Heat sinks

The analysis of the heat exchanger network first identifies sources of heat (termed hot streams) and sinks (termed cold streams) from the material and energy balance. Consider first a very simple problem with just one hot stream (heat source) and one cold stream (heat sink). The initial temperature (termed supply temperature), final temperature (termed target temperature), and enthalpy change of both streams are given in Table 6.1. 

Fig. 6.7a. Above the pinch (in temperature terms), the process is in heat balance with the minimum hot utility Qnmin- Heat is received from hot utility, and no heat is rejected. The process acts as a heat sink. Below the pinch (in temperature terms), the process is in heat balance with the minimum cold utility Qcmin- No heat is received, but heat is rejected to cold utility. The process acts as a heat source.

The point of zero heat flow in the grand composite curve in Fig. 6.24 is the pinch. The open jaws at the top and bottom represent Hmin and Qcmin, respectively. Thus the heat sink above the pinch and heat source below the pinch can be identified as shown in Fig. 

Fundamentally, there are two possible ways to integrate a heat engine exhaust. In Fig. 6.31 the process is represented as a heat sink and heat source separated hy the pinch. Integration of the heat engine across the pinch as shown in Fig. 6.31a is coimterproductive. The process still requires QHmm, and the heat engine performs no... 

In this context, the points correspond to process and utility streams and the lines to heat exchange matches between the heat sources and heat sinks. 

Figure 13.3 shows a process represented simply as a heat sink and heat source divided hy the pinch. Figure 13.3a shows the process with an exothermic reactor integrated above the pinch. The minimum hot utility can be reduced by the heat released by reaction, Qreact-... 

Fig. 14.1a. The background process (which does not include the reboiler and condenser) is represented simply as a heat sink and heat source divided by the pinch. Heat Qreb is taken into the reboiler above pinch temperature and rejected from the condenser at a lower temperature, which is in this case below pinch temperature. Because the process sink above the pinch requires at least Q min to satisfy its...

Some of the early reentry vehicles utilized metallic heat sinks of copper [7440-50-8] or beryllium [7440-41-7] to absorb reentry heat. Other metallic materials that have been evaluated for nosetip appHcations include tungsten [7440-33-7] and molybdenum [7439-98-7]. The melt layers of these materials are beHeved to be very thin because of the high rate at which volatile oxide species are formed. 

In some appHcations, large quantities of waste or low cost heat are generated. The absorption cycle can be directly powered from such heat. It employs two intermediate heat sinks. Its theoretical coefficient of performance is described by... 

A simple cooling cycle serves to illustrate the concepts. Figure 1 shows a temperature—entropy plot for an actual refrigeration cycle. Gas at state 1 enters the compressor and its pressure and temperature are increased to state 2. There is a decrease in efficiency represented by the increase in entropy from state 1 to state 2 caused by friction, heat transfer, and other losses in the compressor. From state 2 to states 3 and 4 the gas is cooled and condensed by contact with a heat sink. Losses occur here because the refrigerant temperature must always be above the heat sink temperature for heat transfer to take... 

Thus, for a successful fluorination process involving elemental fluorine, the number of coUisions must be drasticaUy reduced in the initial stages the rate of fluorination must be slow enough to aUow relaxation processes to occur and a heat sink must be provided to remove the reaction heat. Most direct fluorination reactions with organic compounds are performed at or near room temperature unless reaction rates are so fast that excessive fragmentation, charring, or decomposition occurs and a much lower temperature is desirable. 

In 1954 the surface fluorination of polyethylene sheets by using a soHd CO2 cooled heat sink was patented (44). Later patents covered the fluorination of PVC (45) and polyethylene bottles (46). Studies of surface fluorination of polymer films have been reported (47). The fluorination of polyethylene powder was described (48) as a fiery intense reaction, which was finally controlled by dilution with an inert gas at reduced pressures. Direct fluorination of polymers was achieved in 1970 (8,49). More recently, surface fluorinations of poly(vinyl fluoride), polycarbonates, polystyrene, and poly(methyl methacrylate), and the surface fluorination of containers have been described (50,51). Partially fluorinated poly(ethylene terephthalate) and polyamides such as nylon have excellent soil release properties as well as high wettabiUty (52,53). The most advanced direct fluorination technology in the area of single-compound synthesis and synthesis of high performance fluids is currently practiced by 3M Co. of St. Paul, Minnesota, and by Exfluor Research Corp. of Austin, Texas. 

Eig. 11. Liquid-cooled cold plates or heat sinks have been developed as thermal management solutions to cool components for Hquid-cooled computer systems and other electronic systems where heat removal becomes one of the important design criteria. 

The third characteristic of interest grows directly from the first, ie, the high thermal conductance of the heat pipe can make possible the physical separation of the heat source and the heat consumer (heat sink). Heat pipes >100 m in length have been constmcted and shown to behave predictably (3). Separation of source and sink is especially important in those appHcations in which chemical incompatibilities exist. For example, it may be necessary to inject heat into a reaction vessel. The lowest cost source of heat may be combustion of hydrocarbon fuels. However, contact with an open flame or with the combustion products might jeopardize the desired reaction process. In such a case it might be feasible to carry heat from the flame through the wall of the reaction vessel by use of a heat pipe. 

The vessel, as weU as the wick, must be compatible with the working fluid. Where possible, the wick and vessel are made of the same material to avoid the formation of galvanic corrosion ceUs in which the working fluid can serve as the electrolyte. In addition to its role within the heat pipe, the vessel also serves as the interface with the heat source and the heat sink. 

Fig. 9. High power array of phase coupled GaAs/AlGaAs lasers mounted -side down on a thermal heat sink. The tt/2 shift of the neighboring lasers is indicated by the + and — signs. The output pattern consists of two dominant peaks, each associated with the lasers of the same phase, and much weaker...

AH gg = —43.03 kJ/mol ( — 10.28 kcal/mol) including heat of solution, at standard state m = V) and may require a heat sink to prevent boiling of the reaction mixture. A 30% by weight suspension of MgO in 20°C water boils in the absence of any heat sink. The time to reach boiling is dependent on the reactivity of the MgO raw material, and this time can be only several hours for the more reactive grades of MgO. Investigations of the kinetics of formation of magnesium hydroxide by hydration of MgO have been reported (79). 

Electronic-Grade MMCs. Metal-matrix composites can be tailored to have optimal thermal and physical properties to meet requirements of electronic packaging systems, eg, cotes, substrates, carriers, and housings. A controUed thermal expansion space tmss, ie, one having a high precision dimensional tolerance in space environment, was developed from a carbon fiber (pitch-based)/Al composite. Continuous boron fiber-reinforced aluminum composites made by diffusion bonding have been used as heat sinks in chip carrier multilayer boards. 

A fundamentally different reaction system is under development by Air Products and Chem Systems (23). In this system, synthesis gas is bubbled through a slurry consisting of micrometer-sized methanol catalyst particles suspended in a paraffinic mineral oil. The Hquid phase serves as the heat sink to remove the heat of reaction. 

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