High Temperature Heat Transfer Media

Steam Is the most commonly used heat transfer medium. However, to obtain high temperature with condensing steam requires high pressures. Other heat transfer fluids are available which permit high temperature operation at low or moderate pressures. Both liquid- and vapor-phase systems are used. Liquid systems are almost always more economical liquids are easier to confine smaller transfer lines are required reducing heat losses control is more easily achieved when process temperature levels vary widely. Vapor-phase systems are often used when small heat loads are required or when the heat transfer system is dedicated to a particular service. 

Several types of fluids are available. Some of the more common include eutectic mixtures of organic compounds mineral and petroleum oils liquid metals molten salts gases. 

Caution should be exercised in selecting the proper media for a particular service. Each heat transfer fluid has specific properties which must be evaluated for any application. Some of the things that should be considered include stability at expected operating temperatures cost of the heat transfer fluid heat transfer properties pumping characteristics toxicity flammability ease of handling reclaimability. 

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With reaction temperatures above 300 °C intermediate cooling can still be performed directly with boiling water, whereas in a fixed bed a high-temperature heat-transfer medium must be used as coolant (see next section). 

OTHER COMMENTS used as a high temperature heat transfer medium used as an odorant in perfuming soaps useful in organic synthesis, resulting in epoxy resins, high temperature lubricants, specialty plasticizers, varnishes, artificial sweeteners, fire retardants, and... 

The ideal high level heat-transfer medium would have excellent heat-transfer capabiUty over a wide temperature range, be low in cost, noncorrosive to common materials of constmction, nondammable, ecologically safe, and thermally stable. It also would remain Hquid at winter ambient temperatures and afford high rates of heat transfer. In practice, the value of a heat-transfer medium depends on several factors its physical properties in relation to system efficiency its thermal stabiUty at the service temperature its adaptabiUty to various systems and certain of its physical properties. 

The desirable properties of a high-level heat transfer medium include low cost, non-flammability, nil toxicity, compatibility with common metals, remaining liquid at ambient temperature and most importantly, thermal stability. Materials cannot meet all of these criteria, but some useful ones are discussed below. 

Steam. The steam system serves as the integrating energy system in most chemical process plants. Steam holds this unique position because it is an exceUent heat-transfer medium over a wide range of temperatures. Water gives high heat-transfer coefficients whether in Hquid phase, boiling, or in condensation. In addition, water is safe, nonpolluting, and if proper water treatment is maintained, noncorrosive to carbon steel. 

Convection Heat transfer between higher and lower temperature substances or objects via a moving heat transfer medium. Gas in contact with flame is heated to elevated temperatures. If the high temperature air contacts low temperature objects, it will transfer heat to the object, raising the object s temperature. 

The reactants are phthalic anhydride, urea and copper(n) chloride, which are heated in a high-boiling aromatic solvent such as 1,2,4-trichlorobenzene, nitrobenzene or m-dinitrobenzene in the presence of a catalyst, usually ammonium molybdate. The solvent also acts as a heat-transfer medium. On heating to 120 °C an exothermic reaction begins and this temperature is maintained for about an hour. The temperature is then raised to 160-180 °C and kept constant for 6-12 hours. During this time ammonia and carbon dioxide are evolved, together with some solvent the reaction is complete when ammonia evolution ceases. The remaining solvent is then removed by either steam or vacuum distillation. The yield is 90-95%. For many years the solvent process was in almost exclusive use. 

The heat pipe achieves its high performance through the process of vapor state heat transfer. A volatile liquid employed as the heat-transfer medium absorbs its latent heat of vaporization in the evaporator (input) area. The vapor thus formed moves to the heat output area, where condensation takes place. Energy is stored in the vapor at the input and released at the condenser. The liquid is selected to have a substantial vapor pressure, generally greater than 2.7 kPa (20 mm Hg), at the minimum desired operating temperature. The highest possible latent heat of vaporization is desirable to achieve maximum heat transfer and temperature uniformity with minimum vapor mass flow. 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

The incorporation of turntables and mode mixers to the domestic oven have been employed to reduce non-uniformity within the cavity and consequently reduce the number of hot and cold spots. On scale-up it is important that homogeneity is maintained and the temperature within the bulk medium is consistent. With a single mode reactor, the sample can be located precisely within the cavity where the electric field strength will be maximal. This in turn allows tuning of the power input into the sample and very high internal heat transfer. 

In addition to the shaft power needed to raise the water heat transfer medium from the low to high loop pressure, additional compressor power is needed to overcome frictional pressure drop around the loop. The power associated with this loss has been compared with that of the high temperature process loop in the reference design. To understand the role of the different heat transport fluid -water in the alternate design versus helium in the reference design - the pumping power is expressed as a fraction of the pumping power per unit thermal power transported. This permits a consistent comparison between the two loops even if they differ in total thermal power transported. 

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