Significance of System Effect Factor in HVAC Duct Systems to avoid excess Operational Costs in Building Management

"MENA Construction Economic & Cost Insights

In Oct 2021, the MENA region countries pledged to reduce their carbon emissions to net zero by the year 2050. As a result, all buildings must be net zero carbon by 2050.

Investment in reducing carbon footprint in buildings protects against the effects of climate change and offers benefits beyond carbon reduction such as reduced running costs/utility bills."

As a leading ventilation products manufacturing company in the MEA region, we were obliged to deliver efficient products and also responsible for evaluating Building HVAC systems configurations on the basis of system indicators established by organization standards (AMCA, ASHRAE) in which System effect factor (SEF) that accounts for the effect of conditions adversely influencing fan performance when installed in the air system.

It was an opportunity to troubleshoot the problem with consultants/contractors and explain the significance of SEF, especially for vane axial fan downstream connections without effective duct length, which will lead to an increase in the speed or static pressure of the fan to reach an operating point.

Here is the duct system that illustrates three vane axial fans connected in parallel configuration without minimum effective duct length at outlets and does not account for the SEF in terms of pressure, pascals. The consequence was fans were underperforming and velocity losses of more than 50% at the fan's outlets.

To explain this problem to consultants/contractors simply, we evaluated the SEF with the help of inhouse state-of-art tools.

When three fans run due to the plenums at the fan inlet and outlet, the System Effect loss at the fan outlet is greater than the defined fan outlet velocity. The higher outlet velocity is partially converted to static pressure (Static regain in Vane axial fans) when outlet ducts are used on fan tests. When fans directly connect to plenums or discharge directly, all the velocity energy is lost. In the current case, the fans deliver 10 m/s outlet velocity instead of 19 m/s.

When three identical fans are connected in parallel in such duct systems, the total performance of the multiple fans will be less than the theoretical sum if inlet conditions are restricted or the airflow into the inlets and outlets is not straight. Also, adding a parallel fan to an existing system without modifying the resistance (larger ducts, etc.) will result in lower than anticipated airflow due to increased system resistance.

In such cases, where a system has greater resistance to airflow than designed, an increase in fan speed or increase in pitch angle, or increase in static pressure based on AMCA 201, Section 8.1.1 System effect curves may be necessary to overcome actual system resistance. In all cases the power required increases as the cube of the fan rotational speed ratio, it is very easy to exceed the capacity of the existing motor and that of the available electrical service.

For the case study, the selected motor rating as per fan operating point was 7.5kW. Due to SEF, the static pressure increased by 100Pa for each fan to overcome system resistance and the motor rating increased to 9.2kW. So the excess operational cost of the building as per below:

So HVAC system designer should take the below precautions to prevent deficient performance.

1. Use appropriate allowances in the design calculations when space or other factors dictate the use of less than optimum arrangement of the fan outlet and inlet connections
2. Evaluate the fan performance tolerance and system resistance tolerance to determine if the lower or upper limits of the probable airflow in the system are acceptable to avoid excess operation costs.
3. Include an adequate allowance for the effect of all accessories and appurtenances on the performance of the system and the fan.

"Take into account the fact that the total cost for the building is the original capex cost plus the cost to operate it."

"Green and sustainable building practices can enhance the quality of life for occupants, leading to increased productivity, better health, and higher tenant satisfaction."