Air Blowers selection and calculation

1、 air blowers selection basic knowledge :

(1)Standard state: refers to the air pressure at the inlet of air blowers P = 101325pa, temperature T = 20 ℃, relative humidity φ = 50%.

(2) Specified state: refers to the air intake condition of the air blowers. This includes the local atmospheric pressure or altitude, the pressure of the inlet gas, the temperature of the inlet gas, and the composition and volume percentage concentration of the inlet gas.

3. Flow and flow coefficient of air blowers

(1) Flow: refers to the volume of gas flowing through the inlet of air blowers in unit time. Expressed in Q, usually in m3 / h or m3 / min.

(2) Flow coefficient: φ = q / (900 π D22 × U2), where: φ: flow coefficient, Q: flow, m3 / h, D2: impeller diameter, mu2: linear velocity of impeller outer edge, M / S (U2 = π D2N / 60)

4. Total pressure and total pressure coefficient of air blowers:

(1) Total pressure of blower: the difference between the total pressure on the outlet section and the total pressure on the inlet section. Expressed in PTF, commonly used unit: PA

(2) Total pressure coefficient: ψ t = kpptf / ρ u22, where ψ T: total pressure coefficient Kp: compressibility correction coefficient PTF: total pressure of blower, PA ρ: gas density at inlet of blower, kg / m ^ 3 U2: linear velocity of impeller outer edge, M / s

5. Dynamic pressure of air blowers: the pressure represented by kinetic energy of gas on the outlet section of air blowers, expressed by PD. Unit: Pa

6. Static pressure of blower: the total pressure of air blowers minus the dynamic pressure of air blowers, expressed by PJ. Unit: Pa

7. The relationship among total pressure, static pressure and dynamic pressure of air blowers is as follows

Total pressure of air blowers (PTF) = static pressure of air blowers (PJ) + dynamic pressure of air blowers (PD)

8. Density of gas at the inlet of air blowers: gas density refers to the mass of gas per unit volume, expressed by ρ, commonly used unit: kg / m3

9. The calculation formula of gas density at the inlet of air blowers: ρ = P / RT, where: P: absolute pressure at inlet, PA R: gas constant, J / kg · K. It is related to the type and composition of the gas. T: The Kelvin temperature of the inlet gas, K. The relationship between temperature and centigrade: T = 273 + T

10. Conversion of main parameters between standard state and specified state:

(1) Flow rate: ρ q = ρ 0q0

(2) Total pressure: PTF / ρ = ptf0 / ρ 0

(3) Internal power: Ni / ρ = Ni0 / ρ 0

Note: in the formula, the parameters with the bottom mark “0” are the parameters under the standard state, and those without the bottom mark are the parameters under the specified state.

11. The calculation formula of blower specific speed: ns = 5.54 n Q01 / 2 / (kpptf0) 3 / 4, in which: NS: specific speed of air blowers, important design parameters, specific speed of similar blowers are the same. n: Main shaft speed of blower, R / min Q0: flow at the inlet of blower under standard condition, m3 / s, KP: compressibility correction coefficient ptf0: Fan total pressure under standard state, PA

12. The calculation formula of compressibility correction coefficient is as follows:

KP = K / (k-1) × [(1 + P / P) (k-1) / k-1] × (PTF / P) – 1, where PTF is the absolute pressure at the fan inlet under specified conditions, PA K: gas index, and for air, k = 1.4

13. The calculation formula of air blowers impeller diameter: D2 = (27 / N) × [kpptf0 / (2 ρ 0 ψ T)] 1 / 2

In the formula: D2: impeller outer edge diameter, m n: spindle speed: R / min, KP: compressibility correction coefficient ptf0: total pressure of air blowers under standard condition, unit: PA, ρ 0: gas density at inlet of air blowers under standard state: kg / m3 ψ T: total pressure coefficient of air blowers

14. Pipe network: is connected with the air blowers, airflow through the ventilation duct and all accessories on the pipe.

15. Calculation formula of pipe network resistance: RJ = KQ2, where: RJ: static resistance of pipe network, PA

K: The characteristic coefficient of pipe network is related to the length of pipeline, the type of accessories and the number of other factors. Q: Flow of blower, m3 / S

16. Conversion relationship between common pressure units:

1 mm water column (mmh2o) = 9.807 PA (PA)

17. The approximate relationship between atmospheric pressure and sea level is: P = 101325 – (9.4-11.2) h, where: P: atmospheric pressure PA H: altitude: M

2、 Example of model selection (only one example)

Select an induced draft fan for 2T / h industrial boiler. The performance parameters of the blower and the corresponding air intake conditions at the maximum load are as follows:

Flow: q = 6800 m3 / h, inlet temperature: T1 = 200 ℃

Total pressure: PTF = 2010 PA, inlet absolute pressure P = 96000 PA

Solution: (1) flow rate per second: QS = 6800 / 3600 = 1.89 m3 / s

(2) Air density under specified conditions: ρ = P / RT = 96000 / (287 × (273 + 200)) = 0.707 kg / m3

(3) Converted to total pressure under standard state: ptf0 = PTF × ρ 0 / ρ = 2010 × 1.2/0.707 = 3412 PA

(4) Selected air blowers spindle speed: n = 2800 R / min

(5) Calculation of compressibility correction coefficient:

Kp=K/（K－1）[（1+PtF/P）((k－1)/k)－1]×(PtF/P)－1

=1.4/(1.4－1) ×[(1+2010/96000)(1.4－1)/1.4－1] ×(2010/96000)－1

=0.9926

(6) Calculate the specific speed of the required air blowers:

Ns=5.54 n Q01/2/(KpPtF0)3/4

=5.54×2800×1.89^0.5/(0.9926×3412)3/4

=48

(7) Centrifugal induced draft fan is selected and the highest efficiency point parameters of dimensionless characteristic curve of this type of fan are obtained as follows:

Discharge coefficient: φ = 0.1225 total pressure coefficient: ψ t = 0.536 internal efficiency: η = 0.835

(8) Calculate the impeller outer diameter:

D2=（27/n）×[KpPtF0/（2ρ0ψt ）]1/2

=（27/2800）×[0.9926×3412/（2×1.2×0.536 ）]1/2

=0.497m

(9) Check internal power:

Ni=PtFQs/1000η=2010×1.89/(1000×0.835)=4.5 KW

The reserve coefficient of motor capacity is 1.3, the mechanical efficiency of belt drive is 0.95, and the required power is 6.15kw

The selected motor is: 7.5kw-2 poles