Air Handling Units
Air Handling Units | Best Practices In Maintenance of AHUs

The modern high rise, high density buildings one sees in the large metros are able to house the teeming work force thanks to air conditioning systems, which provide thermal comfort to the occupants _ and allows them to work at high productivity levels. A key component of the air conditioning system is the Air Handling Unit (AHU), which is the most common equipment used to remove the heat load from the work space and transfer conditioned air into the floors. The AHU is an important element of the Heating, Ventilation and Air Conditioning (HVAC) system in a building as it enables large spaces to be cooled simultaneously _ and also aids in maintaining the Indoor Air Quality (IAQ).

While there are usually one or a few chillers in buildings, there are multiple AHUs supporting the chillers. This requires a higher quantum of maintenance as each AHU is required to be monitored, maintained and repaired in case of a break down. While this fact is known to building operators, in practice, it is often seen that the AHUs are not given priority when maintenance plans are developed. The basic maintenance activity of regular cleaning of AHU filters is often missed or not undertaken as per the manufactures’ guidelines. An improperly maintained AHU not only leads to poor IAQ, but also increases operating costs. There is thus a business case to have an effective AHU maintenance plan, which is in turn derived from an understanding of AHU functioning and design.

An overview of AHU

The basic function and components of a typical room AHU is shown in figure 1. The layout represents an AHU, where a portion of the room air is recirculated and some amount of fresh air is introduced in each cycle. Room air is drawn in by the suction of the AHU fan and passed through filters and a heat exchanger _ after which air is reintroduced into the room. The heat exchanger has chilled water flowing and reduces the warm air temperatures and causes cool air to go back into the work spaces, when the temperature is increased by gaining sensible heat of the space.

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 operate, 1.1 being the value for sea level operations.

AHU classification is based on the above formula – for a given space load Q, either the volume (CFM) can be varied or the ΔT is varied to cater to any change in load conditions. When the CFM is constant, the AHU is classified as a Constant Volume (CV) AHU _ and if the CFM is varied to maintain load, it is called a Variable Air Volume (VAV) system. The cooling coil of an AHU is also a critical component and its design directly affects the performance of the system. The key parameter to be considered in the cooling coil design is the velocity of the air across the heat exchange surfaces. The higher the velocity, smaller will be the coil and pressure drop is higher, which therefore requires higher powered fans. Conversely, with a lower coil velocity, the size of the coil increases but the pressure drop is lower and energy required for the fan is lower.

The filters in an AHU ensure that the air entering the work space is fee of dust and other contaminants. The type of filters to be used in an AHU is based on the quality of air required in the work space. Contaminants such as dust, smoke, mites etc. are termed as aerosols and their size measured in microns (micrometer). The ASHRAE standard 52.1 and 52.2 is the commonly used standard to rate performance of filters and the term MERV is used for classifying filtration capacity. Higher the MERV rating (starting from 1, going up to 16), smaller the particles that the filter can capture. Control of cooling capacity in AHUs is achieved by the following approaches:

•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 challenges

Since the AHU is a relatively simple system, it is often assumed that nothing can go wrong in the AHU and one need only to clean the filters and the maintenance is complete. There is however a number of other areas that needs maintenance which is equally critical and important. Table 1 lists the key maintenance requirements of AHUs.

While the maintenance plan in a building will cover most of these areas, based on the authors practical experience in operating and maintaining a large and wide variety of AHUs, the situation however is not so simple. The key challenges or reasons why effective AHU maintenance is not undertaken is as follows:

•No space for maintenance: AHUs are usually housed in AHU rooms on the floor. Since floor space is always at a premium (more the work space, more the rent!), many AHU installations are so poorly designed that there is no space for the maintenance staff to access the filters or the motor belts for maintenance.•
Provision for filter cleaning: Filter cleaning is a labour intensive task and requires space as well as water for effective cleaning. A good AHU room will have earmarked space for cleaning with a water point provided. Most AHU rooms however do not have such arrangement, and the technicians have to take the filters to a designated space for cleaning. If the technicians do not perform this task due to the physical effort required to take the filters, cleaning takes a back seat and maintenance suffers.
Faulty sensors, manual operations: It is common to see AHUs are running at the full capacity irrespective of the load as the sensors have failed, the damper is not operational or the bypass valve is malfunctioning. Since these sub systems are complex, repairs are costly and often postponed by the maintenance team as the system still functions. This results in the AHU running at off design point leading to system deterioration.
Untrained staff: One of the most important reasons for poor AHU maintenance is the lack of knowledge of the staff undertaking the maintenance as well as the maintenance planning staff. The focus is on filter cleaning and other aspects are often neglected, leading to poorly performing AHUs.

AHU Maintenance Best Practices

Considering the thumb rules of 400 CFM per ton 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 wash room 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 to 10% 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 shut down.
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 minimise losses and reduce power consumption.
Install a VFD drive to older AHU motors to improver 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.

Conclusion

AHUs are an important part of the HVAC system in a building, and hence require a high level maintenance and upkeep. A poorly maintained AHU leads to higher energy costs, higher pollutant levels in the work space and inadequate thermal comfort for the occupants.

Although the importance AHUs is understood by the designers and maintainers, AHUs are often neglected as the system is relatively simple and delivers cooled air, even if the system is degraded. Basic aspects such as space for maintenance is often neglected when designing AHU installations, leading to poor maintenance.

The cost of neglecting AHUs is substantial, and building maintenance planners and staff need to give a higher focus on AHU maintenance _ and operation than is currently being delivered. This will not only ensure better air quality for the occupants, it will also lower the owners operating costs due to the lower use of energy in the AHU operations.


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