OPTIMIZING VENTILATION SHAFT VOLUMES AND VARYING OPENING SIZES CAN INCREASE EFFECTIVE TIME PERIOD FOR NATURAL VENTILATION

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1 OPTIMIZING VENTILATION SHAFT VOLUMES AND VARYING OPENING SIZES CAN INCREASE EFFECTIVE TIME PERIOD FOR NATURAL VENTILATION Abhay Nagory University of Southern California Los Angeles, CA Peter Simmonds IBE Consulting Engineers Sherman Oaks, CA ABSTRACT: Ventilation shafts aid natural ventilation in buildings. Natural ventilation reduces the buildings energy requirement and provides thermal comfort for occupants. Natural ventilation is pressure driven having limited operation time. Buoyancy driven ventilation is induced by pressure differentials occurring due to air temperature differences whereas wind induced ventilation depends on prevailing wind speed and wind angle. Generally, a combined effect of buoyancy and wind effects drives the air flow naturally up a ventilation shafts. This study attempts to prove increase in effectiveness of natural ventilation on occupant s thermal comfort by using a ventilation shafts to drive ventilation. While doing so the period of effective natural ventilation also increase. This is achieved by re-configuring the ventilation shaft design to maximize the air flow rates across the occupant zones. The design of openings in the occupant zones which serve as inlets to the ventilation shaft are also considered such that they increase the air flow rates across. When doing so the overall volume of the ventilation shaft is also reduced so that it does not have a strong architectural impact on the building aesthetics. Results from the numerical technique prescribed in the Air Infiltration and Calculation techniques An application guide: June 986 by the Air Infiltration and Ventilation Centre (AIVC) and the computation technique using Multi zone airflow and contaminant Transport analysis (CONTAM) software by National Institute of Standards and Technology (NIST) are compared to derive results of air flow rates, pressure differentials and area of openings. A comparison of air flow rates induced by different temperature and wind conditions in Los Angeles on three models having similar volumes but different ventilation design is performed and reported to verify the effectiveness of natural ventilation..0: INTRODUCTION: Natural ventilation is the process of supplying comfortable air flow rates across habitable spaces without the use of any mechanical ventilation systems. From the ancient Egyptian civilization to the post industrial era, vernacular builders and architects employed ventilation techniques using the immediate atmospheric surroundings of wind and temperature. As a result, various architectural elements have evolved, to assist air to enter, circulate through and condition habitable spaces. Wind scoops were designed to catch wind and provide direct fresh air to the occupants whereas solar chimney relied on air movement induced by thermal differences occurring due to direct solar radiation heat gains on air temperature. With the over growing need to design energy efficient buildings these traditional ventilation elements have transformed to better suit the new age occupant density and thermal comfort. A ventilation shaft looks and works almost similar to a chimney [6]. It features openings at multiple floor levels and an outlet at the roof top. Ventilation shafts rely on buoyancy effects to induce air flow which makes them time and temperature dependent. Therefore their effective use is limited to site and climate specific. Examples of the architectural projects employing ventilation shafts are Frederick Lanchester Library, Coventry, UK; The School

TABLE: PRELIMINARY VENTILATION STUDIES FOR DIFFERENT MODEL TYPES Q = Air flow rate in m /s; W = Wind; B = Buoyancy.

In all thesee projects, the ventilation shaft volumes have architectural impact on the building aesthetics.

This study therefore attempts to increase the effectiveness of natural ventilation induced via ventilation shafts and also

0: BACKGROUND Natural ventilation can be categorized as wind driven and buoyancy driven.

2 TABLE: PRELIMINARY VENTILATION STUDIES FOR DIFFERENT MODEL TYPES Q = Air flow rate in m /s; W = Wind; B = Buoyancy. of Slavonic and East European Studies, London, UK and the Harm A. Weber Library, Elgin near Chicago USA [8]. In all thesee projects, the ventilation shaft volumes have architectural impact on the building aesthetics. This dominating feature of shaft design sometimes poses as a hindrance as a preferred choice among designers. This study therefore attempts to increase the effectiveness of natural ventilation induced via ventilation shafts and also investigates methods to reduce its overall volume. 2.0: BACKGROUND Natural ventilation can be categorized as wind driven and buoyancy driven. Both of these induce pressure differences across openings whichh forces air flow across a space. The fundamental principles governing ventilation are Bernoulli s principle of fluid flow and the Law of conversation of energy. Based on these principles various prediction methodss for estimation of air flow are 2

3 established []. For this study the numerical technique to estimate volumetric air flow rates and pressure differentials prescribed in the Air Infiltration and Calculation techniques An application guide: June 986 by AIVC is adopted. The results obtained from this method were compared to results generated from CONTAM for verification purposes. Both these techniques are based on similar governing and power law relationship of computation and estimation. To quantify the benefits of a ventilation shaft on ventilation a preliminary study of three single zone models having different opening positions, under similar boundary conditions were tested and compared. These tests were conducted to examine the increase in air flow rates under both wind induced and buoyancy induced effects on different ventilation design types [Table ]. For the purposes of this study, uni-directional air flows are examined. The air flow inlets are placed only on one façade and outlets placed on the opposite. The air flow patterns are visualized in the section view of these study models. As seen from the results [Table ] the air flow across the single zone attached to a vertical shaft is greater than the rest of the single models. Similar air flow studies were conducted on three models constructed of three single zones stacked over each other. All three models have similar boundary conditions of temperature and wind speed. For the single sided and cross ventilation model the air flow pattern is regular and convective, where all three zones have direct fresh air flowing inside. The third model which has openings connected to the ventilation shaft have irregular air flow pattern wherein the top zone has air flowing in from the lower two zones. Stale air from the lower zones is not suitable to provide thermal comfort as it is pre heated from heat gains of the lower zones. Moreover this stale air is contaminated and thus cannot maintain the indoor air quality levels for the upper zone. The predicted benefits of natural ventilation are lost in case of this model type. However, the air flow rate up the ventilation shaft is higher than the other two models. TABLE 2: CHANGE IN NEUTRAL PRESSURE PLANE LEVEL FOR DIFFERENT OPENING SIZE: A a A2 b Npl c N 2 N + 42% N 2 N 8% N 2 2 N a: A - Area of Inlet; b: A2 - Area of outlet; c: Npl - Neutral pressure plane level The results of these tests highlight the influence of neutral pressure plane that regulates the air flow pattern across the different floors. Based on these results, additional tests were performed to identify the extent of influence that openings have on the neutral pressure plane levels. The inferences of this study on a single zone model are tabulated below in [Table 2] 2.: Thermal comfort Natural ventilation is effective as long as it provides optimum comfort conditions to the occupants. Modifying the air movement around the human body can make considerable difference in thermal comfort levels. Air movements are instrumental in determining the heat and mass exchange of the human body with the surrounding air [4]. This exchange is primarily through convection. The direct effect of natural ventilation on comfort conditions is to counter the indoor heat gains generated through the building envelope, occupant s body heat, artificial lighting and equipment such as computers. By limiting and controlling the way air changes occur, internal air temperature can be controlled leading to relative thermal comfort. [5]. Therefore to increase the effectiveness of natural ventilation on occupant comfort, the air flow inside the occupant space can be increased such that heat dissipation occurs at a faster rate while the air quality levels are also maintained. Additionally, natural ventilation is effective and possible only till the extent when the cooling effect (created by the air flow) lies within the limits of acceptable operative indoor temperature of heating and cooling. The limits of this operative internal temperature for different external temperatures for adaptive thermal comfort conditions are prescribed in ASHRAE : Internal heat gains: The internal heat gains were calculated in accordance to occupant density and occupant heat dissipation prescribed in ASHRAE 62.[7]. For all the studies the floor plate area is considered to be 50m 2. Considering an occupant density of 5persons / 00m 2 the internal heat gain calculation is tabulated below. Occupant heat 70 W/m 2 20 W Lighting heat 2 W/m W Equipment heat 8 W/m W Total Internal heat gain in Watts 70 W 2.: Temperature difference: Natural ventilation reduces impact of internal heat gains in the occupant space which brings the cooling effect in the space. While doing this the internal temperature also changes. The difference in air temperature is limited to a maximum of. C in a span of 4 hours for spaces

$The actual internal temperature as a result of the induced air flow was later recalculated to identify the effect of induced air flow on thermal comfort[9]. \.$ 0: METHODOLOGY For this study the influence of all physical building parameters which influence air flow, such as height, and openings were examined.

0: METHODOLOGY For this study the influence of all physical building parameters which influence air flow, such as height, and openings were examined.

Each of these building parameters was tested for both buoyancy and wind effects.

depend only on wind speed and wind angle at the openings, but not the height difference between openings.

4 maintaining adaptive thermal comfort conditions [9].The air flow rate calculations conducted for this study were tested assuming a maximum temperature difference of C. The actual internal temperature as a result of the induced air flow was later recalculated to identify the effect of induced air flow on thermal comfort[9]. \.0: METHODOLOGY For this study the influence of all physical building parameters which influence air flow, such as height, and openings were examined. This investigation was to identify the extent and range of influence each of these parameters can have on natural ventilation. Each of these building parameters was tested for both buoyancy and wind effects. A generalized relationship between of these physical parameters on air flow was established whichh further aids in designing the final testing models. depend only on wind speed and wind angle at the openings, but not the height difference between openings..: Opening size: According to the Energy conservation law, the mass of air flowing into an occupant zone will be equal to mass of air flowing out of the occupant zone. This air mass entering and leaving the zone depends on the type and size of openings. Thus opening size regulates air flow inside occupant spaces. This regulatory function of openings is in terms of controlling air flow inside the occupant zones whenn external wind and temperature conditions may not necessarily be preferable for thermal comfort conditions. Test results highlight a trend of increase in air flow rate whenn outlets are greater than inlet opening size..: Occupant Zone height: The overall floor height of the occupant s zone has no influence on airflow rates. In case of wind induced ventilationn where wind induced pressure increases with height the air flows in this case remain similar as the outlets are connected to the ventilation shaft instead to the external atmosphere..2: Opening heights: Fig 2: Air flow ratee with change in opening size. TABLE : CHANGE IN AIR FLOW RATE FOR DIFFERENT OPENING SIZE A a A2 b 2 4 A : A2 4: : 2: : :2 : :4 a: A - Area of Inlet; b: A2 - Area of outlet; c: Q- Air flow rate Q c Fig : Air flow rate with increase in opening heights. Buoyancy induced air flow increases with increase in height. Thus to increase ventilation rates the height difference between the inlets and outlets should be extended as much as possible [Fig ]. These openings, especially the inlets are positioned at levels to allow daylight inside the space and also not to block views. In case of wind induced ventilation the ventilation rate will Conversely when inlets are largerr than outlet openings, the air flow rates are lower when compared to airflow whenn both the openings are of the same size. Inferring from all the above data a relation between different inlets to outlet area configurations to air flow rates was made and generalized. This relationship is constant for both typess of ventilationn mechanisms.[ [Fig 2]. The above relationship of change in air flow rate is constant for different wind speeds, wind angle, opening heights and temperature differences. 4

.4: Ventilation Shaft height Increase in shaft height also increases air flow rates. Wind pressure at shaft outlet increases with increase in shaft height which results in higher in air flow rates.

When the shaft height is equal to zone height, the increase in air flow rates is minimal. Whereas when the shaft heights are equal to twice the zone heights, the air flow rate also doubles [Fig ].

However,, the large size may result as outlets functioning as wind scoops to trap in free wind at higher altitudes.

5 .4: Ventilation Shaft height Increase in shaft height also increases air flow rates. Wind pressure at shaft outlet increases with increase in shaft height which results in higher in air flow rates. Similarly with increase in shaft height, the pressure difference at inlets and outlets also increases, pushing greater air flow. When the shaft height is equal to zone height, the increase in air flow rates is minimal. Whereas when the shaft heights are equal to twice the zone heights, the air flow rate also doubles [Fig ]. This is the prime reason of ventilationn shafts being such tall bold entities on the building mass. Taller shafts render higher air flow. flow movement thee outlet size should be as big as possible. However,, the large size may result as outlets functioning as wind scoops to trap in free wind at higher altitudes. Wind pressure at higher altitudes is large which may obstruct the free convective air flow in the shafts. Fig 4: Pressure distribution for different outlet size. To avoid such an occurrence, inferences from pressure planee calculations of single zone opening analysis are adopted. According to these analyses, the area of inlets should be changed in order to increase the negative pressure on shaft outlets, which can then counter wind induced pressure. While increasing the inlet size, the shaft outlet size decreases. Furthermore, as inferred from neutral pressure plane analysis [Table 2], the air flow pattern will also change as result of change inlet opening size connected to the ventilation shaft. This results in decrease of air flow rates but an increase in pressure differentials across the openings. This increase of pressure at thee outlets assistss in obtaining convective air flow with a shorter shaft. A comparative analysis of change in air flow rate and pressure differentials by changingg the area of thee highest inlet A is the ventilation shaft is illustrated in Figuree 5. Fig : Air flow rate for different shaft heights..5: Ventilation Shaft area The shaft volume should be large enough to accommodate and allow free air mass flow up its height. Thus the shaft area is determined by the actual mass of air or the desired air flow rate required according to the occupant density and site conditions. By determining the optimum shaft height and air flow requirement the shaft area can be re- configured to accommodate the air mass, which can reduce the shaft volume..6: Shaft outlet: By increasing the shaft outlet size, the air flow can be controlled such that all occupant floors experience direct fresh air from the outside wind and temperature conditions [Fig 4]. In case of wind induced pressure, the ventilationn pattern is not irregular as the induced air flow is not relative to height differences between openings. In this case the air flow patterns depend on the orientation of openings, the pressure induced by the prevailing wind speed and the wind angle. From tests conducted on shafts with different outlet sizes, it can be observed that in order to obtain convective air Fig 5: Air flow ratee at shaft outlett for different inlet size. 5

6 .7: Air flow and internal temperature: To ratify influence of increased air flow on reducing air temperature differences, a series of incremental air flow rates was assumed from a single zone. The internal temperature as a result of these air flows was re- temperaturee for calculated. The result of this new internal a constant external air temperature are illustrated in Figure 6.Keeping internal heat gains and external air temperature constant the internal air temperature can be reduced by increasing air flow rates which can enhance thermal comfort conditions. Fig 6: Internal temperature for different air flow rates. By increasing air flow the reduced internal temperature can now be maintained within the operative internal temperatures of heating and cooling. Additional tests of air flow with temperaturee highlight thatt with increase in external temperature the air flow rates decrease marginally, provided the temperature difference is maintained throughout. An increase in air temperaturee is generally not suitable for thermal comfort, thus a strategy to maintainn internal temperature close to the external temperature will suit better in order to increase the effectiveness of ventilation induced by natural mechanisms. Model : This model is a generic representationn of a threee story building. The exterior walls on two opposite ends have 40% of its wall area as openings. The 40% complies with the California TITLE 24 requirement for minimum opening size requirements so as to achieve acceptable daylightt and ventilation. These openings are on the shorter side of the rectangular floor plate of 50m2. The openings are assumed to be 50% operable which computes to 4m2 as their effective opening area. The floorr height is assumed to be 4m. Model 2: This model is model, which is now connected to a ventilation shaft. To establish a workable shaft height for effective stack effect, usually the shaft height is twice the height of the tallest space it is ventilating [7]. The shaftt area was calculated as per the air mass flowing across model.shaft outlet size is a cumulative sum of all threee inlet area sizes. This is done to avoid irregular air flow which occurs due to neutral pressure plane level. Model : The ventilation design in model 2 is refined and reconfigured according to inferences derived from the earlier studies. Thee ventilation shaft height is shorter than that in model 2. The inlets of the occupant zones are spilt in two keeping the total area constant but now placed at different levels. The size of shaft outlet remains the same. 4.: Testing conditions: All these models were tested for four different climatic conditions of temperature and wind flow to obtain air flow results [Table 4]. For this final test the climate on a typical summer day in LOS ANGELES is considered. Temperature: The air flow rates were calculated for two different temperature conditions. The temperatures for this test lie within the heating and cooling set point temperature range of LOS ANGELES, as prescribed in the ASHRAE design guide. [Table 4] ].The heating set point temperature is 8.4 C and the cooling set pointt temperature is 26.7 C. Windd speed and wind angle: The mean wind 4.0: TESTING MODEL: velocity for the summer month of September in LOS ANGELES derivedd from Climate Consultant 5. [7]. The Three models are constructed to test the increase and windd angle is assumed at zero (0) Degree. [Table 4] effect of air flow on internal occupant thermal comfort. TABLE 4: THE FOUR DIFFERENT CLIMATE CONDITIONS CONSIDERED FOR TESTING CONDITONS External Temperature in C Internal Temperature in C Wind speed in m/s Wind Direction TEST 8 2 East to west TESTT 8 2 West to East TEST 2 26 East to west TEST West to East 6

7 TABLE5: THE TESTS MODELS AND THE RESULTS OF AIR FLOW, TEMPERATURE AND WIND DIRECTION A = Area in m 2 : H = Height in m; Q = Air flow rate in m /s; Ti = Internal temperature; To= External Temperature; D= Air flow direction; Ws = Wind speed; Wd= Wind Direction; *= height from individual zone base. 7

5.0: RESULTS: The resultss of air flow rates for these three models are in Table 5. The internal temperature due to induced air flow is re-calculated.

The air flow rates in model 2 and remain almost similar but the shaft volume in model is reduced by almost 20%.

temperature reaches the lower heating set-point temperature of 8.4C. render higher air flow rates with reduced volume size.

Air pressure driven via ventilation shaft has tendency in some cases to negate the direct wind pressure exerted from the opposite direction of airr flow inducedd by pressure developed by buoyancy

Hazim Awbi (edited): Ventilation Systems Design and performance. 2008. 2. Francis Alaard: Natural ventilation in Buildings A design handbook. 998.. Martin W.

0: CONCLUSIONS: The conclusions can serve or aid as a simple approach mechanismm that designers can adopt in order to maximize ventilationn rates induced by natural mechanisms.

This prompts faster cooling effect from natural ventilation, which helps increasing the reliability on natural ventilation mechanisms.

The internal air temperature can brought close to the external air temperature by increasing air flow inside a space.

8 5.0: RESULTS: The resultss of air flow rates for these three models are in Table 5. The internal temperature due to induced air flow is re-calculated. Air flow rates increase exponentially in model 2 and with regular fresh direct air flow in all occupant zones [Fig 7]. The air flow rates in model 2 and remain almost similar but the shaft volume in model is reduced by almost 20%. The internal temperature in model 2 and reach closest to the external air temperature, which aids in providing the benefits of natural ventilation cooling effects as soon as the external air temperature reaches the lower heating set-point temperature of 8.4C. render higher air flow rates with reduced volume size. This also reduced its architectural impact on the building design giving additional space to building designers for otherr building functions. Air pressure driven via ventilation shaft has tendency in some cases to negate the direct wind pressure exerted from the opposite direction of airr flow inducedd by pressure developed by buoyancy effects. 7.0: ACKNOWLEDGMENTS: The authors of this paper appreciate and thank Professor. Murray Milne & Mr. Simon Chiu for their guidance. 8.0: REFERENCES:. Hazim Awbi (edited): Ventilation Systems Design and performance Francis Alaard: Natural ventilation in Buildings A design handbook Martin W. Liddament: Air Infiltration Calculation Techniques An application Guide, June 986: Air Infiltration and Ventilation Centree Fig 7: Air flow rates at occupant zones. 6.0: CONCLUSIONS: The conclusions can serve or aid as a simple approach mechanismm that designers can adopt in order to maximize ventilationn rates induced by natural mechanisms. Higher air flows rates help dissipate internal heat gains faster. This increased rate of heat dissipation aids in obtaining thermal comfort conditions relatively faster. This prompts faster cooling effect from natural ventilation, which helps increasing the reliability on natural ventilation mechanisms. Increased air flow rates also will increase rate of air changes in an occupant zone. This helps in achieving improved air quality levels. The internal air temperature can brought close to the external air temperature by increasing air flow inside a space. This extends the time span in which the internal temperature reaches the cooling set point temperature benchmark, after which generally natural ventilation mechanismm do not renderr thermal comfort conditions. The heating and cooling set point temperatures can vary depending on the target acceptable adaptive thermal comfort conditions to be achieved. Ventilation shafts can 4. CIBSE Application Manual AM 0: Natural ventilation in non-domestic buildings, January Steven J. Enunerich, W. Stuart t Dols (Building and Fire Research Laboratory) James W. Axley (School of Architecture Yale University.) : NISTIR Natural Ventilation Review and Plan for Design and Analysis Tools, Prepared for: Architectural Energy Corporation Shoulder, Coloradoo - August Tommy Kleiven: Natural Ventilation in Buildings - Architectural concepts, consequences and possibilities. March ANSI / ASHRAE Standard Ventilation for acceptable Indoor Air Quality: American Society of Heating, Refrigeration and Air-Conditioning Engineers. Inc. 8. Per Heiselberg, Shuzo Murakami, Claude-Alain Roulet : Ventilation of Large Spaces in Buildings Analysis and prediction techniques, First Edition ANSI / ASHRAE Standard Thermal Environmental Conditions for Human Occupancy: American Society of Heating, Refrigeration and Air- Conditioning Engineers. Inc. 8

PLEA th Conference, Opportunities, Limits & Needs Towards an environmentally responsible architecture Lima, Perú 7-9 November 2012

PLEA th Conference, Opportunities, Limits & Needs Towards an environmentally responsible architecture Lima, Perú 7-9 November 2012 Natural Ventilation using Ventilation shafts Multiple Interconnected Openings in a Ventilation Shaft Reduce the Overall Height of the Shaft While Maintaining the Effectiveness of Natural Ventilation ABHAY