Steam costing

Steam costs vary with the price of fuel. If steam is only generated at low pressure and not used for power generation in steam turbines, then the cost can be estimated from local fuel costs assuming a boiler efficiency of around 75 percent (but can be significantly higher) and distribution losses of perhaps another 10 percent, giving an overall efficiency of around 65 percent. 

The choice of technology, the associated capital, and operating costs for a chlor—alkaU plant are strongly dependent on local factors. Especially important are local energy and transportation costs, as are environmental constraints. The primary difference ia operating costs between diaphragm, mercury, and membrane cell plants results from variations ia electricity requirements for the three processes (Table 25) so that local energy and steam costs are most important. 

Because each effect of an evaporator produces almost as much vapor as the amount it condenses, the total evaporation accompHshed per unit of prime steam, or steam economy, iacreases ia almost direct proportioa to the number of effects used. The total heat load is also spHt up betweea the effects so that each effect has a much lower heat duty than a single effect for the same total evaporation load. However, the total available AT is also spHt up similarly so that each effect of a multiple effect requites about as much heating surface as a single effect operating over the same total temperature difference. Thus ia selecting the number of effects to use ia any iastallatioa, steam cost savings and capital cost of effects have to be balanced. Even before... 

As a general rule, the optimum number of effects increases with an increase in steam cost or plant size. Larger plants favor more effects, partly because they make it easier to install heat-recovery systems that increase the steam economy attainable with a given number of effects. Such recoveiy systems usually do not increase the total surface needed but do require that the heating surface be distributed between a greater number of pieces of equipment. 

Steam cost and value of turbine efficiency, so that consideration can be given to stage and valve options... 

Steam cost, and the value of turbine efficiency. Should it be single-stage or multistage Should it be single-valve or multivalve Is the steam an inexpensive process by-product, or is the entire cost of generating the steam chargeable to the driver ... 

Other advantages of eliminating pulsing are reduced steam costs, more accurate flow measurements possible with a steady flow, and reduced incidence of blowing and seal liquid. Also, the size of the seal drum can sometimes be reduced. 

Assuming water greater than 35°F is needed and that water and steam costs are reasonable ... 

Steam cost (in order to assess the value of machine and cycle efficiencies) ... 

Water. Costs are available from the supplier s invoices but where the operator has reservoirs and/or licenses for extraction from canals or rivers the annual fees and penalties, which are sometimes taxable, should not be forgotten. Projections of future demands will need to be carefully considered where this type of arrangement is made, since such contracts are often available only on long-term bases. Water-treatment plant will produce its own range of costs across the whole field of depreciation, materials, electricity, labor, etc. and these will need to be apportioned to the steam cost departments before final allocation to the user departments. 

What would be the cost of steam saved by coating the pipe with a 50 mm thickness of 85% magnesia lagging of thermal conductivity 0.07 W/m K, if steam costs 0.5 per 100 kg The emissivity of the surface of the bare... 

This simplistic approach to costing steam is often unsatisfactory, especially if the utility system already exists. Steam costs will be considered in more detail in Chapter 23. 

It should also be noted that variation in electricity tariffs that might change through the time of day, day of the week and time of the year will change the optimization and, therefore, the steam costs. An average can be taken according to the relative duration of the tariffs. 

Cooling water cost 0.005 S-kWtr1 Figure 23.56 Site marginal steam costs for the case study. 

Complex steam systems usually feature many important degrees of freedom to be optimized. To establish the steam costs for retrofit of site processes requires an optimization model to be developed. This allows the steam loads for process heating to be gradually decreased and the steam system reoptimized at each setting. The result in cost... 

A pipe containing 150 psi steam in an outdoor location loses 8,400,000 BTU per hour if not insulated. (Unrecovered energy loss = 1,000 BTU/lb vapor.) Assume steam costs 400/1,000 lb, and money is worth 8%. Determine whether it is economically justified to insulate the pipe, and if so, which of the thicknesses given below should be specified assume the insulation will last 20 years. 

The optimal plant operation can be determined by minimizing the total cost function, including steam costs, with respect to P (liquid pumping costs are negligible)... 

Fig. 15.6 The balance showing further savings on addition of reduced steam costs with membrane technology.

C thus, a ton of liquid chlorine will need 140 kg of steam for vaporisation. The 600 tons of chlorine will need 84 tons of steam. At a representative steam cost of US 10 per ton, this comes to around US 300 000 per year. 

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