Simple heater

A simple heater/cooler unit is used to model operations where only the change of state variables of a stream is relevant (temperature, pressure), and not the thermal design of the heat exchanger (Fig. 3.10-left). 

A simple heater is shown in Fig. 4.6. For this system we use the boundaries shown and make the following assumptions ... 

The diagram shows two curves, one for reactants and the other for products. Since any enthalpy is based on an arbitrary datum, we select the enthalpy of the reactants at 77 F (25 0) as zero. Then the enthalpy of the products as 77°F must be i h caaion. 77 f Prom these two data the enthalpy of products or of reactants can be constructed with measurements made in simple heaters such as that of Fig. 4.6 thus we may construct the entire curves of Fig. 4.8. For an isothermal reactor the process can be represented by a vertical line, as shown in the figure the heat removed is equal to the heat of reaction at the temperature of the reactor, which equals the vertical distance between the two curves at that temperature. For an adiabatic reactor, the process can be represented by a horizontal line, as shown in the figure, because the inlet and outlet enthalpies are the same. The outlet temperature of an adiabatic burner is called the adiabatic flame temperature. For most fuels these adiabatic flame temperatures are so high that combustion is not complete [4, p. 498]. 

For simulation purpose, we represent the furnace as a simple heater with a variable duty to match the overflash specification. There are many detailed models for the fired heaters in process simulators. Using these fired heater models can be sophisticated and is not the focus of this text. We will, however, model the crude column rigorously and include all side operations. 

Several forms of apparatus employing electrical heati iig wi 11 be described. A simple form may be readily constructed from a domestic electric iron of 400-500 watts rating. The handle is removed, and two holes of 8 mm. diameter are drilled through the base (ca. 11 mm. thick) so that they meet in the centre of the block. One hole is for a 360° thermometer (small bulb) the other hole is spare and can be used for comparison with a standard thermometer. The heater is mounted on a sheet of thick asbestos board which is fixed to an appropriate wooden base. The wires from the heating unit are connected to two insulated terminals fitted on the board (Fig. 11, 11, 1). The rate of heating is controlled by either of the following methods ... 

PBT resins are very fluid in the melt and sometimes drooling from the injection no22le can be a problem in machines that do not have melt decompression. A simple free-flow no22le can be used satisfactorily to minimise drooling if its temperature can be adjusted by a separate heater band. PBT does not drool as badly as nylons. 

Simple heat losses through the furnace walls are also significant. This follows from the high temperatures and large size of fired heaters, but these losses are not inevitable. In an optimized system, losses through insulation (1) are roughly proportional to... 

Equipment Tests. Procedures for rigorous, detailed efficiency determination are available (ASME Test Codes) but are rarely used. For the objective of defining conservation potentials, relatively simple measurements are adequate. For fired heaters, stack temperature and excess O2 ia stack should be measured for turbiaes, pressures (ia and out) and temperatures (ia and out) are needed. 

A simple vei tical cylindrical heater has vertical tubes arrayed along the walls of a combustion chamber fired vertically from the floor. This type of heater does not include a convection sec tion and is inexpensive. It nas a small footprint but low efficiency, and it is usually selec ted for small-duty applications (0.5 to 21 GJ/h [0.5 to 20 10 Btii/hj). 

Vei tical cylindrical helical coil heaters are hybrid designs that are classified as vertical heaters, but their in-tube characteristics are like those of horizontal heaters. There is no convection section. In addition to the advantages of simple vertical cylindrical heaters, the helical coil heaters are easy to drain. They are limited to smaU-duty applications 5 to 21 Gl/h (5 to 20 10 Btu/h). 

The nomenclature introduced by Hawthorne and Davis [4] is adopted and gas turbine cycles are referred to as follows CHT, CBT, CHTX, CBTX, where C denotes compressor H, air heater B, burner (combustion) T, turbine X, heat exchanger. R and I indicate reversible and irreversible. The subscripts U and C refer to uncooled and cooled turbines in a cycle, and subscripts 1,2, M indicate the number of cooling steps (one, two or multi-step cooling). Thus, for example, [CHT] C2 indicates an irreversible cooled simple cycle with two steps of turbine cooling. The subscript T is also used to indicate that the cooling air has been throttled from the compressor delivery pres.sure. 

A starter or contactor with manual push-button or thermostatic operation to start and stop the fan normally controls simple systems. More complex systems that incorporate components that need control or monitoring are normally operated from purpose-built central control panels. The most common functions provided are fan motor stop, start and speed control, damper control, filter-condition indication and heater battery control. For optimum control, the system should be automatically controlled from thermostats or other sensors and a timeswitch. 

This specialized form of supply air system is often used in large open industrial spaces. It comprises a modular system of components that can be built up into simple systems. A typical system might have a roof inlet cowl, a recirculation damper, a heater battery, a fan, one or two outlet grilles and short sections of connecting ductwork, and would handle airflows up to 3-4m /s, depending on size. A number of individual systems would be used to provide the total airflow required in the space (Figure 28.4). Systems are normally manufactured with aluminum casings to reduce the roof load. 

Electric storage hot water heaters and instantaneous water heaters are used for simple domestic applications and are not considered in this text. For many smaller HW and lower pressure steam generation applications (including bakery ovens and proofers, jacketed kettles, fish pots and other cooking equipment, wineries, breweries, textiles, laundries, phosphatizing processes, humidification, steam baths, clean rooms, and pharmaceuticals), electric boilers offer significant advantages over fossil-fuel boilers and are often the product of choice. 

For all types of boilers, from the most simple HW heaters to immense power generating plants, one of the most fundamental objectives of any water treatment program is (as stressed several times) to minimize boiler section waterside corrosion, especially those common types of corrosion involving oxygen. The actions taken to achieve this objective are generally the same, irrespective of boiler size or design. These actions include ... 

The heating curve of a substance, like that in Fig. 6.26, shows how its temperature changes as heat is supplied at a constant rate, usually at constant pressure. How are these curves produced Simple laboratory heaters can be used to obtain a crude estimate of a heating curve. However, for accuracy, one of two related techniques is normally used. 

The solution of these problems is based on a simple idea the developed laboratory-scale process is used for manufacturing of a chemical product by parallelization of many small units. Although promising great advantages over scale-up, this procedure, denoted numbering-up , is not trivial by far. It cannot be carried out in a simple way due to the tremendous technological effort necessary a chemical plant with hundreds or even thousands of small-scaled vessels, stirrers, heaters, pumps. 

The simple feedback control system below consists of a continuous-flow stirred tank, a temperature measurement device, a controller and a heater. 

Figure 5.156. Feedback control of a simple continuous water heater.

With the stirred-tank heater, we know quite well by now that we want to manipulate the heating coil temperature to control the tank temperature. The process function Gp is defined based on this decision. In this simple illustration, the inlet temperature is the only disturbance, and the load function is defined accordingly. From Section 2.8.2 and Eq. (2-49b) on page 2-25, we have the first order process model ... 

In such cases, radiant heat transfer is used from the combustion of fuel in a fired heater ox furnace. Sometimes the function is to purely provide heat sometimes the fired heater is also a reactor and provides heat of reaction. The special case of steam generation in a fired heater (a steam boiler) will be dealt with in Chapter 23. Fired heater designs vary according to the function, heating duty, type of fuel and the method of introducing combustion air. However, process furnaces have a number of features in common. A simple design is illustrated in Figure 15.19. The chamber where combustion takes place, the radiant section... 

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