these cases, a weighted average of zone signals can recover the
ben-efit at the expense of some comfort in specific zones
Ventilation dampers (OA, RA, and EA) are controlled for
cool-ing, with outdoor air as the first stage of cooling in sequence with
the cooling coil from the cold deck discharge temperature
control-ler Control is similar to that in single-duct systems A more accurate
OA flow-measuring system can replace the minimum positioning
switch
Dual supply fan systems (Figure 51) use separate supply fans
for the heating and cooling ducts Static pressure control is similar
to that for VAV dual-duct single-supply fan systems, except that
each supply fan has its own static pressure sensor and control If the
system has a return air fan, volume control is similar to that
described in the section on Fans in the paragraph on Return Fan
Static Control Temperature, ventilation, and humidity control are
similar to those for VAV dual-duct single supply fan systems
Chillers
The manufacturer almost always supplies chillers with an
auto-matic control package installed Control functions fall into two
cat-egories: capacity and safety
Because of the wide variety of chiller types, sizes, drives,
man-ufacturers, piping configurations, pumps, cooling towers,
distribu-tion systems, and loads, most central chiller plants, including their
controls, are designed on a custom basis Chapter 43 of the 1998
ASHRAE Handbook—Refrigeration describes various chillers (e.g.,
centrifugal and reciprocating) Chapter 11 of the 2000 ASHRAE
Handbook—Systems and Equipment covers variations in piping
configurations (e.g., series and parallel chilled water flow) and
some associated control concepts
Chiller plants are generally one of two types: variable flow
(Fig-ure 52 and Fig(Fig-ure 53) or constant flow (Fig(Fig-ure 54) The fig(Fig-ures
show a parallel-flow piping configuration Control of the remote
load determines which type should be used Throttling coil valves
vary the flow in response to the load and a temperature differential
that tends to remain near the design temperature differential The
chilled water supply temperature typically establishes the base flow
rate To improve energy efficiency, the set point is reset for the zone
with the greatest load (load reset) or other variances
The constant flow system (Figure 54) is only constant flow under
each combination of chillers on line; a major upset occurs whenever
a chiller is added or dropped The load reset function ensures that
the zone with the largest load is satisfied, while supply or return
water control treats average zone load
Fig 52 Variable Flow Chilled Water System
Fig 53 Variable Flow Chilled Water System
Fig 54 Constant Flow Chilled Water System
Trang 8Design and Application of Controls 45.15
Refrigerant Pressure Optimization
Chiller efficiency is a function of the percent of full load on the
chiller and the difference in refrigerant pressure between the
con-denser and the evaporator In practice, the pressure is represented by
condenser water exit temperature minus chilled water supply
tem-perature To reduce the refrigerant pressure, the chilled water supply
temperature must be increased and/or the condenser water
temper-ature decreased An energy saving of about 3% is obtained for each
degree 1 K reduction
The following methods are used to reduce refrigerant pressure:
1 Use chilled water load reset to raise the supply set point as load
decreases Figure 55 shows the basic function of this method
Varying degrees of sophistication are available, including
com-puter control
2 Lower condenser temperature to the lowest safe temperature
(use manufacturer’s recommendations) by keeping the cooling
tower bypass valve closed, operating at full condenser water
pump capacity, and maintaining full airflow in all cells of the
cooling tower until water temperature is within about 2 K of the
outdoor air wet-bulb temperature However, the additional pump
and fan power as well as the fan power of the VAV air handlers
must be considered in calculating net energy savings
Operation Optimization
Multiple-chiller plants should be operated at the most efficient
point on the part-load curve Figure 56 shows a typical part-load
curve for a centrifugal chiller operated at design conditions Figure
57 shows similar curves at different pressure-limiting conditions
Figure 58 indicates the point at which a chiller should be added or
dropped in a two-unit plant In general, the part-load curves are
plot-ted for all combinations of chillers; then, the break-even point
between n and n + 1 chillers can be determined.
Daily start-up of the chiller plant should be optimized to
mini-mize run time based on start-up time of the air-handling units.
Chillers are generally started at the same time as the first fan system
Chillers may be started early if the water distribution loop has great
thermal mass; they may be started later if outdoor air can provide
cooling to fan systems at start-up
The condenser water circuit and control arrangement for the
central plant are shown in Figure 59 The control system designer
works with liquid chiller control when the equipment is integrated
into the central chiller plant Typically, cooling tower, chiller pump,
and condenser pump control must be considered if the overall plant
is to be stable and energy-efficient
With centrifugal chillers, condenser supply water temperature isallowed to float as long as the temperature remains above a lowlimit The manufacturer should specify the minimum entering con-denser water temperature required for satisfactory performance ofthe particular chiller The control schematic in Figure 59 works asfollows: for a condenser supply temperature (e.g., above a set point
of 24°C), the valve is open to the tower, the bypass valve is closed,and the tower fan or fans are operating As water temperaturedecreases (e.g., to 18°C), tower fan speed can be reduced to low-speed operation if a two-speed motor is used On a further decrease
in condenser water supply temperature, the tower fan or fans stopand the bypass valve begins to modulate to maintain the acceptableminimum water temperature
Water Heating
A basic constant volume hydronic system is shown in Figure 60
A variable speed drive could be added to the pump motor and the
Fig 55 Chilled Water Load Reset
Fig 56 Chiller Part-Load Characteristics at Design
Refrigerant Pressure
Fig 57 Chiller Part-Load Characteristics with
Variable Pressure
Trang 9Design and Application of Controls 45.17
Duct static limit control prevents excessive duct pressures,
usu-ally at the discharge of the supply fan Two variations are used: (1)
the fan shutdown type, which is a safety high-limit control that turns
the fans off; and (2) the controlling high-limit type (Figure 28),
which is used in systems having zone fire dampers When the zone
fire damper closes, duct pressure drops, causing the duct static
con-trol to increase fan modulation; however, the concon-trolling high limit
will override
Steam or hot water exchangers tend to be self-regulating and, in
that respect, differ from electrical resistance heat transfer devices
For example, if airflow through a steam or hot water coil stops, coil
surfaces approach the temperature of the entering steam or hot
water, but cannot exceed it Convection or radiation losses from the
steam or hot water to the surrounding area take place, so the coil is
not usually damaged Electric coils and heaters, on the other hand,
can be damaged when air stops flowing around them Therefore,
control and power circuits must interlock with heat transfer devices
(pumps and fans) to shut off electrical energy when the device shuts
down Flow or differential pressure switches may be used for this
purpose; however, they should be calibrated to energize only when
there is airflow This precaution shuts off power in case a fire
damper closes or some duct lining blocks the air passage Limit
thermostats should also be installed to turn off the heaters when
temperatures exceed safe operating levels
Duct Heaters
The current in individual elements of electric duct heaters is
nor-mally limited to a maximum safe value established by the National
Electrical Code or local codes Two safety devices in addition to the
airflow interlock device are usually applied to duct heaters (Figure
62) The automatic reset high-limit thermostat normally turns off
the control circuit If the control circuit has an inherent time delay or
uses solid-state switching devices, a separate safety contactor may
be desirable The manual reset backup high-limit safety device is
generally set independently to interrupt all current to the heater in
case other control devices fail An electric heater must have a
min-imum airflow switch and two high-temperature limit sensors; one
with manual reset and one with automatic reset
DESIGN CONSIDERATIONS AND PRINCIPLES
In designing and selecting the HVAC system for the entire
build-ing, the type, size, use, and operation of the structure must be
con-sidered Subsystems such as fan and water supply are normally
controlled by local automatic control or a local loop control A local
loop control includes the sensors, controllers, and controlled
devices used with a single HVAC system and excludes any
supervi-sory or remote functions such as reset and start-stop However, local
control is frequently extended to a central control point to diagnose
malfunctions that might result in damage from delay, and to reduce
labor and energy costs
Distributed processing using microprocessors has augmented
computer use at many locations other than the central control point
The local loop controller can be a direct digital controller (DDC)
instead of a pneumatic or electric thermostat, and some energy agement functions may be performed by a DDC
man-Because HVAC systems are designed to meet maximum designconditions, they nearly always function at partial capacity Becausethe system must be adjusted and operated for many years, the sim-plest control that produces the necessary results is usually the best
Mechanical and Electrical Coordination
Even a pneumatic control includes wiring, conduit, switchgear,and electrical distribution for many electrical devices The mechan-ical designer must inform the electrical designer of the total electri-cal requirements if the controls are to be wired by the electricalcontractor Requirements include (1) the devices to be furnishedand/or connected, (2) electrical load, (3) location of electrical items,and (4) a description of each control function
Coordination is essential Proper coordination should produce acontrol diagram that shows the interface with other control elements
to form a complete and usable system As an option, the controlengineer may develop a complete performance specification andrequire the control contractor to install all wiring related to the spec-ified sequence The control designer must run the final checks ofdrawings and specifications Both mechanical and electrical speci-fications must be checked for compatibility and uniformity
Building and System Subdivision
The following factors must be considered in the building andmechanical system subdivision:
• Heating and cooling loads as they vary—the ability to heat or coolthe interior or exterior areas of a building at any time
• Occupancy schedules and the flexibility to meet needs withoutundue initial and/or operating costs
• Fire and smoke control and possibly compartmentation thatmatches the air-handling layout and operation
Control Principles for Energy Conservation Temperature and Ventilation Control VAV systems are typi-
cally designed to supply constant temperature air at all times Toconserve central plant energy, the temperature of the supply air can
be raised in response to demand from the zone with the greatest load(load analyzer control) However, because more cool air must then
be supplied to match a given load, the mechanical cooling energysaved may be offset by an increase in fan energy Equipment oper-ating efficiency should be studied closely before implementing tem-perature reset in cooling-only VAV systems
Fig 61 High-Limit Static Pressure Controller
Fig 62 Duct Heater Control
Trang 1045.18 1999 ASHRAE Applications Handbook (SI)
Outdoor air (OA), return air (RA), and exhaust air (EA)
ventila-tion dampers are controlled by the discharge air temperature
con-troller to provide free cooling as the first stage in the cooling
sequence When outdoor air temperature rises to the point that it can
no longer be used for cooling, an outdoor air limit (economizer)
control overrides the discharge controller and moves ventilation
dampers to the minimum ventilation position An enthalpy control
system can replace outdoor air limit control in some climatic areas
After the general needs of a building have been established, and
the building and system subdivision has been made, the mechanical
system and its control approach can be considered Designing
sys-tems that conserve energy requires knowledge of (1) the building,
(2) its operating schedule, (3) the systems to be installed, and (4)
ASHRAE Standard 90.1 The principles or approaches that
con-serve energy are as follows:
1 Run equipment only when needed Schedule HVAC unit
opera-tion for occupied periods Run heat at night only to maintain
internal temperature between 10 and 13°C to prevent freezing
Start morning warm-up as late as possible to achieve design
internal temperature by occupancy time, considering residual
space temperature, outdoor temperature, and equipment
capac-ity (optimum start control) Under most conditions, equipment
can be shut down some time before the end of occupancy,
depending on internal and external load and space temperature
(optimum stop control) Calculate shutdown time so that space
temperature does not drift out of the selected comfort zone
before the end of occupancy
2 Sequence heating and cooling Do not supply heating and
cool-ing simultaneously Central fan systems should use cool outdoor
air in sequence between heating and cooling Zoning and system
selection should eliminate, or at least minimize, simultaneous
heating and cooling Also, humidification and dehumidification
should not take place concurrently
3 Provide only the heating or cooling actually needed Reset the
supply temperature of hot and cold air (or water)
4 Supply heating and cooling from the most efficient source Use
free or low-cost energy sources first, then higher cost sources as
necessary
5 Apply outdoor air control When on minimum outdoor air, use
no less than that recommended by ASHRAE Standard 62 In
areas where it is cost-effective, use enthalpy rather than dry-bulb
temperature to determine whether outdoor or return air is the
most energy-efficient air source for the cooling mode
System Selection
The mechanical system significantly affects the control of zones
and subsystems The type of system and the number and location of
zones influence the amount of simultaneous heating and cooling
that occurs For exterior building sections, heating and cooling
should be controlled in sequence to minimize simultaneous heating
and cooling In general, this sequencing must be accomplished by
the control system because only a few mechanical systems (e.g.,
two-pipe systems and single-coil systems) have the ability to
pre-vent simultaneous heating and cooling Systems that require
engi-neered control systems to minimize simultaneous heating and
cooling include the following:
• VAV cooling with zone reheat Reduce cooling energy and/or air
volume to a minimum before applying reheat
• Four-pipe heating and cooling for unitary equipment Sequence
heating and cooling
• Dual-duct systems Condition only one duct (either hot or cold) at
a time The other duct should supply a mixture of outdoor and
return air
• Single-zone heating/cooling Sequence heating and cooling.
Some exceptions exist, such as of dehumidification with reheat
Control zones are determined by the location of the thermostat ortemperature sensor that sets the requirements for heating and cool-ing supplied to the space Typically, control zones are for a room or
an open area of a floor
Many jurisdictions in the United States no longer permit constantvolume systems that reheat cold air or that mix heated and cooledair Such systems should be avoided If selected, they should bedesigned for minimal use of the reheat function through zoning tomatch actual dynamic loads and resetting cold and warm air tem-peratures based on the zone(s) with the greatest demand Heatingand cooling supply zones should be structured to cover areas of sim-ilar load Areas with different exterior exposures should have dif-ferent supply zones
Systems that provide changeover switching between heating andcooling prevent simultaneous heating and cooling Some examplesare hot or cold secondary water for fan coils or single-zone fan sys-tems They usually require small operational zones, which have lowload diversity, to permit changeover from warm to cold water with-out occupant dissatisfaction
Systems for building interiors usually require year-round coolingand are somewhat simpler to control than exterior systems Theseinterior areas normally use all-air systems with a constant supply airtemperature, with or without VAV control Proper control tech-niques and operational understanding can reduce the energy used totreat these areas Reheat should be avoided General load character-istics of different parts of a building may lead to selecting differentsystems for each
Load Matching
With individual room control, the environment in a space can becontrolled more accurately and energy can be conserved if the entiresystem can be controlled in response to the major factor influencingthe load Thus, water temperature in a water heating system, steamtemperature or pressure in a steam heating system, or delivered airtemperature in a central fan system can be varied as building loadvaries Control on the entire system relieves individual space con-trols of part of their burden and provides more accurate space con-trol Also, modifying the basic rate of heating or cooling input inaccordance with the entire system load reduces losses in the distri-bution system
The system must always satisfy the area or room with the est demand Individual controls handle demand variations in thearea the system serves The more accurate the system zoning, thegreater is the control, the smaller are the distribution losses, andthe more effectively space conditions are maintained by individualcontrols
great-Buildings or zones with a modular arrangement can be designedfor subdivision to meet occupant needs Before subdivision, operat-ing inefficiencies can occur if a zone has more than one thermostat
In an area where one thermostat activates heating while anotheractivates cooling, the terminals should be controlled from a singlethermostat until the area is properly subdivided
Size of Controlled Area
No individually controlled area should exceed about 500 m2because the difficulty of obtaining good distribution and of finding
a representative location for the space control increases with zonearea Each individually controlled area must have similar loadcharacteristics throughout Equitable distribution, providedthrough competent engineering design, careful equipment sizing,and proper system balancing, is necessary to maintain uniformconditions throughout an area The control can measure conditionsonly at its location; it cannot compensate for nonuniform condi-tions caused by improper distribution or inadequate design Areas
or rooms having dissimilar load characteristics or different tions to be maintained should be controlled individually The
Trang 11condi-Design and Application of Controls 45.19
smaller the controlled area, the better the control and the better the
performance and flexibility
Location of Space Sensors
Space sensors and controllers must be located where they
accu-rately sense the variables they control and where the condition is
representative of the area (zone) they serve In large open areas
hav-ing more than one zone, thermostats should be located in the middle
of their zones to prevent them from sensing conditions in
surround-ing zones Typically, space temperature controllers or sensors are
placed in the following locations
• Wall-mounted thermostats or sensors are usually placed on
inside walls or columns in the space they serve Avoid outside
wall locations Mount thermostats where they will not be affected
by heat from sources such as direct sun rays; wall pipes or ducts;
convectors; or direct air currents from diffusers or equipment
(e.g., copy machines, coffee makers, or refrigerators) Air
circu-lation should be ample and unimpeded by furniture or other
obstructions, and the thermostat should be protected against
mechanical injury Thermostats located in spaces such as
corri-dors, lobbies, or foyers should be used to control those areas only
• Return air thermostats can control floor-mounted unitary
con-ditioners such as induction or fan-coil units and unit ventilators
On induction and fan-coil units, the sensing element is behind the
return air grille On classroom unit ventilators that use up to 100%
outdoor air for natural cooling, however, a forced flow sampling
chamber should be provided for the sensing element The sensing
element should be located carefully to avoid radiant effect and to
ensure adequate air velocity across the element
If return air sensing is used with a central fan system, locate the
sensing element as near as possible to the space being controlled
to eliminate any influence from other spaces and the effect of any
heat gain or loss in the duct Where supply/return light fixtures areused to return air to a ceiling plenum, the return air sensing ele-ment can be located in the return air opening Be sure to offset theset point to compensate for the heat from the light fixtures
• Diffuser-mounted thermostats usually have sensing elements
mounted on circular or square ceiling supply diffusers and depend
on aspiration of room air into the supply airstream They should
be used only on high-aspiration diffusers adjusted for a horizontalair pattern The diffuser on which the element is mounted should
be in the center of the occupied area of the controlled zone
Lowered Night Temperature
When temperatures during unoccupied periods are lower thanthose normally maintained during occupied periods, an automatictimer often establishes the proper day and night temperature timecycle Allow sufficient time in the morning to pick up the condition-ing load well before there is any heavy increase Night setback tem-peratures are often monitored and controlled more closely withcontrol systems These computer based systems take into accountvariables such as outdoor temperature, system capacity, and build-ing mass to determine optimal start-up and shutdown times
REFERENCES
ASHRAE 1989 Energy efficient design of new buildings except low-rise
residential buildings ANSI/ASHRAE Standard 90.1-1989.
ASHRAE 1989 Ventilation for acceptable indoor air quality ANSI/
ASH-RAE Standard 62-1989.
Englander, S.L and L.K Norford 1992 Saving fan energy in VAV
sys-tems—Part 2: Supply fan control for static pressure minimization RAE Transactions 98(1):19-32.
ASH-NFPA 1996 National electrical code ANSI/NFPA Standard 70-96.
National Fire Protection Association, Quincy, MA.
Trang 12CHAPTER 46 SOUND AND VIBRATION CONTROL
Data Reliability 46.1
SOUND 46.1
Acoustical Design of HVAC Systems 46.1
Basic Design Techniques 46.2
Equipment Sound Levels 46.4
Duct Element Sound Attenuation 46.11
Use of Fiberglass Products in HVAC Systems 46.17
Sound Radiation Through Duct Walls 46.17
Receiver Room Sound Correction 46.20
Indoor Sound Criteria 46.22
Outdoor Sound Criteria 46.26
Mechanical Equipment Room Sound Isolation 46.27
Sound Transmission in Return-Air Systems 46.31
Sound Transmission Through Ceilings 46.31
Fume Hood Duct Design 46.32
Sound Control for Outdoor Equipment 46.32 Design Procedures 46.34 VIBRATION ISOLATION AND CONTROL 46.37 Equipment Vibration 46.37 Vibration Criteria 46.37 Specification of Vibration Isolators 46.37 Isolation of Vibration and Noise in Piping Systems 46.42 Isolating Duct Vibration 46.44 Seismic Protection 46.45 Vibration Investigations 46.45 TROUBLESHOOTING 46.45 Determining Problem Source 46.45 Determining Problem Type 46.45 Standards 46.47
ECHANICAL equipment is one of the major sources of
Msound in a building Primary considerations often given to
the selection and use of mechanical equipment in buildings have
generally been those directly related to the intended use of the
equipment, like cooling, heating, and ventilation However, for
environmental considerations in critical listening spaces, like
con-ference rooms and auditoria, and for many other spaces with light
building structures and variable-volume air distribution systems,
the sound generated by mechanical equipment and its effects on the
overall acoustical environment in a building must be considered
Thus, the selection of mechanical equipment and the design of
equipment spaces should be undertaken with an emphasis on (1) the
intended uses of the equipment and (2) the goal of providing
accept-able sound and vibration levels in occupied spaces of the building in
which the equipment is located
The system concept of noise control is used throughout, in that
each of the components is related to the source-path-receiver chain
The noise generation is the source; it travels from the source via a
path, which can be through the air (airborne) or through the
struc-ture (strucstruc-ture-borne) until it reaches the ear of the receiver When
the combination of this chain is complex, it can be referred to it as
a system effect So, noise propagates from the sources through the
air distribution ducts, through the structure, and through
combina-tions of paths, reaching the occupants All mechanical components,
from dampers to diffusers to junctions, may produce sound by the
nature of the airflow through and around them As a result, almost
all components must be considered Since sound travels effectively
in the same or opposite direction of airflow, upstream and
down-stream paths are often equally important
Adequate noise and vibration control in a heating, ventilating,
and air-conditioning (HVAC) system is not difficult to achieve
dur-ing the design phase of the system, providdur-ing basic noise and
vibra-tion control principles are understood This chapter discusses basic
sound and vibration principles and data needed by HVAC designers
Divided into two main sections, one on sound, the other on
vibra-tion, this chapter is organized differently than versions in
Hand-books prior to 1995 This chapter includes more information on
acoustic design guidelines and system design requirements Most of
the equations associated with sound and vibration control design in
HVAC systems have been replaced by related tables and simpler
design procedures The equations that have been removed can be
found in the 1991 and 1992 ASHRAE Handbooks In addition,
tech-nical discussions and detailed HVAC component and system design
examples can be found in Algorithms for HVAC Acoustics
(Rey-nolds and Bledsoe 1991)
Other publications that cover sound and vibration control in
HVAC systems include the 1997 ASHRAE tals, which covers fundamentals associated with sound and vibra-
Handbook—Fundamen-tion in HVAC; Schaffer (1991), who provides specific guidelinesfor the acoustic design and related construction phases associatedwith HVAC systems, troubleshooting sound and vibration prob-lems, and HVAC sound and vibration specifications; Ebbing andBlazier (1998), who interpret and clarify how users can make thebest use of HVAC manufacturers’ acoustical data and applicationinformation; and Reynolds and Bevirt (1994), who cover instrumentrequirements, instrument and measurement calibration procedures,measurement procedures, and specification and construction instal-lation review procedures associated with sound and vibration mea-surements relative to HVAC systems
DATA RELIABILITY
The data in this chapter comes both from consulting experienceand research studies When applying the data, especially to situa-tions that extrapolate from the original data, use caution While spe-cific uncertainties are not stated for each data set, the sound levels
or attenuation data are probably within 2 dB of measured orexpected results However, significantly greater variations mayoccur, especially in the low frequency ranges and particularly in the
63 Hz octave band While specific data sets may have a wide tainty range, experience has demonstrated the usefulness of com-bining data sets for estimating the sound level If done correctly,these estimates usually result in space sound pressure levels within
uncer-5 dB of measured levels
SOUND
ACOUSTICAL DESIGN OF HVAC SYSTEMS
The solution to nearly every HVAC system noise and vibrationcontrol problem involves examining the sound sources, the soundtransmission paths, and the receivers For most HVAC systems, thesound sources are associated with the building mechanical and elec-trical equipment As indicated in Figure 1, sound travels between asource and receiver through many possible sound and/or vibration
The preparation of this chapter is assigned to TC 2.6, Sound and Vibration
Control.
Trang 13Sound and Vibration Control 46.3
3 Design duct connections at both the fan inlet and outlet for
uni-form and straight air flow Failure to do this can result in severe
turbulence at the fan inlet and outlet and in flow separation at
the fan blades Both of these can significantly increase the
noise generated by the fan
4 Select duct silencers that do not significantly increase the
required fan total static pressure Duct silencers can
signifi-cantly increase the required fan static pressure if improperly
selected Selecting silencers with static pressure losses of
87 Pa or less can minimize silencer airflow regenerated
noise
5 Place fan-powered mixing boxes associated with
variable-vol-ume air distribution systems away from noise-sensitive areas
6 Minimize flow-generated noise by elbows or duct branch
take-offs, whenever possible, by locating them at least four to five
duct diameters from each other For high velocity systems, it
may be necessary to increase this distance to up to ten duct
diameters in critical noise areas The use of flow straighteners
or honeycomb grids, often called “egg crates”, in the necks of
short-length takeoffs that lead directly to grilles, registers, and
diffusers is preferred to the use of volume extractors that
pro-trude into the main duct airflow
7 Keep airflow velocity in the duct as low as possible (7.5 m/s or
less) near critical noise areas by expanding the duct
cross-sec-tion area However, do not exceed an included expansion angle
of greater than 15° Flow separation, resulting from expansion
angles greater than 15°, may produce rumble noise Expanding
the duct cross-section area will reduce potential flow noise
associated with turbulence in these areas
8 Use turning vanes in large 90° rectangular elbows and branchtakeoffs This provides a smoother transition in which the aircan change flow direction, thus reducing turbulence
9 Place grilles, diffusers and registers into occupied spaces as far
as possible from elbows and branch takeoffs
10 Minimize the use of volume dampers near grills, diffusers andregisters in acoustically critical situations
11 Vibration isolate all vibrating reciprocating and rotating ment if mechanical equipment is located on upper floors or isroof-mounted Also, it is usually necessary to vibration isolatethe mechanical equipment that is located in the basement of abuilding as well as piping supported from the ceiling slab of abasement, directly below tenant space It may be necessary touse flexible piping connectors and flexible electrical conduitbetween rotating or reciprocating equipment and pipes andducts that are connected to the equipment
equip-12 Vibration isolate ducts and pipes, using spring and/or neoprenehangers for at least the first 15 m from the vibration-isolatedequipment
13 Use barriers near outdoor equipment when noise associatedwith the equipment will disturb adjacent properties if barriersare not used In normal practice, barriers typically produce nomore than 15 dB of sound attenuation in the mid frequencyrange
Table 1 lists several common sound sources associated withmechanical equipment noise Anticipated sound transmission pathsand recommended noise reduction methods are also listed in thetable Airborne and/or structure-borne sound can follow any or all
of the transmission paths associated with a specified sound source.Schaffer (1991) has more detailed information in this area
Table 1 Sound Sources, Transmission Paths, and Recommended Noise Reduction Methods
Circulating fans; grilles; registers; diffusers; unitary equipment in room 1
Unitary equipment located outside of room served; remotely located air-handling equipment,
such as fans, blowers, dampers, duct fittings, and air washers
2, 3 Compressors, pumps, and other reciprocating and rotating equipment (excluding air-handling equipment) 4, 5, 6
1 Direct sound radiated from sound source to ear Direct sound can be controlled only by selecting quiet equipment Reflected sound from walls, ceiling, and floor Reflected sound is controlled by adding sound absorption to the room
and to equipment location.
2 Air- and structure-borne sound radiated from casings and through walls of
ducts and plenums is transmitted through walls and ceiling into room
Design duct and fittings for low turbulence; locate high velocity ducts in noncritical areas; isolate ducts and sound plenums from structure with neoprene or spring hangers.
3 Airborne sound radiated through supply and return air ducts to diffusers in
room and then to listener by Path 1
Select fans for minimum sound power; use ducts lined with sound-absorbing material; use duct silencers or sound plenums
in supply and return air ducts.
4 Noise transmitted through equipment room walls and floors to adjacent
rooms
Locate equipment rooms away from critical areas; use masonry blocks or concrete for equipment room walls and floor.
5 Vibration transmitted via building structure to adjacent walls and ceilings,
from which it radiates as noise into room by Path 1
Mount all machines on properly designed vibration isolators; design mechanical equipment room for dynamic loads; balance rotating and reciprocating equipment.
6 Vibration transmission along pipes and duct walls Isolate pipe and ducts from structure with neoprene or spring hangers;
install flexible connectors between pipes, ducts, and vibrating machines.
7 Noise radiated to outside enters room windows Locate equipment away from critical areas; use barriers and covers to
interrupt noise paths; select quiet equipment.
8 Inside noise follows Path 1 Select quiet equipment.
9 Noise transmitted to an air diffuser in a room, into a duct, and out through
an air diffuser in another room
Design and install duct attenuation to match transmission loss of wall between rooms.
10 Sound transmission through, over, and around room partition Extend partition to ceiling slab and tightly seal all around; seal all pipe,
conduit, duct, and other partition penetrations.
Trang 14Sound and Vibration Control 46.5
possible sound power levels, commensurate with other fan
selec-tion requirements
• Many fans generate tones at the blade passage frequency and its
harmonics that may require additional acoustical treatment The
amplitude of these tones can be affected by resonance in the duct
system, fan design, and inlet flow distortions caused by poor inlet
duct design, or by the operation of an inlet volume control
damper When possible, variable speed control for volume
con-trol is preferable to volume concon-trol dampers to concon-trol fan noise
• Design duct connections at both the fan inlet and outlet for
uni-form and straight airflow Avoid unstable, turbulent, and swirling
inlet airflow Deviation from acceptable practice can severely
degrade both the aerodynamic and acoustic performance of any
fan and invalidate the manufacturer’s ratings or other
perfor-mance predictions
Variable Air Volume (VAV) Systems
General Design Factors VAV systems can significantly reduce
energy cost due to their ability to modulate air capacity But they can
be the source of fan noise that is very difficult to mitigate To avoid
these potential problems, the designer should carefully design the
ductwork and the static pressure control systems and select the fan
or air handling unit and its air modulation device
As in other aspects of HVAC design, the duct system should be
designed for the lowest practical static pressure loss, especially in
the ductwork closest to the fan or air handling unit High airflow
velocity and convoluted duct routing can cause airflow distortions
that result in excessive pressure drop and fan instabilities that are
responsible for excessive noise, fan stall, or both
Many VAV noise complaints have been traced to control
prob-lems While most of the problems are associated with improper
installation, many are caused by poor design The designer should
specify high-quality fans or air handling units that will operate in
their optimum ranges, not at the edge of their operation ranges
where low system tolerances can lead to inaccurate fan flow
capac-ity control Also, the in-duct static pressure sensors should be placed
in duct sections having the lowest possible air turbulence; that is, at
least three equivalent duct diameters from any elbow, takeoff,
tran-sition, offset, or damper
VAV noise problems have been traced to improper air balancing
For example, air balance contractors commonly balance an air
dis-tribution system by setting all damper positions without considering
the possibility of reducing the fan speed The end result is a duct
sys-tem in which no damper is completely open and the fan is delivering
air at a higher static pressure than would otherwise be necessary If
the duct system is balanced with at least one balancing damper wide
open, the fan speed could be reduced with a corresponding
reduc-tion in fan noise Lower sound levels will occur if most balancing
dampers are wide open or eliminated
Fan Selection For constant-volume systems, fans should be
selected to operate at maximum efficiency at the fan design airflow
rate However, VAV systems must be selected to operate with
effi-ciency and stability throughout its range of modulation For
exam-ple, a fan selected for peak efficiency at full output may
aerodynamically stall at an operating point of 50% of full output
resulting in significantly increased low frequency noise Similarly,
a fan selected to operate at the 50% output point may be very
inef-ficient at full output, resulting in substantially increased fan noise at
all frequencies In general, a fan selected for a VAV system should
be selected for a peak efficiency at an operating point of around 70
to 80% of the maximum required system capacity This usually
means selecting a fan that is one size smaller than that required for
peak efficiency at 100% of maximum required system capacity
(Figure 6) When the smaller fan is operated at higher capacities, it
will produce up to 5 dB more noise This occasional increase in
sound level is usually more tolerable than the stall-related sound
problems that can occur with a larger fan operating at less than100% design capacity most of the time
Air Modulation Devices Variable capacity control methods can
be divided into three general categories: (1) variable inlet vanes(sometimes called inlet guide vanes) or discharge dampers, whichyield a new fan system curve at each vane or damper setting; (2)variable pitch fan blades (usually used on in-line axial fans), whichadjust the blade angle for optimum efficiency at varying capacityrequirements; and (3) variable speed motor drives where the motorspeed is varied by modulation of the power line frequency or bymechanical means such as gears or continuous belt adjustment.While inlet vane and discharge damper volume controls can addnoise to a fan system at reduced capacities, variable speed motordrives and variable-pitch fan blade systems are quieter at reducedair output than at full air output
Variable Inlet Vanes and Discharge Dampers Variable inlet
vanes vary airflow capacity by changing the inlet airflow to a fanwheel This type of air modulation varies the total air volume andpressure at the fan while the fan speed remains constant While, fanpressure and air volume reductions at the fan result in duct systemnoise reductions by reduced air velocity and pressures in the ductwork, there is an associated increase in fan noise caused by the air-flow turbulence and flow distortions at the inlet vanes acting as a faninlet obstruction Fan manufacturers’ test data have shown that, onairfoil type centrifugal fans, as vanes mounted inside the fan inlet(nested inlet vanes) close, the sound level at the blade passing fre-quency of the fan increases by 2 to 8 dB, depending on the amount
of total air volume restricted For inlet vanes that are mounted nally the increase is on the order of 2 to 3 dB Forward curved fanwheels with inlet vanes are about 1 to 2 dB quieter than airfoil fanwheels In-line axial type fans with inlet vanes generate increasednoise levels of 2 to 8 dB in the low frequency octave bands for a25% to 50% closed vane position
exter-Discharge dampers are typically located immediately stream of the supply air fan and reduce airflow and increase pressuredrop across the fan while the fan speed remains constant Because ofthe air turbulence and flow distortions created by the high-pressuredrop across discharge dampers there is a high probability that ductrumble will occur near the damper location If the dampers are throt-tled to a very low flow, a stall condition can occur at the fan alsoresulting in an increase in low-frequency noise
down-Variable Pitch Fan Blades for Capacity Control down-Variable pitch
fan blade controls vary the fan blade angle in order to reduce theoverall airflow through the fan This type of capacity control system
Fig 6 Basis for Fan Selection in VAV Systems