This analysis presumes that there is similar coupling of heat from the two streams.
On the output (water) side the coupling is from water to brass, which is efficient.
On the input (steam) side we have an unknown selection of any/all
a) Superheated 120C (1 bar) steam (efficient)
b) 100C (1 bar) or 120C (2 bar) vapour (inefficient)
c) 100C (1 bar) or 120C (2 bar) fluid (efficient)
which have a different "coupling coefficient" to brass (I can't think of the technical term),which limits the heat transfer from one side to the other. In a circuit simulation like Spice I could use a current source (= heat) rather than a voltage source (= temperature).
The "coupling coefficient" term is "convection heat transfer coefficient "
http://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html
1 / U A = 1 / h1 A1 + dxw / k A + 1 / h2 A2 (1)
