Heat exchanger tube wall coefficient

The mass transfer coefficients Kg and K/ are overall coefficients analogous to an overall heat transfer coefficient in a shell-and-tube heat exchanger. The overall coefficient in a heat exchanger has three components, an inside coefficient, a wall resistance, and an outside coefficient. Analogs exist in mass transfer. For the inside coefficient, we consider the driving force between the bulk liquid concentration and liquid concentration at the interface ... 

Introduction. The use of fins or extended surfaces on the outside of a heat exchanger pipe wall to give relatively high heat-transfer coefficients in the exchanger is quite common. An automobile radiator is such a device, where hot water passes inside through a bank of tubes and loses heat to the air. On the outside of the tubes, extended surfaces receive heat from the tube walls and transmit it to the air by forced convection. 

Freeboard Heat Transfer. In some applications, e.g., fluidized combustors, heat exchanger tubes are located in the freeboard space above the bed, as well as within the bed. The coefficients / , h, and h, defined by Eqs. (15, 16, and 42), are also appropriate to represent the heat transfer process at the surface of such freeboard tubes. If the tube is placed so that it sees mostly bed material (not the vessel walls), Eqs. (44) and (45) may be used to estimate the radiative... 

Correlations for Convective Heat Transfer. In the design or sizing of a heat exchanger, the heat-transfer coefficients on the inner and outer walls of the tube and the friction coefficient in the tube must be calculated. Summaries of the various correlations for convective heat-transfer coefficients for internal and external flows are given in Tables 3 and 4, respectively, in terms of the Nusselt number. In addition, the friction coefficient is given for the deterrnination of the pumping requirement. 

Effect of Uncertainties in Thermal Design Parameters. The parameters that are used ia the basic siting calculations of a heat exchanger iaclude heat-transfer coefficients tube dimensions, eg, tube diameter and wall thickness and physical properties, eg, thermal conductivity, density, viscosity, and specific heat. Nominal or mean values of these parameters are used ia the basic siting calculations. In reaUty, there are uncertainties ia these nominal values. For example, heat-transfer correlations from which one computes convective heat-transfer coefficients have data spreads around the mean values. Because heat-transfer tubes caimot be produced ia precise dimensions, tube wall thickness varies over a range of the mean value. In addition, the thermal conductivity of tube wall material cannot be measured exactiy, a dding to the uncertainty ia the design and performance calculations. 

The values of CJs are experimentally determined for all uncertain parameters. The larger the value of O, the larger the data spread, and the greater the level of uncertainty. This effect of data spread must be incorporated into the design of a heat exchanger. For example, consider the convective heat-transfer coefficient, where the probabiUty of the tme value of h falling below the mean value h is of concern. Or consider the effect of tube wall thickness, /, where a value of /greater than the mean value /is of concern. 

The calculations are made as follows. The exchanger is divided into small increments to allow numerical integrations. A tube wall temperature is first calculated and then QAV. The gas temperature and composition from an increment can then be calculated. If the gas composition is above saturation for the temperature, any excess condensation can occur as a fog. This allows the degree of fogging tendency to be quantified. Whenever possible, experimental data should be used to determine the ratio of heat transfer to m.ass transfer coefficients. This can be done with a simple wet and dry bulb temperature measurement using the components involved. 

For the three heat exchangers from Exercise 1, make a first estimate of the order of magnitude of the overall heat coefficients from tabulated values of film transfer coefficients and fouling coefficients. Neglect the resistance from the tube walls. 

Table 5.17 gives estimates of the film transfer coefficients of an existing shell-and-tube heat exchanger, assuming the tube-wall resistance to be negligible. 

The overall heat transfer coefficient for thermal energy exchange between the tube wall and the reacting fluid may be taken as 1.0 x 10 3 cal/cm2-sec-°K. The effective thermal conductivity of the catalyst pellets may be taken as equal to 6.5 x 1CT4 cal/(sec-cm-°C). 

In BSCR heat can be removed through the walls of immersed heat exchanger s tubes. The heat transfer coefficient in BSCR can be correlated, supposing that the hydrodynamic conditions may be satisfactorily represented by ug, // /, psi, Cp si, and As/ as ... 

For many years I have carried around with me two sheets of notepaper that contain heat-exchange coefficient equations. These equations calculate the inside and outside tube wall heat-transfer coefficients for... 

The heat exchanger is quite large because of the low heat transfer coefficient found in these gas-phase systems. Therefore the mass of metal in the tubes is quite significant in terms of thermal capacitance. Table 7.3 gives design details of the heat exchanger. The tube diameter is 0.0254 m, length is 5 m, wall thickness is 0.000524 m, and heat capacity is 0.05 kJ kg-1 KT1. 

Repeat the analysis of hydrocarbon cracking in Example 5.6 for the case where there is external heat exchange. Suppose the reaction is conducted in tubes that have an i.d. of 0.012m and are 3m long. The inside heat transfer coefficient is 9.5cal/(K m2 s) and the wall temperature is 525°C. The inerts are present. 

Viscosity Correction for the Dirty Exchanger. These overall coefficients are based upon the uncorrected film heat transfer coefficients, i.e. those without the viscosity correction parameter (gfgw)014. As the temperature difference across a layer is proportional to the layer s thermal resistance (see Equations (2) and (10)), the relative resistances, above, allow estimation of the mean tube wall temperature, and hence evaluation of the viscosity corrections (jib/hw)° 14 to the film coefficients. Mean bulk temperature difference ... 

In addition, the heat transport at the boundary between the fixed bed and the heat exchange surface is also decisive for the heat exchange. The latter heat transport is generally described by a wall heat-transfer coefficient otB.. It lumps the complex interplay between convective flow at the tube wall and conduction transport by contact between the fixed bed and the heat exchange surface. Heat transport in packed tubes has been investigated and discussed in detail [8, 21]. How-... 

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... 

Hot water at 90°C flows on the inside of a 2.5-cm-ID steel tube with 0.8-mm wall thickness at a velocity of 4 m/s. This tube forms the inside of a double-pipe heat exchanger. The outer pipe has a 3.75-cm ID, and engine oil at 20°C flows in the annular space at a velocity of 7 m/s. Calculate the overall heat-transfer coefficient for this arrangement. The tube length is 6.0 m. 

Saturated steam at 100 lb/in2 abs is to be used to heat carbon dioxide in a cross-flow heat exchanger consisting of four hundred 1-in-OD brass tubes in a square in-line array. The distance between tube centers is j in, in both the normal- and parallel-flow directions. The carbon dioxide flows across the tube bank, while the steam is condensed on the inside of the tubes. A flow rate of I lb ,/s of CO at 15 lb/in2 abs and 70°F is to be heated to 200°F. Estimate the length of the tubes to accomplish this heating. Assume that the steam-side heat-transfer coefficient is 1000 Btu/h ft2 °F, and neglect the thermal resistance of the tube wall. 

Heat-transfer coefficient for cross flow over an ideal tube bank Fouling coefficient on outside of tube Heat-transfer coefficient in a plate heat exchanger Shell-side heat-transfer coefficient Heat transfer coefficient to vessel wall or coil Heat transfer factor defined by equation 12.14 Heat-transfer factor defined by equation 12.15 Friction factor... 

Note that UjA-, - U A, but D, t unless A = A . Therefore, the overall heat transfer coefficient U of a heat exchanger is meaningless unless the area on which it is based is specified. This is especially the case when one side of the tube wall is finned and the other side is not, since the surface area of the finned side is several times that of the unfiiined side. 

A counter-flov/ double-pipe heat exchanger is to heat water from 20°C to 80°C at a rate of 1.2 kg/s. The heating is to be accomplished by geothermal water available at 160°C at a mass flovr rate of 2 kg/s. The inner tube is thin-walled and has a diameter of 1.5 cm. If the overall beat transfer coefficient of the heat exchanger is 640 W/m C, determine the length of the heat exchanger required to achieve the desired heating. 

Hot oil is to be cooled by water in a 1-shell-pass and 8-lube-passes heat exchanger. The tubes are thin-walled and are made of copper with an internal diameter of 1.4 cm. The length of each tube pass in the heat exchanger Is 5 m, and the overall heat transfer coefficient is 310 W/m °C. Water flows through the tubes at a rate of 0.2 kg/s, and the oil through the shell at a rate of 0.3 kg/s. The v/ater and the oil enter at temperatures of 20 C and 150°C, respectively. Determine the rate of heat transfer in the heat exchanger and the outlet temperatures of the water and the oil. 

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