HEAT TRANSFER IN SURFACE CONDENSERS

In the past, the heat transfer values for steam surface condensers have not been calculated by the resistance method commonly used for shell and tube heat exchangers. The reason is that steam flow patterns differ throughout the condenser and in some areas the outside tube surface is flooded with condensate making it impossible to determine the shell side resistance ("rs"-see Figure 8). Therefore the Heat Exchange Institute (HEI), a Trade Association of surface condenser manufactures, performed calorimeter heat transfer tests using tube materials then in common use (Copper Alloys). The results were compiled into tables and curves and use "metal correction factors" to accommodate the change in heat transfer due to the difference in thermal conductivity of the different copper tube materials.

The formula for calculating the heat transfer rate evolved as:^(20)

U = C (square root of v) Ft x Fm x Fc where:

U = Overall Heat Transfer Coefficient, BTU/hr ft^2 degrees F

C = Diametral tube constant (Table 8)

v = Average water velocity inside tube, ft/sec

Ft = Inlet Water Temperature Correction Factor (Figure 9)

Fm = Metal Correction Factor

Fc = Cleanliness Factor for Copper Alloys = 0.85. For titanium
and stainless alloys = 0.90.

The suggested metal correction factors severely penalized materials such as titanium and the stainless steels and were increased in value twice in subsequent editions of the HEI Standards for Steam Surface Condensers^(20). There remained, however, extreme dissatisfaction with the metal correction factors within the industry.

Consequently, in 1991, a joint effort was formed by the Electric Power Research Institute (EPRI) and the Empire State Electric Energy Research Corporation (ESEERCO) to construct a test facility at the Rochester Institute of Technology (RIT). The work was completed in 1993 and the results were published in 1993 in a report titled Effect of Tube Material on Steam Condensation^(21). The report's conclusion is that there is no difference in the heat transfer ability of different tube materials other than the wall resistance (rm) due to their thermal conductivity. This method results in the elimination of the metal correction factor Fm and the imposition of the difference in metal resistance of the tube being used over the resistance of 7/8" O.D. x 18 BWG Admiralty, which is included in the original diametral constants.

The revised clean heat transfer rate is then determined as follows:

U = C (square root of v) x Ft

1
--- = R'
U'

R" = R' - .00006767 + rm

where rm is the metal resistance of the tube size and material being used.

The revised service heat transfer:

1
U = --- x Fc
R"

where Fc is the cleanliness factor, usually 0.85 for copper alloys and 0.90 for titanium and stainless alloys.

Metal resistances of commonly used surface condenser tube materials are shown in Table 10 (Page 24). The wall thickness used to calculate these values is based on the nominal thickness. The thermal conductivity values used in the calculations are at the top of each material column and the values for titanium and the stainless steels were determined by certified tests. The values published in the HEI Standards for Steam Surface Condensers, 9th Edition were not determined by testing. The heat transfer values obtained will rank in the order of the thermal conductivity of each material. 17% Chrome Steel (439) was not included in the tables. It has a thermal conductivity very close to titanium.

In Example 1, it should be noted that the calculation does not take into account that fouling with titanium will be less than with the Admiralty brass. The fouling has to be reduced only the amount of the increased metal resistance (.00015200) in order to have the same overall heat transfer rate with titanium.

Example 2 shows the same effect. Due to the cleanliness factor being higher for titanium, switching to titanium from aluminum brass actually results in better performance, providing the water velocity remains the same.

Example 3 illustrates use of the EPRI recommended method.

Example 4 is another example of the EPRI method showing the effect of lower thermal conductivity of the tube material. The thermal conductivity of AL-6XN is 51% that of titanium, resulting in the need for 12% more AL-6XN heat transfer surface in order to have the same performance as with titanium at a water velocity of 7.5 ft./sec.

A simplification of the EPRI method can be made by calculating the heat transfer rate for each tube material and gage (tube wall thickness) and comparing the result(s) to the rate for Admiralty Brass. The resulting metal correction factor, Fm, from Table 9 can then be used in the formula on Page 21. This method has some built in theoretical inaccuracies, but is currently considered, by HEI, to be adequate for calculating heat transfer rates for surface condensers.