Yokogawa EJA115 - Manual (Page 34)

IM 01C22K01-01E 7-4
7. OPERATION
᭿ Equivalent Air Flow Calculation
Q
o
= 0.5356 . Qn ρn . . (2)
273.15 + t
101.325+ p
Zf
Zn
Where, Qo: Air equivalent volumetric flow at 0 °C,
1 atm (Nm 3 /h)
Qn: Volumetric gas flow at 0 °C, 1 atm (Nm 3 /h)
␳n: Specific gas density at 0°C, 1 atm (kg/Nm 3 )
Zn: Compression factor of gas at 0 °C, 1 atm
Zf: Compression factor of gas at operations
conditions (t °C, p kPa)
(b) Obtain a differential pressure from the above
equivalent water or air flow using the nomograph
shown in Figure 7.5.1 or 7.5.2. In this procedure,
multiply Qw or Qo by 1000/60 to convert the flow
unit into liter/min.
(c) Select an orifice bore, taking into considerations
pressure loss, etc.
(d) As necessary, calculate Reynolds number at normal
flow rate and correct the differential pressure
obtained from the procedure (b).
᭿ Reynolds Number Calculation
Re = 354 (3)
W
D. ␮
Where, Re: Reynolds number at normal flow rate
W: Weight flow at normal flow rate (kg/h) (Note)
D: Orifice bore (mm)
␮: Viscosity (mPa·s)
Note: Determination of W
· For liquid, W=Qf· ␳f
· For gas, W=Qn· ␳n
᭿ Differential Pressure Correction using
Reynolds Number
⌬P =
2
. ⌬P
0
1
Kaf/Ka
Where,
⌬P: Corrected differential pressure
⌬P
0
: Differential pressure obtained from proce-
dure (b)
Kaf/ka: Correction factor obtained from Figure
7.5.3
For details concerning determination of differential
pressure correction using Reynolds number, pressure loss,
etc., refer to TI 01C20K00-01E.
F0713.EPS
7.5.2 Example of Calculation
F0714.EPS
Fluid: N
2
gas (Nitrogen gas)
Flow range: 0 to 25 Nm 3 /h (flow rate at 0°C, 1 atm)
Normal flow rate: 18 Nm 3 /h
Specific density: 1.251 kg/Nm 3 (specific density at 0°C,
1 atm)
Temperature: 30°C
Pressure: 100
kPa
Viscosity: 0.018 mPa·s
From Equation (2), air equivalent volumetric flow Qo
is:
Q
o
= 0.5356 Ч 25 1.251 Ч
= 18.38 Nm 3 /h = 306.3 Nl/min
273.15 + 30
101.325 + 100
A differential pressure range of 0 to 2400 mmH
2
O is
obtained from Figure 7.5.2 applying an orifice bore of
6.350 mm (where, Zf/Zn=1 is assumed).
From Equation (3), Reynolds number at normal flow
rate Re is:
Re = 354 Ч
= 6.97 Ч 10 4
18 Ч 1.251
6.35 Ч 0.018
Since the correction factor (1.00) is constant at this
Reynolds number, no differential pressure correction is
required. Consequently, the differential pressure range
is determined as 0 to 2400 mmH
2
O.