HOW TO WORK AHU

 HOW TO WORK AHU

Air handling units!

Air handling units, which usually have the acronym of AHU, are found in medium to large commercial and industrial buildings. They are usually located in the basement, on the roof, or on the floors of the building. And many large buildings will likely have a mixture of all of these. AHU will serve a specified area or zone within a building such as the  side or office areas from floors one to 10, or perhaps a single purpose such as just the building's toilets, therefore it's very common to find multiple AHUs around a building. Some buildings, particularly old high-rise buildings, will have just one large AHU which is usually located on the roof. These will supply the entire building. They might not have a return duct. Some older designs rely on the air just simply leaking out of the building. But this design is not so common any more in new buildings becauseit's very inefficient. Now it's most common to have multiple smaller AHU supplying different zones to give better control and higher quality space conditioning. Buildings are now also much more air tight, so we need to have a return duct to regulate the pressure inside the building.

 Purpose of an air handling unit!

 Air handling units  distribute air within a building. They take fresh  air from outside, and  clean this, heat it or cool it, and then they'll force it through some duct work around to the designated areas within a building. Most units will have an additional duct run to then pull this dirty used air out from the rooms, back to the AHU where a fan will discharge it back into the atmosphere. thermal energy can be extracted and fed into the fresh air supply in take to also save energy. In this very basic model we have two AHU housings for flow and return air. At the very front on the inlet and the outlet of each housing we have a grill to prevent objects and wildlife from entering into the mechanical components inside the AHU. At the inlet of a fresh air housing, and the discharge of the return air housing, we have some dampers. The dampers are multiple sheets of metal which can rotate. They can close to prevent air from entering or exiting the AHU. They can open fully to fully allow air in or out. And they can also vary their position somewhere in between to restrict the amount of air which can enter or exit. I'll also show you some examples here of real world dampers in AHUs. The one on the left has themotorized controller visible which changes a position of the dampers. After the dampers,we'll have some filters. These are there to try and catch all the dirt and the dust  from entering the AHU, and also the building. If we don't have these filters, the dust is going to buildup inside the ductwork and within the mechanical equipment. It's also going to enter the building and be breathed in by the occupants, as well as make the building dirty. So we want to remove as much of this as possible. Across each of the filter banks, we'll have a pressure sensor. As the filters pick updirt, the amount of air that can flow through them is restricted, and this causes a pressure drop. Typically, we'll have some panel filters, or pre-filters to catch largest dust particles. Then we'll have some bag filters to catch the smaller dust particles. 

 Cooling and heating coils!

These are there to cool or heat the air. This designed temperature is called the set point. If the air temperatureis below this value, the heating coil will add heat to increase the air temperature and bring it up to set point. If the air is too hot,then the cooling coil will remove heat to lower the air temperature and also reach the set point. The coils are heat exchangers. Inside the coil is a hot or cold fluid, usually something like a heated or chilled water, refrigerant or perhaps steam. 

Fan!

This is going to pull air in from outside and then through the dampers,the filters, the coils, and then push this out through the ductwork and around the building. Centrifugal fans are very common in old and existing AHU, but EC fans are now being installed and also retrofitted for increased energy efficiency. if the fan is running. If it is running, then it will create a pressure difference, and we can use this to detect a failure in the equipment and warn the engineers of a problem. We also  have a duct pressure sensor. This will read the static pressure and in some AHU, the speed of the fan is controlled as a result of the pressure in the duct. This will also very often find a variable speed drive connected to the fan for variable volume systems. Then we have the duct work which sends the air around the building to the designated areas. We'll also have some ductwork coming back, which is bringing all the used air from the building back to a separate part of the AHU. The return AHU in its simplest form has just a fan and a damper inside. The fan is pulling air in from around the building, and then pushing it all the way out of the building into the atmosphere.

Humidity control!

Pharmaceutical plant need to control the humidity of the air they supply into the building. humidity sensor measure the moisture in the air supply. This will also have a set point for how much moisture should be in the air by design. If the air's moisture content is below this value, then we need to introduce moisture into the air using a humidifier. This is usually one of the last things in the AHU. This device will usually either add steam or a spray of water mist into the air. The cooling coil can be used to further reduce the moisture content by removing more heat, but of course this will decrease the air temperature below the supply set point. If this occurs, then the heating coil can be turned on to bring the temperature back up. This uses a coil in both AHUs, and a pump circulates water between the two. This will pick up waste heat from the extract AHU and add this to the supply AHU. This will reduce the heating demand on the heating coil when the outside air temperature is below the supply set point temperature and the return air temperature is higher than the set point. The heat would otherwise be wasted as it is simply rejected to atmosphere. As the pump will consume electricity, it is only cost effective to turn on if the energy saved is more than the pump will consume. Another very common version we'll come across is to have a duct sit between the exhaust and the fresh air intake. This allows some of the exhaust air to be recirculated back into the fresh air intake to off set the heating and cooling demand. An additional damper sits within the connecting duct to control how much air can be recirculated. This is safe and healthy to do so, but you will need to ensure that the exhaust air has a low CO2 count, so we need some CO2 sensors to monitor that. If the CO2 level is too high,then the air can't be reused. The mixing damper will close and all the return air will be rejected from the building. When in recirculation mode, the main inlet and outlet dampers will not fully close in this setup because we will still need a minimum amount of fresh air to enter the building. We can use this in the winter if the return air is warmer than the outside air. And we can also use this in the summer if the return air is coolert han the outside air, respective to the supply set point temperature. This is very common in newer compact AHU. This uses a large rotating wheel. Half of it sits with in the exhaust air stream, and half of it sits with in the fresh air intake.

Another version we might come across is the air plate heat exchanger. This uses thin sheets of metal to separate the two streams of air so that they do not come into direct contact or mix at all. The temperature difference between the two air streams will cause the heat to transfer over from the hot exhaust stream,through the metal walls of the heat exchanger, and into the cold intake stream. The two air streams need to crossover for this to occur.