Add some description for the first step

docs/tutorials/hrsg_process_steam.rst

@@ -27,6 +27,22 @@ gas (e.g. from a gas turbine) to generate process steam and electricity. The
 figure above shows the structure of the system and the 4 steps we take to build
 the system.
 
+It is a simplified version of an actual plant with some unique features:
+
+- The economizer bypass (connection 28) actually represents a performance
+  downgrade for the boiler. The technical reason is to allow it to produce less
+  than 30 % of the nominal steam mass flow. With this measure the CHP plant
+  can produce almost any steam flow between 40 t/h and 210 t/h, which is a
+  unusual wide range.
+- The real economizer has eight stages. To simplify, we only model there here.
+- Interestingly, the boiler is operated with what could be called a "negative
+  approach" (the temperature coming out of the economizer was higher than that
+  of the HP drum). This issue is managed by increasing the economizer pressure.
+  Although this solution is inefficient from an energy standpoint, it was
+  chosen for operational safety, likely to reduce the risk of economizer
+  steaming.
+- The sweetwater condenser is a solution for HP steam desuperheating.
+
 ...
 
 - Table on boundary conditions
@@ -54,21 +70,30 @@ Steam supply and turbine section
 
     Figure: First part of the system
 
+In the first part we will model the steam supply part and the turbine. We start
+by importing the :code:`Network` class and creating an instance of it.
+
 .. literalinclude:: /../tutorial/advanced/hrsg.py
     :language: python
     :start-after: [sec_1]
     :end-before: [sec_2]
 
-Components
-++++++++++
+Components and Connections
+++++++++++++++++++++++++++
+
+For the components, we need to import the respective classes as shown in this
+part of the flowsheet and create the respective instances.
 
 .. literalinclude:: /../tutorial/advanced/hrsg.py
     :language: python
     :start-after: [sec_2]
     :end-before: [sec_3]
 
-Connections
-+++++++++++
+The same applies for the connections. The extraction steam turbine is connected
+to the live steam source. At a later stage, this connection will be replaced by
+one that directly links the high pressure superheater 2. Similarly, the
+connection 15 connects to a sink, which will be replaced by the low pressure
+feed pump later.
 
 .. literalinclude:: /../tutorial/advanced/hrsg.py
     :language: python
@@ -78,6 +103,26 @@ Connections
 Parametrization & solving
 +++++++++++++++++++++++++
 
+For the live steam at the turbine inlet we have to assume a steam mass flow. In
+the completed model it will be governed from the flue gas mass flow. On top of
+that, we need temperature and pressure information. For the steam supply a
+pressure of 13 bar and 10 °C of superheating is required. There also is a
+specific mass flow to be provided, which however cannot be directly as this
+specification would cause convergence issues. Instead, massflow 3 is fixed
+initially. The outlet of the condenser is saturated liquid at a temperature of
+35 °C. The condensate pump pumps the pressure back to ambient pressure. It is
+assumed that 75 % of the process steam leaving the plant returns in the form of
+condensate. The remaining mass flow is replaced by the makeup water.
+
+.. literalinclude:: /../tutorial/advanced/hrsg.py
+    :language: python
+    :start-after: [sec_4]
+    :end-before: [sec_5]
+
+For the components we can specify the isentropic efficiencies of the turbines
+and the pumps as well as the pressure ratio of the condensing turbine inlet
+valve as well as the condenser pressure loss.
+
 .. literalinclude:: /../tutorial/advanced/hrsg.py
     :language: python
     :start-after: [sec_5]

tutorial/advanced/hrsg.py

@@ -77,7 +77,8 @@
 
 condensate_return_fraction = 0.75
 c13.set_attr(fluid={"water": 1}, x=0, m=Ref(c11, condensate_return_fraction, 0))
-c14.set_attr(fluid={"water": 1}, m=1, T=Tamb)
+c14.set_attr(fluid={"water": 1}, T=Tamb)
+c15.set_attr(m=200)
 # %%[sec_5]
 extraction_turbine.set_attr(eta_s=0.9)
 condensing_turbine.set_attr(eta_s=0.9)
@@ -88,6 +89,12 @@
 steam_cond_pump.set_attr(eta_s=0.7)
 
 nw.solve("design")
+
+c3.set_attr(m=None)
+c11.set_attr(m=extraction_mass_flow)
+
+nw.solve("design")
+
 nw.print_results()
 # %%[sec_6]
 from tespy.components import Drum
@@ -134,8 +141,6 @@
 
 nw.add_conns(c15, c16, c17, c18, c19, c20, c21, c22, c23, c24, c25, c26, c27, c28, c29)
 # %%[sec_7]
-c14.set_attr(m=None)
-
 c16.set_attr(p=2.3, T=55)
 c17.set_attr(T=100, p0=2.3)
 c20.set_attr(x=.05)