Mechanical Ventilation PDF

School of Architecture & the Built Environment **MECHANICAL SERVICES BE6812** **[Mechanical Ventilation]** **Objectives :** To differentiate between various mechanical ventilation systems and know the equipment involved in such systems. **4.1 Introduction** This is ventilation provided by elect...

School of Architecture & the Built Environment **MECHANICAL SERVICES BE6812** **[Mechanical Ventilation]** **Objectives :** To differentiate between various mechanical ventilation systems and know the equipment involved in such systems. **4.1 Introduction** This is ventilation provided by electrically driven fans. This form of ventilation offers a number of advantages over natural ventilation :- 1\) It \_\_\_\_\_\_\_\_\_\_\_\_\_ irrespective of wind conditions. 2\) Specific air change (or ventilation rate) can be determined. 3\) The air can be filtered. 4\) \_\_\_\_\_\_\_\_\_\_\_\_ and \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ can be created. **4.2 Mechanical Ventilation Methods** Mechanical ventilation can be achieved by the following methods :- a\) Natural inlet - mechanical extract. b\) Mechanical inlet - natural extract. c\) Mechanical inlet - mechanical extract. **a) Natural Inlet -- Mechanical Extract** This is the most common system of mechanical ventilation used in hotel toilet, workshop & kitchen. It is required for its negative pressure application. For example -- to prevent the toilet's odour or the kitchen's cooking fumes from entering the ballroom area. ***Fig. 1 -- Plan View*** ***Fig.2 -- Mechanical Extract System*** In this system, - a mechanical exhaust fan is installed at the outlet to draw air out of the kitchen area. - this creates a \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ region in the kitchen area. - the air pressure in the ballroom area will \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_, i.e. positive pressure. - air therefore \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ the ballroom\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_, through the inlet positions, to replace the extracted air. - An exhaust fan can similarly be installed in the toilet. When it is switched on, it creates a negative pressure inside the toilet. This can stop the toilet's odour from entering into the ballroom area. The **mechanical extract** can be in the form of :- a\) an exhaust fan fixed across an opening in an external wall/window. b\) a fan in a length of extract duct for high level discharge or low level extract. A filter may be fixed at the mechanical extract. **b) Mechanical Inlet -- Natural Extract** This method of ventilation is mainly for areas that require positive pressure to avoid contamination from adjacent spaces, e.g. fire escape staircases. In the event of a fire, the inlet fan will be automatically switched on. This creates a positive pressure in the stairwell, i.e. the stairwell is now at a higher pressure than the rest of the building. Smoke from the fire will hence be prevented from entering the stairwell. This will allow occupants to escape. ***Fig. 4 -- Staircase Pressurisation (Section View)*** **c) Mechanical Inlet -- Mechanical Extract** This system has the widest application. It allows pressure in a space to be controlled (either +ve or -ve). It is also possible to control the air distribution in the space by using supply and exhaust ductwork. Of the three methods of mechanical ventilation this is the most expensive. One of the most common applications for this method of ventilation is in basement carparks where it will not be possible to provide openings to act as natural inlets or extracts. **4.3 Special requirements for Basement Carparks by Building Authority.** 1\. Must be mechanically ventilated and consist of a supply and an exhaust part. Each part designed to be operated in two sections, i.e. at least 2 inlets and 2 outlets. 2\. Each section to provide for half the air change or fresh air supply required. 3\. The two sections may operate through a common ductwork. 4\. Each section to have separate control so that in the event of failure of one section, the other continues in operation. 5\. Exhaust and supply of each section to be electrically interlocked such that failure of exhaust shall automatically shut down the corresponding supply. This is to maintain the basement carpark under negative pressure. 6\. The system must be connected to an alternate supply (e.g. standby generator) so that the ventilating system continues to operate in the event of a failure in the principal source of electrical supply. 7\. If the opening for natural ventilation is at least 2% of the floor area, then such natural ventilation may be considered as a satisfactory substitute for the supply part of the mechanical ventilation system. 8\. The exhaust is to discharge sufficiently far away from openings (e.g. fresh air intakes, windows, entrance etc.) This is to prevent re-entry of objectionable odour, toxic gases and flammable vapours into an enclosed space. 9. At least 50% of the exhaust air is to be extracted from a position not exceeding 650 mm above finished floor level.  10. The carpark must be under negative pressure at all times. 11. The ventilation rate is to be 6 air changes per hour. **4.4 Mechanical Ventilation Equipment** There are 3 major types of fans used in air distributions systems:- a\) Centrifugal b\) Axial c\) Propeller **a) Centrifugal Fan** This works as its name suggests, it "throws" the air away from its centre, i.e. it sweeps air from the centre and then blows it out at the side. It can have a single inlet or double outlet. There is a motor to drive the fan. This can be coupled to the fan through a shaft or a drive belt. Some centrifugal fans have the motor incorporated in the fan itself. A centrifugal fan must be mounted on anti-vibration mountings and connected to ductwork with flexible connections. Advantages 1\. Low noise. 2\. Smooth air flow. 3\. Adaptable to various application. Disadvantages 1\. Bulky. 2\. Not very efficient **b) Axial Fan** These consist of impeller blades rotating inside a cylindrical casing. In this fan the air flows parallel to the axis of the fan, thus, the name \"axial fan\". This is usually used as an exhaust fan e.g. in carparks. The fan is easily installed. ***Fig.8 - Axial fan (Section view)*** Advantages 1\. Efficient. 2\. Compact. 3\. Simple to install. 4\. Cost less than centrifugal fan. Disadvantages 1\. High noise level. Axial fans have their motors in the air stream, which can be both an advantage and disadvantage. Whilst the moving air cools the motor down, if there is high temperature or corrosive elements present, then this will be detrimental to the life of the fan motor. A variation of an axial fan is a bifurcated fan used for extracting hot air or gases. ***Fig.9 -- Bifurcated Fan*** The air stream is diverted either side of the motor compartment and then rejoins again downstream. The fan can be used for extracting large volume of dusty or corrosive air, at ambient or elevated temperatures. **c) Propeller Fan** These have two or more blades. They develop low pressure only. They are normally called \"exhaust fans\". Generally, it is used without ductwork at openings in walls or windows. Advantages 1\. Large volume extraction. 2\. Economic to run. 3\. Economic to install. Disadvantages 1\. Cannot be used with ductwork. 2\. Can be noisy. 3\. Resistance causes backflow. **d) Filters** Purpose :- to remove contaminants from the air e.g. dust, dirt etc. Filtering of air prevents :- \- Dirt build-up inside the ventilated or air-conditioned space. \- Dirt accumulation on fans, coils, etc. which will reduce their efficiency. \- Staining of decorations. The efficiency of filters is determined by the percentage of particles retained by the filter. Air filters operate on a number of methods :- i\) Dry fabric arrestment. ii\) Electrostatic precipitation. iii\) Grease filter. i\) Dry Fabric Arrestment This can be cleanable or disposable. It consists of a fine fabric through which the air pass, any particles larger than the mesh of the fabric will be caught on the fabric. When the fabric becomes clogged the filter is cleaned or changed. There are 3 types available : Screen, Bag & Roller ii. Electrostatic Filter Particles passing through the high voltage charge plate are being subjected to an electrostatic ionisation charge and will be attracted to the negative charge plates. The dusts collected on the plates are periodically removed by washing with hot water jets. These filters are good for removing very fine particles but they are very expensive. iii. Baffle Grease Filter (Kitchen cooker hood) Baffle grease filters are used in commercial cooker hoods. This filters work by forcing the grease laden air through several changes in direction as it passes through the filter. The grease droplets are unable to change direction as quickly as the air in which they are carried. The grease will become separated from the air and are deposited on the grease filter blades and drained into drip trays.  Baffle filter  **4.5 Types of Motors** ***AC Induction Motors*** -- The power in the rotor (rotating part) is induced through the moving magnetic fields in the stator (the windings in the stator produce the magnetic field). AC induction motors are: - Most common type of motors used for building services equipment. - Rugged - Simple - Reliable - Low cost ***DC Motors*** -- Direct current motors are sometimes found in variable speed HVAC applications. They are generally more efficient and easier to control than AC motors. **4.6 Components of AC Motor** - ***Stator*** -- fixed part of the motor with windings to produce magnetic field. - ***Rotor --*** the rotor is attached to a shaft the spins and provide mechanical output to equipment connected to it. - ***Enclosure*** -- protect the above components and provide a base for fixing the pump  **4.7 Maintenance of Motors** Regular maintenance of motors will include: - ***Cleaning*** -- of motor surfaces and ventilation openings. Heavy accumulation of dirt will result in overheating and premature failure. - ***Lubrication*** -- of moving parts will avoid unnecessary wear. - ***Keep motor couplings aligned*** -- correct shaft alignment will ensure smooth, efficient transmission of power from motor to the load. Incorrect alignment puts strain on bearing and shafts, shortening their lives and reducing system efficiency. - ***Avoid painting motors*** -- paint will act as insulation, increasing operating temperatures and shortening motor life.