Everything about the structure and use of AIR HANDLING UNITS (AHU)

Air handling units, abbreviated as AHU, are enclosed and factory-assembled equipment that includes a fan(s) and other main components according to relevant standards and one or more applications of air circulation, air cleaning, and heating, They perform cooling, air humidification, air dehumidification, air mixing, etc. in medium to large office and commercial buildings and industrial units.

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In other words, air conditioners are used to control one of the following parameters in space.

• humidity
• air movement
• air purification

Air conditioners move conditioned air to the desired spaces in a building. The role of an air conditioner is ventilation and air distribution inside a building to achieve satisfactory indoor air quality and indoor comfort conditions for residents (including temperature, humidity, and air filtration).

Air conditioners have several pieces of equipment, which include mixing dampers (which combine outdoor air with return air from the building), air filters, heating coils, cooling coils, fans, motors, and noise attenuators (installed in exhaust ducts and air distribution). These components are examined in detail below.

In air conditioners, air humidity is taken from the conditioned air by cooling coils that provide their coolness from a chiller. As soon as the conditioned air is cooled to a temperature lower than its dew point, the moisture in the air is removed. Humidifiers are installed in some air conditioners to increase the amount of humidity, although such air conditioners are rarely seen in today's high-performance buildings due to high energy consumption and high costs. The heating required by the consumer in air conditioners is often provided by heating coils that have hot water inside them from the heating source, which are usually the boilers of the building's central heating system. Electric coils have been installed in some older buildings and are obsolete in modern buildings due to their high concentration of greenhouse gas.

Physically, air conditioners have two types horizontal and vertical air conditioners. Also, in terms of performance, air conditioners generally include two types air conditioners with constant air volume (Constant Air Volume) and air conditioners with variable air volume (Variable Air Volume: VAV). The volume of incoming air in systems with variable air volume is adjusted based on the amount of cooling and heating required, and it reduces the energy consumption of the fan and the energy loss related to reheating (Reheating) when the heating and cooling system interferes. For office buildings with variable air conditioning loads based on season and occupancy, variable air volume (VAV) ventilators are much more efficient than fixed air volume ventilators if properly installed and operated. While most modern office buildings are ventilated by variable volume air conditioners, these systems sometimes operate with high energy waste due to low design efficiency, lack of proper command and operation, and poor maintenance. The final result is that many systems with variable air volume work like systems with fixed air volume and cause energy loss and discomfort to residents. Of course, one of the common ways to control the cooling or heating output from any air conditioner is to install a motorized control valve at the inlet of the cold water flow of the chiller to the cooling coil or the hot water of the boiler to the heating coil of the air conditioner. Naturally, the steering thermostat of these motorized valves is installed inside the desired spaces of the building or even the air distribution channels of the air conditioner.

The energy consumption of an air conditioner is mainly related to the supply air supply fan as well as the return air fan in larger buildings. Air conditioners also have the potential to significantly affect the energy used by other air conditioning equipment such as installed chillers, boilers, and humidifiers, as improper operation of air conditioners increases the demand for ventilation. Proper design, operation, and maintenance are essential for the efficient operation of air conditioners and related units.

Air conditioners are usually placed in the basement, on the roof, or inside the floors of a building. Air conditioners sometimes ventilate a specific area of ​​a building, such as the eastern part or floors 1 to 10 of the building, or they may be used only to ventilate the building's restrooms. Therefore, it is very common to see several air conditioners around and inside a building.

Figure 1- Placement of air conditioners in buildings

Some buildings, especially old and high-rise buildings, have only one big air conditioner, which is usually located on the roof, and this air conditioner covers all areas of the building. These air conditioners may not have a return air duct, in such old designs, they only rely on air leakage from the building. This design is not common in new buildings due to its inefficiency, and today it is more common to use several smaller air conditioners to supply air to different areas. Buildings also have very little air exit compared to the past, so it is necessary to use the return air duct to adjust the air pressure inside the building.

 What happens inside an air handling unit?

An air handling unit (AHU) ventilates the air and distributes it within a building. They usually receive fresh ambient air from the outside, clean it and heat or cool it, they may even humidify the air and then transfer it through channels to the designed areas inside the building. Usually, hygienic air conditioners have an additional duct to remove the used air from inside the rooms, which discharges this air into the atmosphere with the help of a fan. A part of this return air may be returned to the incoming fresh air to save energy.

Figure 2- A view from the inside of an air conditioner

How the air conditioner works and the equipment in it can be seen in the picture below. In the following, we will examine the performance of air conditioners and take a look at the most important components in the structure of air conditioners.

Figure 3- Schematic representation of how the air conditioner works

Housing

In the basic model of the air conditioner, there are two chambers (Housing) for the flow of fresh air and return air. Precisely in front of the entrance and exit of each compartment, nets are installed to prevent objects and animals from entering the mechanical components inside the air conditioner. As you can see in the picture below, in the absence of a net, a large amount of suspended matter may accumulate in the air inlet of the air conditioner, and this shows the importance of placing a net in the inlet and outlet of the air conditioner.

Figure 4 - A blocked mesh of the air intake chamber

One of the important points of the air conditioner body when choosing and buying is the issue of the thickness of the air conditioner body sheet and the other issue of air sealing and sealing of different parts of the body in terms of the absence of open seams for air to pass through the screws and nuts and elements of the air conditioner body.

Dampers

There are dampers at the entrance of the fresh air chamber and the return air discharge chamber of the air conditioners. These dampers include several metal plates that can rotate and change angles. They can be set or even closed based on the damper motor command or manually to prevent the entry or exit of air, also the dampers can be fully opened to allow maximum air entry or exit. Dampers in the intermediate state between closed and open also can change their position to limit the amount of air entering or exiting.

Figure 5- Schematic representation of dampers in air conditioner

Filters

Different filters are built into each air conditioner. Air conditioner filters are designed to trap and absorb all the pollution and dust, etc., and prevent them from entering the air conditioner and the building. If these filters do not exist, dust enters the duct network and mechanical equipment and damages them in the long run. It also eventually enters the building and, in addition to polluting the environment inside the building, it is transmitted to the residents through breathing, so this should be prevented as much as possible. Along the network of filters, there is a pressure sensor. This sensor measures how dirty the filters are and notifies the engineers when it's time to replace the filters. When the pollution is removed by the filters, the amount of circulating air is limited and this problem causes a pressure drop along the filters. In the structure of the air conditioner, there are mainly several panel filters or pre-filters to trap larger dust particles. Finally, several bag filters have been placed to separate and absorb smaller dust particles.

Figure 6- Schematic display of filters in the air conditioner

Heating and Cooling Coils

Another part of the air conditioner system is the heating coils and cooling coils. These coils are installed for heating and cooling the air. The temperature of the incoming air is measured before it leaves the air conditioner and enters the duct network. To maintain the indoor comfort conditions of people inside the building, a pre-designed temperature should be considered, this pre-designed temperature is called a set point or indoor design conditions. If the indoor air temperature is lower than this value in the winter, the three-way motor valve installed in the hot water inlet to the heating coils opens the flow of hot water from the boiler to the coil to increase the temperature of the air leaving the air conditioner and it It reaches the set value and if the air temperature is too high in the summer, by increasing the cold water flow of the chiller to the cooling coils, it reduces the air temperature of the building to bring the air temperature to the set value. Coils are heat exchangers, in which cold or warm water (liquid) flows, such as water heated in an engine room boiler or cooled water in a chiller, refrigerant, or steam.

Figure 7- Schematic representation of heating and cooling coils in the air conditioner

 Fans

In the structure of the air conditioners, there is an air fan after the heating and cooling coils. The presence of this fan is necessary to suck air from the outside and pass it through dampers, filters, and coils, finally, the ventilated air is sent into the duct network of the building. Centrifugal fans are very important in air conditioners, but nowadays EC fans (Electronically Commutated Fans) are installed in air conditioners and they are also replaced to increase energy efficiency in old systems. A pressure sensor is also installed in the air outlet from the fan, this sensor registers the activity of the fan. If the fan is working, it creates a pressure difference, and this pressure difference is used to detect malfunctions in the air-conditioning equipment and warn the engineers and operators of the facilities. Also, in many systems, a duct pressure sensor is placed immediately after the fan, this sensor measures the static pressure of the duct, and in some air conditioners, the speed of the fan and the air pressure inside the duct are controlled, so in most cases and systems Variable Volume (VAV) A variable speed actuator is connected to the fan.

Figure 8- Schematic representation of the fan supplying the inlet air in the air conditioner

 Air ducts

In the structure of the air conditioner, after the ventilation fan, air channels or ducts (Ductwork) are installed, which transfer the output from the air conditioner to the designed areas inside the building. There are also channels for exiting the used air from inside the building to a separate part of the air conditioner. The calculation and implementation of air ducts for the correct transfer of forward and reverse air and the air evacuation of buildings play a vital role, and it is necessary to calculate the dimensions, the angle of change of direction, the radius of the internal arc, the speed of air passage, etc. in all The main and secondary routes of the airflow in the channels should be designed based on accurate engineering calculations using a typewriter and based on one of the calculation methods and only by experienced facility engineers.

Anti-freeze coils (Frost Protection Class)

In cold regions where the air temperature reaches the freezing point or close to it in the cold seasons of the year, a pre-heater is placed in the fresh air inlet of the air conditioner, which is usually an electric heater. When the outside air reaches about 6 degrees Celsius (42.8 degrees Fahrenheit), this heater starts working and heats the air to protect the indoor equipment from freezing. Otherwise, the internal heating and cooling coils may freeze and burst!

Figure 9- Schematic representation of antifreeze coils in the air conditioner

Humidifier

In some buildings, the humidity of the air entering the building must be controlled. For this purpose, humidity sensors are installed in the air inlet and outlet of the air conditioner to control the humidity level of the air transferred to the building spaces. During the design, a certain value (Setpoint) is considered for the humidity level of the inlet air. If the amount of air humidity is less than this value, it is necessary to inject moisture into the air with the help of a humidifier in the final stages of the air conditioning process in the air conditioner. This device usually increases the humidity of the air by adding steam or spraying water.

Figure 10- Schematic representation of the female humidity system in the air conditioner

Also, if the humidity of the incoming air is higher than the value determined in the project design, this excess humidity is reduced by the cooling coil. As the air passes over the cooling coil of the air conditioner, the cold surface condenses the moisture in the air and absorbs it. For this purpose, a drain pan is installed under the cooling coil to collect distilled water. The cooling coil can be used to further reduce the humidity by removing more heat, but this will bring the air temperature below the set temperature, and in this case, the heating coil will also be turned on to raise the temperature. Although this is practical, it is very energy intensive.

Figure 11- Schematic representation of the dehumidification process with the help of cooling coils in the air conditioner

Air Economizer

In common air conditioner models, there is a connecting duct (duct sit) between the air outlet and the fresh air inlet. The duct allows a part of the exhaust air to be returned to the fresh air intake and meet the heating or cooling demand. This work is safe and healthy, but it is necessary to ensure that the amount of carbon dioxide in the exhaust air is low, for this purpose, several carbon dioxide sensors are installed to show its amount in the exhaust air. If the carbon dioxide level is high, the air cannot be reused, so the mixing dampers are closed and all the return air is directed outside the building. In the used air return mode, the main inlet and outlet dampers are not completely closed, because in this case we also need some fresh air to enter the building. This possibility is used in the winter season when the return air is warmer than the outside air and in the summer season when the return air is cooler than the outside air and according to the set temperature of the inlet air, so there are several temperature sensors in the inlet areas. , returned and placed right after the mixing area. Some buildings require 100% fresh air and this strategy is not applicable everywhere.

Figure 12- Schematic representation of the air economizer in the air conditioner

Plate Heat Exchangers

The last equipment used in some air conditioners and we will discuss are plate heat exchangers. In this case, thin metal sheets are used to separate the two air streams so that they do not come into contact with each other or mix. The temperature difference between the two air flows causes the heat transfer from the outgoing flow to the cool incoming flow through the metal walls of the heat exchanger.

Figure 13- Schematic representation of plate heat exchangers in the air conditioner

You can also read the article COMPARISON OF TYPES OF AIR HANDLING UNIT (AHU) for more information.

Final speech

The specialized collection (Damatajhiz) (the first and most reliable authority in the country in the field of building facilities equipment and all air conditioning devices), offers you all kinds of sand filters and hundreds of other products, guaranteeing the originality of the product, valid warranty, and reasonable price. In addition, if you have any questions about choosing and comparing the brands of this product, you can get free advice from the heating equipment experts.

In Iran, the first and most famous specialized group in the field of building installation equipment, air conditioners, and all equipment for swimming pools, saunas, and jacuzzis since 2004, DamaTajhiz is a specialized group. Refer to the time of this group. For more information, you can get advice from the experts of DamaTajhiz Engineering and Production Company by calling 021-88822550.

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