Question 7.5: The schematic diagram for a steam boiler is presented in Fig......
The schematic diagram for a steam boiler is presented in Fig. E.7.7.
(a) Define F, P and L.
(b) Undertake an exergy balance in the boiler.
(c) Determine the exergy efficiency.
(a) The product of the boiler is obviously the transformation of the water flow into a steam flow, so that
P={\dot{B}}_{\nu}-{\dot{B}}_{w}For obtaining that product, the boiler needs the contribution of combustible and combustion air plus the work of the auxiliaries. Since it is atmospheric air, its exergy is null, and the consumption of auxiliaries can be considered negligible compared to the exergy of the combustible, so that
F={\dot{B}}_{f}+{\dot{B}}_{a}+{\dot{W}}\cong{\dot{B}}_{f}The combustion gases exergy is finally destroyed in the environment. Likewise, the heat transferred by convection and radiation through the boiler surface is part of the loss flow. As for the blowdown, we assume that they are intermittent and, therefore, they are not utilized, and so they are also part of the boiler losses. Therefore,
{L}=\dot{B}_{\rm g}+\dot{B}_{Q}+\dot{B}_{b d}(b) Undertaking an exergy balance we have
\dot{B}_{f}=\left(\dot{B}_{\nu}-\dot{B}_{w}\right)+\left(\dot{B}_{\rm g}+\dot{B}_{Q}+\dot{B}_{b d}\right)+\dot{D}(c) The exergy efficiency is
\varphi={\frac{{\dot{B}}_{\nu}-{\dot{B}}_{w}}{{\dot{B}}_{f}}}=1-{\frac{\left({{\dot{B}}_{\rm g}}+{\dot{B}}_{Q}+{\dot{B}}_{b d}\right)+{\dot{D}}}{{\dot{B}}_{f}}}