Heat transfer coefficients clean conditions

Ganapathy, V, Nomograph Relates Clean and Dirty Heat Transfer Coefficients, Fouling Factor, Heating/Piping/Air Conditioning, Jan. (1979) p. 127. 

Estimate the heat transfer coefficient at the vessel wall and the overall coefficient in the clean condition. 

Calculate the heat-transfer coefficient for a fluid with the properties listed in Example 7.18 if the fluid is flowing across a tube bundle with the following geometry. The fluid flows at a rate of 50,000 lb/h (22,679.5 kg/h). Calculate the heat-transfer coefficient for both clean and fouled conditions. 

The objective of a rating problem is to determine if an existing process unit will satisfy process conditions. To arrive at an approximate calculation procedure for rating a heat exchanger, first define a clean overall heat-transfer coefficient, i.e., in the absence of any fouling. Therefore, Rfj andRfo = 0 in Equation 4.15. 

In one run, sodium sulfate and calcium chloride were added to a 200-gallon batch of sea water to give a sulfate concentration of 5200 p.p.m. and a calcium concentration of 640 p.p.m. An addition was made of 5 p.p.m. of low molecular weight polyacrylic acid and the treated sea water was evaporated under standard conditions. The over-all heat transfer coefficient averaged 3600 B.t.u./sq. ft. hr. ° F. for a 5-hour period, with no detectable falloff during the period, and the tube was found to be clean, except for the plastic film. In contrast, evaporation of a similar batch of calcium sulfate-enriched sea water, with no polyacrylic acid, resulted in a marked decrease in heat transfer coefficient to less than 800 B.t.u./sq. ft. hr. ° F. after 2 hours. 

If steam is condensing at a temperature of 111°C on the outside of a tube and an organic liquid at a temperature of 52°C is flowing at 1 m/s through the tube it is possible to estimate the temperature distribution across the surface for clean conditions assuming knowledge of the heat transfer coefficients on either side of the tube and the thermal resistance of the tube wall. If the heat transfer coefficients on the steam and liquid side are 8460 and 1462 W/mi K respectively and the thermal resistance of the tube wall is 0.0000042 m K/W (copper 1.65 mm thick) the total resistance to heat flow Rj. (using Equation 2.5) is ... 

Overall heat transfer coefficient for clean conditions... 

Set the highest liquid level to match the expected liquid level in the reboiler under the most severe turndown conditions required. Keep in mind that these may occur under startup conditions, when the reboiler is clean and the heat transfer coefficient is high. 

Let us define the U value first based on Figure 6.1 where and are film coefficients for fluids inside and outside of the tube, and they can be calculated from the physical form of the heat exchanger, physical properties of streams, and process conditions of streams. Thus, a clean overall heat transfer coefficient (Uc) can be determined based on ... 

Given the uncertainties associated with the calculations, especially those on the shell-side, a sensible design basis for the heat transfer area specification would be the shell-side flow characterized by the clean condition. Of course, the fouling coefficients for the shell-side and tube-side should be included to account for the surface fouling resistance. 

The only factor which affects the overall coefficient is the scale formation. The liquid enters the evaporator at the boiling point, and the temperature and heat of vaporization are constant. At the operating conditions, 990 Btu are required to vaporize 1 lb of water, the heat-transfer area is 400 ft2, and the temperature-difference driving force is 70°F. The time required to shut down, clean, and get back on stream is 4 h for each shutdown, and the total cost for this cleaning operation is 100 per cycle. The labor costs during operation of the evaporator are 20 per hour. Determine the total time per cycle for minimum total cost under the following conditions ... 

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