Heat exchangers natural convection

Direct contact heat exchange natural convection of Pb-Bi, assisted by lift pump Test programme ongoing in RLNL TITech... 

Convective heat exchange, natural or forced Radiation heat transfer, e.g. in furnaces Evaporation Condensation... 

Convective heat exchange, natural or forced Radiant heat transfer, e.g. furnaces Evaporation, e.g. in evaporators Condensation, e.g. in shell and tube heat exchanges Heat transfer to boiling liquids, e.g. in vaporizers, boilers, re-boilers ... 

Decay heat removal loops. The normal route for the removal of decay heat in a fast reactor is via the secondary sodium circuits and the steam plant. Should this route not be available, decay heat in PFR could be rejected by one or more of three thermal syphon loops, each filled with eutectic sodium/potassium alloy. Each loop extracted heat through an immersed coil, intercq>ting some of the primary sodium as it flowed from the core towards an intermediate heat exchanger, and delivered the heat by natural convection to the outside atmosphere through a sodium-potassium/air heat exchanger built into the wall of the secondary containment building. 

This shutdown was preceded by two safety tests in September which allowed, on the one hand, to confirm the ability of Superphenix to evacuate decay heat by natural convection of the primary circuit and the secondary loops after reactor shutdown, and on the other hand, to demonstrate the absence of risk of a reactivity accident, caused by a passage of argon gas in the core following depressurization of an intermediate heat exchanger argon-filled bell. 

Air cooled heat exchangers will continue to operate (but at reduced capacity) due to radiation and natural convection air circulation should a power failure occur. 

Convection The mechanism of heat transfer due to different temperatures, and hence different densities in fluids. It may be natural, dependent only on thermal forces, or forced, when use is made of a rotodynamic device to improve the rate of heat exchange. 

Convection is heat transfer between portions of a fluid existing under a thermal gradient. The rate of convection heat transfer is often slow for natural or free convection to rapid for forced convection when artificial means are used to mix or agitate the fluid. The basic equation for designing heat exchangers is... 

The transfer of heat by convection is also an important component of the indirect cooling of the process. Natural convection currents result from localized heating/cooling effects and the tendency of hot fluids to rise above colder fluids, while forced convection, utilizing a pump, enables higher rates of heat transfer to occur (within the limits of the heat-exchanger design). 

There are several possible mechanisms for the heat exchange between a reacting medium and a heat carrier radiation, conduction and forced or natural convection. Here we shall consider convection only. Other mechanisms are considered in the chapter on heat accumulation. The heat exchanged with a heat carrier (q ) across the reactor wall by forced convection is proportional to the heat exchange area (A) and to the driving force, that is, the temperature difference between the reaction medium and the heat carrier. The proportionality coefficient is the overall heat transfer coefficient (U) ... 

When a liquid warms up, its density decreases, which results in buoyancy and an ascendant flow is induced. Thus, a reactive liquid will flow upwards in the center of a container and flow downwards at the walls, where it cools this flow is called natural convection. Thus, at the wall, heat exchange may occur to a certain degree. This situation may correspond to a stirred tank reactor after loss of agitation. The exact mathematical description requires the simultaneous solution of heat and impulse transfer equations. Nevertheless, it is possible to use a simplified approach based on physical similitude. The mode of heat transfer within a fluid can be characterized by a dimensionless criterion, the Rayleigh number (Ra). As the Reynolds number does for forced convection, the Rayleigh number characterizes the flow regime in natural convection ... 

The temperature of the tube ends of the upper insulation module was higher than that of the tube ends in the lower insulation module, which is a result of additional heat transfer from the reaction zone by natural convection in the upper module. In both cases, the heat loss is sufficient to prevent the rotary valves next to the heat exchanger modules from being damaged by overheating. 

A heat pump system utilizes a heat exchanger buried in water-saturated soil as a heat source. The heat exchanger basically consists of a series of vertical plates with height of 30 cm and a width of 10 cm. These plates are effective Is at a uniform temperature of 5°C. The soil can be assumed to have a permeability of 10 0 nr and apparent thermal conductivity of 0.1 W/m-K. The temperature of the saturated soil far from the heat exchanger is 30°C. Assuming natural convective flow and that there is no interference between the flows over the individual plates, find the mean heat transfer rate to a plate. 

The use of fins is most effective in applications involving a low convection heat transfer coefficient. Thus, the use of fins is more easily justified when the medium is a gas instead of a liquid and the heat transfer is by natural convection instead of by forced convection. Therefore, it is no coincidence that in liquid-to-gas heat exchangers such as the car radiator, fins are placed on the gas side. 

Heat exchangers are complicated devices, and the results obtained with the simplifled approaches presented above should be used with care. For example, we assumed that the overall heat transfer coefficient V is constant throughout the heat exchanger and tliat the convection heat transfer coefficients can he predicted using the convection correlations. However, it should be kept in mind that the uncertainty in the predicted value of U can exceed 30 percent. Thus, it is natural to tend to overdesign the hear exchangers in order to avoid unpleasant surprises. 

Convection. The transfer of heat by the mixing or movements of fluids or fluids with a solid. Mixing may occur as a result of density difference alone, as in natural convection. Alternatively, mechanically induced agitation may produce forced convection, as in turbulent flow in a heat exchanger tube, or to the heat transfer fluid in the jacket of an agitated vessel. The rate of heat transfer is ... 

The schemes with natural reflux are operated based on the principle of natural convection (i.e. circulation based on density difference between cold and heated fluid streams) to create a thermo-siphon. The hotter stream has the lower density. In natural reflux, the heated feed passes to the top of the heat exchanger and then to the bottom of tower by itself while the colder stream drains from the tower bottom to the heater. Forced reflux is based on using pumps for the circulation. 

The first section presents some fundamental ideas that are frequently referred to in the remainder of the chapter. The next three sections deal with the major topics in natural convection. The first of these addresses problems of heat exchange between a body and an extensive quiescent ambient fluid, such as that depicted in Fig. 4.1a. Open cavity problems, such as natural convection in fin arrays or through cooling slots (Fig. 4.1fe), are considered next. The last major section deals with natural convection in enclosures, such as in the annulus between cylinders (Fig. 4.1c). The remaining sections present results for special topics including transient convection, natural convection with internal heat generation, mixed convection, and natural convection in porous media. 

---