The Role of Duct Design in Efficient HVAC Systems

The Role of Duct Design in Efficient HVAC Systems

Industrial HVAC Systems, whether it is Air conditioning or Ventilation system or an centralized fume exhaust system play a pivotal role in maintaining the comfort and safe working environment of a factory. Although the Energy efficiency of the HVAC system or the centrifugal fan greatly depends on the quality of the machine, the design of ducts plays a key role in determining the efficiency of the centrifugal fan or the HVAC system as such.

Who is responsible for the energy efficiency of a centrifugal fan which are used in HVAC and Industrial Ventilation system; whether it Fan or the man who designed it? The answer “Neither of the two” is true to a very large extent. Such is the importance of duct and in turn in the well-thought-out duct design in a HVAC system. How? Let us discuss this in the Following paragraphs.

Imagine driving a car on a road with countless intersections and detours, would you reach your destination efficiently? Similarly, a poorly designed ducting system can create unnecessary twist & turn leading to increased resistance or Static pressure. The increase in static pressure will alter the flow rate and in-turn the efficiency of the fan.

fan.

Design Considerations:

Air flow Requirements: Arriving at the optimal air flow is critical for designing the duct work. Accurate airflow calculations help determine the ideal size and layout of the ducts, preventing issues like inadequate air supply, excessive noise, or uneven temperature distribution.

Pressure Loss: Minimizing pressure loss is essential to maintain efficient airflow. Properly sized ductwork, smooth transitions between duct sections, and well-designed fittings and joints can significantly reduce pressure losses, improving system performance and reducing energy consumption.

We design engineers are habituated to add factor of safety. While designing or sizing the duct, considering unforeseen in the ducting layout and size, we tend to add factor of safety in arrive at the static pressure. The following real-time case study (Conducted by us) will help in understanding the impact of adding factor of safety while arriving at the static pressure.

Case Study

Industrial Air conditioning system in a tyre industry has got an AHU, whose name plate detail of the centrifugal fan is as follows.

Flow Rate Q = 40,000 cmh

Static Pressure (Stp) = 85mm of wg

The mechanical Efficiency of the Centrifugal fan at the selected point is 81% (refer Figure 1)- Selected speed of the impeller is 750 RPM

Since it being a running plant, we were asked to conduct an energy audit. Following are the measured parameters.

Actual Static Pressure of the Fan = 35mm of wg

Since the size of the duct is oversized, i.e. while designing, the actual static pressure has come down from a level of 85mm of wg to 35mm of wg. For sure this is not a one-off case in any industry.

Since the actual static pressure much less than selected static pressure of the fan, the fan will give more flow but at the same time the mechanical efficiency of the fan will drop down to 50%. This in-turn will increase the energy required to drive the fan (BKW). Refer Figure 2.

Note: Please refer the dashed line (Both Figure 1 &2) in orange and the efficiencies corresponding to that.

It is evident from the above case study that good design practices will lead to an energy efficient HVAC system.

Concluding Thought

In the empire of HVAC systems, duct design is the unsung hero that can either elevate efficiency to new heights or drag it down. An HVAC system demands the seamless coordination of its components, with duct design orchestrating the flow of conditioned air. If we want to operate a HVAC system with optimal energy, our focus should also be on the optimal design of duct and its system.