Feedwater heating, cost

The economic analysis to follow depends upon the evaluation of the various available-energy supplies for feedwater heating and, in turn, the costs associated with those supplies. In particular, the costs of interest, for each case, are those required to take the feedwater from the conditions at the inlet to heater number 4 to the normal temperature of feedwater entering the boiler. These costs include the cost of bleeder steam, which is used to increase the temperature of feedwater in the heater and, under the conditions of Case C, the cost of the additional boiler fuel required when the heater is out of service and the temperature of the feedwater is below normal. The hourly cost of feedwater heating for Cases A and B is given by... 

Exampie A.3.1 The pressures for three steam mains have been set to the conditions given in Table A.l. Medium- and low-pressure steam are generated by expanding high-pressure steam through a steam turbine with an isentropic efficiency of 80 percent. The cost of fuel is 4.00 GJ and the cost of electricity is 0.07 h. Boiler feedwater is available at 100°C with a heat capacity... 

Selection of the high pressure steam conditions is an economic optimisation based on energy savings and equipment costs. Heat recovery iato the high pressure system is usually available from the process ia the secondary reformer and ammonia converter effluents, and the flue gas ia the reformer convection section. Recovery is ia the form of latent, superheat, or high pressure boiler feedwater sensible heat. Low level heat recovery is limited by the operating conditions of the deaerator. 

Oxygen can also be removed from feedwater by thermal de-aeration, or partially removed by skilful design of the feed heating system and blowdown recovery. These processes run without cost to the operator, but save chemicals, and, by reducing the required dose of sulfite into the system, decrease the amount of non-volatile solids added into 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). 

Excessive corrosion of the condensate system can lead not only to costly equipment failure and increased maintenance costs, but can also cause deposition of metal oxide corrosion products on boiler heat transfer surfaces if the condensate is recovered as feedwater. Metal oxide deposition on boiler heat transfer surfaces will result in lower fuel to steam efficiency and higher fuel costs. The deposition may also lead to tube failure due to long-term overheating. 

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