Thermal design overall heat -transfer coefficient

The rate of heat-transfer q through the jacket or cod heat-transfer areaM is estimated from log mean temperature difference AT by = UAAT The overall heat-transfer coefficient U depends on thermal conductivity of metal, fouling factors, and heat-transfer coefficients on service and process sides. The process side heat-transfer coefficient depends on the mixing system design (17) and can be calculated from the correlations for turbines in Figure 35a. 

Higher overall heat transfer coefficients are obtained with the plate heat exchanger compared with a tubular for a similar loss of pressure because the shell side of a tubular exchanger is basically a poor design from a thermal point of view. Considerable pressure drop is used without much benefit in heat transfer efficiency. This is due to the turbulence in the separated region at the rear of the tube. Additionally, large areas of tubes even in a well-designed tubular unit are partially bypassed by liquid and low heat transfer areas are thus created. 

The overall heat transfer coefficient is a composite number. It depends on the individual heat transfer coefficients on each side of the tube and the thermal conductivity of the tube material. The individual heat transfer coefficient in turn depends on the fluid flow rate, physical properties of the fluid, and dirt factor. The temperature along the tube is not uniform. The hot and the cold fluids may flow in the same (cocurrent) or in opposite (countercurrent) directions. Generally the hot and cold fluids come in contact only once, and such an exchanger is called single pass. In a multipass exchanger, the design of the... 

The thermal performance of the designed heat exchanger can be checked by calculating the overall heat transfer coefficient. This required calculating the tube side and shell side heat transfer coefficients, the tube wall contribution to the resistance, and the appropriate fouling resistance. The overall heat transfer coefficient, based on the outside surface area of the tubes is... 

The thermal conductivity of carbon steel is 30 Btu/h ft °F. The overall heat transfer coefficient based on the design specifications. 

As a typical design, we consider a superficial gas velocity of 0.25 m/s, 6-mm particles, a tube diameter of 50 mm, as well as a density of 1.39 kg/m3, a viscosity of 1.44 x l(T5Ns/m3, a thermal conductivity of 7.40 x 10 5kW/m/K, and a heat capacity of 2.32 kj/kg/K. One gets Rep = 145, which gives heat-transfer coefficients between 70-100W/m2K (Table 5.12). The conclusion is that the gas side indeed controls the overall heat transfer. 

From the standpoint of heat-exchanger design the plane wall is of infrequent application a more important case for consideration would be that of a doublepipe heat exchanger, as shown in Fig. 10-2. In this application one fluid flows on the inside of the smaller tube while the other fluid flows in the annular space between the two tubes. The convection coefficients are calculated by the methods described in previous chapters, and the overall heat transfer is obtained from the thermal network of Fig. 10-2h as... 

As we learned with the examples of the present chapter, we need to know the overall coefficient of heat transfer for the (thermal) design or performance of a heat exchanger. Since this coefficient usually depends on unknown exit temperatures, we are faced with a trial-and-error procedure. For a first trial, an order-of-magnitude value of the heat transfer coefficient is usually satisfactory. Table 7.2, taken from Ref. 7, gives the value of the overall coefficient for a number of frequently encountered cases. This table, which includes the effect of a fouling factor, is more conservative than Table 1,2. 

A fundamental aspect in the reactor design is the contribution of different thermal resistances in achieving a highly efficient heat transfer. The overall heat coefficient U is given by the relation ... 

The heat transferred between two streams may be found by using an overall coefficient of heat transfer, U==Q/ A AT), By assuming negligible thermal resistance of the material separating the two streams, due to thin walls and high thermal conductivity of the material, and by designating the warmer stream by the subscript h and the cooler stream by the subscript c, the overall coefficient of heat transfer based on the warm-side heat transfer area, A, is defined as... 

---