Heat transfer coefficient mass flow rate

Overall heat transfer coefficient Mass flow rate... 

One manner in which size may be computed, for estimating purposes, is by employing a volumetric heat-transfer concept as used for rotary diyers. It it is assumed that contacting efficiency is in the same order as that provided by efficient lifters in a rotaiy dryer and that the velocity difference between gas and solids controls, Eq. (12-52) may be employed to estimate a volumetric heat-transfer coefficient. By assuming a duct diameter of 0.3 m (D) and a gas velocity of 23 m/s, if the solids velocity is taken as 80 percent of this speed, the velocity difference between the two would be 4.6 m/s. If the exit gas has a density of 1 kg/m, the relative mass flow rate of the gas G becomes 4.8 kg/(s m the volumetric heat-transfer coefficient is 2235 J/(m s K). This is not far different from many coefficients found in commercial installations however, it is usually not possible to predict accurately the acdual difference in velocity between gas and soRds. Furthermore, the coefficient is influenced by the sohds-to-gas loading and particle size, which control the total solids surface exposed to the gas. Therefore, the figure given is only an approximation. 

In the case of a temperature probe, the capacity is a heat capacity C == me, where m is the mass and c the material heat capacity, and the resistance is a thermal resistance R = l/(hA), where h is the heat transfer coefficient and A is the sensor surface area. Thus the time constant of a temperature probe is T = mc/ hA). Note that the time constant depends not only on the probe, but also on the environment in which the probe is located. According to the same principle, the time constant, for example, of the flow cell of a gas analyzer is r = Vwhere V is the volume of the cell and the sample flow rate. 

The essential feature of a Jluidized-bed reactor is that the solids are held in suspension by the upward flow of the reacting fluid this promotes high mass and heat transfer rates and good mixing. Heat transfer coefficients in the order of 200 W/m-°C between jackets and internal coils are typically obtained. The solids may be a catalyst, a reactant (in some fluidized combustion processes), or an inert powder added to promote heat transfer. 

The thesis of Steward indicates that the overall liquid film and mass transfer coefficients were functions of the gas flow rate and the column pressure and are independent of the liquid flow rate and inlet air temperature. The gas film heat transfer coefficient was found to be a function only of the air flow rate. 

Wall-to-bed heat-transfer coefficients were also measured by Viswanathan et al. (V6). The bed diameter was 2 in. and the media used were air, water, and quartz particles of 0.649- and 0.928-mm mean diameter. All experiments were carried out with constant bed height, whereas the amount of solid particles as well as the gas and liquid flow rates were varied. The results are presented in that paper as plots of heat-transfer coefficient versus the ratio between mass flow rate of gas and mass flow rate of liquid. The heat-transfer coefficient increased sharply to a maximum value, which was reached for relatively low gas-liquid ratios, and further increase of the ratio led to a reduction of the heat-transfer coefficient. It was also observed that the maximum value of the heat-transfer coefficient depends on the amount of solid particles in the column. Thus, for 0.928-mm particles, the maximum value of the heat-transfer coefficient obtained in experiments with 750-gm solids was approximately 40% higher than those obtained in experiments with 250- and 1250-gm solids. 

Fig. 4.25 represents a steady-state, single-pass, shell-and-tube heat exchanger. For this problem W is the mass flow rate (kg/s), T is the temperature (K), Cp is the specific heat capacity (kJ/m s), A (= 7i D Z) is the heat transfer surface area (m ), and U is the overall heat transfer coefficient (kJ/m s K). Subscripts c and h refer to the cold and hot fluids, respectively. 

Steady-State Heat Exchanger Constant U=1.5 Heat transfer coefficient [kJ/s m2 K] Constant CP=4.18 Specific heat capacity [kJ/kg K] Constant WH=8.5 Mass flow rate of hot fluid [kg/s] Constant WC=4.17 Mass flow rate,cold fluid [kg/s] Constant TCIN = 323 Inlet cold water temp.[K] Constant TCOUT = 343 Outlet cold water temp. [K]... 

Three different principles govern the design of bench-scale calorimetric units heat flow, heat balance, and power consumption. The RC1 [184], for example, is based on the heat-flow principle, by measuring the temperature difference between the reaction mixture and the heat transfer fluid in the reactor jacket. In order to determine the heat release rate, the heat transfer coefficient and area must be known. The Contalab [185], as originally marketed by Contraves, is based on the heat balance principle, by measuring the difference between the temperature of the heat transfer fluid at the jacket inlet and the outlet. Knowledge of the characteristics of the heat transfer fluid, such as mass flow rates and the specific heat, is required. ThermoMetric instruments, such as the CPA [188], are designed on the power compensation principle (i.e., the supply or removal of heat to or from the reactor vessel to maintain reactor contents at a prescribed temperature is measured). 

It is more convenient to express the mass transfer coefficient in terms of a humidity difference, so that IcgA(Ps — P, ) kA(M, — M). The rate of drying is thus determined by the values of h, AT and A, and is not influenced by the conditions inside the solid, h depends on the air velocity and the direction of flow of the air, and it has been found that h = CG 0 S where G is the mass rate of flow of air in kg/s m2. For air flowing parallel to plane surfaces, Shepherd et alnv> have given the value of C as 14.5 where the heat transfer coefficient is expressed in W/m2 K. 

We have assumed the overall heat transfer coefficient U is constant It may be a function of the coolant flow rate Fj or the composition of the reaction mass, giving one more variable but also one more equation. 

For liquids the situation is more complicated for all these flow geometries, and mass and heat transfer coefficients cannot be found from the same relations. One major difference with liquids is that Sc, Pr, and Le are frequently not close to unity so they cannot be ignored in these expressions. Correlations developed for heat transfer in gases are frequently not directly applicable to mass transfer for liquids. One should find suitable sources of correlations for any particular fluid and geometry if accurate estimations of mass and heat transfer rates are needed. 

A = area for heat transfer t = specific heat capacity of medium H = heat content of steam relative to initial medium temperature = mass flow rate of steam At = initial mass of medium q= rate of heat transfer t = time T = temperature = initial temperature of medium = temperature of heat source and U = overall heat transfer coefficient. 

The mass flow rate of particles affects significantly the heat and mass transfer coefficients. 

U A = heat transfer coefficient and area of environment - cal/s °C V = volume of jacket - liters w = mass flow rate of jacket water - g/s 0 = dead, delay or lag time - seconds... 

The time constant of any process is the result of its capacitance and resistance. Usually, the heat exchanger outlet temperature is the controlled variable, and the flow rate of the heat transfer fluid is the manipulated variable. The time constant of an exchanger is a function of the mass and the specific heat of the tube material, the mass flow, and the specific heat of the process and utility streams and their heat transfer coefficients. 

In a countercurrent packed column, n-butanol flows down at the rate of 0.25 kg/m2 s and is cooled from 330 to 295 K. Air at 290 K, initially free of n-butanol vapour, is passed up the column at the rate of 0.7 m3/m2 s. Calculate the required height of tower and the condition of the exit air. Data Mass transfer coefficient per unit volume, hDa = 0.1 s 1. Psychrometric ratio, (h/hDpAs) = 2.34. Heat transfer coefficients, hL = 3hG. Latent heat of vaporisation of n-butanol, A = 590 kJ/kg. Specific heat capacity of liquid n-butanol, Cl = 2.5 kJ/kg K. Humid heat of gas , s = 1.05 kJ/kg K. 

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