Heat transfer coefficients average coefficient

Film condensation on a horizontal tube. For a curvilinear surface, in particular, for a horizontal circular cylinder along which a condensate film flows, the angle 6 is a nonconstant variable. By taking into account the fact that 6(6) d, where d is the diameter of a circular cylinder, and proceeding by analogy with (5.7.7), one can readily obtain the following formula for the heat transfer coefficient averaged over the external surface of the tube provided that the flow in the condensate film is laminar [200] ... 

The heat-transfer coefficient averaged over each time interval is, by definition,... 

Overall heat transfer coefficient averaged over ... 

For flow of a fluid at right angle to a circular cylinder, the average heat-transfer coefficient-averaged around the periphery of the cylinder—is given by (Eckert and Drake, 1959)... 

In one run, sodium sulfate and calcium chloride were added to a 200-gallon batch of sea water to give a sulfate concentration of 5200 p.p.m. and a calcium concentration of 640 p.p.m. An addition was made of 5 p.p.m. of low molecular weight polyacrylic acid and the treated sea water was evaporated under standard conditions. The over-all heat transfer coefficient averaged 3600 B.t.u./sq. ft. hr. ° F. for a 5-hour period, with no detectable falloff during the period, and the tube was found to be clean, except for the plastic film. In contrast, evaporation of a similar batch of calcium sulfate-enriched sea water, with no polyacrylic acid, resulted in a marked decrease in heat transfer coefficient to less than 800 B.t.u./sq. ft. hr. ° F. after 2 hours. 

For the BFB operating at the above conditions, what would be the heat transfer coefficient (average) around... 

The effective thermal conductivity of a Hquid—soHd suspension has been reported to be (46) larger than that of a pure Hquid. The phenomenon was attributed to the microconvection around soHd particles, resulting in an increased convective heat-transfer coefficient. For example, a 30-fold increase in the effective thermal conductivity and a 10-fold increase in the heat-transfer coefficient were predicted for a 30% suspension of 1-mm particles in a 10-mm diameter pipe at an average velocity of 10 m/s (45). 

Frequently experimental data report average heat-transfer coefficients based upon an arbitrarily denned temperature difference, the two most common being... 

In general, the average heat-transfer coefficient on immersed bodies is predicted by... 

Panel Coils Immersed in Liquid Overall Average Heat-Transfer Coefficients ... 

For low values of the Reynolds number, such as 10, where sn eamline flow should certainly apply, the Nusselt number has a value of about 2, and a typical value of the average heat transfer coefficient is 10 ". For a Reynolds number of 104, where the gas is certainly in turbulent flow, the value of the Nusselt number is typically 20. Hence there is only a difference of a factor of ten in the heat transfer coefficient between tlrese two extreme cases. 

This approach is much easier to use than the first, but may give less accurate results since it assumes an average or typical value for the heat transfer coefficient for all heated source geometries. 

Figure 10-50C. Tube-side (inside tubes) liquid film heat transfer coefficient for Dowtherm . A fluid inside pipes/tubes, turbulent flow only. Note h= average film coefficient, Btu/hr-ft -°F d = inside tube diameter, in. G = mass velocity, Ib/sec/ft v = fluid velocity, ft/sec k = thermal conductivity, Btu/hr (ft )(°F/ft) n, = viscosity, lb/(hr)(ft) Cp = specific heat, Btu/(lb)(°F). (Used by permission Engineering Manual for Dowtherm Heat Transfer Fluids, 1991. The Dow Chemical Co.)...

Providing that the flow rates are steady, the heat transfer coefficients do notvary and the specific heat capacities are constant throughout the working range, the average temperature difference over the length of the curve is given hy ... 

In experiments with bubble-columns containing suspended sand particles with average diameter 0.12 mm, an increase in heat-transfer coefficient was observed with increasing sand concentration, maximum values of 6000 kcal/m2-hr-°C being measured for suspensions containing 50% sand (based on the liquid volume). 

A high degree of turbulence is obtained even at low flowrates and the high heat transfer coefficients obtained are illustrated by the data in Table 9.23. These refer to the heating of cold water by the equal flow of hot water in an HF type exchanger (aluminium or copper), at an average temperature of 310 K. 

From both of these equations, it will be noted that the heat transfer coefficient is noi a function of the temperature difference. Here Re" — (d N pj x-) and Re = (udvp/n), where dv is the tube diameter and u is the average velocity of the liquid in the film in the axial direction. 

It should be emphasized that under conditions of energy dissipation the definition of the heat transfer coefficient as k dT/dr)/ T — T ), where T is the average fluid temperature and 7 is the wall temperature, does not characterize the acmal heat transfer properly (Kays and Crawford 1993 Schlichfing 2000). 

It may be used, the relation of the time-averaged heat transfer coefficients on the top and bottom, as a criterion for determination of dryout. It was assumed that the relation he/hi < 1 indicates dryout, i.e., the surface superheat Tw -7f is greater than that, when the surface contacts single-phase water only (hg is the heat transfer at the bottom of the channel). This method can be applied to connect dryout with hydraulic conditions, if the value of he may be associated with intermittent flow parameters. 

It was observed that at the same boiling number and inlet temperature, an increase in diameter shifts the ONB further from the inlet. The region of the local dryout decreases and the average heated surface temperature decreases as well. Under this condition the heat transfer coefficient increases with increased hydraulic diameter. 

U = overall heat transfer coefficient V = reactor volume = number average DP Xyf = weight average DP... 

Optimized molecular weight-conversion relationship is related to the system heat transfer coefficient. The degree of conversion improvement from improved heat transfer depends on the average molecular weights of polymer being produced for a given initiator system. 

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