Heat flux coefficient

One operating concern for a rich combustor is the occurrence of high combustor wall temperatures. In a fuel-rich combustor, air cannot be used to film-cool the walls and other techniques (e.g., fin cooling) must be employed. The temperature rise of the primary combustor coolant was measured and normalized to form a heat flux coefficient which included both convective and radiative heat loads. Figure 7 displays the dependence of this heat flux coefficient on primary combustor equivalence ratio. These data were acquired in tests in which the combustor airflow was kept constant. If convective heat transfer were the dominant mechanism a constant heat flux coefficient of approximately 25 Btu/ft -hr-deg F would be expected. The higher values of heat flux and its convex character indicate that radiative heat transfer was an important mechanism. 

The average heat flux coefficient to the primary combustor wall is plotted for the fuels in Figure 9. The results in general displayed a convex character as was observed with NO. 2 fuel. The level of the heat transfer coefficient and its convex trend indicates the importance of radiative heat transfer for these fuels. The maximum value of the coefficient for SCR-II fuel exceeded the maximum for other fuels by 30%. The hydrogen content of SCR-II was less than that for the other fuels tested which apparently resulted in a more intense radiating medium. 

Internal Regenerator Bed Colls. Internal cods generate high overall heat-transfer coefficients [550 W / (m -K)] and typically produce saturated steam up to 4.6 MPa (667 psi). Lower heat fluxes are attained when producing superheated steam. The tube banks are normally arranged horizontally in rows of three or four, but because of their location in a continuously active bubbling or turbulent bed, they offer limited duty flexibdity with no shutdown or start-up potential. 

External Dilute-Phase Upflow Cooler. The external ddute-phase upflow design (68) offers some control in the range of heat removal duties but generates relatively low heat-transfer coefficients [60—170 W/(m K)]- This design substantially increases the surface area requirement and thereby reduces the ultimate duty that can be achieved from a single bundle. In addition, poor mechanical rehabdity has been continuously experienced because of excessive erosion at the lower tube sheets as a result of the high catalyst fluxes and gas velocities imposed. 

Heat transfer by nucleate boiling is an important mechanism in the vaporization of liqmds. It occurs in the vaporization of liquids in kettle-type and natural-circulation reboilers commonly usea in the process industries. High rates of heat transfer per unit of area (heat flux) are obtained as a result of bubble formation at the liquid-solid interface rather than from mechanical devices external to the heat exchanger. There are available several expressions from which reasonable values of the film coefficients may be obtained. 

Only fair overall coefficient values may be expected, although heat-flux values are good. 

The Mix-R-Step type in Fig. ll-62e is an adaptation of avibratoiy conveyor. It features better heat-transfer rates, practically doubling the coefficient values of the standard flat surface and trebling heat-flux values, as the layer depth can be increased from the norm 13 to 25 and 32 mm (V2 to 1 and F in). It mav be provided on decks jacketed for air, steam, or water spray. It is also often apphcable when an infrared heat source is mounted overhead to supplement the indirect or as the sole heat source. 

Heat-transfer coefficients in steam-tube dryers range from 30 to 85 J/(m s K). Coefficients will increasewith increasing steam temperature because of increased heat transfer by radiation. In units carrying saturated steam at 420 to 450 K, the heat flux UAT will range from 6300 J/(m s) for difficult-to-diy and organic solids and to 1890 to 3790 J/(m s) for finely divided inorganic materials. The effect of steam pressure on heat-transfer rates up to 8.6 X 10 Pa is illustrated in Fig. 12-71. 

For overall tubeside plus shellside fouling use experience factors or 0.002 for most services and 0.004 for extremely fouling materials. Neglect metal wall resistance for overall heat transfer coefficient less than 200 or heat flux less than 20,000. These will suffice for ballpark work. 

Consider a single-zone jacket where there is an increase in the jacket flow, and a corresponding increase in the outside film coefficient because hj =f(Njjg, G). Therefore, a two-fold increase in the jacket flow results in an increase in hjj by 2 h . The overall heat transfer coefficient U = l/[FpoL + 1/hj], and a larger outside coefficient subsequently increases the overall heat transfer coefficient. The overall heat flux will increase due to the combined effects of the increased flow and lower jacket outlet temperature. The net result is an increase in the pressure drop. 

The coefficients in the equations differ slightly in different references, depending on the entrainment coefficients used. The convective heat flux , in W or W/m from the heat source, can be estimated from the energy consumption of the heat source by... 

A fire tube contains a flame burning inside a piece of pipe which is in turn surrounded by the process fluid. In this situation, there is radiant and convective heat transfer from the flame to the inside surface of the fire tube, conductive heat transfer through the wall thickness of the tube, and convective heat transfer from the outside surface of that tube to the oil being treated. It would be difficult in such a simation to solve for the heat transfer in terms of an overall heat transfer coefficient. Rather, what is most often done is to size the fire tube by using a heat flux rate. The heat flux rate represents the amount of heat that can be transferred from the fire tube to the process per unit area of outside surface of the fire tube. Common heat flux rates are given in Table 2-11. 

Recommended limiting maximum heat flux for the tube density coefficient ... 

At a convection heat transfer surface the heat flux (heat transfer rate per unit area) is related to the temperature difference between fluid and surface by a heat transfer coefficient. Newton s law of cooling defines this ... 

In the convective section, the gas-side heat transfer coefficient controls the heat flux distribution since the... 

From various studies" " it is becoming clear that in spite of a heat flux, the overriding parameter is the temperature at the interface between the metal electrode and the solution, which has an effect on diffusion coefficients and viscosity. If the variations of these parameters with temperature are known, then / l (and ) can be calculated from the hydrodynamic equations. 

The work of Porter et al. has shown that for copper in phosphoric acid the interfacial temperature was the main factor, and furthermore this was the case for positive or negative heat flux. Activation energies were determined for this system they indicated that concentration polarisation was the rate-determining process, and by adjustment of the diffusion coefficient and viscosity for the temperature at the interface and the application of dimensional group analysis it was found that ... 

In addition, manufacturers boiler operational manuals provide hardcopy data ratings for heat transfer coefficients, local heat flux, fuel utilization, furnace heat release rates, maximum continuous rating... 

Wall temperatures drop after reaching the maximum in the case of the two highest heat flux levels in Fig. 8, and this is due to increasing convective heat transfer through the steam film, which now completely blankets the surface. The improved heat transfer is caused by the higher flow velocities in the tube as more entrained liquid is evaporated. Finally, at about 100% quality, based on the assumption of thermal equilibrium, only steam is present, and wall temperatures rise once more due to decreasing heat-transfer coefficients as the steam becomes superheated. 

The values of the heat transfer coefficients for low values of temperature difference are given by equation 9.185. Figure 9.53 shows the values of h and for q for water boiling on a submerged surface. Whilst the actual values vary somewhat between investigations, they all give a maximum for a temperature difference of about 22 deg K. The maximum value of h is about 50 kW/m2 K and the maximum flux is about 1100 kW/m2. 

Similar results have been obtained by Bonilla and Perry 79>, Insinger and Bliss 801, and others for a number of organic liquids such as benzene, alcohols, acetone, and carbon tetrachloride. The data in Table 9.9 for liquids boiling at atmospheric pressure show that tile maximum heat flux is much smaller with organic liquids than with water and the temperature difference at this condition is rather higher. In practice the critical value of AT may be exceeded. Sauer et al.m] found that the overall transfer coefficient U for boiling ethyl acetate with steam at 377 kN/m2 was only 14 per cent of that when the steam pressure was reduced to 115 kN/m2. 

Figure 9.53. Effect of temperature difference on heat flux and heat transfer coefficient to water boiling at...

Another important case is where the heat flux, as opposed to the temperature at the surface, is constant this may occur where the surface is electrically heated. Then, the temperature difference 9S — o will increase in the direction of flow (x-direction) as the value of the heat transfer coefficient decreases due to the thickening of the thermal boundary layer. The equation for the temperature profile in the boundary layer becomes ... 

By comparing equations 11.61 and 11.66, it is seen that the local Nusselt number and the heat transfer coefficient are both some 36 per cent higher for a constant surface heat flux as compared with a constant surface temperature. 

---