Double-fired heater

In the vei tical-tube single-row double-fired heater, a single row of vertical tubes is arrayed along the center plane of the radiant section that is fired from both sides. Usually this type of heater has an overhead horizontal convec tion bank. Although it is the most expensive of the fired heater designs, it provides the most uniform heat transfer to the tubes. Duties are 21 to 132 GJ/h (20 to 125 10 Btu/h) per cell (twin-cell designs are not unusual). 

The flux distribution around the tube is not uniform as well. As indicated in Figure 5.3, the radiating plane is the flame. The diagram on the left shows the flux profile for a single-fired heater. The front of the tube facing the fire picks up most of the heat. The diagram on the right shows the profile for a double-fired heater with flames on both sides of the tubes. The flux pattern is close to uniform. 

The nonuniformity of heat flux around mbe is described by the circumferential flux factor, which is the ratio of peak flux to average flux. Peak flux determines the maximum TWT. The peak flux is typically 1.5-1.8 times the average for a single-fired heater, while it is 1.2 times the average for a double-fired heater. That explains why the double-fired heater has longer run length as it has lower flux rate and hence lower TWT than the single-fired heater. 

Double-fired heaters are usually TWT-limited. However, flux limits are specified for revamps, which depend on specific services. The limits are provided by heater specialists. 

Figure 3-20. Vertical-tube-fired heaters con be identified by the vertical arrangement of the radiant-section coil, (a) Vertical- lindrical all radiant, (b) Vertical-cylindrical helical coil, (c) Vertical-cylindrical, with cross-flow-convection section. d) Vertical-cylindrical, with integral-convection section, (e) Arbor or wicket type, (f) Vertical-tube, single-row, double-fired. [From Chem. Eng, 100-101 (June 19, 1978).]...

Double-fired, multi-point steam (or water) injection, online spalling, bi-direction steam/air decoking and other techniques enable a three-year run length for the heater and 5% savings. 

The 16DF direct-fired, double effect, absorption chiller/heater can also be operated in a non-simultaneous heating (only) mode to provide 140 F (60 C) hot water for-space heating or other purposes without any additional components. In this mode, the cycle follows a different vapor flow path than that undertaken for cooling and does not use the absorption process. ... 

Ancient Romans used underground heaters (hypo-causts) to distribute warmth from fires through the double-layered floors and walls of public baths. This technoiogy is considered one of the earliest heating, ventilation, and air-conditioning systems. 

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