A key component of a building or office air conditioning system is the Air Handling Unit (AHU). While there is a large amount of design considerations that go into the selection of the main chiller and the associated systems, AHUs usually do not get the same attention. The AHU with the matching ventilation characteristics is chosen, installed and put into operation. This is, however, not the best way to get the best out of the overall HVAC system of the building. Taking the case of a typical commercial building running a central water-cooled chiller, there will be 2 – 3 chillers but the number of AHUs will be many, typically, one per floor and in some cases where the floor plate is large, there may be even two per floor. Thus, the energy consumption as well as the impact on the occupants is larger for the AHUs.
In addition to the selection of the AHU, a key component of getting the right output from an AHU is the installation, commissioning and subsequently, the performance management of the unit. An incorrectly installed AHU can lead to various deficiencies such as higher noise, poor air flow, higher energy costs etc. Similarly, once the AHU is put into use, there occur changes in the space that the AHU ventilates. This could be due to changes in the space usage, increased occupancy etc and thus, the AHU then operates at different conditions from the design. The performance will, thus, get impacted leading to poor or improper air flow rates as well as higher energy costs.
AHUs are critical to the efficient operation of the overall HVAC system and the O&M team needs to pay special attention right from the installation stage up to the day to day operations. This article describes the key factors that impact the performance of an AHU and the approach to exploit the unit in the most optimal manner.
Overview of AHUs
Double skinned AHUs, with insulated panels and sophisticated measurement and control systems are now standard in the grade A building that we see. The key components are the supply fan which circulates the air, filter, the fan motor, the cooling coil for heat transfer from the air to the chilled water, damper and the casing. AHUs are usually of the draw through type where the fan is after the filter and cooling coils. Approximately 15 per cent of the energy used in a building is consumed by fans and hence, it is important that the AHUs are not only specified correctly but also installed and operated in the right manner.
Installation Aspects of AHUs
This is the start point of a good AHU operation. The AHUs is typically installed towards the end of the construction phase and in most of the cases, in a hurried manner resulting in physical damage to the coils or the filters. At times, due to poor planning, the AHU room does not have enough space for a maintenance engineer to move freely or allow easy access to the inside of the AHU for maintenance as well as monitoring activities. Some of the key aspects to keep in mind when installing AHUs are:
- Vibration insolation devices should be correctly positioned
- Access doors should be accessible
- Motor should be safeguarded from the water in the air which can lead to condensate settling.
- Drain points and the slope is very critical to prevent micro biological growth in the unit
- Correct alignment of the Fan and motor
- Belt tension
- Clean the filters and coils before go live
Testing and Balancing (TAB)
A planned and robust TAB cannot be over emphasised. Most problems occur due to the installation team not carrying out a full testing of the unit and making corrections to the air flow. This can lead to long drawn out discussions between the operations team and the project implementation team as well as impact on the occupants. Poor balancing will lead to either low or high air flow at the exit points which in turn will create hot or cold spots. If the fan speed is inadequate, it will lead to wastage of energy or in cases where flow is low, the air quality will get impacted. 5- 10 per cent of energy can be wasted if the system is poorly balanced. The key aspect to keep in mind during the commissioning phase are
- Total system static pressure should be less than 500 – 625 Pa
- Air velocity should be in the range of 2000 -2400 fpm
- Adequate test openings should be provided in the AHU to allow for testing the system during the later phases of the AHU life when any changes in the system are carried out or a performance test of the AHU is needed.
Handling Changes in the Work Space
The AHU is selected based on certain design guidelines -no of people, space usage, load, static pressure, no of openings etc. In most cases, the design usage changes – sometimes very quickly at the start of the AHU life and in other instances, later in the life of the building. Since the AHU has a direct impact on occupant comfort which in turn effects user productivity, changes are required to be undertaken to the AHU by the operational team. The factors that impact the AHU performance the most are the fan speed and the cooing coil efficiency.
Checking Adequacy of Fan Speed
At the time of system design, the flow rates are designed with some safety margins in terms of the pressure drops in the ducting. When the system is put into use, if the estimation is off the mark or the space layout has changed, there will be excessive air flow at the end points which will lead to higher noise levels. The solution to this may be change the fan or motor in extreme cases. However, by calculation the new system requirements, this can be avoided. The CFM is a function of the rpm (direct), static pressure (square) and motor current (cube). Using these relationships, and the fan curves, the speed of the fan can be changed to reduce the air flow and hence, the noise.
Checking the Cooling Coil Performance
When the space load changes due to increase or decrease in the occupancy, or addition of more equipment in the space, the performance of the cooling coil should be checked to see if it is adequate to meet the requirements. This typically happens when the building use changes or there is a change in the number of people operating from the space. Cooling coil performance should also be checked if the occupants complain of the space being too hot or cold on a frequent basis. The O&M team can easily calculate the performance (cooling capacity) of the AHU by measuring the temperature of air entering the coil and temperature of water entering. Using these two parameters and the cooling capacity for a known operating point (obtained from the operating manuals), the operator can find the additional cooling capacity needed.
While there are many critical components in the HVAC system of a building, the AHU has a very important role to play in both the system operating efficiently as well as in managing occupant comfort at the desired levels. A breakdown in the chiller may not be noticed by the occupants immediately as the air flow is maintained by the AHU. However, if the air flow stops or is inadequate, there is instant occupant awareness. On the other hand, there are many factors that can affect the functioning of the AHU such as change in the occupancy, change in use of the space etc. Thus, the O&M engineer should have a thorough knowledge of the AHU system and continuously monitor the performance of the system to see that the design specifications are achieved. The AHU is a simple device and a robust planned preventive maintenance plan, enhanced with a performance assessment of the coiling coils and fan speed at regular intervals will go a long way in saving operating costs as well increase the occupant comfort.