The overall system energy efficiency can be defined as the ratio of the energy delivered to and used by all end-users, and the energy supplied to the boiler by fuel.
Energy Used by End-Users Eeu
Energy Supplied to System by Fuel EFUEL
Energy efficiency defined in this way is in fact an energy performance indicator of the observed steam system (Fig. 2.1). Performance indicators are discussed in details in Part I Chapter 2. Energy delivered to end-users can be defined as follows:
Eeu = J2 (ms.n • hs,n - mcn • 4.21 • tc,n) (n = 1, 2, 3, N) (2.2)
ms n = flow rate of in-coming steam to end-user No. n, [t/h];
hsn = enthalpy of in-coming steam, [kJ/kg] (enthalpy of water at 0.1 °C and 0.006113 bar (triple pint of water) is zero). n = flow rate of out-coming condensate from end-user No. n, [t/h]; tc n = temperature of out-coming condensate, [° C];
4.21 = isobaric specific heat of water, [kJ/(kg °C)] (This value will be used for all calculations in this chapter. It is the average value of water specific heat for the temperature range which appears in industrial steam systems).
The mass flow rate of steam is equal or greater than the mass flow rate of out-coming condensate. The condensate flow rate can be zero if steam is used in the process and in that case it has to be fully compensated by make-up water.
The chemical energy of fuel used for steam production is as follows:
In this equation, MFUEL is the flow rate of fuel and GCV is the Gross Calorific Value of fuel used for running the boiler. A more precise definition of fuel energy is discussed in Part III Toolbox 5.
Finally, we can rewrite the equation for the calculation of steam system energy efficiency (Eq. 2.1) as follows:
Similarly, it is possible to define the energy efficiency of the subsystem shown in Figure 2.1. In this way, the following efficiencies (performance indicators) are obtained:
msteam X (hsteam
• For STEAM DISTRIBUTION SYSTEM
msteam • hsteam
• For CONDENSATE RETURN SYSTEM
msteam = mass flow rate of steam, [t/h]; hsteam = steam enthalpy, [kJ/kg]; mcR = mass flow rate of condensate, [t/h]; t<:R = temperature of condensate, [° C]; mMU = mass flow rate of make-up water, [t/h]; tMU = temperature of make-up water, [°C].
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