Applied Building Services Lecture 2 PDF

Applied Building Services Dr. Ahmed Hassan  Lecture 2 Air Distribution Systems  Air Distribution System Design All-air systems Air-water systems All-water systems The conditioned air is produced Air based and water-b...

Applied Building Services Dr. Ahmed Hassan  Lecture 2 Air Distribution Systems  Air Distribution System Design All-air systems Air-water systems All-water systems The conditioned air is produced Air based and water-based cooling are Hot or chilled water is used to centrally at AHU located in a combined. transfer heat to or from a conditioned mechanical room and distributed space or process, through piping via ductwork. connecting a boiler, water heater, or chiller with suitable terminal heat FCU transfer units located at the space or process. AHU  Air Handling Units Central heating and cooling systems use the air-handling units (AHU’s) and ductwork to distribute conditioned air to every space in the building. Functions of AHU: Draw in outside air and return air. Mix them, condition the mixed air. Blow the conditioned air into the space. Exhaust any excess air to outside.  Air Handling Units Components: Inlet louver screen Relief air damper Heating coil Supply fan Parallel blade damper Actuator Cooling coil Return fan Opposed blade damper Mixed temperature sensor Humidifier Filters Air Handling Units Air Handling Units  Air Distribution System Design All-air systems The conditioned air is produced centrally at AHU located in a mechanical room and distributed via ductwork. AHU  All-air systems Advantage: All Air Systems is a recommended option for applications with high occupancy or places with high public gathering such as cinemas, theaters, auditorium, function halls, retail stores, meeting rooms, airports, hotel lobbies etc. Disadvantage: The biggest drawback of all air systems is that it would require a high airflow rate resulting in large ducts, ceiling space and high energy costs for the air distribution system.  Air Distribution System Design All-air systems Single Zone Systems Multiple Zone Systems  Single Zone Systems Energy Supply Building A: This unit has only an electrical supply. This single electrical supply provides all the power for heating, cooling, humidifying, and for driving the fans. Building B: This unit has the electrical supply for cooling, humidifying, and for driving the fans, while the gas line, shown as “gas supply,” provides heating.  Single Zone Systems Energy Supply Building C: This unit has the electrical supply for cooling, humidifying, and for driving the fans. It also has supply and return hot-water pipes, coming from a boiler room in another building. The unit contains a hot-water heating coil and control valve, which together take as much heat as needed from the hot water supply system. Building D: In the same way, there may be a central chiller plant that produces cold water at 5.5 − 9 C. This chilled water is piped around the building to provide the air-handling units with cooling. Like the heating coil and control valve in Building C, there will be a cooling coil and control valve in each unit, to provide the cooling and dehumidification. Single Zone Systems  Single Zone Systems Air inlet screen The inlet louver and screen restrict entry into the system. The inlet louver is designed to minimize the entry of rain and snow. Maintaining slow air-speed through the louver avoids drawing rain into the system. Ex: galvanized-iron mesh. Return air damper: Some air will be exhausted directly to the outside from washrooms and other specific sources, like kitchens. The remainder will be drawn back through the return air duct by the return air fan and either used as return air, or exhausted to outside through the relief air damper.  Single Zone Systems Mixing section A parallel blade damper that direct the air streams toward each other, causing turbulence and mixing. Mixing the air streams is extremely important in very cold climates, since the outside air could freeze coils. Actuators: Link the outside-air damper, the return-air damper, and the relief-air dampers. When the system is “off,” the outside-air and relief-air dampers are fully closed, and the return-air damper is fully open. The system can be started, and all the air will recirculate through the return damper. As the damper actuator drives the three dampers, the outside-air and relief-air dampers open in unison as the return-air damper closes.  Single Zone Systems Mixed Temperature Sensor: The control system needs to know the temperature of the mixed air for temperature control. The mixed temperature sensor is usually installed downstream of the filter. At start, there is no outside air in the flow, hence the mixed-air temperature is equal to the return-air temperature. When the dampers open, outside air is brought into the system, upstream of the mixed-air sensor. It is common to set the control system to provide a mixed-air temperature somewhere between 13 and 16 C. The control system can simply adjust the position of the dampers to maintain the set mixed temperature.  Single Zone Systems Mixed Temperature Sensor - Example: Consider a system with a required mixed temperature of 13C and return temperature of 23C. ‣ When the outside temperature is 13C, 100% outside air will provide the required 13C. ‣ When the outside air temperature is below 13C, the required mixed temperature of 13C can be achieved by mixing outside air and return air. ‣ As the outside temperature drops, the percentage required to maintain 13C will decrease.  Single Zone Systems Heating Coil Its function is to raise the temperature of the mixed air. It is required in cases of very high proportions or 100% outside air is needed. The heat for the heating coil can be provided by electricity, gas, water, or steam. ❖The electric coil is the simplest choice, but the cost of electricity often makes it an uneconomic one. ❖A gas-fired heater often has the advantage of lower fuel cost, but control can be an issue. ❖Hot water coils are the most controllable, but there is a possibility that they will freeze in cold weather.  Single Zone Systems Cooling Coil Cooling is usually achieved with a coil cooled by cold water, or a refrigerant. The cold water is normally between 5.5C and 9C. The coil will normally be cooler than the dew point of the air and thus condensation will occur on the coil. Humidifier: A device for adding moisture to the air by either injecting water-spray or steam into the air. The water-spray consists of very fine droplets, which evaporate into the air. The humidifier will normally be controlled by a humidistat, which is mounted in the space or in the return airflow from the space. Fan The fan provides the energy to drive the air through the system.  Single Zone Systems System Control Both the packaged rooftop unit and the inside, single-zone unit produce the same output: a supply of treated air at a particular temperature. The heating or cooling effect of this treated airflow, when it enters a zone, is dependent upon two factors: The flow rate. The temperature difference between the supply air and the zone temperature. When the unit is supplying one space, or zone, the temperature in the zone can be controlled by: Changing the air volume flow rate to the space. Changing the supply air temperature. Changing both air volume flow and supply air temperature.  Multiple zones air system In many buildings, the unit must serve several zones, and each zone has its own varying load. To maintain temperature control, each zone has an individual thermostat that controls the volume and/or temperature of the air coming into the zone.  Multiple zones air system 1. Single-Duct, Zoned-Reheat, Constant-Volume Systems: The reheat system permits zone control by reheating the cool airflow to the temperature required for a particular zone. The reheat coil is located close to the zone, and it is controlled by the zone thermostat.  Multiple zones air system 2. Single-Duct, Variable-Air-Volume Systems (VAV) It is used for cooling only cases (hot/warm climates). It would be ideal to supply only as much cooling and ventilation as the zone actually requires. The total supply-airflow rate can be varied by varying the flow rate of the supply fan. This damper acts as a throttle to allow more or less cool air into the zone.  Multiple zones air system Single-Duct, Variable-Air-Volume Systems (VAV) Advantages: Low initial costs Low operating costs. Limitations : Poor air circulation in the conditioned space at lower flows; leading to inadequate fresh air supplied to the zone.  Multiple zones air system 3. Constant-Volume, Dual-Duct, All-Air Systems In a dual-duct system, cooling and heating coils are placed in separate ducts, and the hot and cold airflow streams are mixed, as needed, for temperature control within each zone. Constant air flow is maintained by the supply fan.  Multiple zones air system Constant-Volume, Dual-Duct, All-Air Systems  Multiple zones air system Constant-Volume, Dual-Duct, All-Air Systems  Multiple zones air system Constant-Volume, Dual-Duct, All-Air Systems: Advantages: Can serve highly variable sensible-heat loads as in hospitals and laboratories. Variable loads can be due to different occupancy levels, varying equipment loads, or differing orientations No reheat coils near the zones, so the problems of leaking hot water coils is avoided. Limitations: High initial cost, since it requires two supply ducts. Need additional space above the ceiling for the second supply duct and connections. Air Distribution System Design All-water systems Hot or chilled water is used to transfer heat to or from a conditioned space or process, through piping connecting a boiler, water heater, or chiller with suitable terminal heat FCU transfer units located at the space or process.  Fan Coil Unit (FCU) It is a terminal unit that drives air past a finned tube containing hot or cold fluid. The air is usually drawn from the space to be conditioned and returned to it. The unit consists of heating and cooling coils, circulation fan, and proper control system. For central systems, the fan-coil units are connected to boilers to produce heating and to water chillers to produce cooling to the conditioned space. They are rather efficient since they obviate the need to blow large quantities of air around a building, but do require decentralized maintenance (filter changes, condensate pan cleaning, etc.).  Fan Coil Unit (FCU) These units are available in various configurations to fit under windowsills, above furred ceilings, in vertical pilasters built into walls, etc. These are mainly used as ductless systems but can be connected to ductwork also. Fan Coil Unit (FCU)  All-water systems All water system provide individual room control as required in hotels, multi storey apartments, offices etc. Advantages: Improved efficiency of space utilization; lower energy costs; individual temperature control; and architectural flexibility: with the ability to install units in the floor, ceilings or walls/façades, there is always an option to meet specific requirements. Disadvantages: Do not filter or ventilate effectively; do not satisfy ventilation air intake or exhaust rate needs; lack humidity control; must be switched over for seasonal operation; and require a great deal of maintenance.  Fan Coil Unit (FCU) Air-water systems Air Cooled Chiller + FCU  Air-water systems Air based and water-based cooling can be combined if: The air-based system does not have sufficient cooling capacity. The air system is used to provide the outside air for ventilation and building pressurization while terminal units utilizing fan coil units are used as in all water system. Applications: It is much suitable for retrofitting of existing buildings, where there is space restriction; It is very good solution for perimeter zone buildings with large sensible loads; It can be used where close control of humidity in the space is not necessary; and It serves well for large office buildings, hotels, lodges etc.  Air-water systems Advantages of air-water systems: Air water systems require comparatively low air flow rates (only for ventilation). This means that the required air supply and extract duct cross sectional areas are significantly reduced; Provides ventilation and humidity control; and All advantages of All-water systems. Disadvantages of air-water systems: Between-the-season operation is difficult to manage; Normally limited to exterior zones.