Heat balance, determination

THE HEAT BALANCE DETERMINES THE NET ENERGY YIELD AND FLOWRATE... 

System heat balances determine component heat loads and operating conditions and, thus, provide the basis of system mass estimates. Section 4.5.3 describes the heat balances and piping arrangements that were assumed for mass estimating purposes. 

The impact that variations in coke content and burning conditions can have on the overall heat of coke combustion is shown in Table 2. Because the heat balance dictates the amount of heat that is required from burning coke, the heat of combustion then determines the amount of coke that must be burned. 

For heat transfer directly to solids, predictive equations give directly the volume V or the heat-transfer area A, as determined by heat balance and airflow rate. For devices with gas flow normal to a fluidized-solids bed,... 

If the instruments are in, the meter run was available, and gas composi-tion can be accurately determined as needed, one last minor hurdle should be addressed. This is power measurement. The indirect method, such as heat balance can be used. In fact, it should be used as a redundant method. 

In contrast, most equipment can safely tolerate higher degrees of heat density than those defined for personnel. However, if anything vulnerable to overheating problems is involved, such as low melting point construction materials (e.g., aluminum or plastic), heat-sensitive streams, flammable vapor spaces, or electrical equipment, then the effect of radiant heat on them may need to be evaluated. When this evaluation is required, the necessary heat balance is performed to determine the resulting surface temperature, for comparison with acceptable temperatures for the equipment. 

The rate constant k is expressed as k = exp(15.32 - 7,550/T), sec Determine the operating points for both the mass and heat balance equations. 

The feed temperature Tg for the adiabatic operation at the optimal temperature T p is determined from the heat balance, -GAli + Q = 0, where G = pu and... 

When a damp cloth is laid in an air flow, it settles after a certain time ic an equilibrium temperature, the so-called wet bulb temperature (0 ), which is determined through heat and mass transfer. Negotiating the heat flow obtained by radiation and conduction, the heat balance of the wet cloth in a stationary situation can be expressed as... 

In wall heat balance Eqs. (8.14) and (8.15), the radiation heat flows T and V from the heat sources and V 0 y2i from upper zone wall surfaces to lower zone wall surfaces are assumed to increase the temperature of the walls. In practical cases it is quite complicated to determine how much of the radiation flow rate will be distributed to outer walls and to other surfaces. 

Because the specific heat capacity of the water vapor is different from that of the dry air, the true dry-bulb mixed-stream air temperature can be determined only by means of a heat balance. 

The noncontact measurement principle, usually called optical or radiation temperature measurement, is based on detecting electromagnetic radiation emitted from an object. In ventilation applications this method of measurement is used to determine surface temperatures in the infrared region. The advantage is that the measurement can be carried out from a distance, without contact with the surface, which possibly influences the heat balance and the temperatures. The disadvantages are that neither air (or other fluid) temperature nor internal temperature of a material can be measured. Also the temper-... 

If an evaporation temperature (Pc) is pre-selected as a parametric independent variable, then the temperatures and enthalpies at c and e are found from (b) above the temperature T(, is also determined. If there is no heat loss, the heat balance in the HRSG between gas states 4 and 6 is... 

Kehlhofer explains that the pre-heating loop must be designed so that the heat extracted is. sufficient to raise the temperature of the feed water flow from condenser temperature T to Ta (see Fig. 7.6). The available heat increases with live steam pressure Ipf), for selected 7 b(= Ta) and given gas turbine conditions, but the heat required to preheat the feed water is set by (Ta — T. ). The live steam pressure is thus determined from the heat balance in the pre-heater if the heating of the feed water by bled steam is to be avoided but the optimum (low) live steam pressure may not be achievable because of the requirement. set by this heat balance. 

Using the operating data from the case study. Example 5-5 shows heat balance calculations around the stripper-regenerator. The results are used to determine the catalyst circulation rate and the delta coke. Delta coke is the difference between coke on the spent catalyst and coke on the regenerated catalyst. 

The calculation of heat balance around the reactor is illustrated in Example 5-6. As shown, the unknown is the heat of reaction. It is calculated as the net heat from the heat balance divided by the feed flow in weight units. This approach to determining the heat of reaction is acceptable for unit monitoring. However, in designing a new cat cracker, a correlation is needed to calculate the heat of reaction. The heat of reaction is needed to specify other operating parameters, such... 

The only proper method to evaluate the performance of a cat cracker is by conducting a material and heat balance. One balance will tell where the unit is a series of daily or weekly balances will tell where the unit is going. The heat and weight balance can be used to evaluate previous changes or predict the result of future changes. As discussed in the next chapter, material and heat balances are the foundation for determining the effects of operating variables. 

The heat balance exercise provides a tool for in-depth analysis of the unit operation. Heat balance surveys determine catalyst circulation rate, delta coke, and heat of reaction. The procedures described in this chapter can be easily programmed into a spreadsheet program to calculate the balances on a routine basis. 

All planned heat balance measurements have been established and heat loss from both reactor and from vapour stack determined, the former more and the latter rather less satisfactorily. 

Regions of stable and unstable operation determined by numerical simulation of mass and heat balances equations first- and second-order, autocatalytic, and product-inhibited kinetics graphically presented boundaries in co-ordinates in practice. safe operation if l/5e>2. Equality of heat generation and heat removal rates Semenov approach modified for first-order kinetics. 

The heating period begins with FLAG set initially to zero. When Xy > 1 then FLAG becomes 1, and the distillation period begins at statement 10. At each time interval the subroutine TCALC is used to make the iterative bubble point calculation. The component mass balance determines the removal of volatiles in the vapour, where the total molar flow rate, V, is determined from the energy balance. 

For a given inlet and outlet cold water temperature, the outlet hot water temperature is determined by an overall heat balance where... 

For organic solutions the heat of mixing is usually small compared with the other heat quantities, and can usually be neglected when carrying out a heat balance to determine the process heating or cooling requirements. 

It is often possible to make a material balance round a unit independently of the heat balance. The process temperatures may be set by other process considerations, and the energy balance can then be made separately to determine the energy requirements to maintain the specified temperatures. For other processes the energy input will determine the process stream flows and compositions, and the two balances must be made simultaneously for instance, in flash distillation or partial condensation see also Example 4.1. 

If the specification is incomplete, determine the unknown fluid temperature or fluid flow-rate from a heat balance. 

The output of each power range channel is directly proportional to reactor power and typically covers a range from 0% to 125% of full power, but varies with each reactor. The output of each channel is displayed on a meter in terms of power level in percent of full rated power. The gain of each instrument is adjustable which provides a means for calibrating the output. This adjustment is normally determined by using a plant heat balance. Protective actions may be initiated by high power level on any two channels this is termed coincidence operation. 

Figure 8. Measurements in a heat treatment chamber for hardboard. In a few channels of the carload of 100 single-laid boards, the air velocity and the temperature increase from ingoing air to outgoing air were measured as indicated. Also, measuring points used to determine the total heat balance in the chamber are indicated. (Reproduced with permission from ref. 10. Copyright 1989 De Gruyter.)...

Three different principles govern the design of bench-scale calorimetric units heat flow, heat balance, and power consumption. The RC1 [184], for example, is based on the heat-flow principle, by measuring the temperature difference between the reaction mixture and the heat transfer fluid in the reactor jacket. In order to determine the heat release rate, the heat transfer coefficient and area must be known. The Contalab [185], as originally marketed by Contraves, is based on the heat balance principle, by measuring the difference between the temperature of the heat transfer fluid at the jacket inlet and the outlet. Knowledge of the characteristics of the heat transfer fluid, such as mass flow rates and the specific heat, is required. ThermoMetric instruments, such as the CPA [188], are designed on the power compensation principle (i.e., the supply or removal of heat to or from the reactor vessel to maintain reactor contents at a prescribed temperature is measured). 

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