Fired heaters velocity

Volume 1, Chapter 9 explains the criteria for choosing a diameter and wall thickness of pipe. This procedure can be applied to choosing a coil diameter in an indirect fired heater. Erosional flow criteria will almost always govern in choosing the diameter. Sometimes it is necessary to check for pressure drop in the coil. Typically, pressure drop will not be important since the whole purpose of the line heater is to allow a large pressure drop that must be taken. The allowable erosional velocity is ffiven bv ... 

Fired heaters radiant rate, 12,000 Btu/(hr)(sqft) convection rate, 4000 cold oil tube velocity, 6 ft/sec approx equal transfers of heat in the two sections thermal efficiency 70-75% flue gas temperature 250-350°F above feed inlet stack gas temperature 650-950°F. 

Compared to a forced-circulation reboiler, a fired heater often operates at a higher pressure drop, higher velocities, and a larger fractional vaporization. It is common to have fired reboilers operate at 30 to 50 percent vaporization. 

Fig. 4.25. Forced draft heater for petro-chem processing—may be cylindrical with one burner as shown, or a circie of vertically up-fired, high-velocity type H burners (fig. 6.2) or rectangular (a cabin heater) with rows of up-fired burners, or rows of side-fired type E flat-flame burners, shown in fig. 4.26 and 6.2.

Heuristic 57 Typical heat fluxes in flred heaters are 12,000 Btu/hr-ff in the radiant section and 4,000 Btu/hr-fi in the convection section, with approximately equal heat duties in the two sections. Typical process liquid velocity in the tubes is 6 ft/s. Thermal efficiency for modem fired heaters is 80-90%, while older fired heaters may have thermal efficiencies of only 70-75%. 

The contents of the stirred-tank heating system shown in Figure E4.10 are heated at a constant rate of Q(Btu/h) using a gas-fired heater. The fiow rate w(lb/h) and volume y(ft ) are constant, but the heat loss to the surroundings Q/ (Btu/h) varies with the wind velocity v (ft/s) according to the expressions... 

For corrosion and safety reasons, the condensate recovered from these sources is best not returned to the deaerator for use as boiler feedwater. However, depending on the contaminant, the condensate may be reused for a number of services. Our favorite reuse of such contaminated condensate is as a replacement for velocity steam in the heater-tube passes of a fired furnace. 

Liquid drains, manifolded and taken to a heater for vaporizing, process safety valves, vents and thaw lines, together with the major casing vents, all culminate in a vent stack to the atmosphere. This stack should have its exit point well above surrounding equipment to prevent it from being a hazard to personnel in the event of a fire. A continuous flow of nitrogen at a velocity of 1 ft/sec can be supplied at the bottom of the vent stack to blanket all process and casing equipment from air. A check valve placed in this line provides added protection. 

The basic airflow system for flotation dryers is shown in Figure 40.10. It consists of three basic components (1) supply fan (2) exhaust fan and (3) heater. The supply fan is sized for the air volume required by the open area of the dryer nozzles and the maximum nozzle outlet velocity. It blows hot air through the air-bar nozzles onto the coated web surface. The spent air is exhausted by an exhaust fan. The heater can be a direct-fired gas burner for air temperatures up to 400°C or high-steam coils for air temperatures up to 200°C. When drying aqueous coatings, the exhaust is led to a recirculation system where part is bled to the atmosphere and the remainder reheated for recirculation. When solvents are involved, this spent air may be incinerated or led to a solvent-recovery unit. 

Scale frequently deposits in the oil formation near the well bore, in the perforations, or even on the face of the formation. Scale can form over the inlet ports of a rod pump or a Reda pump, starving them of fluid and possibly causing the Reda to get hot and bum out. Scale can form in the pump itself, even though the velocity of fluid movement is high. Fire tubes in aU types of heaters fail prematurely when scale formation results in overheating. Corrosion is often more severe under a scale deposit. 

---