Heat transfer feedwater heater

In fossil fuel-fired boilers there are two regions defined by the mode of heat transfer. Fuel is burned in the furnace or radiant section of the boiler. The walls of this section of the boiler are constmcted of vertical, or near vertical, tubes in which water is boiled. Heat is transferred radiatively from the fire to the waterwaH of the boiler. When the hot gas leaves the radiant section of the boiler, it goes to the convective section. In the convective section, heat is transferred to tubes in the gas path. Superheating and reheating are in the convective section of the boiler. The economizer, which can be considered as a gas-heated feedwater heater, is the last element in the convective zone of the boiler. 

As an example, let us consider a feedwater heater, such as illustrated by Component No. 6 in Figure No. 1. Let Z represent the annualized capital cost (say in dollars per year) of owning and operating the feedwater heater (including maintenance, overhead, etc., as well as interest). Also, let X represent the unit cost of each type of lost work, while T0 represents the lost work, where represents the rate of entropy creation (or production) corresponding to each type of lost work in the feedwater heater (2, 6,7). Then let A, Ag, and Ah represeht the unit costs of lost work Td a T0 b, and T h due respectively to head loss (pressure drop) in the feedwater A, head loss in the condensing steam B, and heat transfer (temperature drop) from the condensing steam.to the feedwater, denoted by H, so that the total annualized cost T attributable to the feedwater heater is,... 

Under certain assumptions (discussed below), the cost per transfer unit Yx is independent of the number of transfer units x, as shown following Eqn. (20) below. Figure 4 displays the minimum of this dimensionless total cost for a feedwater heater with constant parameters Yx, y, Xu, Tc, M, cp, and U. Similar curves result for condensers and boilers. The values used for the dimensionless parameters y and Yx/XfjTQ are shown in the figure. From the definition x = UA/Mcp, it is seen that x is directly proportional to the heat transfer area A (since U, M, and Cp are constant). 

Figure 4. Determination of the optimum heat transfer area for the feedwater heater.

The steam supplied to a given feedwater heater must be at a pressure hi enough that its saturation temperature is higher than the temperature of feedwater stream leaving the heater. We have here presumed a minimum ter perature difference for heat transfer of no less than 5°C, and have chose extraction steam pressures such that the tsM values shown in each feedwat heater are at least 5 C greater than the exit temperature of the feedwater strear The condensate from each feedwater heater is flashed through a throttle valv to the heater at the next lower pressure, and the collected condensate in the fin heater of the series is flashed into the condenser. Thus, all condensate retu from the condenser to the boiler by way of the feedwater heaters. 

Although the steam generation rate is higher than was found in Example 8.1, the heat-transfer rates in the boiler and condenser are appreciably less, because their functions are partly taken over by the feedwater heaters. 

Feedwater heaters increase plaint efficiency by increasing the average temperature at which heat is supplied to the cycle H2O from the products of combustion (7, articles 15.1-6) that is, inasmuch as the available-energy consumption for driving heat transfer processes increases with the temperature difference... 

Nuclear heat transfer to desalination plant The nuclear power plant supplies steam to three turbogenerators and to a desalination plant (80 000 tonnes of desalinated water per day). To transfer the heat from the reactor to the intermediate heat exchangers six primary parallel sodium loops and six independent secondary sodium loops are provided. The turbine exhaust steam under the pressure of 0.6 MPa (6 bar) is supplied to the desalination plant, and the condensate produced with the temperature of about 100°C flows to a heater and a deaerator and is pumped by feedwater pumps to the natural circulation steam generators (Fig. 4). 

Noncontact heat exchangers, which transfer heat between fluids through a conductive barrier (Figure 23.9), are used in a wide variety of applications, including nuclear steam generators, condensers, and closed feedwater heaters. 

Copper alloys (Admiralty, copper-nickel) and austenitic stainless steels are the most commonly used materials for feedwater heater tubing based upon their resistance to general and localized corrosion, erosion-corrosion, and SCC, and adequate heat transfer performance [1,2]. Carbon and low-alloy steels are most often used for the shells of such heaters for economy and availability. 

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