Operating limits heat pipe

Although there are several limits which apply to heat pipe operation, these generally lend themselves to specific design solutions or occur at sufficiently high levels of performance to permit a wide latitude of practical appHcations. The envelope of these limits is shown generically in Figure 3. 

The temperatures Tm of the heat transfer medium required at the operating limits 110 and 120 of the reaction are found by making Qr = Q. The results are tabulated. Plots are shown for 25 mm and 75 mm. In the largest pipe Tm... 

Figure 12.36 illustrates the relationship between the static liquid and static vapor pressures in an operating heat pipe. As shown, the capillary pressure gradient across a liquid-vapor interface is equal to the pressure difference between the liquid and vapor phases at any given axial position. For a heat pipe to function properly, the net capillary pressure difference between the wet and dry points, identified in Fig. 12.36, must be greater than the summation of all the pressure losses occurring throughout the liquid and vapor flow paths. This relationship, referred to as the capillary limitation, can be expressed mathematically as... 

The test facility needs to be established to test the heat transfer performance and heat transport limitations. Both transient and steady-state tests should be conducted for a micro heat pipe. For low-temperamre heat pipes, however, the steady-state test is of most concerned. A typical experimental system for low-temperature heat pipes similar to the one shown in Fig. 3 would normally be used. The test facility shown in Fig. 3 consists of the heat pipe, a heat power supply and measuring unit, a cooling unit, and a data acquisition unit for the temperature measurements. The operating temperature of a heat pipe can be controlled by a cooling block connected to a cooling bath, where the temperature of the coolant is maintained at a constant temperature based on the designed operating temperature. The heat source is directly connected to the evaporator. 

When liquids are to be evaporated on a small scale, the operation is often accomplished in some form of jacketed kettle. This may be a batch or continuous operation. The rate of heat transfer is generally lower than for other types of evaporators and only a limited heat transfer surface is available. The kettles may or may not be agitated. Jackets may be of several types conventional jackets (formed with another cylinder concentric to the vessel), dimpled jackets, patterned plate jackets, and half-pipe coil jackets. (See Figure 11-1.) This variety provides a great deal of flexibility in the choice of heat transfer medium. 

Microheat pipes are subjected to the same operating limits as the conventional heat pipe. The capillary limitation is the most important operating limit of the microheat pipe. 

At low temperatures, viscous forces are dominant in the vapor flow down the heat pipe. At very low operating temperature, the vapor pressure difference between the closed ends of the evaporator (the high-pressure region) may be extremely small. Because of the small pressure difference, the viscous forces within the vapor region may prove to be dominant and hence limit the heat pipe operation. From the 2-D analysis by Busse,... 

Possible limiting range of a heat pipe as a function of operating temperature 496... 

PWRs operate differendy from BWRs. In PWRs, no boiling takes place in the primary heat-transfer loop. Instead, only heating of highly pressurized water occurs. In a separate heat-exchanger vessel, heat is transferred from the pressurized water circuit to a secondary water circuit that operates at a lower pressure and therefore enables boiling. Because of thermal transfer limitations, ultimate steam conditions in PWR power plants ate similar to those in BWR plants. For this reason, materials used in nuclear plant steam turbines and piping must be more resistant to erosion and thermal stresses than those used in conventional units. 

---