Just as passages in the human respiratory system transfer air from the environment into the lungs, ducting systems help circulate the air within workspaces to and from the Air Handling Unit. While the technology that drives chiller plants and associated systems has advanced considerably over the last few decades, ducting systems have unfortunately not changed much in this time period. The improvements in duct design have been incremental, with minor design and structural changes that have generally not kept pace with the tremendousdevelopments in the main HVAC systems.
Duct systems, unlike the chiller and auxiliary system components cannot be easily changed and once the ducting has been installed, especially where false ceilings are used, there is not much change that can be done to the ducting layout. On the other hand, chiller pumps, chilled water pipes, values etc. can easily be changed for newer components or in case of breakdown. There is thus an enhanced need to design, install and then maintain the duct system to very high standards so that the air conditioning system functions as per the design.
This is however not the situation on the ground – duct maintenance is usually the last priority on the building managers mind. Poor maintenance of duct has impacts on not only the efficiency of the system, but also the health of the occupants and hence a high level of duct maintenance is essential.
OVERVIEW OF DUCTING SYSTEMS
Duct Types: Ducting can come in many forms – the basic galvanized steel sheet ducting which is the most common method used due to lower cost, the prefabricated and insulated ducts made from Aluminum or Fiberglass as well as flexible tubing ducting systems
Ducting System Components: Figure 1 shows the main components of a typical ducting system. Each ducting system design will vary based on the requirements of the design, layout of the workspace, cost considerations etc.
Vibration Isolators: The AHU fan generates vibrations due to imbalance, misalignment or other mechanical faults which can get transferred to the ducting system and cause ducting system joint failures. The vibration isolators are flexible sections just after the AHU in the supply line and just before the AHU in the return line.
Dampers: To allow the operator to vary flow for modifying temperature or for air balancing purposes, dampers are installed at selected locations. The Dampers control the volume of air that flows to a work space. Modern dampers are electrically controlled using system input parameters in the Variable Air Volume (VAV) type of systems.
Diffusers: The conditioned air exits the duct work through diffusers. The diffusers help is distributing the air and giving it maximum travel and also minimize the noise as air passes across a varying diameter orifice.
Duct Installation Standards
The most common standard used for duct design is the Sheet Metal and Air Conditioning Contractor’sNational Association (SMACNA) which lists various guidelines and standards covering aspects such as duct thickness for pressure classes, reinforcement spacing values etc.
A key parameter that impacts the Operations and maintenance team is the integrity of the system – the amount of leakages that are there in the system. The SMACNA standards allow a 5% air volume leakage. The Energy Conservation Building Code (ECBC) also lays down guidelines1 (Figure 2) for ductwork sealing.
Duct Testing: The most common standard used for testing the ducting system is the SMACNA performance Duct performance test standards. It has four main criteria – Burst Pressure capacity, Collapse Pressure capacity, Wall deflection measurement and Leakage testing. Another standard used for leakage testing is the DW 144 standard (Adopted by Building Engineering Services Association- BESA) which classifies the leakage percentages based on the class of ducts. Duct classification is a function of the operating pressure in the system. Class A systems do not require testing and 6% leakage is permitted, while Class B system need to have 10% of duct work tested and 3% leakage is permitted and so on. The Indian standard for Ducting IS 655 does not specify any testing and works on the principle that is the ducting is designed as per the standards, then the system will function as per design.
WHY DUCT MAINTENANCE IS IMPORTANT AND NECESSARY
Ducting systems are in operation continuously, and there is no “back up” of the duct as other equipment and systems have. The duct systems transport air to the occupants of the building and hence impact the wellbeing of all the users of the work space. Ducting system is thus critical for business operation in any office or workplace and the following are the key reason for the building maintenance team to keep duct maintenance a top priority
Contamination of Air: The key types of contamination that can occur in the ducting system are:
Dust contamination: The most common duct systems are the Galvanized ducts which are fabricated onsite. The construction site is full of dust and construction debris in the environment. All this dust settles in the ducts during the fabrication. While basic duct cleaning is undertaken in many duct installation works, the dust is difficult to fully take out and there is re accumulation of dust on the duct walls from the workspace dust, AHU dust etc. This dust eventually travels into the work space and impacts the air quality of the work space.
Microbiological Contamination: Poorly maintained ducts will have growth of mound on the inside surfaces of the ducting. There would be telltale signs of mold formation at the diffusers where black streaks will be visible. The moldhas harmful bacteria that pass onto the work spaces and clause allergies and health related issues to the occupants.
Leakage of air: Dust systems are designed for a small amount of system volume leakage as the joints of the ducts are not air tight. Higher levels of leakage will result in higher energy usage by way of the AHU fan running longer and faster to maintain the static pressure in the ducting. Leakages can also impact the air balancing of the system which in turn will lead to hot and cold spots, further impacting the overall system efficiency.
Lowering of System Efficiency: Accumulation of dust and mold on the inner walls of the ducting causes the amount of friction that the air experiences to increase. This causes a drop in static pressure, resulting in the Fan running at higher speed and consequently higher energy usage.
MAINTENANCE BEST PRACTICES FOR DUCTING SYSTEMS
The dusting system is typically a reliable, low maintenancesystem in the overall building systems environment. Since there are no moving parts except the dampers, the maintained is relatively easy and not expensive. This is one of the reasons that the basic maintained of the ducting systems is often neglected by the building maintenance teams. A few good maintenancepractices for ducting systems are as follows:
Check for Leakages: Leakages can reduce air flow and increase cost of operation, so the site should be checked for leakages once in six months. This can be easily done by visual inspection method.
Air flow testing: The ducting system is usually installed with tap off points that can be used for air flow measurements. A good practice is the measure the air flow at least once a year and compares the data with the design values. Lower flow rates are indicative of higher friction or blocked dampers or air flow passages.
Duct cleaning: It is not easy to estimate the dust and microbiological levels in ducts as most of the ducting is concealed. A good practice is to have the ducts inspected at least once in 2 years by an external agency and then assess requirement of cleaning. Advanced duct cleaning systems allow for quick and reliable duct cleaning are an expense well spent.
Vibration testing: Higher levels of vibrations in the AHU as well as the system can lead to higher noise levels and also possibility of fatigue failure of ducting supports and joints. An annual vibration analysis of the key joints and fasteners will help reduce such damage.
Indoor Air Quality Tests: The best way to assess the quality of the ducting system is to carry out IAQ tests that will bring out the values of various indicators such as particulate matter, microbiological content etc. in the air. IAQ test should be undertaken at least once I six months and especially when there is a major change in internal design of additional head count uses the work space.
CONCLUSION
Ducting systems play an important role in providing conferrable air to the building occupants. Since all the occupants breathe the air from the ducting systems, any deterioration of the air quality will have a negative impact on a large part of the workforce. The sick building syndrome is a classic example of what poor duct system can lead to. Since ducts are always hidden from the user’s eyes, there is often a tendency to postpone or altogether eliminate duct maintenance schedules. This can be damaging to the air quality and also have an impact on the operating costs of the HVAC systems as the system will be operating at a higher design point and lower efficacy due to friction losses in the duct work. Thus, the building’s HVAC teams should have a well-developed maintenance plan for the ducting system and ensure that it is executed as per manufactures guidelines to have a healthy and efficient ducting system.
