Ventilation Systems - Assignment Example

Student Name: Tutor: Title: Ventilation Systems Date: ©2016 Table of Contents Table of Figures 2 1.1Introduction 2 1.2Natural Smoke Ventilation Systems 3 1.2.1Description 3 1.2.2Operations of Natural Ventilation 6 1.2.3Control Requirements 6 1.2.4Fire Service Controls in Natural Ventilation 7 1.2.5Merits and Demerits 7 1.3Mechanical Smoke Ventilation Systems 8 1.3.1Description 8 1.3.2Design 9 1.3.3Fire Service Controls 9 1.3.4Merits and Demerits 10 2.0Computational Fluid Dynamics (CFD) Modelling 11 2.1Assumptions 11 2.2Aim of Modelling 11 2.3Scenarios 12 2.3.1 Scenario A: Wind- driven Ventilation 12 2.3.2Scenario B: Mechanical Ventilation (Pressurized) 13 2.3.3 Scenario C: Combined Ventilation 13 3.0Conclusions 14 References 15 Table of Figures Figure 1: Natural Ventilation Fire Scenario 12 Figure 2: Fire Scenario B under Mechanized Pressure Systems 13 Poor circulation of air within an enclosed structure causes buildup of carbon dioxide and heat, resulting in the structure being stuffy and uncomfortable for the residents. It is therefore, important to provide pathways through which cool ambient fresh air can enter the structure and displace the stuffy air in sufficient quantities so as to maintain a level the residents find comfortable without excessive draft being generated. This process is replacing stale or noxious air with fresh is referred to as ventilation [Hus06]. Ventilation is generally used to control the quality of air inside a structure by either displacing or diluting indoor pollutants to concentrations not regarded as harmful to the occupants. This is of added importance in the event of a fire where buildup of smoke could compromise the escape route endangering the lives of the occupants of the structure. Effective ventilation may be attained either by mechanized or natural systems. 1.1 Natural Smoke Ventilation Systems 1.1.1 Description Natural ventilation facilitates the replacement of air inside building with fresh air form outside without the use of any mechanical systems or power [Jia03]. There has been increased interest in its use for non- domestic buildings, becoming an attractive design alternative rather than the traditional mechanical ventilation systems [Mar00]. Adequate fresh air and high thermal comfort in spaces that are well ventilated but with minimal or no use for ventilations systems nor active cooling, means that there is successful wind ventilation [Aut13]. The flow of air between the outside and inside of the structure is propagated by pressure differentials caused by natural forces. The natural forces of interest are wind and temperature respectively responsible for wind and buoyancy driven ventilations. The applicability is only limited to a small range of climates, preferably suited for mild climates [Lid96]. Wind driven ventilation The wind that acts directly on a building face results in the windward side having positive pressure and a corresponding negative pressure on the opposite side [Kha08]. The pressure gradient created, as well as pressure differences within the building itself, drive the airflow throughout the building. Other factors influencing its performance are wind behavior, interactions with building envelopes and air exchange devices or openings. Wind driven ventilation may be classified as; A. Single sided ventilation Occurs when only one opening on the building’s side allows for air to flow into the structure. This method is adopted when there is fixed or structural partitioning that cannot allow for an opening to be provided for [Mar00]. The open space could be located on either the windward or leeward side of the building. Buildings are mostly designed with the windows functioning as the openings for ventilation purposes. The calculation of the ventilation rate for this approach is tasking because of the bi- directional flow and strong turbulent effect. This is because the external air and the indoor air get into and out of the building through openings of close proximity. However, Wang and Chen [Wan12] say that by combining different models for eddy penetration, mean and pulsating flow, the ventilation rate can be accurately predicted. B. Cross ventilation This method provides for two openings in the structure, one on either side of the building. This method achieves the highest air exchange rates and can ventilate larger floor spaces than the single- sided ventilation method [Mar00]. It is the most recommended method of ventilation due its high effectiveness, and should be designed such that there are no obstructions between the openings on either side of the structure to optimize the path the air follows [Aut13]. Although placing the openings directly opposite each other will provide the most ventilation, this should be avoided as it will result in zones that are not sufficiently ventilated. The openings should be stepped to ensure there’s proper circulation within the structure before the air stream leaves at the other end. In a structure with a complex interior, cross ventilation may be curtailed by the closing of openings by the buildings occupants. Construction of vents and duct systems to channel fresh air throughout the building is generally used by most construction companies to overcome this challenge [Wis16]. Due to its effectiveness, cross ventilation may also result in increased energy costs, especially in cold climates. This is due to installation of heating appliances to curb the cooling effect brought about by ventilation. Wind driven ventilation has significant limitations in its operation, such as; 1. There is difficulty in harnessing the wind due to its unpredictability caused by variations in speed and direction 2. Close proximity of the structure to an industrial area could result in contaminated air being filtered into the building Buoyancy driven ventilation This type of ventilation is dictated primarily by temperature which influences the density of air. Hotter air is less dense, thus buoyant and is displaced by the cooler air, creating an upward stream. For proper ventilation, there needs to be significant difference between the temperatures inside and outside the structure. Ventilation may occur either by displacement, where cooler air enters through the bottom and warmer air exits through the top apertures in the building, or by mixing, where both cooler and warmer air pass through top apertures. Through the two streams mix, the cooler air will still reach the bottom due to its higher density. The windows should be designed to be tall, preferably with a top and bottom opening to allow the formation of a convection circulation due to the local stack effect [Mar00]. Advantages of buoyancy driven ventilation include [Lin99]; 1. Wind independent – takes place even on still, hot summer days 2. Air flow stability – no gusts 3. Sustainability – no dependence on wind, only temperature difference Disadvantages of this system are; 1. Lower magnitude – the exchange of air is a gradual process 2. Temperature gradient – if there is no temperature gradient between the outside and inside, no air exchange will take place 1.1.2 Operations of Natural Ventilation There are various strategies and aspects that need to be considered when implementing natural ventilation in a structure. These are [Aut131]; 1. Thinner buildings – these increase the surface area to volume ratio of the structure, making natural ventilation easier. The surface area for heat loss is also increased 2. Tall buildings – wind speed is higher at greater heights and vice versa due to the frictional drag effect. This means that at higher elevations, ventilation occurs at much faster rates 3. Orientation strategies – the building should be oriented to maximize benefits form cooling breezes. A wind rose diagram can be used to determine which winds are desirable. For most wind ventilation, the width of the building ought to be aligned with the prevailing winds. 1.1.3 Control Requirements The flow of air through the building can be facilitated by installing various components and devices, some of which are; 1. Openable windows – provision of such allows for free air circulation in the structure. These are advantageous in that the occupants can choose the degree of opening depending on the prevailing conditions 2. Ventilation louvres – these are small openings, usually metallic that allow air exchange in a structure. They may be openable or not depending on the design 3. Rooftop vents – these are used to ventilate steep- slope roof assemblies by allowing outside air to enter and exit attics and ventilation spaces. There are various types of roof vents, such as ridge vents, static vents, gable end vents and turbine vents. The rate of ventilation achieved by these vents is increased significantly when there is wind [Nat16]. 4. Ceiling cavities – the roof space can capture as much as 100c and radiate back into the house if not properly ventilated. Whirlybirds, soffit and drip edge vents are wind- driven ventilation mechanisms that could be installed to facilitate the escape of this heat. These mechanisms also prevent buildup of dampness in the ceiling cavity [Bui16] 1.1.4 Fire Service Controls in Natural Ventilation Ventilation plays a very crucial role in the event of a fire outbreak, as it sucks out the smoke from the building allowing the occupants a means of escape with a clear line of site out of the building. Accumulation of smoke may result in choking hazards, and even propagate fire spread as the smoke rids the environment of any moisture that could curtail the fire spread. The building’s ventilation should be designed such that [The10]; a. A stairway that have protected views shall only be served by exclusive ventilation ducts. This is to prevent smoke being removed from other parts of the building finding its way to these areas which are places of refuge. b. External wall vents or smoke shafts should be provided for interior rooms and hallways where the occupants may need to pass through to escape from the fire. c. All corridors adjoining stairs should be provided with a vent located as high as practicable (paragraph 2.26). All vents should be designed such that they open simultaneously in the event of a fire 1.1.5 Merits and Demerits Natural ventilation has various advantages, some of which are; a. Energy saving – the energy utilized for air ventilation, heating and air-conditioning systems in a building can be rerouted for other purposes of the natural ventilation achieves acceptable levels of air quality and thermal comfort [Lid96] b. Cost – the capital cost is reduced by from equivalent air- conditioning units [Mar00]. The cost of operation is also significantly lowered. c. Environment – the system has little environmental impact since no mechanical systems are employed [Mar00]. The building also acts as a primary climate modifier, thus making environmental control easier and manageable. However, the system experiences various shortcomings, such as; a. Draughts – since the process governing the flow of air is natural, there is a possibility of the pressure gradient being too steep, causing draughts to develop b. Summer overheating – during the summer, the temperature difference between the air indoors and outdoors is very small. This means that there is no pressure gradient to force the outside air in, resulting in a buildup of heat within the structure. c. Energy consumption – there have been complaints that natural ventilation systems have higher than expected energy consumption [Mar00] d. Complicated design – the ventilation performance depends on the building’s form, its surrounding and climate. This makes the prediction of natural ventilation difficult, as well as development of a successful design [Jia03] e. Inconsistent – it cannot be used alone, especially in areas where winds do not blow continuously. This will require it be used in conjunction with mechanical systems to complement.[Cha16] These shortcomings can be overcome by proper management and operation of natural ventilation strategy. 1.2 Mechanical Smoke Ventilation Systems 1.2.1 Description Mechanical smoke ventilations refer to appliances and systems which actively supply and exhaust air from a structure to provide proper air quality and thermal comfort levels. These provide the ultimate in smoke control as they remove smoke along the escape route, allowing the occupants safe passage out [Fir14]. A typical mechanical ventilation is powered by Air Handling Units, comprising of a box housing a fan and filter chambers. Other components, such as cooling and heating elements, may be installed depending on the location of the structure and the desired result. Despite the fact that natural ventilation is preferred, mechanical ventilation comes in handy in some situations, such as [Des15]; 1. Deep buildings where perimeter based ventilation is impractical. 2. Poor local air quality, such as, close to a busy road 3. Location of building is sheltered from direct winds 4. Restricted opening of windows – may be due to security, ambient noise levels 5. High density of occupation – high levels of heat and contaminants From the above, mechanical systems are generally installed in bustling urban areas full of activity and obstructions to winds, but there is also significant application in residential dwellings. 1.2.2 Design There are various types of mechanical systems employed to maintain air quality in today’s structures. Some of the most commonly used types are [Des15]; A. Internal air movement system – this is a circulation system that causes movement of air but does not introduce ‘fresh’ air into the building, e.g. ceiling fan. B. Positive pressure system – cool and ‘fresh’ air gets introduced into the building by inlet fans, increasing the internal pressure C. Negative pressure system – this is a suction system that removes the ‘stale’ air in the building through exhaust fans, lowering the pressure inside the building D. Hybrid pressure system – this is a system that applies both inlet and exhaust fans, maintaining the building’s internal pressure at the same level as the outside air E. Localized exhaust system – this is a system that ensures source-removal of heat and other contaminants e.g. fume cupboards and the hoods in a cooker. The selection and design of a system to be installed in a structure is a specialist task done by the building services engineer, especially when including additional components to cater for humidity, frost, among others. However, general rules of thumb apply for simple situations. Mechanical systems are usually operated automatically with calibrated sensors determining the intensity and duration of ventilation required to maximize occupant comfort and minimize energy consumption. Manual overrides may be installed when there is a malfunction of the sensors. 1.2.3 Fire Service Controls The design of mechanical ventilation systems should be such that [The10]; a. Ductwork do not interfere with the transfer of smoke or fire in the building, with any exhaust points being located away from final exits (paragraph 5.46) b. Separate ventilation systems ought to be provided for each protected stairway and escape route (paragraph 5.47) c. Smoke sensors are placed in recirculation systems so as to shut them down should there be smoke being carried back into the building (paragraph 5.49) The rate of ventilation achieved by these systems needs to be high enough to keep up with the pace of the fire, as well as preventing the sick building syndrome, as this may lead to incapacitation of the occupants [Jaa95]. 1.2.4 Merits and Demerits Mechanical ventilation systems have various advantages, such as; 1. Independent of weather conditions – the lack of wind or temperature gradient does not affect the working of mechanical systems 2. Magnitude – since the air flow is propagated by fans, the magnitude of flow is significantly higher than natural ventilation 3. Control – mechanical systems offer a degree of control to the occupants as the intensity of operation can be adjusted accordingly to achieve the desired effect 4. Better air control – the sensors can measure the contaminants in the air and start the ventilation should the concentration exceed acceptable levels The disadvantages of this system are; 1. Cost – the installation, operation and maintenance of the mechanical systems is an expensive affair, compared to natural ventilation 2. Energy consumption – mechanical systems require electrical energy for the machinery required to ventilate the structure 2.0 Computational Fluid Dynamics (CFD) Modelling The use of Computational Fluid Dynamics has become a common method for calculating ventilation flows in ventilation systems design especially for new constructions [Hus12]. It provides a clear review of the patterns involved in air flow that would otherwise be impractical with traditional techniques or just too expensive. CFD is the application of mathematics, physics and computational software to represent the flow of a liquid, and the effect this liquid has on the objects it flows past. It uses Navier-Stokes equations relating velocity, pressure, density and temperature of a liquid [Rou14]. CFD assists system administrators in the identification of hotspots and the optimization of the use of cold air in cooling or is mixing [Rou14]. This modelling technique has the potential to provide detailed flow patterns and temperature distribution giving it an operational advantage. The calculations can also include all likely physical processes such as transient behavior and surface heat transfer. CFD modelling was used to demonstrate the evacuation of occupants and prevention of smoke spreading between floors in the 15 storey structure. 2.1 Assumptions The structure is supported by reinforced concrete elements (beams, columns and slabs) and concrete block masonry. The plan provided is identical for all 15 floors of the apartment block. The pressurization fan is located on the ground floor. The staircase dimensions are. It is also assumed that all the windows of the building are open. 2.2 Aim of Modelling CFD modelling is employed in building ventilation in an effort to [Cun15]; a. Improve the efficiency of existing cooling systems and predict the efficacy of a specific layout. b. Assess and rule out any risk of recirculation of ‘stale’ air into the ‘fresh’ air intake system c. Optimize the design of ventilation by modelling different configurations and operational modes and choosing the most efficient one d. Assess the distribution of temperature and optimize the composition of walls to achieve the best performance 2.3 Scenarios 2.3.1 Scenario A: Wind- driven Ventilation In this scenario, fire commences in the middle room of the first floor of the apartment block containing 15 floors. Modelling ought to pay keen attention to the first floor, the stairwell and the second floor. Below is a layout plan for the aforementioned scenario; Figure 1: Natural Ventilation Fire Scenario Some of the parameters required for the modelling of heat production and release rates include; Room ceiling height – Floor slab and ceiling thickness – This scenario depicts a situation where the building is located in a secluded area with direct winds hitting the longer dimension perpendicularly. 2.3.2 Scenario B: Mechanical Ventilation (Pressurized) It is assumed the fire starts at the same location as for Scenario A. This is done so as to compare the effectiveness of each of the ventilation techniques in extracting heat and smoke from the building. It is also assumed the windward side windows are closed, so that no natural wind- driven ventilation is taking place. The leeward windows are left open to allow for some buoyancy- driven ventilation. Figure 2: Fire Scenario B under Mechanized Pressure Systems This scenario depicts the building being located in an area where there are no direct winds and the rate of buoyancy- driven ventilation is too low to cater for smoke removal and thermal comfort issues. 2.3.3 Scenario C: Combined Ventilation The location of the fire source remains at the centre room on the first floor. In this scenario, all the windows are open and a mechanical ventilation system is also provided. This model will help determine if the ventilation rate is significantly improved when both natural and mechanical ventilation systems are in operation. 3.0 Conclusions The Computational Fluid Dynamics modelling should provide an accurate depiction of the ventilation rates of each of the mechanisms and whether it would be enough to ensure the safety of the building’s occupants by efficient removal of smoke from the means of escape. The extended corridor should be provided with additional fire protection to maintain its integrity, as the maximum travel distance of the occupants exceeds the limits set forth in the regulations [The10]. Design changes to the plan should also be considered to help safeguard the safety of the occupants. An alternative escape route should be designed so that the occupants have another means of escape should one of them be compromised by the fire. References Hus06: , [1], Jia03: , [2], Mar00: , [3], Aut13: , [4], Lid96: , [5], Kha08: , [6], Wan12: , [7], Wis16: , [8], Lin99: , [9], Aut131: , [10], Nat16: , [11], Bui16: , [12], The10: , [13], Cha16: , [14], Fir14: , [15], Des15: , [16], Jaa95: , [17], Hus12: , [18], Rou14: , [19], Cun15: , [20], [1] A. V. Hussain and S. K. Muhammed, Handbook of Heating, Ventilation and Air Conditioning, New York: Industrial Press, 2006. [2] Y. Jiang, D. Alexander, H. Jenkins, R. Arthur and Q. 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