Fan selection is a basic skill that wind turbine designers must master. It requires us to use some basic knowledge to select the fan that meets the conditions of use simply, quickly and accurately. Wrought Iron Balcony Railing,Wrought Iron Railings,Iron Railing,Metal Railing MINGYI METAL CRAFTS CO.,LIMITED , https://www.mingyi-ironware.com
Fan selection is a basic skill that wind turbine designers must master. It requires us to use some basic knowledge to select the fan that meets the conditions of use simply, quickly and accurately. Below, Dongguan Ruitian Electromechanical "target=_blank> Electromechanical Technology Co., Ltd. will explain a few issues that should be paid attention to during the selection process of the Reitz fan.
First, flow and pressure
The main performance parameters of the fan include flow, pressure, speed, power, efficiency and so on. When selecting the fan, the atmospheric pressure, medium temperature and gas composition under the conditions of the fan should be clearly specified.
The so-called fan flow rate refers to the volume of gas flowing through the inlet of the fan per unit time. It is expressed by m3/h, m3/min, and m3/s. M3/h is commonly used on ventilator samples and nameplates, while m3/min is commonly used in blowers, but m3/s is used in ventilator design calculations and performance calculations.
The flow rate includes volume flow rate and mass flow rate. The volume flow rate is usually measured by the volume flow rate in the standard atmospheric state. The unit flow rate is called standard cubic meter per hour (Nm3/h), or standard cubic meter per minute (Nm3/). Min).
The standard atmospheric state is defined as follows:
The state of dry air at a pressure of 101,325 Pa, a temperature of 273 K, and a gravitational acceleration of 9.81 m/s 2 is referred to as a standard atmospheric state. The density of air in the standard atmospheric state is 1.29 kg/m3.
K is the absolute temperature unit. When the fan is selected and the performance is converted, the gas temperature is in absolute temperature.
The relationship between absolute temperature and Celsius temperature is as follows:
T = 273 + t
t - Celsius temperature
T - absolute temperature
This standard atmospheric state is the atmospheric state in the physical sense. In the chemical and metallurgical industries, the amount of air to be treated is generally measured.
When the user gives the nominal flow, it needs to be converted into the flow under the working condition. This flow is the flow parameter required for the selection of our fan. The conversion formula is as follows:
- Working condition flow
- standard flow
t - working temperature
- local atmospheric pressure
The standard state of the inlet of the ventilator specified in China refers to the condition that the pressure is 101,325 Pa, the temperature is 293 K, the medium is air, and the relative humidity is 50%. Its density is 1.2 kg/m3. When using the specific number of revolutions, the parameters in the working condition state should be converted to the parameters under the standard state of the fan inlet, and the parameters in the standard atmospheric state cannot be directly converted to the parameters in the standard inlet state of the fan. type.
Mass flow is commonly used in transport compressors and gas turbine plants. Its unit is usually expressed in kg/s. The chemical industry often uses mass flow of gas.
When the user gives a mass flow, the gas density or gas composition should also be given. Then converted to gas volume flow:
Gas volume flow =
For normal ventilation, pressure is required to overcome the resistance of the pipe, which must be generated by the fan. The pressure of the fan is divided into static pressure, dynamic pressure and full pressure. The pressure that overcomes the resistance of the pipeline is static pressure; the form of converting the required kinetic energy into the pressure in the gas flow is dynamic pressure. In practice, static pressure and dynamic pressure are required for the purpose of air supply.
The full pressure of the fan refers to the total pressure increase given by the fan, that is, the difference between the total pressure between the outlet and the inlet of the fan. The so-called static pressure of the fan means that the dynamic pressure of the fan outlet is subtracted from the full pressure of the fan.
The pressure is generally measured by a pressure gauge. The pressure value read on the pressure gauge is the difference between the pressure of the gas at the measurement and the external atmospheric pressure, which is called gauge pressure. The actual pressure of the gas at the measurement site is absolute atmospheric pressure.
Second, density and temperature
In the selection of the fan, the density of the transport gas is often used. The density determines the pressure, which is related to the atmospheric pressure, the gas constant and the gas temperature. When both the gas density and the gas temperature are given, we can directly use the density for pressure conversion, and the temperature is used to select the material.
When the user gives the temperature of the transport gas, the average molecular weight of the gas, and the local atmospheric pressure, the gas density conversion should be performed using the following formula:
Gas density Ï = R is the gas constant
R=
- General gas constant, the value is always equal to 8314J / (mol? K)
- Average molecular weight of gas
For example, the atmospheric pressure is 101,325 Pa, the gas temperature is 20 ° C, and the gas average molecular weight is 29, and the density of the gas is calculated.
First find the gas constant R, R = J / Kg? K
Then the gas density Ï=
Third, the speed and the number of revolutions
The so-called specific number of revolutions ns is a fan that is similar in geometry to the fan, and is defined by a virtual airflow of 1 m3/sec and a full-pressure energy head expressed as a hypothetical speed of 1 m.
Ns=5.54n
Where Q - flow (m3 / s)
P——full pressure (Pa)
n——Speed ​​(r/min)
Ï——Gas density at the inlet of the ventilator (kg/m3)
When the inlet of the fan is the standard intake condition and the gaseous medium is air, Ï=1.2 kg/m3 in the formula, so it can be written as
Ns=5.54n
As can be seen from the definition of ns, in the same type of impeller, the general geometry of the impeller can be inferred from the size of ns. In order to determine the rotational speed, in the centrifugal impeller, the blade flow path width is determined according to the air volume, and the diameter of the impeller is determined according to the energy head. Thus, the blade having a smaller ns has a narrower blade width than the diameter, while the blade having a larger ns has a wider blade width. In the axial flow impeller, the ns with a large ns is an impeller having a correspondingly longer blade length than the hub. From the perspective of the impeller strength, the impeller with a narrow blade width or a short length is good, so the strength of the impeller should be considered when determining the specific rotation number ns.
Generally, the specific number of revolutions of the centrifugal fan is ns=15-80; the mixed flow fan is ns=80-120; the axial flow fan is ns=100-500.
For a certain fan, the flow and pressure (or flow coefficient and pressure coefficient) are changing as the operating conditions change. Therefore, in general, each operating point can calculate a specific number of revolutions, that is, a fan has many ratios of revolutions. However, for the sake of comparison, the specified number of revolutions of the highest efficiency point of the fan is taken as the specific number of revolutions of the fan. For example, the specific speed of the 4-72 fan is 72.
In addition, the specific number of revolutions of the ventilator refers to the specific number of revolutions when single-stage single suction. Therefore, for the double suction single-stage fan, the upper type can be written as
Ns=5.54n
Two-stage single suction fan can be written
Ns=5.54n
The specific number of revolutions can be used as a basis for the classification, serialization and similar design of the ventilator, and is therefore a very important parameter of the ventilator.
The fan speed directly affects the performance parameters of the fan. In the existing fan samples, except for the belt drive, the fan speed is the asynchronous speed of the motor, namely 960, 1450, 2900 and so on. Nowadays, due to the continuous improvement of the technical level, the rated speed of the motor is very close to the synchronous speed of the motor. Therefore, if we use the motor speed in the fan sample to select the fan, then the selected fan performance parameter error It will be very big.
For example, the current 4KW motor of about 200KW has its rated speed of 1490rpm. With the fan speed on the sample, the actual airflow difference is 2.76%, and the actual wind pressure is 5.6%. The fan standard stipulates that the wind pressure deviation does not exceed 5% under the specified flow rate. That is to say, the fan selected by us has exceeded the standard without considering the influence of other conditions.
Therefore, we should use the rated speed of the motor when selecting the type. If the ratio is selected by the number of revolutions, the number of revolutions of the motor is corrected after the selection.
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Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn