To improve filtering, heating, ventilation, and air condition (HVAC), cross contamination, expenses, and general population health, airflow systems and duct work must be given adequate attention upon installation and during periodic servicing. Here are 10 steps to improve your operations:
# 1 – The lack of reinforced ductwork does not meet standard pressure specifications.
Most facilities’ duct construction including duct accessories and air handling equipment do not meet the minimal industry standards put forth by The Sheet Metal and Air Conditioning Contractors National Association (SMACNA). In some cases, the design engineer may have implemented the project design given the current standards but by the time the facility was constructed the standards may changed.
The SMACNA standards are widely used, accepted, and specified in project documentation. Most tradesmen will design and fabricate ductwork they way in taught a few decades ago based on the older standards. Today, there is a lack of clarification by designers and craftsmen for incorporating the appropriate pressure specifications so that the ducts do not collapse or blow out. Balancing and air flow measurements are crucial to ensure that the design adheres to industry standards and for the verification. You can get more information from industry expert such as Phoenix Controls.
# 2 – Failure to seal test ducts adequately.
Air leakage from ducts which are not properly sealed may exhibit some obvious signs such as little to no airflow at the end of a run of duct. Air may leak out at duct seams and joints and the only correction required may be to seal them using aluminum tape or similar appliqués. Some preferential seam type may include the Snap Lock type, slip, drive, or TDC connector joints. Also, where rods or damper connections are made, an inspection should be made and leaks verified.
Another concern is the bend angles and routing of ducts. As a rule, there should be no sharp turns or angles and, assuming that duct size has not changed, air pressure should remain almost constant. Keep in mind that air leaks out at the path of least resistance or towards ambient conditions.
# 3 – Failure to verify sealed return and exhaust ducts.
Air will leak into the ductwork through the same openings as there are for outwards leaks. In an ideal world, the supply and exhaust ducts should be equal as well as their air volume and flow. Typically, this is not the case because exhaust air flow may be at 15% at the fan which is over designed and the exhaust flow such as a hood location can be 20% under design. An inspection and verification of all seals and joints regardless of air leaking into or out of should be performed.
# 4 – Lack of pressure tested ductwork for verification of effective ducts and joints.
Industry standards have defined the leakage rates for class 24, 12, and 6. What this means is that at 1 inch of test pressure, the ducts are permitted to leak 24, 12, or 6 cubic feet per minute (CFM) per square foot of duct surface. The pressure test determines how well the ducts are constructed or not and the amount of leakage that exists.
# 5 – Failure to understand duct standards in the design and installation process.
While standards are used as a project reference it has been shown that proper techniques are not understood by either engineers or installers. Design engineers often do not allow sufficient room to install proper sized transitions or offsets. In many cases when there is insufficient room, the craftsman may overlook the applicable standard and install transitions or offsets which are do not have proper slope or transition points. The entire system has imposed restrictions that the fan must overcome when standards for fittings are not followed.
# 6 – Failure to properly install turning vanes.
Air flowing in ducts has the same restriction whether it is flowing away from the fan or flowing to the fan. When the air velocity is greater than 1000 feet per minute (fpm), vanes are required. A misconception seems to exist in the industry that implies that return ducts and exhaust ducts do not require turning vanes – this is a myth.
# 7 – Noisy ducts and increased pressure drops for velocities approaching 2500 fpm.
Duct velocity can be calculated by dividing the design air flow in a duct at any location by the area of the duct in that location. When velocities are over 2500 fpm, it is good practice to notify the engineer and voice your concerns. It is much easier to solve a potential problem by increasing duct size before the duct is installed. If potential problems are addressed and corrected before they are found during the test and balance phase of the project, the design engineer and installing contractor cost overruns and unnecessary expenditures can be avoided.
# 8 – The impact on fan capacity caused by fan system effect (not understood by industry).
The fan system will suffer a reduction in capacity whenever installations differ from the design. The airflow leaving a fan does not reach a uniform flow profile for several duct diameters from the fan discharge. The distance required to reach uniform flow is dependent of the velocity of the air. The higher the duct velocity, the greater the distance required before this uniform flow is attained. System effect is the fact that fans are tested with a specific amount of straight duct connected to the inlet or discharge.
# 9 – Outdated drawings.
There are variances regardless when the airflow system is installed, modified, changed, or an expansion is performed when compared to the original design. Undersized ductwork may be installed, the VAV box inlets may have duct sizes omitted from the drawings and the inlet sizes may not be recorded on the drawings
# 10 – The VAV box and insufficient room to allow for this installation.
When sufficient lengths of straight ducts are not installed, the air flow sensors on a VAV box do not read properly. This causes the VAV box to hunt and cause the VAV controller to be hard to control. Unstable control can cause noise and cause the fan to operate at higher static pressures than necessary.