Heat loads

2 Bent Load. If the air leaving, the tower is saturated with-water vapor at 95 F and the air entering is saturated with water yapor at 72 F, then 66,500 lb of dry air per minute is needed to handle the heat load as follows  

Very few changes would be necessary to handle the increased flow of water. The pipe and the flow control valves on top of the tower would need to be enlarged. It would also be necessary to ream out the plastic orifices in the bottom of the hot water basin as the flow increased. 

A decade ago it was not uncommon to specify 85° F cold water and to select exchangers for 88 to 90° F cold water. To some engineers it seemed logical to include a small 3°F safety factor in the calculations. However, this 3°F safety factor quite often turned out to be a 50% safety factor as far as the price and size of the cooling tower were concerned. Table 3-5 indicates that a tower sized to cool 28,500 gpm from 118 to 88° F with an 80° F wet bulb would cost 361,920, or 12.70 per gpm. A cooling tower selected to cool 28,500 gpm with the 3°F safety factor or from 115 to 85° F with an 80°F wet bulb would cost 532,480, or 18.68 per gpm. This is approximately 50% more in cost, length of concrete basin and fan horsepower. 

This is not to say that no safety factor should be included in the heat load calculations. It is recommended that a safety factor be included in the gpm only. If a small safety factor is included in the 

Range is a direct function of the quantity of water circulated and of the heat load. Increasing the range as a result of added heat load does require an increase in tower size. If the cold water temperature is not changed and the range is increased at the top (higher hot water temperature), the tower size should be increased moderately. 

Over-all Length, ft. No. of Fans. Fan Diameter, ft. Total Fan Horsepower.. . .  

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In modern separation design, a significant part of many phase-equilibrium calculations is the mathematical representation of pure-component and mixture enthalpies. Enthalpy estimates are important not only for determination of heat loads, but also for adiabatic flash and distillation computations. Further, mixture enthalpy data, when available, are useful for extending vapor-liquid equilibria to higher (or lower) temperatures, through the Gibbs-Helmholtz equation. ... 

Generally speaking, temperature control in fixed beds is difficult because heat loads vary through the bed. Also, in exothermic reactors, the temperature in the catalyst can become locally excessive. Such hot spots can cause the onset of undesired reactions or catalyst degradation. In tubular devices such as shown in Fig. 2.6a and b, the smaller the diameter of tube, the better is the temperature control. Temperature-control problems also can be overcome by using a mixture of catalyst and inert solid to effectively dilute the catalyst. Varying this mixture allows the rate of reaction in different parts of the bed to be controlled more easily. 

An unnecessary load on the separation, leading to higher heat loads and vapor rates. 

Large heat loads to be transferred result in large reboilers and condensers. 

Large heat loads will cause high vapor rates, and these require large column diameters. 

Thus capital cost considerations reinforce the argument that the nonintegrated sequence with the lowest heat load is that with the lowest total cost. 

When the integration of sequences of simple columns was considered, it was observed that sequences with higher heat loads occurred simultaneously with more extreme levels. Heat integration always benefits from low heat loads and less extreme levels, as we shall see later in Chap. 12. Now consider the effect of thermal coupling arrangements on loads and levels. Figure 5.18 compares a... 

Figure 5.18 Relationship between heat load and level in simple and prefractionator sequences. (From Smith and linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

As pointed out in Chap. 5, replacing simple columns by complex columns tends to reduce the vapor (and heat) load but requires more of the heat to be added or removed at extreme levels. This means that the introduction of complex columns in the design might prejudice heat integration opportunities. Thus the introduction of complex distillation arrangements needs to be considered simultaneously with the heat integration. This can be carried out manually with some trial and error or using an automated procedure such as that of Kakhu and Flower. ... 

Kayihan, F., Optimum Distribution of Heat Load in Distillation Columns Using Intermediate Condensers and Reboilers, AfC/iS Symp. Ser., 192(76) 1, 1980. 

ATlma = log mean temperature diflEerence for enthalpy interval k Qij = heat load on match between hot stream t and cold stream... 

Fig. 8. Heat-exchange network showing heat-load loop.

Fig. 11. Alternative network configurations where the numbers not ia circles represent heat loads of streams ia kW (7). See text.

An alternative starting network is one without stream spHts. The networks from the TI method maximize energy recovery and may introduce heat-load loops. Stream spHts ate not made in the initial steps of network invention. The ED method is proposed to be one in which heuristic rules and strategies would be used to improve the networks developed by the TI method. The importance of a thermodynamic base for evolutionary rules is stressed in this proposal, but there is no expHcit guidance for the evolutionary process. 

L number of heat-load loops in a heat-exchange network... 

Natural-draft cooling towers are extremely sensitive to air-inlet conditions owing to the effects on draft. It can rapidly be estabUshed from these approximate equations that as the air-inlet temperature approaches the water-inlet temperature, the allowable heat load decreases rapidly. For this reason, natural-draft towers are unsuitable in many regions of the United States. Figure 10 shows the effect of air-inlet temperature on the allowable heat load of a natural-draft tower for some arbitrary numerical values and inlet rh of 50%. The trend is typical. 

Scale control can be achieved through operation of the cooling system at subsaturated conditions or through the use of chemical additives. The most direct method of inhibiting formation of scale deposits is operation at subsaturation conditions, where scale-forming salts are soluble. For some salts, it is sufficient to operate at low cycles of concentration and/or control pH. However, in most cases, high blowdown rates and low pH are required so that solubihties are not exceeded at the heat transfer surface. In addition, it is necessary to maintain precise control of pH and concentration cycles. Minor variations in water chemistry or heat load can result in scaling (Fig. 12). 

The main mechanisms of hearth bottom wear are high heat load, chemical attack, erosion from molten Hquids, mechanical and thermal stress, and penetration because of ferrostatic and process pressure. A variety of special purpose carbons have been developed to minimize or eliminate the damage caused by these wear mechanisms. 

The use of chemical agents in battie imposes a significant burden on troops because of the cumbersome nature of the protective clothing and the attendant heat load in hot climate situations. This factor alone imposes a burden on potential target personnel, lowering their effectiveness. U.S. troops in the 1991 Mideast war Desert Storm were provided with protective gear that did not deter them with regard to the outcome of the action. 

Selecting the process stream matches that will participate in the retrofitted network and their heat loads. 

Because each effect of an evaporator produces almost as much vapor as the amount it condenses, the total evaporation accompHshed per unit of prime steam, or steam economy, iacreases ia almost direct proportioa to the number of effects used. The total heat load is also spHt up betweea the effects so that each effect has a much lower heat duty than a single effect for the same total evaporation load. However, the total available AT is also spHt up similarly so that each effect of a multiple effect requites about as much heating surface as a single effect operating over the same total temperature difference. Thus ia selecting the number of effects to use ia any iastallatioa, steam cost savings and capital cost of effects have to be balanced. Even before... 

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