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1. Furnaces/ Pipe Still Heaters CE-609 Petroleum Refinery Engineering Adeel ur Rehman, PhD

2. Pipe Still Heaters Why we need a furnace or Pipe/ tube Still Heater? Refinery processing is basically a fact rested upon effective distillation and condensation. Before the introduction of pipe still heaters (1918) into refinery operations, the direct heating stills (convective type) were used for all heating operations. Than what was the problem? Non-uniform heating and excessive heating could not be abated satisfactorily in these stills. with these inescapable difficulties, the refinery could not be magnified or utilized for high capacities at short notice, thus, one has to be content with small refinery sizes. Pipe still heaters are different from old still, heaters in one way ; the radiant section is separated from convection section. Heat transfer in radiant section as high as 70%. The stack losses and heater losses are brought down to 12% and 5% respectively followed by a close response to control.

3. Pipe Still Heaters Pipe heaters can be conveniently categorized into two types: Horizontal type Vertical type The horizontal type heater usually means a box type heater with the tubes running horizontally along the walls. Vertical type is normally a cylindrical heater containing vertical tubes. Cylindrical heaters require less plot space and are usually less expensive. They also shave better radiant symmetry than the horizontal type. Horizontal box types are preferred for crude oil heaters, although vertical cylindrical heaters have been used in this service. Vacuum unit heaters should have horizontal tubes to eliminate the static head pressure at the bottom of vertical tubes and to reduce the possibility of two-phase slugging in the large exit tubes.

4. Pipe Still Heaters Sections of

5. Pipe Still Heaters Sections of

6. Sections of Pipe Still Heaters Radiant Section The radiant tubes, either horizontal or vertical, are located along the walls in the radiant section. It receives radiant heat direct from the burners. Radiant zone contains refractory lining for thermal insulation. This is also called the firebox. Convection Section The heat transfer in this section is by Convection. The convection section removes heat from the flue gas to preheat the contents of the tubes. It significantly reduces the temperature of the flue gas exiting the stack. Too much heat consumed in the convection section is a sign of high draft. Tube temperature is measured in both convection and radiant sections.

7. Radiant Section

8. Convection Section

9. Sections of Pipe Still Heaters Shield Section In shield section there are tubing which shield the convection tubes from the direct radiant heat. The bridge wall or break wall temperature is the temperature of the flue gas after the radiant heat is removed by the radiant tubes and before it hits the convection section. Measurement of the draft at this point. This determines the heater performance. Breeching Sections and Stack The transition from the convection section to the stack is called the breeching. Measurement of stack emissions for compliance purposes is normally made here.

10. Walls of Heater/ Refractories The outer layer of the walls is typically made of steel sheets. On the inner side, the walls and floor are coated with a layer of refractory and insulating materials. This design not only ensures resistance to the intense heat and high temperatures inside the fired heater but also minimizes energy loss.

11. Airflow and Combustion Products in Fired Heaters Fire heaters are classified into four categories based on the method of air and hot gas flow within the chamber: Natural Draft Forced Draft Induced Draft Balanced Draft

12. Airflow and Combustion Products in Fired Heaters

13. Burner The burners are responsible for burning fuel to supply the heat needed. Fired heater burners may utilize either a natural draft or a forced draft system. This draft is generated by buoyant forces and the difference in density between the gases inside the fired heater and the surrounding air, and it is influenced by the height and temperature of the gases inside. The burners used in fired heaters may be installed either horizontally or vertically. The four common burner installation configurations in fired heaters are shown in the figure below.

14. Burner Operations The burners on a process heater premix the fuel with the primary air. The primary air flow should be maximized without lifting the flame off the burner. The pressure of the fuel gas supply is important since low gas pressure degrades performance. Most of the air (as primary air) is delivered to the burner along with the fuel. Secondary air is introduced and adjusted with the registers. Too much or too little secondary air gives poor combustion.

15. Burner Operations INSUFFICIENT COMBUSTION AIR ► Long flame ► Cooler temperature ► Very little NOx TOO MUCH COMBUSTION AIR ► Short flame ► Cooler temperature ► Wasted heat ► Increased Nox The high temperature in the flame and radiant section, together with combustion turbulence at the burners, causes dissociation of air molecules. The nitrogen and oxygen atoms combine to form nitric oxide (NO) and nitrogen dioxide (NO2), together known as NOx. Hydrogen content — high Excess air —normal low Atomization —steam mechanical air assisted mechanical

16. Types of Fired Heater Configurations Fire heaters are available with different structural configurations and arrangements for the radiant section tubes. The structure may be cylindrical or cubical, while the radiant section tubes can be positioned horizontally, vertically, helically, or in an arbor shape. Examples of these various fired heater configurations are shown in the figure

17. Pipe Still Heaters SHAPES OF DIFFERENT FURNACES

18. Pipe Still Heaters

19. Pipe Still Heaters

20. Pipe Still Heaters

21. Pipe Still Heaters

22. Pipe Still Heaters

23. Fired Heater Design Standards

24. DESIGNING RADIANT SECTION 1. Heat duty Different heat transfer rates are necessary for different types of operations. Heating rates are specified either on the basis of projected area of tube or outside surface area of tube. Care should be excercised not to exceed the heat density either on surface of the tube or in furnace. Heat Duties of various operations Process Projected Area Basis (KJ/hr.Sq.meter) Velocity (M/Sec.) Topping operations 25–50 × 10⁴ 2.5 Vacuum distillations 25–30 × 10⁴ 2–2 Mild cracking 14–20 × 10⁴ 2–3 Severe cracking & coking 25–35 × 10⁴ 2–3 Treatment operations 20–25 × 10⁴ 1–2 Cracking of gases for olefins 13–16 × 10⁴ 8–10

25. Pipe Still Heaters 2. Air fuel ratio The ratio of water vapor to carbon dioxide usually depends upon the air fuel ratio. Excess of air fuel ratio no doubt decreases the attainable maximum temperature, but increases the partial pressure of carbon dioxide and water vapor, hence increases heat transfer. Consequently, a fuel rich in hydrogen gives better radiant heat transfer due to the formation of water vapor after combustion.