BASIC STUDY GUIDE FOR VENTILATION SYSTEM
3.4 FORCED VENTILATION
These systems comprise of fans, air distribution system, filters etc. Various types of systems are described in para 3.6. The types of fans used in ventilation systems are described in the following sub-paras and estimation of fan static pressure is covered in para 3.5.
3.4.1 Propeller fans
These fans are generally mounted on the wall and are designed for low static pressures upto 12.5 mmWG. Propeller fans are not suitable where the air is required to be distributed by an air distribution system consisting of filters, ducting, supply or return air grills, volume control dampers etc. These fans are available in the capacity range from 500 to about 23,500 M3/Hr. The capacity of 23,500 M3/Hr is at a static pressure 2.5 mm WG. The fan impeller sizes generally range from 200 to 900 mm in diameter.
3.4.2 Tube-Axial Fans
These fans are axial flow fans, which can generate static pressures up to 40 mm WG.
Special fans with variable pitch angle for the blades and variable speed can be designed for higher static pressures and air flow control, but these are more expensive and economical only for large capacities of the order of 2,00,000 M3/Hr and larger. Tube axial fans are generally specified for pressurised ventilation systems where the ventilated area is to be supplied with filtered air to maintain a dust-free environment. Tube axial fans are available in the capacity range from about 3,000 to 1,50,000 M3/Hr, with a static pressure of 5 to 110 mm WG. The impeller diameters for these capacities range from 300 to 2,000 mm or more. The fan selection shall ensure that fan motor synchronous speeds do not exceed 1,500 RPM for impeller diameters up to 450 mm and 1,000 RPM for impeller diameters greater than 450 mm.
Tube axial fans are generally noisier than propeller or centrifugal fans. These fans shall be selected for operating speeds within the speed limits given above so as to ensure that noise levels are maintained within acceptable limits. The efficiency of tube axial fans is generally lower than that of centrifugal fans and considering that the noise level also is
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generally much higher, it is recommended that tube axial fan static pressures be limited to 40 mm WG.
A variation of this fan configuration is the roof extractor, which is primarily a tube axial fan provided with a hood for protection from the rain or sunshine. Roof extractors are available in the capacity range of 5,000 to 45,000 M3/Hr with static pressure upto 7 mm WG.
3.4.3 Centrifugal Fans
These fans are used for applications requiring fan static pressures of 40 mmWG and higher. The impellers for these fans may be of the following types:
(a) Airfoil Backward Curved Vanes
These fans have non-over loading characteristics and have the highest efficiency of all types of centrifugal impeller types.
(b) Backward Curved or Inclined Vanes
These impellers like airfoil backward curved vanes have non-overloading characteristics and efficiencies only slightly lower than airfoil vanes. These fans are relatively cheaper than airfoil impeller fans and are generally preferred over other types of impellers.
(c) Forward Curved or Inclined Vanes
These fans are generally used for smaller capacities upto about 1,000 M3/Hr or for static pressures of 75 mm WG or less. These fans are also used for AHUs. The motor rating shall be at least 15% above the power requirement at duty point or 10% higher than the maximum power requirement at selected speed whichever is higher, considering that the power consumption rises continuously from shut-off to free delivery.
(d) Radial Vanes
These fans are generally used in material handling applications or for applications requiring high static pressure of around 1,000 mm WG, e.g. fans for vacuum cleaning system. These fans are generally not used for ventilation applications.
Centrifugal fans can be Single Inlet-Single Width (SISW) or Double Inlet- Double Width (DIDW). These fans are generally belt driven by an electric motor drive.
SISW fans have a single impeller with air intake from one side of the fan casing with the fan drive arrangement located on the other side of the casing. The process gas or air handled may be ducted to the inlet of the fan; i.e. the fan may be used for systems where
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the process gas or air to or from the process or ventilated area is ducted to the fan suction. This is advantageous, as the drive motor for this fan is completely external to the fan gas circuit and can be approached for routine maintenance without exposure to the gas. The single impeller of the SISW fan necessitates a thrust bearing to take care of the thrust generated by the one-sided air intake.
DIDW fans are provided with a double impeller placed back-to-back, allowing air intake from both sides of the fan casing. As a result of this configuration, the process air or gas cannot be ducted to the DIDW fan inlet due to the drive arrangement being located on either of sides of the casing. The DIDW fans along with the belt drive and motor necessarily have to be located within the process gas or ventilated air stream. Due to the air intake from both sides of the casing, DIDW fans are balanced as compared to SISW fans, obviating the need for thrust bearings and resulting in relatively lower maintenance.
For a given air flow and static pressure the impeller diameter for DIDW fans is smaller, having a smaller height but larger floor space as compared to SISW fans. For commercially available fans, SISW fans generally have an efficiency about 5% to 20%
higher than that for DIDW fans, depending upon the capacity and static pressure and outlet velocity selected for the fan. As compared to SISW fans, which are ducted at the intake, for DIDW fans, the fan heat gain to the process air or gas may be as much as 18%
higher than the fan brake power. The break-up of this heat gain is given in the table at the end of this para.
The type of fan whether SISW or DIDW may be selected considering the above and the requirements of the particular application.
Centrifugal fans are available for a capacity range from 500 to 400,000 M3/Hr or higher, with static pressures ranging from 40 to 4,000 mm WG.
SL. NO. ITEM HEAT GAIN (%
OF FAN BKW) REMARKS
1. Fan efficiency 5* * Assumed for the purpose of this table
2. Belt drive loss to process air
3 For DIDW fans the belt drive
package, i.e. the belts, pulleys etc., and the fan motor are located in the air or gas stream.
3. Motor heat loss 10 Heat loss from motor corresponds to a motor efficiency of about 90 %
Total heat gain 18
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