Fluid Heaters and Film Temperature

Industrial heat transfer fluids are often heated by exhaust gases from incinerators, turbine exhaust, or waste flue gas from various sources. It is basically a coil or tube bundle like an economizer consisting of tubes in parallel- or counter-flow direction with plain tubes or finned tubes if the duty is large. The fin geometry is selected with great care as one should

be concerned about the heat flux inside the tubes, which can increase the film temperature as well as the tube wall temperature! Hence, one of the important considerations in the design of these hot oil or fluid heaters is the film temperature limitation. Many fluids such as glycol–water solutions, hydrocarbon oils, silicone oils, molten salts, and liquid metals have limitations on both bulk fluid temperature and film temperature. If the film temperature is exceeded by using improper fin geometry, then thermal degradation of the fluid can set it. Two fluid heaters may operate at the same bulk temperature or have the same duty, but one can have a much higher film temperature due to the use of poor fin geometry or high gas velocity, which results in high heat flux inside the tubes. Appendix E discusses the effect of fin geometry on superheater tube wall temperatures.

The fluid film temperature is not usually measured directly, and it must be estimated by the designer of the exchanger. Two different heaters may operate at the same heat duty, fluid flow rate, and bulk fluid outlet temperature, and yet they may have significantly different film temperature profiles. The film temperature is an important value as fuel degradation begins in the fluid film, as this is where the fluid is hottest. For typical thermal oils, the fluid decomposition rate doubles for roughly every 10°C increase in fluid temperature. It is not unusual for the maximum fluid film temperature to exceed the heater outlet

228 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

temperature by 40°C or more, with degradation rates in the film exceeding those in the bulk fluid by a factor of 20 or more. Shortened fluid life can be a costly result since the fluid is often used elsewhere and recirculated. Suppliers of thermic fluids provide the safe film temperatures to be maintained through proper design.

Economical designs of fluid heaters using waste flue gas have a combination of plain and finned tubes depending on gas inlet temperature, heat flux, and film temperature in each zone. It has been shown in Appendix E that finned tubes increase heat flux inside the tubes and hence the film temperature. Finned tubes should be used with caution.

Film temperature = fluid temperature + heat flux inside tube/tube-side heat transfer coefficient.

Example 4.6

A thermal fluid of 450,000 kg/h has to be heated from 313°C to 340°C. Duty is 9040 kW. The heat source is 45,000 kg/h of clean incinerator exhaust at 1000°C. Flue gas analysis

is % volume CO 2 = 4.5, H 2 O = 10, N 2 = 75, O 2 = 10.5. Flue gas is clean. Fouling factors used = 0.00017 m 2 K/W on gas and fluid sides. The following design was provided by a vendor. Check the film temperature.

The information from the vendor is shown in Figure 4.38. To check the results, one must first compute the gas and fluid properties. The following fluid properties were

Coil performance Surface Geometry

Gas temp in ±10°F

Tube OD, in. Tube ID, in.

Fin width, in. Fin conductivity Tubes/row No deep Length, ft

Long pitch Streams Parl = 0, countr = 1 Arrangement Fluid ﬂow, Ib/h % volume-ﬂuegas

Tr pitch, in.

Fin thk, in.

Fin height, in.

Fins/in

Fluid in

Out

Flue gas

H 2 O = 10, N 2 = 75, O 2 = 10.5

Gas temp out ±10°F Gas sp ht, Btu/Ib°F Duty, MM Btu/h

U, Btu/ft 2 h°F Surface area, ft 2 LMTD, °F

Gas pr drop-in wc Max gas vel-ft/s Fluid temp in, F Fluid temp out±10°F FLuid pr drop, psi Fluid velocity, ft/s Fluid ht tr coeﬀt Max ﬁlm temp, °F

Max heat ﬂux Max tube temp, °F Max ﬁn tip temp

Gas ﬂow, Ib/h

Gas foul factor

Coil3 Coil2 Coil1 Coil3 Coil2 Coil1

FIGURE 4.38

Thermal performance information for fluid heater.

Waste Heat Boilers 229

TABLE 4.21

Properties of Thermic Fluid

Thermic Fluid

Flue Gas

Temperature

400 Specific heat, J/kg K

1150 Viscosity, kg/m h

0.1127 Conductivity, W/m K

Density, kg/m 3 820

obtained from the fluid manufacturer, and the flue gas properties were estimated using method discussed in Appendix F and are shown in Table 4.21.

The coil is divided into three sections. The hot front end has plain tubes, while the next section has tubes with 1.5 fins/in. and the last section has 3 fins/in.

One may use the methods discussed in Appendices C and E to evaluate the gas- side heat transfer coefficients. The tube-side coefficient at the hot end exit as shown in 0.8 i 18 Appendix B is estimated using h . i = 0.0278 w C/d

Flow per tube (with 16 streams) w = 455,000/16/3,600 = 7.9 kg/s C 04 .

h i = .0278 × 7.90 .8 × 151/.0774 1.8 = 2194 W/m 2 K (385 Btu/ft 2 h °F). This is an average value

for the entire coil, but Figure 4.39 shows the breakup for each of the three sections. Let us use these values.

The gas-side heat transfer coefficient at the highest gas temperature may be estimated by the methods described in Appendices C and D. Heat flux inside the tubes for the bare tube section = 11.01 × (1,832 − 645) × 3.5/3.046 = 15,016 Btu/ft 2 h. Hence, film temperature = 15,016/394 + 645 = 683°F. Similarly, one may compute the film temperatures in each section. One may also check the gas-side heat transfer coefficients using Appendices C and

E for plain and finned tubes and Appendix D for nonluminous heat transfer coefficient.

Gas A B C D E Steam

Heat flux = 34348 kcal/m 2 h Clean

Gas A B C D E Steam

185 Heat flux = 29295 kcal/m 2 h Scale formed

FIGURE 4.39

Temperature profile in clean and fouled conditions. Note: (A) Gas film, (B) gas-fouled layer, (C) tube wall, (D) steam-fouled layer, (E) steam wall.

230 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

The maximum film temperature must be checked against the allowable film temperature provided by the supplier of the oil or fluid. A margin of 10°C–15°C below the allowable value may be prudent.

Fluid Heaters and Film Temperature