Air Handling & Air Distribution
in HVAC System
Air Handling Unit (AHU) is basically used to condition indoor air with their heating, cooling, humidification and de-humidification functions in a HVAC system…
- Bijan Kumar Mandal,
Air conditioning is the process of conditioning of indoor air to maintain its temperature, humidity, velocity and cleanliness inside the room or desired location at a desired level for human comfort or any industrial process. Air handler or air handling unit (AHU) is one of the most essential and major parts of an air conditioning system. AHU is a large metal box that consists of a blower, heating or cooling elements, filter racks or chambers, sound attenuators and dampers. AHU is basically used to condition indoor air with their heating, cooling, humidification and de-humidification functions in a HVAC system.
Basic Components of an Air Handling Unit
An air handling unit consists of few important components. Those are as follows:
Casing is the insulated enclose of an AHU which keeps all the system components of an AHU safe inside and prevent the heat gain or loss from it. A schematic diagram of a typical air handling unit is shown in Fig. 1.
Two types of ducts are used in an AHU. These are the supply air duct and return air duct. The cool and conditioned air is supplied to desired locations from the AHU by the supply air duct, while the hot air from the room is again returned back to the air handling unit through return air duct. There is one main supply duct that is then divided into various small ducts those lead to all the rooms that are to be maintained at a specific condition. Similarly, the return ducts from all the rooms also end into one main duct. Insulating materials are used to cover those ducts to prevent the heat gain or loss from these ducts and the ducts are designed in such a fashion that the distribution should be equal to all the rooms and wastage should be minimal.
The Mixing Chamber
In this section, the fresh air from the environment is mixed with return air from the room. This is done to minimize energy consumption by taking advantage of the heating/cooling ability of the return air. In this system, fresh air is only required to supply the air change rate needed for comfort.
Damper openings are generally controlled manually or by servo motor. But the main issue is to allow air to circulate freely and facilitates installation in closed ducts. Thus, dampers are designed to allow synchronous control. The three-way mixing chamber has been designed for similar purposes. However, this system includes a fan for return air. The desired portion of return air is given back to the system and the remainder can be exhausted.
Filter is one of the very important components in an air handling unit. Filters are basically used to remove particles and contaminants of various sizes from the air. These air filters are usually placed at initial stage of an AHU to keep the downstream components clean.Generally, filters are placed in two or more successive stages with a coarse grade filter in front of a fine grade filter. Sometimes, final filtration medium is also there for further cleaning of the air.
Different types of air filters are used in an AHU and their performances are listed in table 1. The type of air filter being used is very much dependent on the application of the system.
Air filters may also be classified according to their applications such as:
• Panel Filter
These types of filters are flat and rectangular in shape and provide a low efficiency filtration. The high velocity filters are arranged vertically whereas the low velocity filters are arranged in V shape. Typical air velocity through these types of filters ranges between 2-3 m/s.
• HEPA Filter
HEPA Filters are very efficient and is able to achieve efficiency up to 99.97%. These filters are efficient in removing minute particles and airborne bacteria from the air. It is usually used in clean room applications such as semiconductor production floor, operation theaters and places undergoing critical processes.
• Electrostatic Filter
Electrostatic Filter is used to remove particles from the air by using highly charged electrodes that ionized the air. Bag Filter is able to remove dust particles and is thrown away after use. Roll Filter is used for high velocity filtration where the used part is rolled up automatically or manually.
Fig 2: A Heating Unit Figure 3: A Cooling unit
Cooling / Heating Arrangements
Temperature control is one of the major factors in an air handling unit. The perfect temperature for human comfort generally varied between 18°C – 23°C. So, for human comfort this temperature must be maintained by an AHU. Therefore, a cooling or heating or arrangement for both purposes is provided with the AHU. Heating and cooling is, generally, done by either direct type heat exchangers or indirect heat exchangers. Direct type heat exchanger includes burning of gaseous fuel in the air stream for heating or evaporator for cooling purpose. Electrical heater or heat pump can also be used to serve the purpose. While, in indirect heat exchangers hot water or steam or chilled water in pipe line can be used. These pipe lines are manufactured from copper and fins are provided which is made from copper or aluminum. Typical heating and cooling units have been shown in figure 2 and figure 3 respectively.
Humidifier / Dehumidifiers
Another very important parameter while designing an air handling unit is humidity. A humidity comfort level in the range of 45% - 55% relative humidity (RH) should be maintained. The humidity of the air sometimes goes very low causing discomfort to the occupants during the peak season of winter or the vice versa during summer. That makes the manufacturers to think about the necessity of humidifier in air conditioning system. Figure 4 shows a pictorial view of a humidifier. The humidity of the air can be increased or decreased according to the requirement using the humidifiers or de-humidifiers. Various types of humidifier are commonly used in an AHU. Those are:
• Spray Type
Spray type humidifier has a header and spray nozzles that spray water with a pressure of 15 psi or more.
• Steam Pan Type
Steam Pan Type humidifier has a pan and a heating coil to heat up the water of the pan. The evaporation of water caused by the heating will increase the humidity level of the surrounding air.
• Steam Grid Type
Steam Grid type humidifier has tiny holes on the pipe to distribute the steam that flows through it. In this case, the water that is heated up to produce the steam to be supplied to the grid is conditioned to prevent odor being discharged to the room.
Fig 4: Humidifier
Another very important component in an air handling unit is fan or blower. The hot return air from the room is first sucked and then blows it over the cooling coil where the hot air is cooled and then that is sent to the room to be conditioned. This is done by the fan or blower arrangements in an AHU. In general, two types of arrangement of fan are there in an AHU: draw though arrangement and blow through arrangement. The return air is sucked through the filter, the cooling coil and humidifier in the draw through arrangement. While passing through the filters, humidifier and cooling coil, the air gets conditioned and then it is sent to the required location. But, in case of the blow through arrangement the fan absorbs the return air and blows it over the air filter and the cooling coil. The air then flows to the rooms to be air conditioned. The draw through arrangement is used more commonly due to its compactness. The fans that are used in AHU are basically of centrifugal types. Figure 5 shows the pictorial view of fan unit used in an air handling unit.
Fig 5: Fan unit
Difference between Air Handling Unit and Fan Coil Unit
Though both air handling unit (AHU) and fan coil unit (FCU) serves the same basic function of cooling in a HVAC system, there are few differences between those two units. Those are:
• AHU is generally bigger system than a FCU.
• AHU is more complex than FCU.
• AHU is generally used in bigger establishments
• AHU systems needs ducting whereas, FCU does not need any duct work.
• AHU system takes outside air into the system whereas, FCU just recycles air.
• AHU is used for filtering, heating or cooling and humidification or dehumidification of air while FCU just cools or heats air.
• AHU is less noisy than FCU.
Fig 6: Grills and Registers
Air Distribution System
After transmitting conditioned supply air from the air handling unit to the room, it has to be distributed to the conditioned space. So, it is very important to design the air distribution system properly. It is found that sometimes the efficiency of an air distribution system becomes low in the range of 60 – 75% due to the poor design. There is a scope of improving the efficiency upto 80% or more with proper installation of distribution system. Proper designing and installation of the air distribution system can save money up to 50 – 200$ per year. Moreover, efficient distribution system also reduces the equipment size.
Types of Air Distribution Devices
Different types of air distribution devices are being used in HVAC systems recently. These are:
• Grilles and Registers
Grilles are the outlets for supply air or inlets for return air whereas, registers are the grille with a volume control damper. Figure 6 shows the front view of a supply air grille with horizontal and vertical vanes that are basically used for deflecting airflow. Grilles have a comparatively lower entrainment ratio, greater drop, longer throw and higher air velocities in the occupied zone compared to slot and ceiling diffusers. The performance of the grilles are specified in terms of core size or core area, volumetric flow rate of air, effective air velocity, total pressure drop, throw and noise levels. They can be mounted either on the sidewalls or in the ceiling.
• Ceiling Diffusers
A ceiling diffuser consists of concentric rings or inner cones made up of vanes arranged in fixed directions. Ceiling diffusers can be round, square or rectangular in shape. Square and rectangular ceiling diffuser has been shown in figure 7. A square diffuser is the most commonly used diffuser for supplying air. The supply air is discharged through the concentric air passages in all directions. The adjustable inner cones or the deflecting vanes are provided to change the air distribution pattern. These types of diffusers are normally mounted at the center of the conditioned space and those can provide large entrainment ratio and shorter throw for conditioned spaces with low head space.
• Slot Diffusers
Slot diffusers are made up of plenum box with single or multiple slots and air deflecting vanes. These types of diffusers are mounted on either side of walls or in the ceiling. Linear slot diffusers are mounted on the sidewalls of the conditioned room. These are used for supplying both supply air and return air. These diffusers are particularly suitable for large open-spaces as long as 30 meters in length that require flexibility to suit changing occupant distribution. Figure 8 shows the photograph of conditioned space with linear slot diffusers mounted on the ceiling.
Fig 7: Ceiling Diffuser
Fig 8: Slot Diffuser
Types of Air Distribution Systems
Two types of air distribution system are commonly used for maintaining the room conditions. Those are:
• Constant Volume
Constant-volume systems are operated at a constant airflow rate and only the temperature varies to maintain the zone set point. Constant-volume units can be used in single-zone or multi zone applications. This type of system can use single duct or dual duct for the distribution of air. A single-duct system provides ventilation and cooling to the conditioned space. If heating is required then a heating unit is provided in the terminal unit or a separate system for heating may be introduced. A dual-duct system can distribute both hot and cold air by using a single fan to move air through both cooling and heating coils in the air handler. The supply air can be distributed by separate duct to the desired locations depending on the zone requirements. Another constant-volume system is the multi zone unit. The multi zone unit supplies air to several zones from a centrally located air-handling unit. Individual zone requirements are met by mixing cold and warm air through dampers in the air handler. The conditioned air is then distributed to the zones via single ducts.
• Variable Air Volume (VAV)
A variable air volume (VAV) can vary the air flow at a constant temperature. In a VAV system one supply duct distributes supply air at a constant temperature. As the supply air temperature is constant, the air flow rate must vary to meet the rising and falling heat gains or losses within the thermal zone served. This type of system has many advantages over constant volume system. In this system, the supply air flow rate can be varied. The precise control of temperature, reduced compressor wear can be achieved in this system. This system consumes less power to run the system fans and makes less noise compared to constant volume system.
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