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局部排氣導管設計

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局部排氣導管設計. 基本假設. 非壓縮性流,流體密度不變,質量守恆→風量(流量)守恆 導管無洩漏. 造成靜壓變化的因素. 管壁摩擦(同時造成全壓損失) 紊流(同時造成全壓損失) 流速改變(不造成全壓損失). 管壁摩擦. 紊流壓損. D P S = F P V < 0. 流速改變. 靜壓下降. 全壓幾乎不變. 導管開口(含氣罩)的壓力損失. 風速: 0 -> 導管風速 動壓:增加 若無其他損失: 靜壓下降:且下降量 = 動壓增加量 全壓不變 考慮摩擦與紊流損失 全壓下降 靜壓下降,且下降量 > 動壓增加量. 若無摩擦與紊流損失

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slide2

基本假設

  • 非壓縮性流,流體密度不變,質量守恆→風量(流量)守恆
  • 導管無洩漏
slide7

造成靜壓變化的因素

  • 管壁摩擦(同時造成全壓損失)
  • 紊流(同時造成全壓損失)
  • 流速改變(不造成全壓損失)
slide14

紊流壓損

DPS= F PV < 0

slide17

流速改變

靜壓下降

全壓幾乎不變

slide19
導管開口(含氣罩)的壓力損失
  • 風速:0 -> 導管風速
  • 動壓:增加
  • 若無其他損失:
    • 靜壓下降:且下降量 = 動壓增加量
    • 全壓不變
  • 考慮摩擦與紊流損失
    • 全壓下降
    • 靜壓下降,且下降量 > 動壓增加量
slide20

若無摩擦與紊流損失

Pv = 25 mmH2O

Ps = -25 mmH2O

Pt = 0

考慮摩擦與紊流損失

Pv = 25 mmH2O

Ps = -40 mmH2O

Pt = -15 mmH2O

v = 20 m/s

Pv

Ps = 0

Pv = 0

Pt = 0

Pt

Ps

slide21
描述氣罩壓力損失的方法
  • 壓損因數(Entry Loss Factor,Fh)
  • 進口係數(Coefficient of Entry,Ce)
slide22
氣罩壓損係數
  • 氣罩後方 Pt = -FhPv or DPt = FhPv
  • Ps = Pt –Pv = -FhPv –Pv = -(Fh + 1) Pv or DPs = (1 + Fh )Pv
  • 1 :加速因數
slide23

若無摩擦與紊流損失

Pv = 25 mmH2O

Ps = -25 mmH2O

Pt = 0

考慮摩擦與紊流損失

Pv = 25 mmH2O

Ps = -40 mmH2O

Pt = -15 mmH2O

Fh = -Pt/Pv = 15/25 = 0.6

or = -Ps/Pv – 1 = 40/25 – 1 = 0.6

v = 20 m/s

Pv

Ps = 0

Pv = 0

Pt = 0

Pt

Ps

slide24
氣罩進口係數
  • Ce = 氣罩風量/無壓損之流量 =Q/Qmax
slide25

若無摩擦與紊流損失

Pv = 25 mmH2O

Ps = -25 mmH2O

Pt = 0

考慮摩擦與紊流損失

Pv = 25 mmH2O

Ps = -40 mmH2O

Pt = -15 mmH2O

v = 20 m/s

Pv

Ps = 0

Pv = 0

Pt = 0

Pt

Ps

slide26

氣罩前後壓力變化

Fh

Pv

Ps = (1+Fh) Pv

Pt = FhPv

Ce

slide35

如何決定壓損係數

  • 自行測試(壓力計,建議採用)
  • 廠商提供資訊
  • 經驗公式或圖表(最後手段)
slide36

風量

風速

DPt = K×Q2

v = Q/A

Pv = rv2/2

DPt = F×Pv

動壓

slide37

局部排氣導管設計基本要求

  • 每一氣罩風量須高於需求值
  • 每一導管風速須高於搬運風速
  • 其他考慮因素:
    • 導管內物質濃度
    • 風速上限:靜電、管壁摩耗、物料損失
    • 出口排放風速
    • 排氣機進出口管徑
slide38

如何決定氣罩風量

  • 法令要求:控制風速與抑制濃度要求
  • 作業人員個人採樣
  • 煙霧測試或追蹤氣體
  • 方法:
    • 測試(建議採用)
    • 圖表或經驗公式
slide39

log(氣罩設計風量)

log(氣罩風量)

log(容許濃度)

log(個人採樣濃度)

slide40

如何決定搬運風速

污染物

愈濕黏

搬運風速

愈大

slide41

導管設計主要目的

  • 決定各導管管徑
  • 決定排氣機性能要求
slide42
例題
  • 具單一氣罩之局部排氣裝置
  • 氣罩風量:8.5 m3/min
  • 氣罩壓損係數:Fh = 0.85
  • 氣罩至排氣機導管:
    • 管壁摩擦因數:f = 0.024
    • 長度:11.6 m
    • 搬運風速:23 m/s 以上
    • 含有一肘管(Fe = 0.22)
  • 排氣機至出口導管
    • 長度:2.4 m
    • 直徑:10 cm
slide43

0

Pt0 = Pv0 = Ps0 = 0

Ps4 = 0

2

3

4

1

2.4 m

11.6 m

計算方向

計算方向

空氣密度r = 1.2 kg/m3

氣罩風量需求:Q = 8.5 m3/min

氣罩壓損係數:Fh = 0.85

肘管壓損係數:Fe = 0.22

排氣機前導管最低搬運風速:vmin = 23 m/s

排氣機後導管管徑:10 cm

導管摩擦損失因數 f = 0.024

slide44
0-1
  • Q = (8.5 m3/min)(1 min/60 s) = 0.1417 m3/s
  • 導管截面積最大為:Amax = Q/vmin = (0.1417 m3/s)/(23 m/s) = 0.006159 m2
    • 若導管截面積大於此值,導管內風速會小於規定的 23 m/s
  • 導管最大管徑dmax = (4Amax/p)1/2 = [(4)(0.006159 m2)/p]1/2 = 0.08856 m
    • Amax = pdmax2/4
slide45
0-1(續)
  • 若導管管徑只能使用 2、3、4、…cm:取管徑d1 = 8 cm = 0.08 m
    • 若以四捨五入取d1 = 9 cm > dmax,會使導管內風速小於規定的 23 m/s
    • 若導管管徑可以訂做(自由選取),則令d1 = dmax = 0.08856 m
    • 若管徑d1已事先給定,則無需上述步驟
slide46
0-1(續)
  • 以管徑d1 = 0.08 m 繼續以下計算:
    • 導管截面積:A1 = pd12/4 = p (0.08 m)2/4 = 0.005027 m2
    • v1 = Q/A1 = (0.1417 m3/s)/(0.005027 m2) = 28.18 m/s > vmin = 23 m/s
    • Pv1 = rv12/2 = (1.2 kg m/s)(28.18 m/s)2/2 = 476.6 Pa
      • 在 20°C 時,若要求單位為 mmAq,則可使用Pv1 = (28.18/4.04)2 = 48.7 mmAq
slide47
0-1(續)

氣罩後方導管動壓

  • 氣罩壓損:DPth = FhPv1 = (0.85)(476.6 Pa) = 405.1 Pa
  • 氣罩後方(點 1):
    • Pt1 = 0 – DPth = -405.1 Pa
    • Ps1 = Pt1 – Pv1 = -405.1 Pa – 476.6 Pa = -881.7 Pa
      • 或Ps1 = -(1 + Fh) Pv1 = (1 + 0.85)(476.6 Pa) = -881.7 Pa

acceleration factor

slide48
1-2
  • 導管壓損:f(L12/d1)Pv1 = (0.024)(11.6 m)/(0.08 m)(476.6 Pa) = 1659 Pa
  • 肘管壓損:FePv1 = (0.22)(476.6 Pa) = 104.9 Pa
  • 1-2 的總壓損DPt1-2 = 1659 Pa + 104.9 Pa = 1763 Pa
  • 通式:DPt1-2 = (fL12/d1 + SF)Pv1 =[(0.024)(11.6)/(0.08) + 0.22](476.6 Pa) = 1763 Pa = DPs1-2

導管上配件壓損係數總和

導管摩擦壓損係數

A1 = A2,v1 = v2,Pv1 = Pv2,DPt 1-2 = DPs1-2

slide49
1-2(續)
  • 排氣機入口(點 2):
    • Pv2 = Pv1 = 476.6 Pa
    • Pt2 = Pt1 – DPt1-2 = -401.5 Pa – 1763 Pa = -2168 Pa
    • Ps2 = Pt2 – Pv2 = -2168 Pa – 476.6 Pa = -2645 Pa
      • 或Ps2 = Ps1 – DPs1-2 = -881.7 Pa – 1763 Pa = -2645 Pa
slide50
4-3
  • 排氣機後方導管:由出口往前算
  • 排氣機後方導管管徑已給定:
    • d3 = d4 = 10 cm = 0.1 m
    • A3 = A4 = pd42/4 = pd32/4 = p (0.1 m)2/4 = 0.007854 m2
    • v3 = v4 = Q/A3 = Q/A4 = (0.1417 m3/s)/(0.007854 m2) = 18.04 m/s
    • Pv3 = Pv4 = rv32/2 = rv42/2 = (1.2 kg/m3)(18.04 m/s)2/2 = 195.2 Pa
slide51
4-3(續)
  • 出口(點 4)
    • Ps4 = 0(大氣)
    • Pt4 = Ps4 + Pv4 = 0 + 195.2 Pa = 195.2 Pa
  • 導管 3-4 壓損:
    • DPt3-4 = fL34/d3Pv3 = (0.024)(2.4 m)/(0.1 m)(195.2 Pa) = 112.4 Pa = DPs3-4

A3 = A4,v3 = v4,Pv3 = Pv4,DPt 3-4 = DPs3-4

slide52
4-3(續)
  • 排氣機出口(點 3)
    • Pt3 = Pt4 + DPt3-4 = 195.2 Pa + 112.4 Pa = 307.7 Pa
    • Ps3 = Pt3 – Pv3 = 307.3 Pa – 195.2 Pa = 112.4 Pa
      • 其實就是DPs3-4,因為Ps3 = Ps4 + DPs3-4 = 0 + DPs3-4
slide53

Pv

Pt

FTP

Ps

FSP

ftp fsp
FTP 與 FSP
  • 排氣機全壓(FTP) = 排氣機出口全壓 – 排氣機入口全壓 = Pt3 – Pt2 = 307.7 Pa – (-2168 Pa) = 2476 Pa
    • 用來計算耗電量
  • 排氣機靜壓(FSP):
    • = FTP – 排氣機出口動壓(Pt3 – Pt2 – Pv3)
    • = 排氣機出口靜壓 – 排氣機入口全壓(Ps3 –Pt2)= 112.4 Pa – (-2168 Pa) = 2281 Pa
    • 排氣機標準規格(可以提高多少壓力)

FTP

slide55
排氣機耗電量

總效率

h

P = (0.1417 m3/s)(2476 Pa)/0.4 = 877 W = 0.877 kW

每運轉一小時耗電 0.877 度

slide56
系統需求曲線

FSP ≈Q2

設計點:

FSP = 2281 Pa @8.5 m3/min

排氣機風量