Characteristics of Centrifugal Blower and Its Effective Use in High Static Pressure Area

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1 Characteristics of Centrifugal Blower and Its Effective Use in High Static Pressure Area Masayuki TAKAHASHI With small fans, selecting the right fan that most fits the purpose is extremely important from the standpoint of energy efficiency, noise reduction, and weight reduction. However, the number of small fan types is large. In addition, their purposes vary widely ranging from cooling and ventilation, to air extraction. Therefore, we must select a fan that most fits its purpose. Here, we introduce one small fan type, the Sirocco fan. We will explain how the motor is able to perform with high efficiency and low noise in a high static pressure environment and how this fan is most suitable for cooling the recent high density mounting equipment. 1. Introduction With small fans, selecting the right fan (air flow method) that fits its purpose is extremely important from the standpoint of energy efficiency, noise reduction, and weight reduction. However, there are a great number of types of small fans. In addition, their purposes range widely from cooling, air blasting, and ventilation. Therefore, we must select a fan that most fits its purpose. Here, we introduce one small fan type, the Sirocco fan. We will explain its characteristic of high efficiency and low noise in a high static pressure environment and its appropriateness for cooling recent high density mounting equipment. 2. Types and Characteristics of Small Fans 2.1 Types Small fans can be divided into three types by the air flow method; propeller fan, sirocco fan, and cross flow fan. The structure and characteristics of these fans are given in the Table 1. Fan Type Structure and Characteristics The propeller (blade vanes) in the circular flow path between cylindrical hub and housing compresses and sends air and Propeller generates air stream in the direction of rotation axis. Fan Because air flows along the rotation axis, this type has (Axial) compact structure. In addition, the fan's capacity to obtain great airflow makes it suitable for ventilation/cooling an entire section inside machine. The centrifugal force created by the runner (forward curved vanes) aligned in cylindrical form creates spiral flow almost Sirocco Fan perpendicular to the rotation axis. The scroll rectifies the spiral (Centrifugal flow in one direction and thus increases pressure. with multiple This fan limits the opening where air can come out and unidirectionally concentrates airflow. These characteristics lead the fan to be used for blades) local cooling. With its high static pressure, this fan is also suitable for cooling in ill-ventilated system or for air flow using ducts. Cross Flow Fan (Through flow, Cross flow) Table 1 Fan Structure and Characteristics This fan has a runner similar to that of sirocco fans. However, two sides of the runner are covered with side plates and therefore, no air flows in from axial direction. This creates through flow that passes through the runner. The cross flow fan uses this through flow. Air is sent by the long cylindrical runner, allowing us to obtain wide airflow. The air is sent out horizontally along the runner circumference, resulting in uniform airflow. This fan is suitable for applications such as uniform cooling of circuit boards mounted in systems, or air curtain. Because of airflow bent at right angle and its rectangular parallelepiped shape, this fan can be installed at a corner of a system, making an effective use of space. Figure 1 Figure 2 Sirocco Fan Figure 3 Cross Flow Fan 1 RENGA No.162

2 2.2 Specific Speed and Efficiency Specific speed Two fans with structural similarity shown in the Figure 4 demonstrate similar performance. w1 c1 u1 w2 c2 u2 Maximum Total Pressure Efficiency max [%] Centrifugal Fan with Narrow Straight Vanes Turbo Blower Radial Fan Turbo Fan Axial Fan with Streamlined Stator Vanes 6 5 Fan with Multiple Vanes Axial Fan with Stator Vanes 4 Tubelike Axial Fan Specific Speed ns Figure 5 Relationship between Specific Speed and Efficiency (By Method) While the airflow of a fan is proportional to the third power of the runner diameter and to the rotational speed, pressure is proportional to the square of the runner diameter and to the square of the rotational speed. This can be expressed in the following equations. 3 Q 1 D 1 N = 1 Q 2 D 2 N 2 P 1 /( 1 g) P 2 /( 2 g)...(1)... (2) Q: Airflow D: Runner diameter : Density P: Static pressure N: Rotational speed g: Gravitational acceleration By eliminating D1/D2 from equations (1) and (2), we can obtain an index that does not depend on fan size.... (3) The equation (3) expresses specific rate and generally represented by ns. n s ( ) Figure 4 Fans with Structure Similarity Q 1 1/2 2 2 D = 1 N 1 D ( ) ( ) N 2 2 1/2 Q N 1 = N 1 [P 1 /( 1 g)] 3/4 2 [P 2 /( 2 g)] 3/4 Q 1 1/2 = N [P 1 /( g)] 3/4... (4) Using past design examples, we can acquire important data in designing fans such as relationship between specific rate and efficiency Relationship between specific speed and efficiency The relationship between specific speed and efficiency by the air flow method is shown in the Figure 5. The best efficiency point (hereinafter referred to as optimum specific speed) is found at ns = 2 for propeller fans and ns = 4 for sirocco fans Relationship between specific speed and airflow - static pressure characteristic In general small fans, the rotational speed N is nearly constant at around 3 r/min. The equation (4) shows that the known specific speed ns that gives high efficiency for each air flow method and constant rotational speed N determine the relationship between the optimum airflow and static pressure for each blast method. In other words, there is an operation zone that is optimum for each blast method. The relationship among the optimum ns, airflow, and static pressure for sirocco fans and propeller fans is shown in the Figure A Specific Speed n s, = 4 Maximum Efficiency for Sirocco Fan (V)5(Hz) MB14-B (V)5(Hz) MRS16-BUL (V)5(Hz) (V)5(Hz) Specific Speed n s, = 2 Maximum Efficiency for Figure 6 Relationship between Specific Speed ns and Airflow-Static Pressure Characteristic (By Method) For a sirocco fan (optimum specific speed ns = 4), the curve A represents the relationship between the optimum airflow and static pressure. From this curve, we can see that the high efficiency zone for sirocco fans is on high static pressure side. For a propeller fan (optimum specific speed ns = 2), the curve B represents the relationship between the optimum airflow and static pressure. From this curve, we can see that the high efficiency zone for propeller fans is on high airflow side. B RENGA No.162 2

3 This is because while centrifugal-flow sirocco fans are suitable for increasing pressure, axial-flow propeller fans are suitable for increasing airflow. The characteristics of sirocco fans MB14-B and, and that of propeller fans MRS16-BUL and are also shown in the Figure 6. Sirocco fans show high static pressure characteristics and propeller fans shows high airflow characteristics. In other words, both fans show characteristic that agrees with what we saw with the optimum specific speed ns. The intersection of the airflow-static pressure characteristic curve with curve A and B is the optimum operating point. The area shown with oval around the point is the recommended operation zone. 2.3 Input Airflow-Input characteristic of sirocco fan and propeller fan MRS16-BUL is shown in the Figure 7. Input [W] 12 8 Sirocco Fan To prevent such issues, we need to know the noise level at the operating point in advance. The noise level at the operating point is measured with loaded noise measuring system (Figure 8). This structure allows us to control the static pressure in the chamber with the aperture mounted to the outlet in order to measure the noise level of the fan with a load. The system is also soundproof, absorbs sound, and is vibration-proof to avoid measuring a noise level caused by other factors than the fan (such as sound reflected inside enclosure and sound emitted from enclosure due to vibration transmitted from the fan) Result of measuring loaded noise level The result of measuring loaded noise level of the sirocco fan and propeller fan MRS16-BUL is given in Figure 9. While the propeller fan shows lower noise level with higher airflow which means low static pressure, the sirocco fan shows lower noise level with higher static pressure. In terms of noise level, small fans have optimum operation zone. Propeller fans will function better at low static pressure while the sirocco fans at high static pressure Figure 7 Airflow-Input Relationship (By Method) Noise Level [db(a)] 7 6 Sirocco Fan While the propeller fan shows smaller input with higher airflow, the sirocco fan gives smaller input with lower airflow, which means higher static pressure. This is because the sirocco fan is more efficient at higher static pressure while the propeller fan is more efficient with higher airflow. 2.4 Noise Level How to measure loaded noise level The noise level given in catalogues is the one at maximum airflow and is not the noise level of the fan actually installed in a system, which means not of the fan with a load (at operating point). For this reason, the fan actually installed to a system does not yield the expected noise level. Microphone 1m Frame Sound Absorbing Material Chamber Figure 8 Loaded Noise Measuring System Aperture Outlet Figure 9 Loaded Noise Level of and Sirocco Fan The noise level of the sirocco fans arises greatly from pressure noise *1 and this results in lower noise level at high static pressure area where airflow is lower. On the other hand, the noise level of the propeller fans arises greatly from turbulent noise *2 and the turbulent noise is greater at high static pressure. 3. Fan Selection 3.1 Fan Selection Procedure There are several methods to select a fan. A fan shall be selected with the following procedure. 1. Find required ventilation flow rate and airflow from the upper limit of the temperature rise in the system, generated calorie, and required passing air speed. 2. Find pressure loss (resistance curve) of the system. 3. Calculate required airflow of the fan from the step 1 and 2, and select the best fan. 3 RENGA No.162

4 Here we skip step 1, supposing that these values are known and describe from step 2. See (1), (2), and (3) in the bibliography for the step 1 and other selection examples and methods. 3.2 Pressure Loss (Resistance Curve) If an object is placed in the air current, the object acts as resistance that blocks the air current. For example, when ventilating inside the enclosure, internal components, flow path, and the shape of outlet act as resistance. If a fan is to be used for cooling, the object to be cooled and ducts connected act as resistance. The loss caused by such resistance is called pressure loss and represented by static pressure [Pa]. Pressure loss is almost proportional to the square of air speed or airflow and therefore, it can be expressed with quadratic curve. If airflow doubles, the pressure loss quadruples. The curve that plots the relationship between airflow and pressure loss is called resistance curve. This resistance is inherent in each system. The pressure loss (resistance curve) of 35 mm and 6 mm square straight pipe ( mm in length) is shown in the Figure 1. Defining pressure loss (resistance curve) should be a key point in selecting a fan. 3.3 How to Calculate Pressure Loss (Resistance Curve) Here we describe two major calculation methods. 1. Measurement with multi-nozzle airflow measuring system (double chamber, *3 ) 2. Calculation with CFD (Computational Fluid Dynamics) Calculation with actual measurement Pressure loss of the device under test (DUT) is measured with multi-nozzle airflow measuring system. An auxiliary fan flows air into the DUT and the resulting pressure loss is measured. With this, data with highest accuracy can be obtained. Fan under Test Auxiliary Blower Nozzle Chamber A Chamber B Ps P Manometers Resistance Curve for 35 mm mm Square Straight Pipe Resistance Curve for 6 mm mm Square Straight Pipe Figure 1 Pressure Loss Compared to 6 mm pipe, 35 mm pipe has narrower flow path and thus greater resistance, resulting in greater pressure loss. The pressure loss produced by flowing air at airflow of 2.5m 3 /min is 3 Pa for 6 mm pipe while it is 38 Pa for 35 mm pipe, which is more than 1 times greater. A fan shall be selected according to data on pressure loss (Figure 1). Let's think about flowing air into each pipe at airflow of 2.5m 3 /min. As for 6 mm straight pipe, we shall look at airflowstatic pressure characteristic of fans and select a fan that generates static pressure of 3 Pa or higher at airflow of 2.5m 3 /min. With 35 mm pipe, we need a fan that generates static pressure of 38 Pa or higher at airflow of 2.5m 3 /min. Figure 11 Multi-Nozzle Airflow Measuring System Calculation with Computational Fluid Dynamics This method uses Computational Fluid Dynamics (CFD) to analyze pressure distribution in the system. We used PHOENICS (5), major CFD software, for this analysis. CFD divides the model into meshes and calculates heat equation and fluid equation for each mesh. This method is very convenient for relative comparison such as seeing change in pressure loss upon change on shape or internal structure. This method is also used when simplified equations cannot be applied due to complicated shape. However, such absolute evaluation requires verification experiment on actual model. Besides pressure loss, CFD also allows us to find airflow distribution and temperature distribution in the system. 4. Fan Selection Examples Since its shape and characteristics provide limited application, selecting a cross flow fan is relatively easy. By contrast, we need to select the right fan type, referring to airflow-static pressure characteristics and it may often be difficult to choose either a propeller fan or sirocco fan for right application. Here we will give a selection example for a sirocco fan, as well as the result of choosing a propeller fan for an application in which a sirocco fan was to be used. RENGA No.162 4

5 4.1 Selection Conditions Here is an example where a fan is to be selected for the model that supplies air via square duct (Figure 12). The duct is equipped with a square gate valve to adjust outlet air speed. 2. Calculation with CFD The result of analyzing the pressure loss generated at airflow of 2.5 m 3 /min is shown in the Figure 15. Gate Valve Figure 12 Selection Model Required airflow shall be known. Here we calculate pressure loss (resistance curve) and select a fan. 1. Required airflow 2.5 [m 3 /min] 2. Duct Square straight pipe X L42 [mm] 3. Gate valve Clearance: 25 [mm] Position: 8 [mm] from outlet 4.2 Calculating Pressure Loss (Resistance Curve) 1. Actual measurement using multi-nozzle airflow measuring system We measured pressure loss in the model with multinozzle airflow measuring system. A photo of mounted system is shown in the Figure 13, and the measurement result of the pressure loss (resistance curve) in the Figure 14. Figure 15 Result of Pressure Loss Analysis with CFD The Figure 15 is a contour map. The color in the model shows the distribution of the pressure loss and the value of the color bar (on the left of the Figure 15) corresponding to the color in the model represents the scale of the pressure loss. The maximum value of the color bar indicates the maximum pressure loss. We can see that the maximum pressure loss of 353 Pa is taking place near the air inlet. Varying airflow for more pressure loss analysis gives us a resistance curve. This resistance curve is also given in the Figure 14. The result of actual measurement with multi-nozzle airflow measuring system and that of calculation with CFD is almost identical, and thus, we can see that simulation is highly reliable. 4.3 Fan Selection Now we are to select a fan according to the resistance curve obtained by actual measurement with multi-nozzle airflow measuring system. From measurement result given in the Figure 14, the resistance curve is sharp, increasing more as static pressure increases. For this reason, we can expect that selecting a sirocco fan will be advantageous since it is highly efficient with lower noise level at higher static pressure. Figure 13 Nozzle 1. Selecting a sirocco fan To flow required airflow of 2.5m 3 /min, static pressure of 32 Pa or higher is required CFD Actual Measurement Resistance Curve 2 Units in Series Figure 14 Pressure Loss (Resistance Curve) Required Characteristic Figure 16 Selection 5 RENGA No.162

6 The characteristic of, a sirocco fan, is given in the Figure 16. We can see that this fan gives the required characteristic. 2. Selecting a propeller fan Installing two units of in serial meets the required characteristic (Figure 16). 4.4 Comparison Table 2 shows the comparison of input, noise level, size, and weight of the selected sirocco fan and propeller fans. Table 2 Difference in Input and Noise Level between the Fans Selected Fan Selected Fan Sirocco Fan 2 units in serial Operating Point Input W Maximum Airflow Noise Level db(a) Operating Maximum Point Airflow Mass kg Conclusion Along with miniaturization and technical advantages of recent systems, the pressure loss of the system where a fan is to be used tends to be higher and higher. Sirocco fans have characteristics of higher efficiency and lower noise level at higher static pressure. And we found that sirocco fans are more advantageous than propeller fans in terms of efficiency, noise level, and weight if the pressure loss is great and operating point is in high static pressure area. When selecting a fan, finding pressure loss (resistance curve) of the system and defining the operating point is essential. *1: Noise caused by impact with turbulent flow vanes *2: Noise caused by stream turbulence and swirl of turbulent flows *3: Compliant with AMCA (The Air Moving and Conditioning Association) STANDARD 21 Although the value of airflow-static pressure at operating point is around the same in both fans, the sirocco fan gives lower input and noise level. As described before, sirocco fans have lower input and noise level at higher static pressure while propeller fans have greater input and noise level at higher static pressure. Especially, while the propeller fans have lower noise level in catalogue (at maximum airflow), the value is lower with sirocco fan at operating point. The result is reversed. As is the case with this example, if the operating point is at high static pressure, selection of a sirocco fan is advantageous in terms of input, noise level, and weight. Conversely, if the operating point is at higher airflow, propeller fans will be advantageous. Reference documents (1) "Denshi kiki sekkei no tameno fan motor to souon netsu taisaku (21)", Kogyo Chosakai Publishing Co., Ltd. (2) "Koumitsudo jissou kiki ni okeru netsu taisaku" by Takahiro Ito of Oriental Motor, Internepcon Lecture Paper, (21) (3) "Koumitsudo jissou souchi ni taiou suru cooling fan no kashikoi sentaku", extract from Oriental Motor brochure (4) Concentration, Heat & Momentum Limited Website (5)"Soufuuki to asshukuki" by Takefumi Ikui and Masahiro Inoue (1986), 122, Corona Publishing, Co., Ltd. Writer Masayuki TAKAHASHI TMS Company RENGA No.162 (Japanese Original Edition): Published in October 22 Copyright 24 ORIENTAL MOTOR CO., LTD. RENGA No.162 6