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© 2006 American Standard All rights rerved
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volume 35–2
keeping cool with outdoor air …
Airside Economizers
Y ou’re in your kitchen on a beautiful, breezy fall day, with a pie baking in the oven. Would you rather cool off the kitchen by opening your windows or by turning on your central air conditioner? By opening the windows, of cour! Why? If the outdoor air is cool and dry, it can cool the space nicely without using electricity for compressors and fans, and it ventilates the space with fresh outdoor air in the barg
ain. The same logic holds in non-residential buildings. In fact, it not only makes n to introduce outdoor air when conditions are “right” but it’s also required by ASHRAE Standard 90.1–2004[1] (Standard 90) and, in many jurisdictions, by the building code.
Let’s take a high-level look at airside economizing:what it is, what it requires, and how it’s done.
How does economizer cooling work?
Most commercial buildings have some spaces that need cooling all year long. If it’s colder outdoors than indoors, it often makes energy-n to
“economize” by bringing in more-than-minimum outdoor air to reduce the hours of mechanical cooling system operation.
A typical “mixed-air” air handler
includes dampers for outdoor air, return air, and relief air (Figure 1). The dampers can be controlled to provide airside economizing. Usually, an air handler with economizer controls has four operating modes.
Heating mode. During very cold weather, the air handler brings in minimum outdoor airflow (for
ventilation) and mixes it with return air.The mixed air is then heated as necessary to maintain the desired supply-air (or space) temperature.ASHRAE Standard 62.1–2004[2] (Standard 62) or building code
requirements determine the minimum intake of outdoor air. This minimum may be ret downward from the design value (to save heating energy) using various dynamic ret
approaches. But when it’s cold outside, no more than minimum outdoor air enters the building, whether or not the system includes airside economizer controls.
Modulated economizer mode. During “cool” weather (30°F to 55°F [1°C to 13°C], for example), the required space temperature can be maintained without any mechanical cooling or heating by simply adjusting the mix of outdoor air and return air.In this mode, the economizer system adjusts both the outdoor- and return-air dampers, modulating the airflows to
Figure 1.T ypical air handler
from the editor …
For the denizens of commercial
四六级官网登陆入口
buildings, comfort cooling is one of life’s basic necessities. Without it, we quickly become irritable, lethargic, and unable to concentrate. But the cost of keeping us comfortably cool ris with each passing day … and that cost isn’t
confined to the person or organization who pays the building utility bill.
Using outdoor air to help cool a building isn’t a new concept. Thanks to energy standards and local building codes, it’s no longer optional in many areas, either .In this EN, Dennis Stanke (Trane staff engineer and chair of ASHRAE SSPC 62.1) reviews ASHRAE Standard 90.1’s requirements for airside economizers and discuss the underlying design
decisions and benefits of effective airside economizer systems.
match cooling capacity with cooling load—without mechanical cooling. Intake airflow varies between the minimum tting and a maximum value (100% of supply airflow) to maintain the supply-air (or space) temperature at tpoint.
Integrated economizer mode. During mild weather (55°F to 75°F [13°C to 24°C], for example), outdoor air can provide some cooling capacity, but not enough to satisfy the load, so mechanical coo
ling supplements the economizer cooling provided by the wide-open outdoor-air damper. We refer to this mode as integrated economizer becau it combines “free” cooling (100% outdoor air)
with mechanical cooling to meet the required cooling capacity. The system
stays in integrated economizer mode until outdoor conditions reach the high-limit shutoff tting (discusd in more detail on p.5), or until the outdoor conditions fall to the point where modulated economizer operation
can handle the cooling load. Mechanical cooling mode. Economizer operation is disabled during warm weather, when outdoor conditions exceed the high-limit shutoff tting. Minimum outdoor air for ventilation (determined by Standard 62 or the local building code) mixes with return air. The mixed air then is mechanically cooled as needed to maintain the supply-air (or space) temperature at tpoint.
As in the heating mode, the minimum outdoor airflow may be ret downward from its design value to save cooling energy. But, when it’s hot outside, no more than minimum outdoor air enters the building
whether the system includes an
airside economizer or not.
Note: Specific control quences for
the preceding operating modes may
vary with equipment configuration,
type of economizer control, and high-
limit ttings.
What does
Standard 90 require?
According to Standard 90,
Section6.5.1, economizer cooling
systems (either airside or waterside)
must be ud in all cooling systems
with fans. There’s also a long list of
exceptions to this requirement (e
int, p.3) becau the value of
economizer cooling depends on
climate, building type, system type,
and control ttings. Nevertheless,
Standard 90 requires economizer
cooling in many locations for many
buildings and many different systems.
Figure2 shows the U.S. climate
zones* defined in Appendix B of the
Standard. Figure3 shows regional
economizer requirements, which aim
to minimize the ratio of economizer-
related costs to energy-related savings.
The requirements are bad on
cooling system capacity and the
expected number of hours with
outdoor conditions that are appropriate
for economizing.
Potential energy savings are highest
where the weather is dry or marine, so
systems with capacities larger than
≈5tons (65,000Btu/h [19kW]) in
the locations must include
economizer cooling. Moist, cool
climates provide fewer opportunities
for “free” cooling, so the economizer
requirement only applies to systems
with capacities larger than ≈11tons
(135,000Btu/h [40kW]).
Potential benefits are lowest where
the weather is moist and warm.
Economizer cooling is not required in
the climates (although it is allowed)
becau ASHRAE studies indicate that
the potential savings in mechanical
cooling energy may not be sufficient to
justify the additional cost of
implementing it.
*Appendix B of Standard 90 also identifies
climate zones for various cities elwhere in
North America and around the world.
Figure 2.U.S. climate zones
2●Trane Engineers Newsletter volume 35–2providing insights for today’s HVAC system designermarquee
Exceptions to Standard 90’s economizer requirement
Section 6.5.1 requires economizers for
all systems in all locations. But it also identifies the nine exceptions listed below. Note:Economizer sy
stems may be
ud even if not required, provided that the economizer system conforms to the requirements in Sections 6.5.1.1 through
6.5.1.4.
(a)Systems using fan-cooling units
with individual capacities less than
65,000Btu/h (19 kW) in dry climates, less than 135,000Btu/h (40 kW) in cool-moist climates, and with any capacity, large or small, in warm-moist climates (Figure3).
(b)Systems with gas-pha outdoor air cleaning to meet ASHRAE Standard 62. (c)Systems that deliver more than 25% of the supply air to spaces humidified above 35°F dew point for process needs.(d)Systems with condenr heat recovery.
(e)Any residential space system with a capacity that’s less than five times the applicable limit listed in Exception (a). (f)Systems with space nsible cooling loads (excluding transmission and infiltration loads) equal to or less than transmission and infiltration loads at 60°F.
(g)Systems that are expected to operate less than 20 hours per week.
(h)Supermarket systems where outdoor air for cooling affects open refrigerated cas.
日语能力测试(i)Systems with high mechanical cooling efficiencies (equal to or exceeding the requirements of Table 6.3.2).•
Section 6.5.1.1.1:Design capacity. Airside-economizer systems must include outdoor- and return-air dampers that are sized and modulated so that up to 100% of design supply airflow can be outdoor air. In other words, the system must be designed to allow outdoor airflow ranging from the minimum required for ventilation to the maximum delivered by the
supply fan.
For VAV systems, the supply fan usually delivers less than cooling-design airflow during the integrated economizer mode—even though the outdoor-air damper is wide open. Of cour, supply fan airflow (and therefore, intake airflow) increas if the control rets the supply-air temperature upward at part load.†
†Warmer supply air increas the delivered airflow (therefore fan energy) and space humidity levels
while reducing both mechanical cooling and local reheat energy. Thoroughly analyze the effects of a supply-air-temperature ret strategy before making it part of the system design.Section 6.5.1.1.2:Control signal.
Airside economizer operation must
be appropriately quenced with
mechanical cooling to maximize energy
savings while avoiding wasteful
simultaneous cooling and heating.
To help assure proper quencing,
control of the economizer dampers
shouldn’t be bad on mixed-air
conditions alone.
Figure4a and Figure4b (p.4) showindustry是什么意思
typical economizer quencing for
constant- and variable-volume
systems. Standard 90 does not
specifically require the operating
modes, but they result from logical
quencing of mechanical and
economizer cooling. The diagrams
halo 歌词expand on the single diagram
xna
prented in the Standard 90.1–2004
Ur’s Manual.[3]
Figure4a:Constant-volume
systems.In heating mode, minimum
outdoor-air intake flow enters the
system. Heating capacity decreas as
the outdoor-air heating load decreas.
As the weather warms, when the
system no longer needs heating, it
enters the modulated economizer
mode. Outdoor air and recirculated
return air modulate to maintain space
(or supply-air) temperature at tpoint.
Outdoor air provides the needed
cooling capacity without any
mechanical cooling. The black area in
Figure4a reprents the mechanical
cooling energy that’s saved during
modulated economizer operation or
“free cooling.”
As the cooling load increas, the
outdoor-air damper eventually opens
to100% and the return-air damper
clos completely. The system enters
integrated economizer mode, where
100% outdoor airflow provides part of Figure 3.Standard 90’s regional economizer requirements bad on cooling system capacity
providing insights for today’s HVAC system designer Trane Engineers Newsletter volume 35–2●3
the required cooling capacity and mechanical cooling provides the balance, modulating or cycling as necessary to maintain the required space (or supply-air) temperature. The red area in Figure4a reprents the mechanical cooling energy that’s saved during integrated economizer operation.
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The system stays in integrated economizer mode until the outdoor-air condition reaches the high-limit shutoff tting. At this point, the controls disable economizer operation and the system enters the mechanical cooling mode, where a water valve modulates or a compressor cycles to provide all cooling capacity needed to maintain space (or supply-air) temperature. In this mode, the outdoor-air damper
clos to allow only minimum
intake airflow.
In some locations, direct expansion (DX) systems may be designed to enter the mechanical cooling mode directly from the modulated economizer mode. If 100% outdoor air is unable to provide the required cooling capacity, then the outdoor-air damper clos to its minimum position and mechanical cooling modulates to provide all of the needed cooling capacity. This “non-integrated economizer” approach avoids unstable refrigerant system operation and coil frosting, which can occur when a DX system cycles at low loads. But it also reduces the potential savings in mechanical cooling energy reprented by the red-shaded area in Figure4a. Figure 4b:Variable-volume systems.Consider a single-duct, chilled water VAV system with reheat terminals:In heating mode, mi
nimum outdoor airflow enters the system and recirculated return air provides the balance of supply airflow. Supply airflow usually decreas as the heating load diminishes becau the reheat terminals need less airflow when cooling than when heating. When the cooling load starts to ri, the modulated economizer mode begins. Supply airflow increas (that
is, intake airflow increas while return
airflow decreas) to maintain the
required supply-air temperature
without mechanical cooling. In the
integrated economizer mode, the
outdoor-air damper stays wide open
摩登家庭第一季下载to provide some cooling capacity
while the mechanical system
modulates to provide the balance.
The system enters mechanical cooling
mode when outdoor air reaches the
high-limit shutoff condition. Intake
airflow drops to the minimum
requirement, and supply and return
airflows increa while the cooling coil
provides the required cooling capacity.
Both constant-volume and VAV
systems u linked outdoor- and
return-air dampers, which are operated
by a single actuator or by multiple
coordinated actuators. However, as
mentioned in the Standard 90 ur’s
manual and in Guideline 16, Selecting
Outdoor, Return and Relief Dampers
for Airside Economizer Systems[4],
Figure 4b.T
ypical economizer control quence for variable-volume (VAV) systems
Figure 4a.T ypical economizer control quence for constant-volume systems
4●Trane Engineers Newsletter volume 35–2providing insights for today’s HVAC system designer
quential or “optimized” damper operation can save supply fan energy by reducing mixing box pres
sure during the modulated economizer mode.[5] Section 6.5.1.1.3:High-limit shutoff. Integrated airside-economizer operation must be disabled (that is, the outdoor-air damper must reduce intake airflow from maximum to the minimum airflow required for ventilation) whenever outdoor air exceeds a prescribed high-limit condition. This makes n:If it starts to rain or if it gets really hot outside, you want to clo the kitchen window, right? Converly, economizer operation must be enabled when outdoor conditions are equal to or below the high-limit shutoff. In other words, the prescribed high-limit shutoff tting acts as an economizer enable/
disable tting.
Above the prescribed high-limit shutoff
—a “best compromi” established by
ASHRAE using computer simulations
for various control types in various
climates—Standard 90 assumes that
the system needs more mechanical
cooling energy to condition 100%
outdoor air than to condition mixed air,
so economizer operation must be
disabled. Below the high-limit shutoff,
the Standard assumes that the system
needs less mechanical cooling to
condition 100% outdoor air. Of cour,
since the prescribed limits are bad
on broad assumptions, the “best”
economizer enable/disable condition
for a given building in a given location
might actually differ from the
prescribed limit.
The prescribed high-limit tting
99宿舍四级查分depends on location and the type of
high-limit that’s ud. Table 6.5.1.1.3A
in the Standard allows six different
control types:fixed dry bulb,
differential dry bulb, fixed enthalpy,
electronic enthalpy, differential
enthalpy, and dew point–and–dry bulb.
In dry and marine climates and in very
cold climates, fixed enthalpy control is
not allowed (Figure5, p.6). Why?
Becau it could result in 100%
outdoor air during many hours when
outdoor conditions are dry but very
warm. If the cooling coil is dry, too,
both nsible load and mechanical
cooling energy increa rather than
decrea. In cold climates, fixed
enthalpy control could disable
economizer operation during cool, rainy毛孔特别粗大怎么办
weather and miss valuable hours of
Decisions, Decisions
Standard 90 includes prescribed minimum requirements for economizer systems. But within the requirements, designers still must make veral economizer-related decisions in addition to choosing the HVAC system type (CV-basic, CV-reheat, VAV) and size:to economize or not to economize, whether to u an integrated or non-integrated economizer, which of six types of high-limit shutoff control to implement, and perhaps an optimum high-limit shutoff tting as well. Economize? Even though economizer cooling may not be required (becau the system is small or becau it’s in a warm, moist climate), a thorough engineering analysis may show that it reduces operating costs enough to be worthwhile. Economizer type? Even though it’s not required in most locations, a designer may choo integrated economizer control anyway. Why u it? It always increas economizer hours and it doesn’t increa first cost in chilled water systems. Why not u it? In simple DX systems, operating with 100% outdoor air may result in coil frosting at low loads. And, at low outdoor air temperatures (low loads), it caus compressor short-cycling, which may lead to refrigerant system instability and possible reliability problems. The common “fix” for low-load cycling usually involves hot gas bypass, which increas the first cost of integrated economizer contr
ol and decreas the benefit.High-limit shutoff type? Depending on
location, designers may choo from five
or six types of high-limit shutoff control,
ranging from lowest cost (fixed dry bulb)
to perhaps lowest energy (differential
enthalpy):
•Fixed dry bulb (not allowed in some
climate zones) disables economizer
operation when outdoor air exceeds a
fixed temperature.
•Differential dry bulb disables
economizer operation when the
temperature outside is warmer than
the return air.
•Fixed enthalpy (not allowed in some
climate zones) disables economizer
operation when outdoor air exceeds a
fixed enthalpy (28 Btu/lb [65kJ/kg]).
•Electronic enthalpy disables economizer
operation when outdoor air exceeds a
lected dry-bulb/dew-point curve,
defined by electronic nsor
manufacturers.
•Differential enthalpy disables
economizer operation when outdoor-air
enthalpy exceeds return-air enthalpy.
•Dew point–and–dry bulb disables
economizer operation when outdoor air
exceeds a fixed dry-bulb (75°F [24°C])
OR a fixed dew-point (55°F [13°C])
temperature.
•Others:Standard 90 doesn’t
permit other control types under the
prescriptive approach to compliance.
Each permitted approach has a unique first
cost, operating cost, maintenance cost,
and space relative-humidity performance.
To properly weigh the tradeoffs and make
an informed choice, designers should u
an economic/performance analysis
program to compare at least fixed dry
bulb, fixed enthalpy, and differential
enthalpy, since the are common and
widely applicable.
High-limit shutoff tting? The
values are prescribed for different control
types in different climates. An economic/
performance analysis may show that
higher or lower values offer better energy
savings or better relative humidity
performance for a particular building
system. The authority having jurisdiction
may agree to approve a variance if such an
analysis shows that a “custom” shutoff
tting results in lower energy u than the
“generic” prescribed tting.
Decisions about economizer systems can
be made spontaneously bad on first-cost
considerations (for example) … or they can
be made deliberately bad on an analysis
of both life-cycle cost and relative-humidity
performance. Many designers and their
customers prefer the latter.•
providing insights for today’s HVAC system designer Trane Engineers Newsletter volume 35–2●5
