Friday, November 17, 2017

Fired Heaters Performance Checking-Sample calculation

From My Previous Post of Fired Heater Performance check Equation, In this Post lets do a sample calculation :

The Equation Is :

Say Heater Duty is 25 MW, Temperature in and Out is 250 °C and 375 °C, Excess Air is 20%  and Radiant Av Flux is 35000 W/h.m2

VC Heater. Single side fired. 2 pass-6 " NB tubes. Exit Gas Approach to inlet fluid temperature is 100 °C
C
Stack Gas Temperature will be = Fluid Out Temperature + Approach = 250 + 100 =350 °C.

Thermal Efficiency fro Chart 1 below :( for 350 °C tem and 20 % Excess air , its coming 83.5 %)

Let say 1.5% is further loss in casing so efficiency is 82 %.

Assume radiant duty is 65% of total.

Radiant inlet temperature = 294°C


Average radiant fluid temperature = (294+375)/2 = 334°C


Tube metal temperature, take 50°C above = 384°C


Radiant gas temperature, read from chart -2 = 940°C


Radiant section thermal efficiency, from chart -1 again = 53%


Deduct 1.0% firebox casing loss, net efficiency = 52%


Radiant section duty = 52/82 = 63.4% Vs 65% assumed = 15.85 kW


If you want to size this heater,


Radiant heat transfer area = 15.85 e6/35,000 = 453 m
2

Radiant coil length = 453/(π*6.625*0.0254) = 856 m

Take 60 tubes, 30 per pass,


even number; top inlet / top outlet

Each tube, effective length = 14.3 m


Credit for 180° bend, tube weld to weld = 13.8 m


Tube Circle Diameter 60 tubes on 12” pitch on circle


= 60*12*0.0254/ π = 5.8 m


L/D ratio = 2.5


Chart -1



Chart-2
Reference : Energy Environment Engineers