Heat exchanger contact resistance

Here I /G. is the heat exchanger contact resistance. The reason for rhe contact resistance is that there exists a resistance to heat flow between the outer surface of the pipe and the collar of the plate tins. Normally, the fins are attached to the pipes by mechanical expansion of the tubes out into rhe plate-fin collars. Because of this manufacturing method, the contact will not be ideal. Small gaps between the pipe surface and rhe collar of the tins will occur. 

It is very difficult to estimate the magnitude of the contact conductance G. Normally the total conductance of the heat exchanger is determined, and G - is calculated from Eq. (9.48). Only in the case that rhe plate fins are welded to the pipes with a metallurgical contact is the contact conductance infinite, leading to zero contact resistance, that is 1 /G,. = 0. 

The sample introduction unit was constructed from inert materials, which minimizes the introduction of metal contamination into the system. The samples or digestion acids make contact only with PTFE, Kel-F, glass, acid-resistant rubber and platinum-iridium (9 + 1) alloy. In addition, the construction materials were Hmited to acid-grade Arborite (ureaformaldehyde laminate). Perspex and stainless-steel. The unit was constructed in three continuous sections a heated sample compartment, a turntable mechanism and heat-exchanger compartment, and a pump compartment. 

Composition 70% copper, 29% zinc, 1% tin, and 0.03% arsenic. It has good corrosion resistance, especially in sea water. Arsenic inhibits the loss of zinc from the alloy. It is used for tubing applications in condensers, preheaters, evaporators, and heat exchangers which contact salt water, oil, steam, and other liquids below 500°F (260°C). 

Additional parameters specified in the numerical model include the electrode exchange current densities and several gap electrical contact resistances. These quantities were determined empirically by comparing FLUENT predictions with stack performance data. The FLUENT model uses the electrode exchange current densities to quantify the magnitude of the activation overpotentials via a Butler-Volmer equation [1], A radiation heat transfer boundary condition was applied around the periphery of the model to simulate the thermal conditions of our experimental stack, situated in a high-temperature electrically heated radiant furnace. The edges ofthe numerical model are treated as a small surface in a large enclosure with an effective emissivity of 1.0, subjected to a radiant temperature of 1 103 K, equal to the gas-inlet temperatures. 

The most generally applicable predictive method for contact resistance is ascribed to Irvine and Taborek in Section 2.4.6 of the Heat Exchanger Design Handbook [1]. Their method requires numerical values of the above properties, which usually do not have high accuracy, and the authors estimate about 25% mean error, with an error spread of a factor of 2 about the actual value. A typical thermal contact resistance is on the order of... 

The contact resistance is also affected by differential thermal expansion when the surfaces are at elevated temperatures or with large temperature differences in the two materials. The problem is aggravated by thermal cycling, especially if one material is heated above its elastic behavior range, and must be considered with finned tubes in air-cooled heat exchangers. 

If there is any contact or bond resistance present between the fin and tube or plate on the hot or cold fluid side, it is included as an added thermal resistance on the right side of Eq. 17.5 or 17.6. For a heat pipe heat exchanger, additional thermal resistances associated with the heat pipe should be included on the right side of Eq. 17.5 or 17.6 these resistances are evaporator resistance at the evaporator section of the heat pipe, viscous vapor flow resistance inside heat pipe (very small), internal wick resistance at the condenser section of the heat pipe, and condensation resistance at the condenser section. 

When two dissimilar metals are immersed in an electrolyte they usually develop different potentials in accordance with the theory already presented. If the metals are in contact the potential difference provides the driving force for corrosion. Severe corrosion often occurs as a result of the contact between two metals. In shell and tube heat exchangers where the tubes are fabricated from a corrosion resistant alloy, and the shell is made from mild steel for instance to reduce the capital cost, corrosion is very likely unless adequate protection is made. The less resistant of the two metals is caused to corrode, or to corrode more rapidly, while the resistant metal or alloy corrodes much less or may be even completely protected. The basis for galvanic corrosion is illustrated on Fig. 10.6. Metal A has a lower electrode potential than metal B. Ions migrate in the conducting solution while electrons flow across the junction of the two metals, as a result metal A is corroded at C. 

Uses Silicone for use in heat-resist, coatings for appliances, engines, mufflers, furnaces, stacks, heat exchangers, nonstick cookware food-contact coatings... 

Chem. Descrip. Methyl phenyl silicone resin in xylene Uses Silicone for use in heat-resist, coatings for appliances, engines, mufflers, furnaces, stacks, heat exchangers food-contact coatings Features Dries soft, flexible, and tack-free at R.T. gains solv. resist, at 260 C, maintains flexibility, film integrity beyond 650 C Regulatory FDA 21 CFR 175.300... 

---