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PPT On HVAC System Selection For

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Published in: Physics
9,358 Views

Selection of suitable HVAC system for buildings.

Mudasir K / Abu Dhabi

10 years of teaching experience

Qualification: M.Sc in Mechanical & Manufacturing Engineering

Teaches: Safety Training Course, 3D Printing, Advanced Maths, Science Projects, Chemistry, Education, Mathematics, Physics, Project Management, Engineering, Operational Research, Product Design, Safety Engineering, PTE, SAT, GED, Maths

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  1. Building Engineering Systems Mohammed Mudasir Kausar
  2. Contents Introduction to conventional technical systems in buildings (HVAC, electrical systems and services-week 1 HVAC system types, working principles and system components- week 3 Cooling load calculation- week 4 & 5 (understandig weather data, understanding heat gain, Understanding Insulations, Learning eQuest, hands on eQuest) System sizing, selection, codes and regulations week 6&7 Midterm Exams, week 8 HVAC system Integration into buildings week 9 & 10 (Integrating Air handling systems, central chillers, Fluid handling systems and ducting systems- Duct designing and distribution, week 11 & 12 System controls and supplies, week 13 (dampers, diffusers, humidifier/dehumidifiers, thermostats and controllers Final Exams week 16
  3. Good to know Refrigerant - It is a substance or mixture used in heat pump and refrigeration cycle. Thermal distribution system — Method of distributing heating and cooling. The systems that blow air through ducts and other systems that use water and electricity. Duct — It is used to deliver and remove air. It is generally made of galvanized steel , aluminum. It is a valve or a plate that stops or regulates the flow of air Dampers - inside a duct.
  4. Cooling Coil Water Supply Dry Air Dampers Water Distribution Water Pane/ Humidified A ir Drain
  5. 20 20 Condensate water
  6. Reheat Coil
  7. Introduction to HVAC system in in A/C plant Thermal Distribution System plant Qbld Conditioned space Fig.36.1: Schematic of a summer air conditioning system with the thermal distribution system All air system, higher energy conservation, (better air control, free heat transfer media, easy to handle, low heat capacity, inefficient heat transfer, requires more space, not easy to retrofit) All water system (higher heat capacity, higher transfer rates, requires less volume, easy to retrofit, water is not free, needs higher pumping power, not easy to handle due to water leakage. Air-Water system-Combines ups ad downs of both, applied in very large buildings, especially sky scrappers
  8. Energy has to be either supplied or extracted from the conditioned space. A/C plant acts either as a heat source (winter) or heat sink (summer). Air, water or refrigerant are used as media to transferring energy from A/C plant to the conditioned space. A thermal distribution system is required to circulate the media between the A/C plant and conditioned space. Also to introduce fresh air in to conditioned space to maintain required indoor air quality (IAQ).
  9. All Air System In these systems, air is processed in the A/C plant and transferred through insulated ducts using blowers and fans to the conditioned space. The return air from the conditioned space is conveyed back to the plant where it is again processed thus completing the cycle. Classification I-Single duct system 2-Multiple duct system Single duct system are further divided into I-Constant volume single zone system 2-Constant volume variable zone system 3-Variable air volume system
  10. All Air HVAC system Advantages I-Air is easy to handle 2- Air is free to use 3-Less power required to flow air compared to water system 4-Good control on temperature and humidity Disadvantages I-Air has lower thermal conductivity and lower heat capacity which decreases heat transfer rate 2-Air needs larger volume 3-Not easy to retrofit due to larger space requirements Applications In smaller and medium buildings
  11. Single duct constant volume Single zone system Humidifier Cooling coil OD air Dampers RC air Exhaust Heating coil Supply air duct with fan Return air duct with fan o Conditioned space 1. Spaces with uniform loads, such as large open areas with small external loads e.g. theatres, auditoria, departmental stores. 2. Spaces requiring precision control such as laboratories
  12. O/D (Outdoor) air for ventilation and R/C (recirculated) air mixed in the required proportions using dampers. As the mixed air flows, the temperature and moisture content brought to the required values. Then this air is supplied to the conditioned space, where it meets the building cooling or heating requirements. The return air leaves the conditioned space, a part of it is recirculated and the remaining part is vented to the atmosphere A thermostat senses the temperature of air in the conditioned space and controls the amount of cooling or heating provided in the coils so that the supply air temperature can be controlled as per requirement. A humidistat measures the humidity ratio in the conditioned space and controls the amount of water vapor added in the humidifier and hence the supply air humidity ratio as per requirement. However, the single zone may consist of a single room or one floor or whole of a building consisting of several rooms. A separate sub-system controls the amount of OD air supplied by controlling the damper position
  13. Single duct constant volume Multi zone system space is conditioned by temperature and humidity of air Reheat coils cc Advantages Zone 1 .0 zone 2 Zone 3 Relatively small space requirement Excellent temperature and humidity control Proper ventilation and air quality in each zone High energy consumption by first cooling and then heating
  14. For very large buildings with several zones of different cooling/heating requirements, it is not economically feasible to provide separate single zone systems for each zone. For such cases, multiple zone systems are suitable. Figure shows a single duct, multiple zone system with terminal reheat coils. In the reheat coil the supply air temperature is increased further to a required level depending upon the load on that particular zone. This is achieved by a zone thermostat which controls the amount of reheat, and hence the supply air temperature. The reheat coil may run on either electricity or hot water. Disadvantages High energy consumption for cooling, as the air is first cooled to a very low temperature and is then heated in the reheat coils b) Simultaneous cooling and heating is not possible.
  15. Single duct variable volume (flow rate) Multi zone systems space is conditioned by amount air flow rate Cooling coil Zone 1 Zone dam ers zone 2 Zone 3 Low cooling energy consumption due to absence of reheat coil Low fan power consumption due to controlled flow rate Less humidity control At lower cooling loads lower volume flow rate can cause air quality problem
  16. The amount of air supplied to each zone is controlled by a zone damper, which in turn is controlled by that zone thermostat as shown in the figure Thus the temperature of supply air to each zone remains constant, whereas its flow rate varies depending upon the load on that particular zone. Compared to constant volume systems: Lower energy consumption in the cooling system as air is not cooled to very low temperatures and then reheated as in constant volume systems. Lower energy consumption also results due to lower fan power input due to lower flow rate, when the load is low. These systems lead to significantly lower power consumption. Disadvantages: Since the flow rate is controlled, there could be problems with ventilation, IAQ and room air distribution when the zone loads are very low. In addition it is difficult to control humidity precisely using VAV systems. Balancing of dampers could be difficult if the airflow rate varies widely. combining VAV systems with terminal reheat it is possible to maintain the air flow rate at a minimum required level to ensure proper ventilation and room air distribution
  17. Dual Duct Constant Volume Multi zone Systems cc Mixing box Zone 1 Cold air Zone 2 Warm air Return air Advantages 1. Since total airflow rate to each zone is constant, it is possible to maintain proper IAQ and room air distribution. 2. Cooling in some zones and heating in other zones can be achieved simultaneously 3. System is very responsive to variations in the zone load, thus it is possible to maintain required conditions precisely. Disadvantages 1. Occupies more space as both cold air and hot air ducts have to be sized to handle all the air flow rate, if required. 2. Not very energy efficient due to the need for simultaneous cooling and heating of the air streams.
  18. Dual Duct Variable Volume Systems Draw it as an exercise in practical session
  19. All water System Advantages 1. The thermal distribution system requires very less space 2. Individual room control is possible, and at the same time the system offers all the benefits of a large central system. 3. Since the temperature of hot water required for space heating is small, it is possible to use solar or waste heat for winter heating. 4. It can be used for new as well existing buildings (retrofitting). 5. Simultaneous cooling and heating is possible with 4-pipe systems Disadvantages 1. Requires higher maintenance compared to all air systems, particularly in the conditioned space. 2. Draining of condensate water can be messy and may also create health problems if water stagnates in the drain tray. This problem can be eliminated, if dehumidification is provided by a central ventilation system, and the cooling coil is used only for sensible cooling of room air. 3. If ventilation is provided by opening windows or wall apertures, then, it is difficult to ensure positive ventilation under all circumstances, as this depends on wind and stack effects. 4. Control of humidity, particularly during summer is difficult using chilled water control valves. Applications Larger buildings Retrofit buildings Individual control buildings
  20. Two Pipe system Heating/cooling coil Classifications Flow control valves Supply water line Pump Zone 1 Return water line Zone 2
  21. Four Pipe All water System Draw and explain as work in practical session
  22. Hybrid Air-Water System Central plant for secondary water Central plant for primary air Used in the largest buildings Secondary water lines Primary air ducts 4 Primary + secondary air oom unit R—zSecondary air Combines features of low space requirements and higher heat transfer rates of water Exploits the possibility of using free heat transfer media i.e. air
  23. Advantages and Disadvantages Advantages 1. Individual zone control is possible. 2. It is possible to provide simultaneous cooling and heating 3. Space requirement is reduced compared all air systems. 4. Positive ventilation can be ensured under all conditions. 5. Cooling coil operates dry and its life thereby increases and have no problems related to odours or fungal 6. The conditioned space can sometimes be heated with the help of the heating coil and secondary air, thus avoiding supply of primary air during winter. 7. Service of indoor units is relatively simpler compared to all water systems. Disadvantages 1. Operation and control are complicated due to the need for handling and controlling both primary air and secondary water. 2. In general these systems are limited to perimeter zones. 3. The secondary water coils in the conditioned space can become dirty if the quality of filters used in the room units is not good. 4. Since a constant amount of primary air is supplied to conditioned space, and room control is only through the control of room cooling/heating coils, shutting down the supply of primary air to unoccupied spaces is not possible. 5. If there is abnormally high latent load on the building, then condensation may take place on the cooling coil of secondary water. 6. Initial cost could be high compared to all air systems.

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