[Home]	Search:
Theme: Select one Steely Old Modern Midnight Ultramarine Swiss Chocolate Traditional

Issue : January-March 2005

Special AHU Designs Aero-acoustic technology can be used to save space, reduce energy as well as noise & vibration in design of air handling units

Dipti Datta is a 1960 graduate of Calcutta University who later studied naval architecture in West Germany. From 1980 onwards he has been president and leader of R&D in M&I Heat Transfer Products. He has concieved, researched, developed and commercialised unique air handling & distribution technologies with 30 patents to his credit.

HVAC systems are required to provide comfort, in terms of temperature and humidity, and sufficient amount of oxygen to occupants in buildings. Due to the increase in energy costs in recent years, there has been more emphasis on varying the rated capacity of a fan to match the volume of air required by the system, thus achieving a reduction in horsepower, for which various control mechanisms are currently in use. Another main concern for modem buildings is the poor Indoor Air Quality (IAQ) and noise. Therefore, air handling units are required to provide quiet operation and decreased fan/motor noise with reduced airflow rate.

• Low frequency noise
• Excessive vibration
• High energy consumption by fan motor

M&I specializes in design of air handling units, using axial, centrifugal or plenum fans that provide major benefits to address these problems.

[top]

Axial Fan Systems

Axial fans, with straight airflow, are the most efficient fans (80 to 85 % efficiency) but are considered to be noisy. A new design approach can be used for air handling systems with axial fans. This design would incorporate noise absorption and flow control within the aerodynamically designed and acoustically treated inlet and outlet components that are directly coupled to the axial fan.

Such a design would offer major benefits:

• Space savings of up to 60%
• Energy efficiency of up to 50%
• Low noise : lower than NC–38 space adjacent to the fan room
• Low vibration : less than 0.1 mil peak displacement
• Integrated system with optimum system performance

Two main components of this design are the inlet flow concentrator/silencer and the outlet regain attenuator:

1. The inlet flow concentrator is computer designed to optimize aerodynamics and acoustics, providing both aerodynamic and absorptive insertion losses. The air is uniformly guided at a perpendicular angle to the fan annulus area. A central pod is incorporated into the design, dimensionally matched to the fan hub, minimizing impact losses at the fan hub. Various internal flowshaping devices are also used to assure uniform air velocity across all coils and filters. The flow-shaping devices are also acoustically treated, thus functioning as a silencer.
2. Outlet regain attenuator is also computer designed as an efficient diffuser. The velocity of air leaving the fan, which is very high, is gradually reduced to the duct velocity using multiple diffusers, thus maximizing the velocity regain of the axial fan's annular velocity energy.

Both inlet and outlet components can be designed in various configurations, such as inline (axial) type or radial type with one, two, three or four sided inlet/outlets, thus offering design solutions for every type of site constraint.

Figure 1 details an axial fan system based on the design concept outlined above, with an inline inlet silencer. The sound data is shown in Table 1. The supply air leaving the fan is turned 90 degrees into the supply duct, through an outlet attenuator. Figure 2 illustrates a different arrangement, where the fan can be installed for vertical up flow and “outlet regain attenuator” can be placed above the fan, thus reducing the space requirement for the system.

The performance of the silencers, directly coupled to the fan versus conventional splitter silencer is detailed in Figure 3. This new design system offers a much better sound performance, especially at low frequencies, with a lower pressure drop.

The axial fan would generate minimum 5 dB less noise due to the aerodynamic system design with uniform, non--turbulent airflow into and out of the fan. In addition, special resonators can be incorporated at the fan inlet and discharge, tuned to the fan's blade passage frequency. This would reduce the pure tone peak sound emitted from the fan. Thus uniform, quiet and pleasing sound can be achieved in the occupied space.

[top]

Centrifugal Fan Systems

SISW or DIDW centrifugal fans are commonly used in air handling units. These fans are designed for angular airflow. They are also less efficient than axial fans; the efficiency is 70 to 75%, provided that a minimum two-diameter straight duct is provided downstream of the fan discharge. The maximum fan capacity is limited to 50–60,000 cfm capacity, due to the shipping limitations and the size of the plenum required to house the centrifugal fan.

Frequently, a blow-through design is required in the air handling unit, where coils and/or final filters and/or silencers are located downstream of the centrifugal fans, as shown in Figure 4. In such applications, the fan discharge is not ducted and the following optional designs are used instead:

• Install a diffuser plate across the air handling unit housing, downstream of the fan: this results in 0.6" to 1" WG pressure loss, increasing energy consumption while still not providing uniform velocity across the coils/filters/ silencers downstream.
• Install a diffuser bag at the fan discharge: the results are similar to the use of diffuser plates.

There are additional losses due to non-uniformity of the velocity across the fan discharge, when the centrifugal fan is used without a ducted discharge, further increasing the energy consumption.

An alternate design for a similar system can incorporate an aerodynamically designed diffuser within the air handling unit, directly coupled to the fan discharge, as shown in Figure 5. The fan's nonuniform discharge velocity can be gradually reduced, thus regaining fan's peak velocity pressure, and consequently reducing the energy consumption, while ensuring uniform velocity across the coils/ filters downstream. This diffuser can also be acoustically treated, thus providing significant sound attenuation without using any additional space in the unit housing.

Another innovative design is shown in Figure 6. This design incorporates acoustic treatment into the fan scroll. System components, such as filters and coils, are mounted on three or four sides of the radial inlet flow concentrator, which also doubles as the inlet silencer. A plenum fan wheel is mounted horizontally above the inlet flow concentrator. The fan scroll is formed from perforated sheet metal with acoustical media behind it. As a result, the fan scroll also acts as a very effective outlet silencer, without causing any pressure drop or requiring any additional space for the silencer. This design enables centrifugal fan systems with capacities as high as 100,000 cfm to be factory built. It also minimizes the footprint and height requirement of the unit while ensuring efficient and quiet operation without the need for any external silencers.

[top]

Plenum Fan Systems

Plenum fans are centrifugal fans without the scroll. These fans are becoming increasingly popular for use in air handling units regardless of their inefficient design (55-65 % efficiency). This is due to their lower cost, reduced size requirement in the fan plenum and design flexibility resulting from the capability to offer multiple take-offs within a short distance from the fan.

Plenum fans are noisy in the low frequency bands (16.5 Hz to 125 Hz), resulting from flow turbulence at the fan inlet and discharge. This noise is difficult to treat.

A plenum fan system, that incorporates the integration of noise absorption and flow control within the inlet and outlet of the fan, can be designed. Such a system would have superior performance to other such systems in the market. A computer designed, acoustical inlet flow concentrator section directs the entering air uniformly over the full width of the fan wheel without localized flow separation. An acoustically treated outlet diffuser is incorporated into the fan discharge, gradually decelerating the air to the discharge velocity at the diffuser perimeter. Refer to Figure 7 and Figure 8.

Aerodynamic inlet and outlet conditions thus created at the fan wheel ensure low vibration generated by the fan. Also, the noise generated by the fan is reduced by 8- 9 dB in the low frequencies. The fan noise is treated right at the source before it is discharged into the air handling unit. Also, special resonators, which are tuned to the fan's blade passage frequency, can be incorporated at the fan inlet and discharge, reducing the pure tone peak sound emitted from the fan. As a result, radiated noise from the unit housing and airborne noise at the unit intake and discharge are very low, as shown in Table 2. The sound requirements in the occupied space can be met without any additional silencers or much shorter silencers than would be required with conventional air handling units for most applications, thus reducing the air handling unit size and lowering the power consumption.

Conclusions

Today's air handling units used in building HVAC systems need to be energy efficient, quiet and space effective. Air handling units can be designs that incorporate aerodynamics and acoustics. This would provide unique engineered solutions for any type of application using axial, centrifugal or plenum fans. Such aeroacoustic technology can address most of the building’s HVAC problems, including fan capacity and energy consumption constraints, accomplished through converting unproductive velocity pressure of the fan into static pressure regain. Similarly, by reducing the noise generated by the fans using aerodynamic designs and treating the noise at the source, the indoor air quality (IAQ) requirements of modern buildings can be achieved.

[top]