Study On Deep Heat Transfer In Large-scale Longitudinal Flow Shell-and-Tube Heat Exchangers

Shell-and-tube heat exchangers (STHXs) are the most important general equipments widely used in power, energy, metallurgy, chemical industries, etc. In STHXs, the ratio, ? (outlet temperature of hot fluid to that of cold one) indicates heat exchange depth. When outlet temperature of hot fluid is lower than that of cold fluid, ? ? 1 deep heat transfer is achieved. Deep heat transfer state is meaningful in some industries; for example, in sulphuric acid manufacture transforming system, ethylene production system, and distillation system. In sulphuric acid industry, STHXs have a trend to be super large as the production scale is getting larger. L/D (ratio of length to diameter) of super large STHXs is getting smaller because the length of STHXs (L) is unchanged but the diameter of STHXs (D) is getting larger. However, deep heat transfer is getting more and more uneasy to be achieved as L/D is smaller and smaller. Setting multi-parallel-channel structure (MPC) in the shell side of super large STHXs is a way to solve the problem. Super large STHX with MPC in the shell side can be seen as a heat exchanger network composed of many parallel-tube-bundle STHXs. In super large STHXs with MPC in the shell side, deep heat transfer can be achieved because of L/W of parallel-tube-bundle STHX is large though L/D of super large STHXs is small. It is necessary to study on L/W and deep heat transfer characteristics of parallel-tube-bundle STHXs. Super large STHXs with MPC in the shell side have been applied in industries, but velocity field in the shell side is unclear. In this thesis, velocity field in the shell side of super large STHXs with MPC also will be studied.In this thesis, super large STHXs with MPC in the shell side can be divided into a lot of parallel-tube-bundle by adding clapboards in the shell side. Based on geometric similar principle, a classical parallel-tube-bundle flow path in the super large STHXs with MPC in the shell side is studied by numerical analysis to get the velocity field in the shell side. As it is similar to the flow path in the shell side of super large STHXs with MPC in the shell side, the research results are meaningful to show the velocity field in STHXs with MPC in the shell side. The influence of L/W of parallel-tube-bundle flow path to deep heat transfer characteristic is also studied by computational fluid dynamics commercial software FLUENT.Super large STHXs with MPC in the shell side can be seemed as a heat exchanger network composed of a lot of parallel-tube-bundle flow path models. Each parallel-tube-bundle flow path model can be seemed as a parallel-tube-bundle STHX. Five parallel-tube-bundle STHXs are made according to the parallel-tube-bundle flow path model. Experimental and numerical study on the five STHXs with different L/W has been done to reveal the influence of L/W on fluid distribution, heat transfer and resistance performances in the shell side. The experimental platform is set up and reliability analysis is carried out on the equipments and test system. Hot air flow in the tube side and cold air flow in the shell side of parallel-tube-bundle STHXs. Numerical results have a good agreement with experimental results. Inlet temperature of hot air in five STHXs is the same, also is the inlet temperature of cold air. The total heat transfer coefficients are obtained. The influence of L/W on deep heat transfer performance in STHXs is summarized. The results show that in the condition of average velocity is 10 m·s^-1 both in shell and tube side, velocity distribution in shell side is more and more uneven with L/W decreasing. At the same time, heat transfer performances decline sharply and pressure drop in shell side increases dramatically with L/W decreasing. Deep heat transfer can be achieved in parallel-tube-bundle STHXs when L/W?4.62. But in parallel-tube-bundle STHXs with L/W?3.08, deep heat transfer performance can't be achieved any more.Temperature field uniformity principle is used to analysis the mechanism of heat transfer performance decline with L/W decreasing. A parallel-tube-bundle STHX can be divided into a lot of sub-elements. This is equivalent to take a whole heat exchanger as a heat exchanger network composed of several sub-STHXs. In sub-STHXs, there is a characteristic hot fluid temperature ( t_h) and a characteristic cold fluid temperature ( t_c), and their difference is named local characteristic temperature difference ( H ). The aggregate of these local characteristic temperature differences forms a temperature difference field (in short: TDF) of the STHX H(x,y,z)=t_h(x,y,z)-t_c(x,y,z) Heat transfer performance of STHXs is determined by the synergy between hot fluid temperature field and cold fluid temperature field, that is, temperature difference field. Hot and cold fluid temperature is a function of space. The closer of their functional form, the better synergy they are. In this thesis, parallel-tube-bundle STHXs are divided into a lot of sub elements. By this method, the internal temperature difference field is shown in the form of 2D figures. In order to quantitatively describe the uniformity of temperature difference field in a STHX, a factor named the uniformity factor of TDF is defined. After analysis, conclusions are obtained: the inherent reason why heat transfer efficiency decreases with L/W decreasing is that the uniformity of temperature difference field decreases with L/W decreasing. It is consistent with the principle of uniformity of temperature difference field proposed by GUO.Heat transfer efficiency depends not only on numbers of transfer units and temperature differences between both the inlet t and outlet temperature of hot fluid and cold fluid, but also on the fluid flow pattern (co-current flow, counter-current flow, cross flow). Countercurrent heat exchanger has the highest heat transfer efficiency depends on the most uniform field of heat transfer temperature difference compared to co-current and cross heat exchangers. At the same condition, compared with TDF in the counter-current flow pattern, the degree of uneven of TDF in cross flow or co-current flow reflects the loss of temperature difference. In this thesis, heat transfer temperature difference in baffle flow pattern is compared with that in counter flow pattern in terms of flow path distribution analyses. The deviation of heat transfer temperature difference between the two flow patterns have been analyzed at different?. In order to make the whole heat transfer temperature difference in STHXs lose under 5% for?<1, the ratio of baffle flow area to the whole area of heat exchangers should be smaller than 0.6 / R_1a,c and R_1a,c is R₁ at critical point of deep heat transfer in countercurrent flow pattern.Super large STHXs with MPC in the shell side have been applied in industrial production systems, and have made great economic benefits by the higher efficiency. In this thesis, velocity field in the shell side of super large STHXs with MPC in the shell side is studies based on geometric similar principle. Five parallel-tube-bundle STHXs have been made to do experimental study on the influence of L/W to deep heat transfer characteristic. The range at with L/W deep heat transfer can be achieved is obtained based on the experimental and numerical study. The mechanism of heat transfer performance decreases with L/W decreasing is analyzed on the basis of the uniformity principle of temperature difference field. Based on flow path analysis research, the deviation of heat transfer temperature difference between baffle flow area and counter flow area for different?is given. In order to achieve deep heat transfer, the reasonable ratio of baffle flow area to the whole heat transfer area has been given. Research work in this thesis provides a theoretical reference for design of super large STHXs with MPC in the shell side, and is useful in industrial applications.