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American Aldes Controlling Stack Effect User Manual

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    Central ventilation and toilet exhaust risers are designed for the 
    purpose of providing mechanically controlled ventilation and 
    protecting against poor indoor air quality. In high-rise buildings 
    (buildings over four stories), exhaust ventilation risers and 
    subsequent fans are often dramatically affected by environmental 
    factors such as stack pressure. When buildings are built tightly to 
    conserve energy, stack pressure has a greater effect on a system’s 
    ability to regulate indoor air quality, ultimately detracting from 
    a building’s energy efficiency. 
    The central duct riser used for air exhaust and/or ventilation air 
    distribution in tall buildings is the main focus for building designers 
    and engineers looking to improve energy efficiency and indoor 
    air quality. Maintaining proper airflow rates in duct risers is the 
    key for both indoor air quality and energy efficiency assurance; 
    Difficulty balancing the system, poor maintenance practices, and 
    fluctuations in system pressure due to stack effect make it very 
    problematic to maintain proper flow rates, let alone minimize 
    energy consumption. 
    One challenge designers face is how to minimize the effect stack 
    pressure has on a particular system, while minimizing fan motor 
    power consumption. To combat seasonal fluctuations in system 
    pressure, designers can either increase fan-induced duct pressure 
    or find a means to modulate the opening at each intake point. In 
    the absence of either solution, these seasonal pressure variations 
    will result in over- or under-ventilation, increased thermal load on 
    the building, and fluctuations in sound levels at the intake points. 
    This application guide discusses how stack pressure is determined, 
    its effect on vertical riser system performance, and what can be 
    done to overcome this ever-present condition.
    Controlling Stack Effect in Ventilation Duct 
    Risers Promotes Energy Efficiency and IAQ
    The difficulty of maintaining proper ventilation system riser airflow 
    balance in areas with large shifts in climatic conditions is mainly 
    due to stack effect. Stack, or hydrostatic pressure, is created when 
    differences exist among air temperature, altitude, and vertical 
    distribution of air from indoor and outdoor conditions. As discussed 
    in ASHRAE Fundamentals Chapter 26, “stack pressure differences 
    are positive when the building is pressurized relative to outdoors, 
    which causes flow out of the building.  Therefore, in the absence of 
    other driving forces, when the indoor air is warmer than outdoors, 
    the base of the building is depressurized and the top is pressurized 
    relative to outdoors; when the indoor air is cooler than outdoors, 
    the reverse is true.”
    Stack effect is unavoidable, and increases with building height 
    and as temperature differences between inside and outside air 
    increase.  The level of stack pressure within vertical chases in a 
    compartmentalized building (multiple floors) is also affected by 
    the degree of air tightness between floors and with the exterior 
    walls.  Tall buildings, larger differences in indoor and outdoor 
    temperatures, and tight construction all contribute to greater 
    pressure differences within elevator shafts, stairwells, and exhaust 
    Since vertical duct risers penetrate the floors of compartmentalized 
    buildings and provide an open vertical chase throughout the 
    length of the duct itself – usually the height of the building – 
    stack pressure within these ducts can be calculated using the 
    following formula:
    ΔPs = C1 · g · p · (T1 - T0) /T1) · H
    Where : 
    Ps=stack pressure, in. of water
    C1=unit conversion factor = 0.00598 
    (in. of water) x ft x s2/lbm
    g=gravitational constant, 32.2 ft/s2
    p=indoor or outdoor air density
    T0=outdoor air temperature (R)
    T1=indoor air temperature (R)
    H=Height (ft.)
    A simple rule of thumb can be derived from the same formula 
    as follows:
    ΔPs = .0000274 in. w.g. per ft x (TF1 - TF0)                                   
    							Example:  A 200 ft. building in Chicago, 0°F winter design condition, 
    70°F indoor temp:
    ΔPs = .0000274 x 200 (70° F - 0°F)
    ΔPs = .00548 (70)
    ΔPs = 0.38 in. w.g.
    If the duct riser extends only partially throughout the building 
    height, apply the same formula for only the length of the duct 
    riser.  To determine the stack pressure at each intake point, apply 
    the same formula for the length of each point to the fan.  This 
    assumes that the fan at the top of the riser is capable of handling 
    the increase in pressure and resulting increase in flow, and the 
    neutral pressure point is at or above the fan itself.
    If the increase in stack pressure and flow results in conditions 
    beyond a fan’s performance capability, the neutral pressure 
    point would be lower than the fan at a point within the duct 
    riser itself.  This would actually result in positive pressure, or 
    outflow of air above the neutral pressure point at the top of the 
    riser, and completely eliminate the ventilation performance in 
    those areas.  This scenario is often identified as a cause of poor 
    indoor air quality.
    The increase in pressure from stack effect results in increased 
    flow through the duct riser.  The flow at each floor’s intake points 
    varies as a square root of the difference in pressure through 
    the opening.  Assuming that the fan can effectively remove this 
    increase in flow, the percentage of change in flow at each intake 
    point is taken as follows:
    Where : 
    Q1 = Q0
    ΔP0 + ΔP1
    Q0=Flow at design ΔP0 in the absence of stack 
    Q1=New Flow under stack pressure conditions
    ΔP0=Pressure in the absence of stack
    ΔP1=Pressure including stack
    Example:  Flow at an exhaust grille located on the first floor is 100 
    CFM, ΔP0 at .10 in. w.g., the increase in flow as a result of increase 
    in stack pressure:
    Q1 = 100.10 + .38
    Based on our original example of a 200 ft. building, and assuming 
    design of 100 CFM per floor with a total of 14 floors, the total system 
    airflow would increase from 1400 to 2286 cfm.  This represents an 
    increase of 63.2% in total flow, or 886 CFM of unwanted ventilation 
    and additional load on the building! 
    Ultimately, the effect stack pressure has within a building relates 
    to the amount of unwanted infiltration of unconditioned air and/
    or exfiltration of conditioned air.  This unwanted movement of air 
    relates to increased thermal load on the building and uncontrolled 
    energy consumption.  Since the mechanical ventilation riser 
    is a contributor to overall building pressure buoyancy, not to 
    mention proper regulation of ventilation for IAQ, it is important to 
    recognize that proper balancing and regulation of these systems 
    has a significant effect on energy consumption. 
    One technique to minimize the effect stack pressure has on exhaust 
    ventilation system prescribed airflows is by increasing the internal 
    duct pressure created by the fan.  The greater the internal duct 
    pressure, the less effect stack pressure can have on the system; 
    however, increased pressure also relates to increased fan motor 
    BHP and relative energy consumption in watts.  To determine the 
    increase in pressure necessary to overcome stack pressure within 
    a tolerance of +/-10% in a balanced static system, the following 
    formula can be applied:
    Q1 / Q0 = 1.1 =ΔP0 + ΔP1
    Squaring both sides to solve
    for ΔP0 : 1.21 =ΔP0 + ΔP1
    ΔP0  =wΔP1= 4.760. 21
    (times the increase in stack effect pressure)
    Where Tolerance factor
    Q1 = Q0 +/- 10 %
    Simply stated, the pressure drop at each grille for static balancing 
    must be 4.76 times the anticipated stack effect at each respective 
    intake point to maintain the airflow within 10% of design values.  
    This is true for all the grilles regardless of elevation within the 
    building.  In the absence of stack effect, the formula does not 
    apply.  When applying the increase in pressure factor of 4.76 to 
    our example, and given the original stack pressure of 0.38 at the 
    first floor grille, the fan must now operate at a level to ensure 1.81 
    Ps in. w.g. at this same grille.  The increase in necessary pressure 
    will not only result in excessive energy consumption, but excessive 
    noise generated at each grille as well.
    Controlling Stack Effect
    = 100√4.8 = 100x2.2 = 220cfm 
    							and modulate the opening to regulate flow in response to these 
    changes.  This will allow the use of lower-pressure fans for energy 
    savings and prevent stack pressure from effecting flow rates and 
    resulting in over- and under-ventilation.  Unfortunately, most 
    modulating dampers on the market today are designed using 
    pitot tube pressure-sensing devices and electric drive motors 
    and controllers to actuate a damper for flow control.  Using one 
    of these devices on every intake point in a riser is often more 
    costly than years of energy penalties on systems without them.
    0 cfm
    0 cfm
    18 cfm
    28 cfm
    52 cfm
    81 cfm
    83 cfm
    94 cfm
    78 cfm
    117 cfm
    132 cfm
    150 cfm
    Unregulated Unbalanced Airflows
    Over-ventilation causes energy waste,
    Under -ventilation causes poor indoor air quality (IAQ)
    After solving for the increase in pressure necessary to maintain 
    balanced flows, simple fan laws can be applied to determine 
    the required increase in fan RPM.  Fan laws show that pressure is 
    proportional to the square of the RPM.  
    SP1 / SP2 = (RPM1 / RPM2)2
    Therefore, using the previous example, the increase in RPM can 
    be determined as follows.  Assuming the original fan RPM is 1000 
    and the pressure to achieve design airflows in the absence of 
    stack pressure is 0.22, derived from 0.10 at each grille and 0.12 to 
    account for duct loss:
    RPM2 = RPM1 
    SP2= 10001.91= 2 9 5 0  rpmSP10.22
    The result is an increase of almost three times the original RPM 
    design in order to prevent changes in airflow due to stack pressure 
    effect.  When applying this increase to energy consumption of fan 
    motors, the increase varies with the cube of the RPM.
    HP1 / HP2 = (RPM1 / RPM2)3
    Following the previous example:
    HP2 / HP1 = (2950 / 1000)3
    HP2 / HP1 = 25.7
    Therefore, the final result is a more than 25-times increase in power 
    consumption to operate a fan at the higher pressure required to 
    ensure proper system balance in the presence of stack effect.
    Through analysis of traditional central duct riser system 
    designs, and factors that effect overall airflow performance, it 
    is determined that excessive energy consumption will increase 
    as stack pressure increases.  Since statically controlled systems 
    have no means of adjusting to fluctuations in stack pressure, 
    the amount of excessive energy consumed will either come in 
    the form of additional thermal load on the building, which will 
    result in increased heating costs, or from increased fan power 
    to control the flows at higher pressure.
    The other negative factors associated with statically controlled 
    risers are excessive noise and duct leakage created by high fan 
    pressures, or the potential for under ventilating portions of the 
    building.  Either scenario can result in an unsuitable environment 
    for the building’s occupants.  
    The only solution to dealing with stack pressure effect on vertical 
    risers is to monitor the pressure at each intake point into the riser 
    Controlling Stack Effect
    							TYPICAL SPECIFICATIONModel CAR-II Constant Airflow Regulators by American ALDES Ventilation Corporation, Bradenton, Florida, shall solely operate on duct pressure and require no external power supply.  Each regulator shall be pre-set and factory calibrated requiring no filed adjustment to the airflows as indicated on the schedule, and shall be rated for use in air temperatures ranging from -25°F to 140°F (-32°C to 60°C).  
    Constant Airflow Regulators shall be capable of maintaining constant airflow within +/-10% of scheduled flow rates (15% for units 50 CFM or less), within the operating range of 0.2 to 0.8 in. w.g. differential pressure, or 0.6 to 2.4 in w.g. on high-pressure models (CAR-II-HP), or 0.1 to 0.42 in. w.g. on low-pressure models (CAR-II-LP).  Sound power levels shall not exceed those for each size and CFM rating as scheduled.  Regulators shall be provided as an assembly consisting of a 94V-0 UL ABS plastic body housed within a round sleeve for mounting in round duct.  Each round sleeve must be fitted with a lip gasket to assure perimeter air tightness with the interior surface of the duct.   All regulators must be classified per UL 2043 and carry the UL mark indicating compliance. All Constant Airflow Regulators will require no maintenance and must be warranted for a period of no less than five years.  Constant Airflow Regulators shall be installed in tight ducting systems in accordance with all applicable codes and manufacturer’s instructions.
    Constant airflow is achieved by controlling the free area 
    through the device.  At minimum static pressure, the aero-wing 
    is parallel to the air stream.  As the static pressure increases, 
    the aero-wing lifts, thereby reducing the amount of free area 
    through the regulator.  At the same time, the higher static 
    pressure increases the air velocity resulting in CONSTANT 
    AIRFLOW.  This occurs regardless of pressure differences in 
    the range of 0.2 to 0.8 in. w.g. (50 to 200 Pa).  The air velocity 
    in the duct is in the range of 60 to 700 ft/min. (0.3 to 3.5 m/s).
    Controlling Stack Effect
    American ALDES Ventilation Corporation  •  4521 19th Street Court East, Suite 104  •  Bradenton, FL 34203 – USA
    941.351.3441  •  800.255.7749  •  941.351.3442 (fax)  •  [email protected]  •  www.aldes.us
    © 2013 American ALDES Ventilation Corporation.  Reproduction or distribution, in whole or in part, of this document, in any form or by any means, without the express written consent of American ALDES Ventilation Corporation, is strictly prohibited. The information contained within this document is subject to change without prior written notice.
    controlling stack effect_application guide_1113   
    Balancing and commissioning of a ventilation riser is usually 
    considered difficult and tedious.  Low airflows through small, 
    often inaccessible, sidewall-mounted registers located on multiple 
    floors, is challenging to any test-and-balance contractor.  It requires 
    special instrumentation and many man hours for typical riser 
    systems.  Even with modulating duct openings, the balancer’s 
    job is compounded by fine-tuning controllers to specified set 
    points before and after airflow measurements. 
    In addition to the physical constraints of balancing vertical risers, 
    the time of year and stage of construction dramatically affect the 
    measurement readings the contractor will record.  This goes back 
    to stack pressure effects on the system.  
    The ultimate solution to ensure proper system balancing and 
    airflow regulation is the American Aldes CAR-II Constant Airflow 
    Regulator.  The CAR-II is a factory-calibrated passive airflow 
    regulator that eliminates the need for balancing airflows at the 
    grilles.  It does not require any external power since if automatically 
    adjusts to the proper airflows in response to duct inlet pressure.
    The active control element of the CAR-II is a unique aerofoil. Using 
    Bernoulli’s Principle, the aero-wing damper lifts in response to 
    increasing static pressure. This operation regulates the free-area 
    opening through the control, resulting in maintenance of velocity 
    and specific airflow setpoints.  
    Because the CAR-II will maintain the prescribed airflows as it 
    adjusts to changes in pressure caused by stack effect, it eliminates 
    over-ventilation caused by the exhaust riser, which saves energy.  
    The use of CAR-IIs also allows for fan operation at the lowest 
    pressure level possible without sacrificing airflow performance, 
    which saves fan energy consumption.  Finally, CAR-IIs eliminate 
    under-ventilation caused by imbalances of the exhaust system, 
    which protects against poor indoor air quality.
    CAR-IIs are employed in thousands of buildings in the United 
    States and around the world.  This well-proven technology was 
    developed to minimize fan energy use in the late 1970s.  Today, 
    CAR-IIs serve as a simple solution to indoor air quality ventilation 
    regulation and energy savings.  The CAR-II by American Aldes 
    continues to lead the industry in economical passive airflow control 
    regulation.  Consult the factory, or an American Aldes certified 
    representative to discuss how CAR-IIs can save money, conserve 
    energy, and protect any building against poor ventilation control.    
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