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P. 57
ction of Rayleigh number (Rad) and Prandtl number 4. Heat transfer across pipe surface is one dimensional
(Pr) which is given below: and is in radial direction.
½2 5. Downstream pipe surface temperature must be
°° above 100 oC to ensure the steam is in super heated
° 1° condition
0.387 *Rad6
Nud °®0.60 ° (8)
° ª 9 º ¾ The only drawback of the method is to remove a portion
° ««1 » ° of insulation at the downstream of the valve.
° 0.55916 » °
¯ ¬« ¼» °
Pr ¿
Rad and Pr can be obtained from: The user inputs which are required for determining the
mass flow rate are:
Rad g * E * (Tav sur Tamb ) * D3 (9) ➤ Uninsulated length of the pipe
out (10)
➤ Outside diameter of the uninsulated portion (Dout)
-*a ➤ Temperature at location 1 (T1) in °C
➤ Temperature at location 2 (T2) in °C
Pr - ➤ Ambient Temperature (Tamb) in °C
a
Where: Financial Losses
g is the gravitation acceleration The energy lost from the leakage flow is given by:
β is the thermal expansion coefficient of air
ϑ is the kinematic viscosity of air Pleak = (ho – href ) * m (12)
α is the thermal diffusivity of air
Equations to calculate the thermo physical properties of Where:
air are given in Appendix.
Pleak is the energy lost from the leakage flow
Substituting Equations (8), (9) and (10) in Equation (7) we
can obtain the convection coefficient. ho is the enthalpy of the leakage steam at source (to be
obtained from steam table)
Thus the mass flow rate of the leakage steam can be href is the enthalpy of the demineralized water make up to
calculated from Equation (4): the system (to be obtained from steam table)
m *V * A*F * (Ta4v sur T4 ) hc * A * (Tav sur Tamb ) (11) m is the mass of the leakage steam obtained from
amb Equation (11)
Cp * (T1 T2 )
The following assumptions are made in the above Cost of Excess Coal
expression:
To maintain full unit load, more fuel needs to be burned
1. The heat transfer rate across the length of un-insulated in order to mitigate the energy lost by the leakage steam.
pipe is uniform throughout the length. Energy required from excess coal is given by:
2. Expansion of steam across the valve and across the Pexcess coal Pleak (13)
un-insulated portions is an isenthalpic process. Thus Kb
any change in kinetic energy is neglected.
Amount of excess coal required is: (14)
3. Change in pipe surface temperature of the Excess coal (per hour per Kg) = Pexcess coal * 3600
un-insulated region is the change in the steam
temperature. Cv
57
(Pr) which is given below: and is in radial direction.
½2 5. Downstream pipe surface temperature must be
°° above 100 oC to ensure the steam is in super heated
° 1° condition
0.387 *Rad6
Nud °®0.60 ° (8)
° ª 9 º ¾ The only drawback of the method is to remove a portion
° ««1 » ° of insulation at the downstream of the valve.
° 0.55916 » °
¯ ¬« ¼» °
Pr ¿
Rad and Pr can be obtained from: The user inputs which are required for determining the
mass flow rate are:
Rad g * E * (Tav sur Tamb ) * D3 (9) ➤ Uninsulated length of the pipe
out (10)
➤ Outside diameter of the uninsulated portion (Dout)
-*a ➤ Temperature at location 1 (T1) in °C
➤ Temperature at location 2 (T2) in °C
Pr - ➤ Ambient Temperature (Tamb) in °C
a
Where: Financial Losses
g is the gravitation acceleration The energy lost from the leakage flow is given by:
β is the thermal expansion coefficient of air
ϑ is the kinematic viscosity of air Pleak = (ho – href ) * m (12)
α is the thermal diffusivity of air
Equations to calculate the thermo physical properties of Where:
air are given in Appendix.
Pleak is the energy lost from the leakage flow
Substituting Equations (8), (9) and (10) in Equation (7) we
can obtain the convection coefficient. ho is the enthalpy of the leakage steam at source (to be
obtained from steam table)
Thus the mass flow rate of the leakage steam can be href is the enthalpy of the demineralized water make up to
calculated from Equation (4): the system (to be obtained from steam table)
m *V * A*F * (Ta4v sur T4 ) hc * A * (Tav sur Tamb ) (11) m is the mass of the leakage steam obtained from
amb Equation (11)
Cp * (T1 T2 )
The following assumptions are made in the above Cost of Excess Coal
expression:
To maintain full unit load, more fuel needs to be burned
1. The heat transfer rate across the length of un-insulated in order to mitigate the energy lost by the leakage steam.
pipe is uniform throughout the length. Energy required from excess coal is given by:
2. Expansion of steam across the valve and across the Pexcess coal Pleak (13)
un-insulated portions is an isenthalpic process. Thus Kb
any change in kinetic energy is neglected.
Amount of excess coal required is: (14)
3. Change in pipe surface temperature of the Excess coal (per hour per Kg) = Pexcess coal * 3600
un-insulated region is the change in the steam
temperature. Cv
57
