Specific area of heat exchangers

In the present work, the transient and steady-state characteristics of a fluidized bed combustor are studied by solving numerically a dynamic model in which lateral solids and gas dispersion, lateral temperature distribution and wide size distribution of coal feed are taken into account. The influences of bubble size, excess air rate, specific area of heat exchangers and coal feed rate on the performance of the fluidized combustor are examined by means of simulation with the model. 

The effects of bubble size and specific areas of heat exchangers on the transient average carbon concentration and bed temperature are presented in Figure 9. It can be seen that the critical bubble size is about 5 cm, which is much smaller than that for the type A combustor. This is because of the relatively small excess air rate used and the large carbon concentration gradient... 

Figure 8. Effect of bubble size and specific area of heat exchangers on the ( )...

Optimize the net power produced by the cycle with variable based on the criterion of (a) net power per unit conductance of heat exchanger, and (b) specific net power per unit surface area of heat exchangers with C/H=C/H = 0.5kW/[m (K)]. 

The rate of heat transfer per unit area of heat exchanger (heat flux), q, will be a function of the temperature driving force AT, tube diameter d, the mean fluid flow velocity u, fluid flow properties density p and viscosity p - and fluid thermal properties - specific heat capacity cp and thermal conductivity k. 

Fire resulted from a rupture in an offline reboiler (which is a specific type of heat exchanger) that was in standby mode and located in the propylene fractionator area of the plant, adjacent to the in-service reboiler. The heat input system of the in-service reboiler was undergoing regular evaluation and analysis work at the time of the incident. Investigation ongoing. 

Objective functions that allow only discrete values of the independent variable ) occur frequently in process design because the process variables assume only specific values rather than continuous ones. Examples are the cost per unit diameter of pipe, the cost per unit area for heat exchanger surface, or the insulation cost considered in Example 1.1. For a pipe, we might represent the installed cost as a function of the pipe diameter as shown in Figure 4.2 [see also Noltie (1978)]. For... 

Equipment Constraints These are the physical constraints for individual pieces of equipment within a unit. Examples of these are flooding and weeping limits in distillation towers, specific pump curves, heat exchanger areas and configurations, and reactor volume limits. Equipment constraints may be imposed when the operation of two pieces of equipment within the unit work together to maintain safety efficiency, or quality. An example of this is the temperature constraint imposed on reactors beyond which heat removal is less than heat generation, leading to the potential of a runaway. While this temperature could be interpreted as a process constraint, it is due to tlie equipment limitations that the temperature is set. 

Table 3.3 Overview of heat exchanger types and their specific surface area. (Adapted from Ref [22].)...

In general the design of heat exchangers involves the determination of the required area A. The necessary heat transfer, the temperatures and the fluids are generally known from the process specification, the individual heat transfer coefficients of the fluids may be calculated, and values of the fouling resistances on either side of the heat exchanger would have to be estimated. It is the latter that can be difficult and if the resistances are incorrectly estimated difficulties in subsequent operation may be manifest. 

Example. Given Shell exit temperature, Tout = 100 F tube fluid temperature, t, at reversal of tubes = 71° F overall heat transfer coefficient, C/, multiplied by the surface area of the exchanger per baffle section, Ag (sq.ft.) or UAg = 2,000 shell flow rate, W, (lb./hr.) multiplied by the specific heat of the sheU fluid, C, (Btu/lb. °F) or WC = 10,000. Ratio of stream heat capacities, JR, equals the tube fluid flow rate, w (Ibs./hr.) times the specific heat of the tube fluid, c, (Btu/lb. °F) divided by WC equals one, or i = wcjWC = 1 magnitude of the by-pass stream expressed as a fraction of total flow, I = 0.6. 

Because of the small thermal diffusion paths and the high specific surface area, micro heat exchangers are used for rapid heat transfer between hot and cold fluids. Compared to conventional heat exchangers, the size can be considerably... 

Microstructured reactors (microreactors) represent a new type of reaction equipment for applications in chemistry. Small dimensions of microchannels provide a short diffusion time, better temperature and pressure control, large specific surface area, high heat exchanging efficiency, and a higher level of safety [1]. 

In comparison to males BMR is generally lower in females and higher in children. BMR is measured 12—14 hours after a meal, by direct or indirect calorimetry. Feeding increases BMR because of the necessary energy expenditure that occurs during the assimilation of nutrients into the body (also known as specific dynamic action). BMR is also very closely related to body surface area, since this is where the majority of heat exchange takes place. 

Heat exchanger cost data can usually be manipulated such that fixed costs, represented by the coefficient a in Eqs. (F.2) to (F.4), do not vary with exchanger specification. Equations (F.3) and (F.4) can now be rearranged to give the modified exchanger area A as a function of actual area A and the cost law coefficients ... 

When heat-exchange surface is being provided in the design of an absorber, the isothermal design procedure can be rendered valid by virtue of the exchanger design specifications. With ample surface area and a close approach, isotherm operation can be guaranteed. 

For the first kind of application, the focus is on certain elements of the HVAC component under consideration. The simulation is used to study and optimize design-specific aspects such as the pipe size and spacing or wetted area and fin geometry in a heat exchanger. This kind of modeling requires detailed knowledge on many input parameters and the related physical processes. 

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