Coolant velocity

With tube side condensation, coefficients are generally lower than for comparable shell side condensers. This phenomenon is attributed to (1) lower coolant velocities outside the tubes than are possible with tube side cooling, and (2) increased film thicknesses, namely, film resistance inside the tubes. 

Effect of surface roughness. CHF for rough surfaces was measured on vertical annular tubes cooled by a downward flow of subcooled water by Durant and Mirshak (1959, 1960). An increase in the apparent critical heat flux of as much as 100% over a smooth surface was obtained at the same coolant velocity, temperature, and pressure. The heated surfaces were 304 SS and Zircaloy-2 tubes about... 

The stabilizing effect of cocurrcnt cooling has hardly been exploited up to now in industrial reactors. This may be due to the fear that, at the required low flow velocity (in the example of Fig. 20B, , = 0.01 m/s). heat transfer will be inadequate and natural convection will occur in the cooling jacket. However, , describes the mean coolant velocity parallel to the tube axis. With a cross-cocurrcnt flow of the coolant, the actual flow velocity may in fact be substantially larger, de-... 

In DCMD, increase in flow rate increases the permeate flux. The shear force generated at high-flow rate reduces concentration polarization. Banat [64] found that the flow rate of cooling water had minimal effect on the permeate flux. Ohta [65] has shown that an increase in coolant velocity from 0.02 to 0.08 m s resulted in 1.5-fold increase in the permeate flux. In the same study, it was found that an increase in velocity of hot feed increased the flux by twofold. 

Mdiere AT is the tenqierature differraice between the outer surface of the can and the coolant (°C) and h the film coefficient for heat transfer betwem can and coolant (W m K ). The film coefficioit is affected by the coolant velocity along the surface, the tenyierature gradient and also by the onset of boiling in a BWR. 

Figure 9. High coolant velocities in unclogged tubes of radiator section caused impingement attack and penetrating failures.

By now, comprehensive studies have been performed and reliable industrial experience has been gained on material corrosion in sodium. Corrosion intensity in sodium is significantly lower than that in water or lead-based coolants [5.9], In sodium, as well as in the other liquid metals, corrosion rate depends on many factors (temperature level, coolant velocity, impurity content, temperature difference, time, etc.). When evaluating corrosion rate, the major part of researchers took into account only the most contributing factors. Empirical equations for corrosion rate were most commonly derived for 316 steel at the coolant velocity of > 4 m/s and oxygen content of < 10 ppm. The most reliable results were obtained in [5.10, 5.11] for corrosion rate K, mg/cm h, that can be expressed as follows ... 

It was found in the tests performed on the experimental and industrial facilities, that corrosion rate for chromium steels in PbBi alloy is 6-60 mg/m at 450-500°C. It can vary with temperature, coolant velocity, oxygen content and other parameters. In lead, this value is about 0.026 mg/m h at 600°C with no mass transfer [5.20]. 

Coolant velocity in the core Core pressure drop Pumping power... 

Maximum permissible coolant velocity (based on corrosion and erosion conditions) is 2.5 m/s ... 

Although the density of the gas coolant is considerably smaller than that of the water or sodium coolant, the allowable temperature rise and coolant velocity can be substantially larger. In addition, of course, the coolant cross-sectional area can be made larger if necessary, with some sacrifice to the power density but without neutron absorption. The temperature rise for the water coolant is generally limited by the amount of boiling allowable, whereas the temperature rise for the sodium coolant is generally limited by thermal-stress problems that can arise in metal... 

The required coolant flow is 94.5 x 10 Ib/hr, of which 94% goes through the core subassemblies. The total effective coolant flow area is 3760 in., giving a core average coolant velocity of 18.3 fps. 

The 42-in. and 30-in. pipe diameter are selected to obtain coolant velocities which provide a reasonable balance between erosion-corrosion, pressure drop, and system volume. The surge line is sized to limit the frictional pressure loss through it during the maximum in-surge so that the pressure differential between the pressurizer and the heat transfer loops is no more than 5 percent of the system design pressure. The spray line sizing is discussed in Section 5.4.10. 

If care is taken in the panel design to prevent low velocity regions near the tops of panels, which can become easily blocked by vapor, and if a sufficiently high coolant velocity is used (>1 m/s... 

Plant Coolant velocity in core (m/s) Pressure drop across core (MPa)... 

The circulation scheme adopted for the main circulation circuit, which provides for free levels of coolant in the upper part of the reactor mono-block and in the SG module channels, coupled with a low coolant velocity in the downcomer sections of the circuit, ensures a reliable separation of the steam-water mixture from the lead-bismuth coolant in the event of an accidental tightness failure in the SGtube system. 

The loop consists of 19 heater tubes, a cooler at the top plenum, and 18 downcomer tubes. Various cases were analyzed. The average mass flow rate and average velocity of the coolant in the coolant tube were found to be 6.7 kg/s and 0.04 m/s respectively. The variation of coolant velocity with internal diameter of fuel channels is shown in Fig. XXIX-8. 

FIG. XXIX-8. Coolant velocity versus internal diameter of coolant channels. 

The high surface to volume ratio of spherical fuel elements ensures excellent heat transfer conditions, which results in low maximum and average fuel temperatures. In case of a coated particle fuel, the situation is even more favourable, since such fuel is designed to operate at very high temperatures. The coolant velocity is about 16 cm/sec. The core is cooled by forced convection, but the residual heat produced in the fuel chamber is removed by natural convection. 

Physical and chemical conditions and durability of the materials in the IMR. More data are required to validate the integrity of fuel cladding under boiling conditions. A strategy of water chemistry must be developed. Flow induced vibration (FIV) related issues could be neglected because of low coolant velocity (less than Im/s in the riser). 

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