Importance of AHUs in Office Environment

It is important that the AHU is operated as well as maintained with the right seriousness and as per laid down procedures. Here’s the detail guidelines.

An Air Handling Unit (AHU) is an important component of the air conditioning system as it enables large spaces to be cooled simultaneously and also aids in maintaining the Indoor Air Quality (IAQ). Each building will have a number of AHUs and the quality of the air that goes into the workspace will depend a great deal on the health of the AHU and its associated components. Another key aspect is how effectively is the AHU maintained – more often than not, this is a low priority area for HVAC system operators which lead to poor IAQ issues inside the workspace.

The CoVID 19 pandemic has put more focus on the AHU due to the concerns related to spread of the infection within the office space and hence it is even more important that the AHU is operated as well as maintained with the right seriousness and as per laid down procedures.

AHU design basics
The quantity of air that needs to be circulated to remove the sensible heat is given in Cubic Feet per Minute (CFM) and can be calculated by the formula Q= 1.1 x CFM x ΔT where Q is the sensible load, ΔT is the temperature difference between the return and supply air and CFM the volume of air to be supplied for maintaining a desired temperature. 1.1 is conversion factor catering to the atmospheric pressure the AHU operates, 1.1 being the value for sea level operations.

The design of the cooling coil of an AHU has a significant effect on the performance of the system and the effectiveness of the cooling of the space. A key contributor in cooling coil design is the velocity of the air across the heat transfer surfaces. The higher the velocity, smaller will be the coil and pressure drop is higher. Lower coil velocity has the converse effect. The fan size will depend on the velocity that is chosen, and this will in turn affect the energy usage of the AHU.

Filters are the heart of the AHU as they prevent dust and other contaminants entering the workspace and the effectiveness of the AHU depends to a great deal on the filtration system in use. Contaminants such as dust, smoke, mites etc. are termed as aerosols and their size measured in microns (micrometer). Filter performance is based on ASHRAE standard 52.1 and 52.2 and the most common filter classification in use in the industry is the MERV (Minimum Efficiency Reporting Value) rating. Higher the MERV rating (starting from 1, going up to 16), smaller the particles that the filter can capture.

Capacity Control of AHUs
The heat load within a workspace will vary during the day as on account of change in occupant levels as well as external loads such as outside temperature. Thus, a mechanism for capacity control is essential to provide uniform cooling to the work environment. This is done by using the following methods:
• Changing the amount of chilled water that is flowing across the heat exchanger by use of a bypass line between the chilled water inlet and out lines. Based on the return air or room temperature, the value on the bypass line is regulated to allow more (when room temp is high) or less (when room temp is low) water to the heat exchanger. The volume of air flow is fixed. This is the most commonly used AHU design due to its simplicity and low initial cost.
• Changing the Volume of the air entering the room by use of damper allows the temperature of chilled water to be constant. The damper opens or closes based on the return air temperature.
• BY using a Variable Frequency Drive (VFD) on the Fan motor, the CFM is varied based on room temperature. This is the most efficient way to vary capacity as the power consumption varies as a cube of the motor speed.

AHU Maintenance
Keeping the AHU healthy and effective is relatively easier as compared to more complex equipment found in buildings. Key aspects to keep in mind regarding AHU upkeep are:
• Filter health which includes both the need for clean filters and also inspection for any damages to the filtration surface. A parameter to check health of the filter is the differential pressure across the filter inlet and outlet.
• Air flow measurement helps in ascertaining the condition of the Fan as well as the filters. The CFM value during operation when compared to the design values will give the maintenance team an idea of how the whole AHU is running
• Cleanliness is very critical for AHUs as the air is going into the workspace. Thus, the HVAC team should ensure that the AHU rooms are free from dust and other contaminant and not used as storage spaces as is the case in many buildings.
• Heath check of the drive system involves checking the condition of the motor and belt.

AHU Maintenance Best Practices
Considering the thumb rules of 400 CFM per tonne air conditioning or 20 CFM per person a 100,000 sq ft, 5-floor building can have anywhere between 10 – 20 AHUs. Effectively maintaining these AHUs is critical for managing the IAQ, providing air at the desired temperature and keeping operating costs down as the AHUs are on the full duration of the time the workforce is in the office.

A few best practices in AHU maintenance that can enhance the overall utilisation of the AHU are as follows:
• Clean AHU drains at regular frequency. This helps in preventing microbiological growth.
• Provide a water point in the AHU room to enable cleaning of AHU filters. If this cannot be provided, then a space in the closest washroom should be ear marked for AHU cleaning.
• Balancing of the System should be undertaken whenever there is a change in the office layout. Proper balancing can save up to 5 – 10 percent of energy.
• CFM measurements must be conducted once a year as part of the pre annual shutdown activities of the HVAC system. This allows any defects in the system such as improper balancing, motor conditions, filter conditions etc. to be identified in advance and can be rectified during the shutdown.
• Record of the filter pressure differential should be maintained and regularly analyses to predict filter clogging before it becomes chocked.
• Identify leakages in the ducting and AHU casing to minimize losses and reduce power consumption.
• Install a VFD drive to older AHU motors to improve energy usage of the AHU.
• Calibrate the AHU linked sensors annually.
• Measure static pressure developed by the AHU annually – this will help identify faults in the motor and AHU system.
• Monitor AHU Motor current on a weekly basis to check for defective motor bearings or impeller imbalance, which can lead to higher power consumptions.

Special Considerations due to CoVID 19
ISHRAE has released guidelines for use of HVAC systems in office spaces during the Pandemic which have been widely adopted in the Industry and by government agencies. The ISHRAE guide is comprehensive and very detailed and covers various aspects of HVAC operations, including AHU operations. Some of the key considerations related to AHUs to be taken based on the ISHRAE recommendations are:
• The fresh air intake should be maximum possible.
• Filter cleaning is recommended to with 5 percent Cresol solution (containing 50 percent Cresol and 50 percent liquid soap solution). Mix 1 litre of this solution in 9 litres of water (above methodology is only for washable filters).
• AHU should be run in fan mode for 2 – 4 hours before start of operations to ventilate the workspace.
• Use of MERV 13 filters is recommended based on the AHU design and capacity.
• UVGI systems may be installed on the AHU coil side to sanitize the coil surfaces.

Conclusion
AHUs play an important role in HVAC systems and in commercial office spaces, a nonoperational AHU, even for a short duration can result in loss of productivity and lead to discomfort to the occupants. It is thus essential the maintenance staff understand AHU design as well operations in sufficient detail and the maintenance is undertaken as per the manufactures guidelines and laid down standard operating procedures. This will not only enhance occupant wellbeing but lower operating costs for the building owners.