Heat-transfer surface area

The heat-transfer surface area determined by the basic sizing or rating method described herein is considered the minimum required area. There are also additional surface area requirements in the final sizing of a heat exchanger. 

Fypass Flow Effects. There are several bypass flows, particularly on the sheUside of a heat exchanger, and these include a bypass flow between the tube bundle and the shell, bypass flow between the baffle plate and the shell, and bypass flow between the shell and the bundle outer shroud. Some high temperature nuclear heat exchangers have shrouds inside the shell to protect the shell from thermal transient effects. The effect of bypass flow is the degradation of the exchanger thermal performance. Therefore additional heat-transfer surface area must be provided to compensate for this performance degradation. 

A heat-transfer surface area, flow area 2 m... 

Provide adequate heat transfer surface area or temperature gradient (keeping in mind that fluid properties and temperature change as the reaction progresses)... 

This Method Quickly Determines Required Heat Transfer Surface Area and How Changing Loads Will Affect Performance... 

Since air has relatively poor thermal transport properties when compared to water, the air cooled heat exchanger could have considerably more heat transfer surface area. A large space requirement may result. 

The fixed-tubesheet exchanger is the most common, and generally has the lowest capital cost per ft of heat-transfer surface area. Fixed-tubesheet exchangers consist of a series of straight tubes sealed between flat, perforated metal tubesheets. 

HEAT TRANSFER SURFACE AREA, m 2 SPECIFIC HEAT OF LIQUID, kJ/kg.K WEIGHT OF BATCH LIQUID, kg. HEATING MEDIUM TEMPERATURE, K INITIAL BATCH TEMPERATURE, K ... 

A = heat transfer surface area c = specific heat of batch liquid C = coolant specific heat M = weight of batch liquid Tj = initial batch temperature Tj = final batch temperature tj = initial coolant temperature U = overall heat transfer coefficient = coolant flowrate 6 = time... 

Investigate and determine thermal parameters involving conditions under which heat transfer takes place and an accurate value of the heat transfer surface area. Additionally, a review of the process is required for thermal stability (see Chapter 6). 

Required effective outside heat transfer surface area based on net exposed tube area. Note. Later in text = A. 

A boiler is a device for heating water or other liquids and is most commonly constructed as a closed pressure vessel, containing a furnace area, purpose-designed heat transfer surface areas, and other functional components. 

Fire tube boilers (shell boilers or shell and tube boilers) convert heat from burning fuel within a furnace (combustion chamber, firebox, or furnace tube) to generate either HW or steam. Fire tube boilers are designed to direct the combustion gases through tubes (held within tube sheets) that are surrounded by BW, thus providing for a greater heat-transfer surface area and improved efficiency. 

A boiler bank is also included. The boiler-bank tube bundle provides sufficient heat transfer surface area to provide the rated capacity for saturated steam. Boiler-bank tube spacing and dimensions are arranged so that a steam-water circulation subsystem connects the top and bottom drums with subcooled water passing down the tubes farthest from the furnace and returning as a steam-water mixture. 

Reactor vessel Reactors produce intense heat, and those employed in large nuclear power boiler installations (typically 800-1,000 MWh) may have 50,000 to 60,000 sq ft of heat transfer surface area with a heat flux of from 150,000 to 500,000 Btu/ft2/hr. 

Heat transfer problems become more severe as reaction rates are increased and water-to-monomer ratios are reduced. In addition, as reactor sizes are increased for improved process economics, the amount of wall heat transfer surface area per unit volume will drop and result in a lower reactor space-time yield. 

Influence of reactor size/shape on process performance via heat transfer. Heat transfer between a reaction mixture and its surroundings strongly depends on the size of the reactor. The amount of heat evolved during an exothermic reaction is proportional to the volume of the reaction mixture (Qgcncrated V), ie. the effective volume of the reactor, whereas the amount of heat removed from the mixture is proportional to the heat-transfer surface area (firemovod A,). Morc precisely, the amount of heat transferred is given by ... 

Evaporation can be performed directly from reactors or kettles provided that substances are thermally stable. Such evaporation is time consuming because of the low heat-transfer surface area per unit volume. In the case of temperature sensitive materials, the residence time in the evaporator must be short and the temperature should be as low as possible. Consequently, continuous vacuum evaporators with a short residence time should be used to treat such materials. Falling-film (thin-film) evaporators are suitable to perform such operations. A typical falling-film evaporators is shown in Fig. 7.2-14. Centrifugal evaporators are also commonly used. 

Table 7.4-2 Volume V, and the ratio of heat-transfer surface area A, to volume V7 for reactors of AE series ...

Fig. 4.25 represents a steady-state, single-pass, shell-and-tube heat exchanger. For this problem W is the mass flow rate (kg/s), T is the temperature (K), Cp is the specific heat capacity (kJ/m s), A (= 7i D Z) is the heat transfer surface area (m ), and U is the overall heat transfer coefficient (kJ/m s K). Subscripts c and h refer to the cold and hot fluids, respectively. 

Heat balances on a small differential element of heat transfer surface area, AA, give... 

However, the simulation of the performance for a heat exchanger with a known heat transfer surface area will demand an iterative split boundary solution approach, based on a guessed value of the temperature of one of the exit streams, as a starting point for the integration. 

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