MECHANICAL VENTILATION
Natural and Mechanical Systems Four (4) possible combinations of natural and mechanical ventilations tions are as follows; Natural inlet and outlet; utilizing open able windows, air bricks, louvers, door-way and chimneys. Provide up to about three air changes per hour but depend upon prevailing wind direction and strength, the stack effect fect of rising warm air currents, and adventitious opening around doors and windows. Natural inlet, mechanical outlet; Mechanical extract fans in windows or roofs and ducted systems where the air is to be discharged away from the occupied space owing to its contamination with heat, fumes, smoke, water vapor and odour.. A slight reduction in air static pressure is caused within the building, and external airs flows inwards. This system can be used in dwellings, offices, factories or public buildings. Mechanical inlet, natural outlet; Air is blown into the building through a fan convector or ducted system to pressurize the internal atmosphere slightly with a heated air supply. The air leaks out of the building through adventitious openings and permanent air bricks or louvers. Mechanical inlet and outlet; When natural ventilation openings unable to cope with large air flow rates without disturbing the architecture or causing uncontrollable draughts. Full mechanical control of air conditioning and ventilation system are in used.
Introduction Indoor/outdoor exchange of air to replenish oxygen and to remove pollutants generated by breathing, indoor activities and emissions from building materials and furnishings. Poor indoor air quality causes or irritate 50% of all illnesses and can lead to a host of health problems for people of all ages. For ages, most buildings relied on openings through the building envelope to provide indoor/outdoor air exchange. Nowadays, mechanical ventilation can make it safe to seal unwanted air leaks and install as much insulation as needed. Ventilation in modern buildings (enclosed space and without natural ventilation) requires additional equipment to create air movement.
The purposes/objectives of mechanical ventilation system: 1. To get enough air (quality and quantity). 2. To control ventilation systems (flow in and out) 3. To control indoor environment (air quality, air velocity, location, way of use). 4. To remove heat and dirty air. Four (4) basic types of whole-house mechanical ventilation systems: Exhaust System Supply System Balanced System Plenum System
(a) Exhaust System An exhaust system may consist of a single fan that is centrally located in a hallway or at the top of the stairs, or it can be an upgraded bathroom fan that has a motor built to run for longer periods of time and is sized to ventilate the whole house. Three (3) mode of operation: 1. Mechanical inlet, natural exhaust (positive/over pressure) 2. Natural inlet, mechanical exhaust (negative/under pressure) 3. Mechanical inlet and exhaust (controlled pressure)
Mechanical Air Supply with Natural Air Exhaust Air supply is controlled mechanically and the air exhaust takes place on a natural way by ventilation openings, windows or shafts. There will be an overpressure in the building. To prevent draught, the air supply in the room has to be placed as high as possible and the air inlet grid must have a possibility to be regulated. By preheating the incoming air, draught problems can also be decreased. An air filter used to clean the incoming air. A ventilator (or fan) controls the air supply and the outdoor air is transported into the building by ducts. By controlling the ventilator it is possible to control the ventilation capacity for the system. Exhaust System
Exhaust System Natural air supply with mechanical air exhaust popular form of ventilation in residential and offices. The mechanical air exhaust system creates an under pressure in the building, the system is less dependent on the weather. The under pressure creates a pressure difference over the ventilation openings encourage the air to suck in. High wind pressure or temperature difference can result in draught problems. To prevent draught the air supply openings have to be placed as high as possible and the air inlet grid must have a possibility to be regulated. Ventilation capacity (air out) controlled by an exhaust ventilator. In residential buildings suction takes place from at least the kitchen, the bathroom and the toilet. (need for suction ducts) In non-residential buildings suction mostly takes place from the corridor. For locations where the sound load is high sometimes baffle filters are used.
Mechanical Supply and Exhaust A mechanical ventilation system can be combined with all sorts of heating and cooling systems. Often the heating, cooling and ventilation of a building are combined in the air-conditioning system (HVAC). In a mechanical ventilation system the supply air and the exhaust air are transported mechanically. Advantages of an Exhaust Ventilation are: Good control of the ventilation capacity; no dependence of the outdoor weather conditions and despite possible noisy environment. The possibility of extracting heat from the exhaust air and use it to preheat the fresh air supply (heat recovery). The possibility of preheating and pre-cooling of the air supply. The possibility of humidify and dehumidify of the air supply. The possibility of cleaning the air by an air filter or supplying the air from a relative clean site of the building. Exhaust System
Exhaust Fresh-up Raising exhaust air decreases room pressure to prevent the leaking of odors or floating bacteria into other rooms
(b) Supply System A supply system slightly pressurizes the house by drawing outdoor air into the house. This pressurization pushes indoor air out through small openings in the building, which helps to prevent outside moisture and pollutants from getting in through these passages. Positive indoor pressure also protects against back drafting.
(b) Supply System In a supply system, a fan pulls outside air into the house, creating positive pressure. Ductwork brings the air into the living spaces that need it most typically the bedrooms and living room. Spot exhaust fans are still needed in high-pollutant areas such as the kitchen, bathroom, and laundry room.
Supply rich mode: Raising the air supply maintains proper room pressure to prevent back-flow to toilet or kitchen odours or moisture inflow. FRESH-UP operation: Both the excessive supply mode and the excessive exhaust mode are selectable. This function allows for a more comfortable air environment whether the occupied space is required to be pressurized or de-pressurized.
(c) Balance System A balanced central system uses two fans (supply and exhaust fans working in cooperation) and creates a neutral, or balanced pressure. One fan exhausts air out of the house, while the other brings the same amount of outside air into the house. These systems are more complex than supply-only or exhaust-only ventilation systems, but provide better fresh air distribution. They may be two wall-mounted fans of equal capacity--one for supply and one for exhaust- -in two different rooms. Alternatively, they may be ducted to supply air to the common living areas, such as living rooms and bedrooms, and to exhaust air from the rooms where pollution is high, such as the kitchen and bathroom In a balanced system, air is supplied to some rooms and exhausted from others. An optional heat- or energy-recovery unit transfers heat or humidity from one air stream to the other. Spot exhaust is included where necessary.
(c) Balance System (benefits) Balanced ventilation systems can provide many benefits including: Improved indoor air quality Improved comfort Improved health Lower utility bills Improved resale position
Balance System (benefits) Improved indoor air quality. Improved comfort. Improved health. Lower utility bills. Improved resale position. Balanced ventilation systems supply fresh air to the living and sleeping areas of homes while exhausting stale air at an equal rate from the bathrooms. This proactive approach to ventilation can result in improved indoor air quality. ENERGY STAR labeled homes with tight construction and balanced ventilation systems can have fewer drafts and a constant supply of outdoor air resulting in improved comfort. Stale air can cause health problems. It can be responsible for symptoms such as headaches, drowsiness, and respiratory problems. These symptoms are more common in homes with poor ventilation and moisture control. Continuously providing fresh air can result in the improved health and well being of the occupants. Less energy is consumed to operate ventilation systems than to heat and cool excessive amounts of outdoor air that infiltrates leaky homes. Additional savings are captured when these systems are equipped with either a sensible or total heat exchanger. This can result in lower utility bills, making homes less expensive to operate. ENERGY STAR labeled homes with balanced ventilation systems can provide the many impressive benefits listed including more comfortable homes with better indoor air quality and lower utility bills. These benefits can translate into higher resale value.
Typical Installation Configurations SA = Supply air RA = Return air OA = Outside air EA = Exhaust air
Plenum is the space that can facilitate air circulation for heating and air conditioning systems, by providing pathways for either heated/ conditioned or return airflows. Space between the structural ceiling and the dropped ceiling or under a raised floor is typically considered plenum. Air pressure in plenum is higher than outside. For buildings, plenum is an enclosed space that are not for human occupancy, but are often used for heating, ventilating and/or air conditioning equipment and airflow, and are for equipment such as cables, piping, and luminaries (d) Plenum System
Types of fan used for ventilation systems The motive power for the distribution of air in ventilation systems is invariably provided by electrically driven fans. There are three main types in use: 1. Propeller fans 2. Centrifugal fans 3. Axial flow fans
Announcement!! Attention to all students BFC 3153 section 3 and 4 who missed the 2 nd Quiz (successfully done during lecture session on 7 th February 2011), chances for you to replace the marks by performing presentation on TYPES of FAN USED FOR VENTILATION SYSTEMS for the next lecture. Please give me your name and sub topic to be present during our next class.
(i) Propeller fans Propeller fans are suitable for situations where no great resistance to airflow has to be overcome. A typical fan The free intake and discharge condition of ventilation fans situated in wall openings giving direct in/out and out/in movement are eminently suited to this type of fan, which under these conditions can move large volumes of air economically and with very low installation costs. Efficiency 60-65% Short duct systems can also be served, provided the resistance of the system is low.
(ii) Centrifugal fans Centrifugal fans can generate high-pressure rises in the gas stream 760Pa. Accordingly, they are well-suited for industrial processes and air pollution control systems. Efficiency 80%, high cost, silence, big size and need more space. A centrifugal fan has a fan wheel composed of a number of fan blades mounted around a hub. The hub turns on a shaft that passes through the fan housing. The gas enters from the side of the fan wheel, turns 90 and accelerates as it passes over the fan blades. The term, centrifugal, refers to the trajectory of the gas stream as it passes out of the fan housing. The centrifugal type generally supplies more cfm per horsepower at static pressures over 4.0-4.5 inches of water and is often used where low noise level is desired
Types of fan wheels Forward-curved fans [Figure 4(a)] use blades that curve toward the direction of rotation of the fan wheel. These are especially sensitive to particulate and are not used extensively in air pollution control systems. Efficiency 50-60%. Backward-curved fan blades [Figure 4(b)] use a straight plate, a curved plate, or a curved airfoil. These types of fan wheels are used in fans designed to handle gas streams with relatively low particulate loadings because they are prone to solids build-up. Backward-curved fans are more energy efficient (80-85%) than radial blade fans. Radial blades are fan wheel blades that extend straight out from the hub. A radial blade fan wheel, as shown in [Figure 4(c)], is often used on particulate-laden gas streams because it is the least sensitive to solids build-up on the blades. Efficiency 45-55%. The fan wheel consists of a hub and a number of fan blades. The fan blades on the hub can be arranged in three different ways: Forward; Backward; Radial
(iii) Axial flow fans This type of fan is becoming popular. Use in the ductwork system Efficiency is high to 75%, simple installation and appearance neat, particularly in a line of ducting. The vane-axial type has a propeller and vanes, and is best for static pressures under 4.0-4.5 inches of water
Advantages of good ventilation 1.0 REDUCTION IN HUMIDITY LEVELS -Dust-mite levels decrease -Mould disappears -Fungus disappears 2.0 REMOVES THE CAUSE OF TIREDNESS AND HEADACHES -High CO² levels are replaced with fresh air -Moist, stuffy air is eliminated -Poisonous particles emitted by artificial building materials are removed 3.0 ELIMINATES RADON PROBLEMS -Radon gas will be ventilated away 4.0 REDUCES THE NEED FOR REDECORATION -Cigarette smoke will not linger -The amount of pollution in the air decreases 5.0 LOWERS THE HEATING COST -A A damp house is impossible to heat (try heating a wet towel)
Example : TURBINE ROOF VENTILATION The Whirling Turbine Action: Tornado Turbine Roof Ventilation Removes Convicted Heat Pollution Build-Up - Improves Air Quality - Water and Dust Proof - Improves Productivity - Removes Heat, Fumes, Steam and Dust
Quiz 1 1. Why proper ventilation is required inside a building? 2. Explain only 3 factors from 6 factors consider to maximize the natural ventilation. 3. Sketch and briefly describe 4 basic system in mechanical ventilation.