Heat input limitation

In Fig. 2.2 a heat input limitation has been shown and a vertical line from this scale meets the particular combined thickness chosen. From this point, horizontal movement meets the vertical from the CE scale to identify the line TT, which gives the minimum preheat temperature required. It should be noted that in making this construction the electrode size and run length are related to heat input by means of Tables 2.1-2.4. Bead sizes less than this maximum may be employed but only at the cost of increasing the preheat temperature and this can be checked in the diagram. 

Other Technologies. As important as dehydrogenation of ethylbenzene is in the production of styrene, it suffers from two theoretical disadvantages it is endothermic and is limited by thermodynamic equiHbrium. The endothermicity requites heat input at high temperature, which is difficult. The thermodynamic limitation necessitates the separation of the unreacted ethylbenzene from styrene, which are close-boiling compounds. The obvious solution is to effect the reaction oxidatively ... 

The use of high or low limits for process variables is another type of selective control, called an override. The feature of anti-reset windup in feedback controllers is a type of override. Another example is a distillation column with lower and upper limits on the heat input to the column reboiler. The minimum level ensures that liquid will remain... 

Abnormal Process Heat Input - The required capacity is the maximum vapor generation rate at PR valve relieving conditions, including any noncondensibles produced from overheating, less the normal condensation or vapor outflow rate. In every case, one should consider the potential behavior of a system and each of its components. For example, the fuel or heating medium control valve or the mbe heat transfer may be the limiting consideration. Consistent with the practice... 

Unfired cycle This cycle is very similar to the mentary-fired case except there is no added fuel heat input. The approach temperature and pinch point are even more critical, and tend to reduce steam pressures somewhat. Similarly, the gas turbine exhaust temperature imposes further limits on final steam temperature. 

If the heat input from a given appliance is increased, both the temperature and flow rate of the gases entering the flue will rise. The net result is that an increased heat input leads to a greater flow rate, but this is more noticeable at low heat inputs. There is a limit to the rate that can be vented into a given flue before the pressure losses overcome the flue draft and spillage (failure of the flue products to be satisfactorily cleared) occurs. 

Concern about hydrogen damage has lead to much debate regarding limits for protection potentials of high-alloy steels. However, it is thought that under normal seawater service and cathodic protection conditions, these materials will not be adversely affected provided that the microstructure has at least 40% austenite present . This latter point is of particular importance to welds and their heat affected zone where careful control of heat input is necessary to maintain a favourable microstructure. 

Boiler designers must be in a position to accurately predict the various heat release rates from furnace and burner designs and to match them to limitation standards for both heat input and boiler constructional steels. Some considerations are ... 

Modules Every module design used in other membrane operations has been tried in pervaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is very low (0.1-1 Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for heat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construction was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capillary bundles. The capillary device with the feed on the inside of the tube has many advantages in principle, such as good vapor-side mass transfer and economical construction, but it is still limited by the availability of membrane in capillary form. 

In addition to the effects of formulation factors on freeze-drying behavior, it is important for the pharmaceutical scientist to understand basic principles of heat and mass transfer in freeze-drying [29,30]. Because of the high heat input required for sublimation (670 cal/g), transfer of heat from the heated shelf to the sublimation front is often the rate-limiting step in the coupled heat... 

Do vaporizers have a limited heat input capacity ... 

For direct flame impingement, as in the case of a fire around a vessel, there is much greater heat input to the vessel. Therefore, higher water application rates are required to control the rate of temperature rise of the liquid contents and limit the relief valve from lifting or the shell above the liquid level from being weakened by high metal temperatures. 

Considerable heat will be used to melt and to vaporize the reaction products. Vaporization of a reaction product is commonly the limiting factor in determining the maximum flame temperature. For example, consider a beaker of water at 25°C. As the water is heated, at one atmosphere pressure, the temperature of the liquid rises rather quickly to a value of 100 c. To heat the water over this temperature range, a heat input of approximately 1 calorie per gram per degree rise in temperature is required. To raise 500 grams of water from 25° to 100°C will require... 

C and D by the direct return of uhcooled first stage reooopresslon vapors from 1, and the use of a heat exchanger (Indicated as H) to heat the liquids from 2 which are pumped to C. The gas conditioning unit A serves as the reflux condenser for this fractionation system. Two vapor pressure controls are available, the first stage pressure (on 1 and D), and the temperature of C and D which can be controlled, within limits, by adjusting the heat input, H. 

In this mode heat input to the reboiler is held constant throughout. In practice it is set to its maximum limit, the value of which depends on the heat exchange system to the reboiler. Domenech and Enjalbert (1974), Greaves et al. (2001), Greaves (2003) used this mode of operation in their laboratory conventional batch column. Cuille and Reklaitis (1986) and Mujtaba (1989) also used this mode of operation in their simulation studies. 

For a constant volume, it can be shown that the surface area is proportional to /l. With a shorter reactor, and hence a smaller area, the radiant temperature must increase in order to provide a constant heat input causing the tubeskin temperature to rise beyond the design limit. 

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