How to Control Air Handling Unit (AHU) Functions to Reduce Energy Consumption & make AHU Energy Efficient.
Table of Content
The definition of air handling unit from ANSI/AHRI Standard 430-2009 states that it is "A factory-made encased assembly consisting of a fan or fans and other necessary equipment to perform one or more of the functions of circulating, cleaning, heating, cooling, humidifying, dehumidifying and mixing of air"
The AHU is used to control the following parameters of the space.
Temperature
Humidity
Air Movement
Air Cleanliness
Control of Air Handling Unit(AHU) functions to reduce Energy Consumption
The way how AHU components are controlled has a major impact on energy consumption. Some of the important control functions impacting energy efficiency are as below mentioned:
1. Air filter monitoring
In Air Handling Unit(AHU), filter becomes contaminated which causes a higher airflow resistance. If the fan capacity is not regulated then the amount of air supplied or extracted from the building will be reduced.The actual decline in air flow depends on the fan curve, however, for the difference between a clean filter and the maximum pressure drop, it can easily amount up to 40%. This obviously leads to the decline of Indoor Air Quality. If the filter is not replaced after reaching the final pressure, the decline in air flow will be even higher.
In turn, when the capacity of a fan is regulated to compensate the higher filter resistance, the fan consumes more energy to deliver the same amount of air. Energy consumption may increase by more than 1.5 times. To mitigate these effects, the contamination of air filters must be monitored to indicate the need for replacement.
For ventilation units, the filter pollution warning system must have a pressure drop indicator for visual signalling or for triggering an alarm in the control system. In the latter case, each filter must be equipped with an air pressure device that sends a signal to the control system once the pressure drop in the filter exceeds its limits.
The most common types of devices are the following:
Pressure relay: It is a cheaper option than pressure transmitter. The relay activates when the pressure drop in the filter exceeds its limit. It is the most common device for filter pollution indication in non-residential ventilation units.
Pressure transmitter: Pressure transmitter indicates a pressure drop in the filter, and therefore provides an actual pollution level to the control device.
Main advantages of pressure sensors:
Lowers maintenance costs by extending the filters' service time
Lowers energy costs by precisely indicating clogging
Indicates filter pollution in a distant control device and therefore eliminates unnecessary checks
2. Control flow rate of Air
To avoid fluctuations in the ventilation air volume due to filter contamination, the fan capacity should be step less regulated by controlling its speed. This function is also necessary to adjust airflow supplied to the building to the current demand (e.g. depending on the number of people currently in the building or office). In addition, the step less fan speed control facilitates adjustment of the airflow rate during commissioning of the system.
The most common way to control Air flow rate are the following:
Use of multi-speed or variable-speed drive fans
3. Control of internal air leakage in Energy Recovery Components
Some types of Energy Recovery Components are prone to the internal air leakage, which may result in the decline indoor air quality and increased energy consumption. The issue concerns in particular rotary heat exchangers. In general, the leakage rate depends on the pressure difference between supply and exhaust air sides. Correct AHU design enables to achieve, at start-up, a pressure difference which eliminates or minimizes the leakage.
However, this pressure difference may vary during regular operation. The AHU control system should provide an optimal pressure difference under any operation conditions.
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Air Handling Unit (AHU) - FAQ
Q: What are the components of an Air Handling Unit?
Ans: The AHU is a large metal box containing separate ventilators for supply and exhaust, heating coil, cooling coil, heating/cooling recovery system, air filter racks or chambers, sound attenuators, mixing chamber, and dampers
Q: What is AHU & FCU?
Ans: AHU and FCU are both included in the HVAC system. AHUs are usually connected to a central HVAC system whereas an FCU can function or be installed itself. Because of this, it is often the AHU that is used to ventilate an entire building whereas FCUs are used in smaller and often local spaces only.
Q: How many filters are there in AHU?
Ans: There are usually three types of filters within an air handling unit: Primary filters. Secondary filters. Final filters
Q: How is AHU air flow measured?
Ans: An anemometer, a test instrument that measures air velocity is used to determine the average airspeed in the duct. Then the average feet per minute is multiplied by the area of the duct in square feet to determine the airflow moving through the duct.
Q: How do you calculate airflow?
Ans: Air velocity or distance traveled per unit of time is most often exhibited in feet per minute (FPM). Multiplying air velocity by the area of a duct determines the volume of air flowing past a point in the duct during specified unit of time. Volume flow is typically measured in cubic feet per minute (CFM).
Q: What is AHU ventilation?
Ans: An Air Handling Unit (AHU) is used to re-condition and circulate air as part of a heating, ventilating and air-conditioning system. AHUs connect to ductwork that distributes the conditioned air through the building and returns it to the AHU.
Q: How do you check for duct leaks?
Ans: Turn your HVAC system on and return to the areas where ductwork is accessible. Check the connections between each section of duct, placing your hand over the metal. If you feel air against your hand, the connection is loose and there is an air leak. A common place for leaks is the duct joints.
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