Ideal heat exchangers

Many other options are conceivable. A good example is the crosscurrent monolith. In theory, such a system allows ideal heat exchange... 

Table 8.2. Set of governing expressions for the ideal heat exchanger as depicted in figure 8.3. Remarks stream 1 corresponds to the hot utility, whereas stream 2 is the cold utility.

Table 8.3. Properties and operational parameters of the ideal heat exchanger system. Remarks most of the operational parameters are taken from Nummedal (2001) Nummedal and Kjelstrup (2001) the nominal case results in a heat duty of Q=60 kW and a flowrate of cold utility p2=0.286 kg xs. ...

Figure 8.4. Response time as a function of the thermal driving force for an idealized heat exchanger. Remarks alternative designs with different heat transfer areas are plotted and compared to the case where Q=60 -10 W (marker ).

Example 4.3. A simple Linde liquefaction system operates between 290 K and 71.9 K and uses nitrogen as the working fluid. The gas is isothermally and reversibly compressed to 10.1 MPa. The low pressure corresponds to the saturation pressure of liquid nitrogen at 71.9 K (0.05 MPa). Assuming ideal heat exchangers and no heat inleak to the system, what is the liquid yield and FOM for this liquefier ... 

A cold-gas refrigeration system with helium as the working fluid operates between 0.101 and 1.52 MPa at 300 K. The 1.52-MPa gas is cooled to 20 K whereupon it is expanded to 0.101 MPa to provide the refrigeration effect. The refrigeration system utilizes an isothermal and reversible compressor, ideal heat exchanger, a reversible adiabatic expander, and an... 

In an ideal Kapitza liquefaction system, nitrogen gas enters the compressor at 0.101 MPa and 295 K and is compressed isothermally and reversibly to 5.05 MPa. The gas enters the adiabatic and reversible expander at a condition of 5.05 MPa and 250 K. The expander handles 60 % of the high-pressure gas. Assuming ideal heat exchangers, determine the liquid yield, the work per unit mass compressed, and the work per unit mass liquefied when the expander work is utilized in the compression process. 

The liquid yield with an ideal heat exchanger would have been 0.0427. The additional work required by the compressor to account for the nonideal heat exchanger is given by either Eq. (5.61) or Eq. (5.62)... 

The FOM for the system with an ideal heat exchanger is 0.0801. Thus, there is a 63% decrease in the FOM for this simple Linde liquefier with a decrease in the effectiveness of the heat exchanger of only 5%. 

Since h = h, both points are located underneath the phase envelope, indicating that partial condensation already has occurred in the cold heat exchanger. At point 5, an actual enthalpy of 99.9 kJ/kg corresponds to a temperature of 119 K. Thus, for an ideal heat exchanger at the cold end of the cycle, the maximum temperature for Ty is also 119 K. From the definition for the effectiveness of the heat exchanger, we can now determine hj, the actual enthalpy at point 7, and the corresponding temperature... 

Flow Maldistribution. One of the principal reasons for heat exchangers failing to achieve the expected thermal performance is that the fluid flow does not foUow the idealized anticipated paths from elementary considerations. This is referred as a flow maldistribution problem. As much as 50% of the fluid can behave differently from what is expected based on a simplistic model (18), resulting in a significant reduction in heat-transfer performance, especially at high or a significant increase in pressure drop. Flow maldistribution is the main culprit for reduced performance of many heat exchangers. 

Minimum Area. The limit of minimum network area is presented in References 2 and 3. If idealized double-pipe exchangers are used, a heat-exchange network having minimum area can quickly be developed for any In the limiting case, where all heat-transfer coefficients are assumed to be equal, the area for this network can easily be obtained from the composite streams by... 

Although the T-s diagram is veiy useful for thermodynamic analysis, the pressure enthalpy diagram is used much more in refrigeration practice due to the fact that both evaporation and condensation are isobaric processes so that heat exchanged is equal to enthalpy difference A( = Ah. For the ideal, isentropic compression, the work could be also presented as enthalpy difference AW = Ah. The vapor compression cycle (Ranldne) is presented in Fig. H-73 in p-h coordinates. 

Heat Flux Tests Removable tube test heat exchangers find an ideal use in the field for monitoring heat flux (corrosion) conditions, NACE TM0286-94 (similar to laboratory test. Fig. 28-4, page 28-12). 

In a recuperative heat exchanger, each element of heat-transferring surface has a constant temperature and, by arranging the gas paths in contra-flow, the temperature distribution in the matrix in the direction of flow is that giving optimum performance for the given heat-transfer conditions. This optimum temperature distribution can be achieved ideally in a con-tra-flow regenerator and approached very closely in a cross-flow regenerator. 

Here I /G. is the heat exchanger contact resistance. The reason for rhe contact resistance is that there exists a resistance to heat flow between the outer surface of the pipe and the collar of the plate tins. Normally, the fins are attached to the pipes by mechanical expansion of the tubes out into rhe plate-fin collars. Because of this manufacturing method, the contact will not be ideal. Small gaps between the pipe surface and rhe collar of the tins will occur. 

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