Adding a Heat Exchanger

Adding a heat exchanger to the cycle will require a change in constraints, 5, 7 and g. For a given set of expressions, [. ], these changes are represented, as shown in Table VI, byH... 

And of course, one would want to consider adding a heat exchanger so the hot oil leaving the distiller could heat the oil/benzene mixture entering the distiller. 

Chlorination of Hydrocarbons or Chlorinated Hydrocarbons. Chlorination at pyrolytic temperatures is often referred to as chlorinolysis because it involves a simultaneous breakdown of the organics and chlorination of the molecular fragments. A number of processes have been described for the production of carbon tetrachloride by the chlorinolysis of various hydrocarbon or chlorinated hydrocarbon waste streams (22—24), but most hterature reports the use of methane as the primary feed. The quantity of carbon tetrachloride produced depends somewhat on the nature of the hydrocarbon starting material but more on the conditions of chlorination. The principal by-product is perchloroethylene with small amounts of hexachloroethane, hexachlorobutadiene, and hexachloroben2ene. In the Hbls process, a 5 1 mixture by volume of chlorine and methane reacts at 650°C the temperature is maintained by control of the gas flow rate. A heat exchanger cools the exit gas to 450°C, and more methane is added to the gas stream in a second reactor. The use of a fluidi2ed-bed-type reactor is known (25,26). Carbon can be chlorinated to carbon tetrachloride in a fluidi2ed bed (27). 

If a heat exchanger is added at low pressure ratio (Fig. 3.4b) then the mean supply temperature is greater than that of the (CHTX)r cycle whereas the temperature of heat rejection will be the same as in the [CHTXJr cycle. Therefore the efficiency of the [CHTHTXJr cycle is greater than that of the [CHTXJr cycle. 

In a typical closed-loop system, liquid from the separator is cooled in a heat exchanger then recirculated. Any excess liquid added by mist and vapor flows out through a level-control valve. ... 

The added capability of calculating unknown values based on measured inputs will greatly enhance the system capabilities. For example, the neither fouling factor nor efficiency of a heat exchanger can be directly measured. A predictive maintenance system that can automatically calculate these values based on the measured flow, pressure and temperature data would enable the program to automatically trend, log and alarm deviations in these unknown, critical parameters. 

Nevertheless, even if phosphate is added directly to the boiler, if this sludge is not removed by BD it eventually settles and acts as a foulant to impede flow in the boiler waterways. Furthermore, without adequate dispersants during boiler operation, or if the boiler is not flushed out when drained down, the sludge may bake onto a heat exchange surface and generate an adherent, heat transfer-resisting deposit. 

A heat exchange term is added to the energy balance. Equation (5.30), to give... 

The hot reactor effluent gases are cooled to 230—265°F in a heat exchanger and passed into a water absorption tower. Formaldehyde is water-soluble and is separated from the remaining gases that exit the column overhead. Formaldehyde concentration in the tower is adjusted by controlling the amount of water added to the top of the tower. Generally, a product containing 37—56% formaldehyde in water is made. Methanol is often added as a stabilizer. 

In the closed cycle, the heat is added to the fluid in a heat exchanger from an external heat source, such as a nuclear reactor, and the fluid is cooled in another heat exchanger after it leaves the turbine and before it enters the compressor. A schematic diagram of a closed Brayton cycle is shown in Fig. 4.3. 

Effect of Uncertainties in Thermal Design Parameters, The parameters that are used in the basic sizing calculations of a heat exchanger include heat-transfer coefficients tube dimensions, eg, tube diameter and wall thickness and physical properties, eg, thermal conductivity, density, viscosity, and specific heat. Nominal or mean values of these parameters are used in the basic sizing calculations. In reality, there are uncertainties in these nominal values. For example, heat-transfer correlations from which one computes convective heat-transfer coefficients have data spreads around the mean values. Because heat-transfer tubes cannot be produced in precise dimensions, tube wall thickness varies over a range of the mean value. In addition, the thermal conductivity of tube wall material cannot be measured exacdy, adding to the uncertainty in the design and performance calculations. 

The cleaning and passivation process, or a simple modification thereof, should be repeated whenever any significant cooling system upset occurs (for example, if the pH dips to below 5.5 due to excess acid being added, or when a heat exchanger is taken out of service for repairs). 

In the secondary loop, a feed water pump circulates water through a heat exchanger where the primary and secondary loops exchange heat. The water in the secondary loop is turned to steam here and feeds a turbine, where electricity is generated. In the boiling-water reactor, there is only one loop, and as a result, the overall efficiencies are higher at the added expense... 

The performance of a heat exchanger depends upon the transfer surfaces being clean and uncorroded. The performance deteriorates with time due to accumulation of deposits on the heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer. In the design of heat exchangers, this added resistance is accounted for by a fouling factor or dirt factor, R. ... 

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