A steam generator consists of an in-line array oftubes,

Chapter 13, Problem 13.126

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A steam generator consists of an in-line array oftubes, each of outer diameter D?10 mm and lengthL?1 m. The longitudinal and transverse pitches areeach SL?ST?20 mm, while the numbers of longi-tudinal and transverse rows are NL?20 and NT?5.Saturated water (liquid) enters the tubes at a pressureof 2.5 bars, and its flow rate is adjusted to ensure that it leaves the tubes as saturated vapor. Boiling thatoccurs in the tubes maintains a uniform tube wall tem-perature of 400 K.(a) Consider case (a) for which products of com-bustion enterthe tube bank with velocity andtemperature of V?10 m/s and Tm,i?1200 K,respectively. Determine the average gas-sideconvection coefficient, the gas outlet tempera-ture, and the rate of steam production in kg/s.Properties of the gas may be approximated to bethose of atmospheric air at an average tempera-ture of 900 K.(b) An alternative steam generator design, case (b),consists of the same tube arrangement, but thegas flow is replaced by an evacuated space withelectrically heated plates inserted between eachline of tubes. If the plates are maintained at auniform temperature of Tp?1200 K, what isthe rate of steam production? The plate and tubesurfaces may be approximated as blackbodies.(c) Consider conditions for which the plates areinstalled, as in case (b), and the high-temperatureproducts of combustion flow over the tubes, as in case (a). The plates are no longer electri-cally heated, but their thermal conductivity is sufficiently large to ensure a uniform plate tem-perature. Comment on factors that influence theplate temperature and the gas temperature distri-bution. Contrast (qualitatively) the gas outlettemperature and the steam generation rate with the results of case (a).