Pressure drop fired heaters

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 ... 

The tube inside heat transfer coefficients and pressure drop can be calculated using the conventional methods for flow inside tubes see Section 12.8, and Volume 1, Chapter 9. If the unit is being used as a vaporiser the existence of two-phase flow in some of the tubes must be taken into account. Bergman (1978b) gives a quick method for estimating two-phase pressure drop in the tubes of fired heaters. 

All of the commercial simulators include models for heaters, coolers, heat exchangers, fired heaters, and air coolers. The models are easy to configure, and the only inputs that are usually required on the process side are the estimated pressure drop and either the outlet temperature or the duty. A good initial estimate of pressure drop is 0.3 to 0.7 bar (5 to lOpsi). 

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. 

Heaters encounter four major design limitations heat flux, process pressure drop, TWT, and BWT. Heat flux limited heaters are usually characterized with high pressure AP (>20psi) and most general service heaters fall into this flux-limited category. Typically, the flux limit for single-fired heaters is around 10,000 Btu/ft h. 

For fired heaters, the limitations could be heat flux or TWT. The former is applied to heaters with pressure drop larger than 20 psi, which is the most common. The latter is for low-pressure drop heaters. When the heater duty must be increased to handle duty much larger than design, the existing heater may be insufficient in meeting the limits. Installing a new heater could be very expensive. The most effective way to avoid this is to increase feed preheating via process heat recovery. 

Because it is known that the flowrate of naphthalene has been reduced by 50%, the new outiet pressure from P-201 can be calculated from Equation 119.71. The feed pressure remains at 80 kPa. At a naphthalene flow of 6.41 Mg h, Equation 119.71 gives a pressure increase of 455.73 kPa, so P3 = 535.73 kPa. Because the flowrate has decreased by a factor of 2, the pressure drop in the fired heater decreases by a factor of 4 (see Equation rE19.7a11. Therefore, P5 = 510.73 kPa. Therefore, the pressure of Stream 6 must be 510.73 kPa. The flowrate of air can now be calculated from the conpressor curve equatiom... 

Once a decision is made to eliminate these two bottlenecks (fired heater and separation system), the analysis of reactor behavior could be modified to consider a number of secondary effects. The case study did not consider the effect of increasing reactor flowrates on the heat transfer coefficient (gas-phase resistance dominates). The inpact of the increased pressure drop over the reactor was not examined. These options were not necessary to confirm that the reactor is capable of providing a 50% increase in acetone production. The option of increasing the HTM flowrate was not appraised. 

For the remainder of this project we will concentrate mostly on the closed fire heater where higher available pressure drops, and higher temperatures close to the fuel bed should offer improved prospects of success. 

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