貨物油タンクの加熱における伝熱上の問題

Cargo-oil tanks of tanker are usually equipped with heating coils. Its ordinary arrangement is shown diagrammatically in Figs. 1 and 2. Steam is supplied downward from one end of the heating coil, and the condensate is returned upward from the other end. In designing the heating system, the heating surface and the steam consumption must be evaluated making use of past experiences. An investigation into the conventional method of the evaluation leads us to the conclusion that a refinement of the evaluation will certainly result in a substantial reduction of the weight of the heating coils as well as the amount of steam consumption. some important points in the above reasoning are as follows. 1. In Calculating heat transfer through heating coils from steam inside to outside, the heating coil must be treated as a combination of separately calculated two parts, namely, the steam condensing section and the liquid-water cooling section, because the mechanism of heat transfer is entirely different. The necessity is demonstrated in a result of a simple model experiment with a glass tube as shown in Figs. 3 and 4. The coefficients of heat transfer for the steam-condensing section and the water-cooling section, k_s and k_w, show tendencies logically understandable. However, if the entire length is treated as a single process of heat transfer, then the coefficient of heat transfer, k_g, is sometimes larger than any of k_s and k_w, sometimes equal to k_s, and sometimes in-between of k_s and k_w, as indicated in Fig. 5. Evidently k_g is a poor measure for evaluating past experiences. An improvement in this point alone may result in a more exact design and thereby in a economy of heating surface. 2. The film coefficient of heat transfer from condensing steam is excellent only when the heating surface is free from accumulation of air and codensate. However, the usual way of arrangement of heating coils as shown in Figs. 1 and 2 will inevitably hold up a considerable amount of air and water as indicated by Fig. 6. 3. The rate of heat transfer at the steam inlet will exceed that of all other part of the coil because, at this point, the steam temperature is high and neither condensate nor air can accumulate. This means that the heating at the bottom of cargo-oil tanks is quite uneven. In another words, the temperature distribution in cargo-oil tanks must be uneven. If the heated medium is water, then water near the free surface will obtain a substantially higher temperature than water at the bottom which might practically be cold. Since the heated medium is not water but crude oil consisting of a variety of substances of different boiling temperatures, the temperature at the free surface may not rise substantially because the evaporated gasoline will absorb latent heat. It is certainly desirable to heat uniformly at the bottom and prevent losses both in heat and gasoline. 4. All of these losses can be avoided if the coil is heated by hot water instead of steam. It must also be remembered that a hot-water piping can carry more heat than a steam piping of the same size. 5. Heat transmission through bottom and side plates of ships was also discussed briefly in the paper.