Considerations while selecting a Centrifugal Fan in a HVAC system

Centrifugal fans are the most versatile air moving equipment. The housing (volute) of the centrifugal fan accelerates the air and changes the direction of the airflow twice, that is 90⁰ before leaving the volute / housing. These fans are relatively quite and reliable. If selected properly, these centrifugal fans are highly energy efficient. The following paragraphs discuss about the considerations to be made while selecting a centrifugal fan for your HVAC application.

A. Selection consideration based on type of Impeller

Radial: These are suitable for high pressure and medium airflow requirements. Typical HVAC systems fall under this category. Even shop ventilation systems, these types of radial fans are suitable and energy efficient.

Forward curve: These are medium pressure, high airflow fans that can be used in both clean airs, ventilating and exhaust applications. Having said, when compared to backward curve (Refer below), these types of impellers have lesser mechanical efficiency.

It can be seen from the figure 1 that the orientation of the impeller vanes is along the direction of rotation.

Backward Curve: These are high-pressure, high flow, high efficiency fans. Power reduces as flow increases over the most efficient area of the system. Typical suitable for Air conditioning and ventilation systems.

It can inferred from Fig 2 that the orientation of impeller vanes is opposite to the direction of rotation.

Aerofoil: These are the highest efficiency fans, best in clean air applications. Please refer the shape of the vane, which looks like a Rain drop

B. Velocity and Pressure Requirement

To choose an appropriate centrifugal fan we need to know the required airflow for the process and the static pressure required. In general, arriving at the required air flow will not create much of a concern, where arriving at the static pressure is where concern arises. Arriving at optimal static pressure is one of the influencing factors in selecting an efficient centrifugal fan.

How to determine the optimum static pressure?

Before selecting the centrifugal fan, we need to decide on the layout of the HVAC system. Based on the airflow to be distributed to various places in a building / shop floor, we need to arrive at the layout of duct. It is important not to alter much in the ducting layout once the fans are selected and manufactured. Drastic changes in ducting layout will affect the required static pressure to a greater extent. Change in static pressure will alter the operating point, which means change in airflow (either increases or decreases) and the static efficiency of the fan.

We designers of the HVAC system generally have a tendency of adding factor of factor of safety to the calculated parameter of the fan (Flowrate of Air & Static pressure). While arriving at the static pressure, it is important not to add much of factor of safety. If suppose, the calculated static pressure of 45mm of Aqua and if we add a 30% factor of safety, the static pressure would be 59mm of Aqua. In reality (Actual after installation) if the static pressure is around 40mm of Aqua, the fan will try to give more airflow and at the same time, the efficiency of the fan will come down, which will result in increase in power than expected.

Hence key to success in arriving at optimum static pressure are.

• Arrive at the proper ducting layout before calculating static pressure!
• Do not add factor of safety (Not more than 8%) to the calculated value of static pressure.

C. Environment

 Corrosive Environment: In wet, humid environments, a stainless-steel blower is recommended, preferably with a stainless wash-duty motor.

Hazardous Environment: Consider a non-sparking blower, such as a radial or high-pressure blower, with an explosion-proof motor.

D. Air Quality / Quality of Fluid Handled

Clean Air: Consider a forward curve, backward inclined or airfoil blower for increased efficiency.

Lightly Dusty Air: Consider a radial or high-pressure blower. For very light, non-abrasive dusts, a backward inclined blower is acceptable.

Corrosive: In wet, humid, corrosive environments, you should opt for a stainless steel blower, preferably with a stainless wash-duty motor.

Combustible: If there's combustible dust or particulate in the air, opt for a non-sparking blower (Example paint booth application), such as a radial or high-pressure blower with an explosion-proof motor.

Air Temperatures: Use belt drive units with steel wheels in higher temperatures. If temperatures climb above 120°C, consider an industrial blower with a heat slinger (For dissipates heat from the fan shaft).

E. Drive arrangements for Centrifugal Fans (AMCA Standard 99-2404-03)

The drive arrangement names use these abbreviations:

SW = Single Width

DW = Double Width

SI = Single Inlet

DI = Double Inlet

In total 10 arrangements can be made with major classification of SISW and DIDW. Since the intent of the article is not to elaborate on the arrangement, it is not mentioned.

Inspection And Testing of Centrifugal Fans

It is important to carry out the following tests before purchasing the centrifugal fan.

• Impeller Wheel Balancing Certificate.
• Operating characteristics Curve showing the centrifugal fan’s performance at different values of static pressure. Testing is carried out confirming to relevant code for Centrifugal Air Blowers.
• Blower noise level at operating point.

Mounting of Centrifugal Fan

          As most of the centrifugal fans in the Air Handling Units (AHUs) are mounted over a metal structure at a height or mounted on a concrete structure at a height, the isolation vibration plays an important role. It is advisable to mount the fan fans over a rigid metal frame with Vibration Isolators with spring. This will have a better life when compared to conventional rubber mounts.

Conclusion

The optimal selection of Centrifugal fan plays a major role in deciding a HVAC system to deliver the required process parameters like temperature and Humidity and the operational efficiency. Poor selection of Centrifugal fan might satisfy the process parameters, but the cost of running or the “Life Cycle Cost” of the plant and capital expenses might go high. Hence a detailed analysis and a meticulous selection of Centrifugal fan will help in reducing the Life Cycle Cost of the HVAC system.