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ASME FEATURE Runge Kutta routine) and iteration on the wall temperature profile (with DNSQE from SLATEC).[8] Relevant heat transfer and pressure drop correlations are obtained from the literature, see Table 1. More details on the heat exchanger framework can be found in Skaugen et al.[9]

Waste heat recovery unit (WHRU) The WHRU is modeled as a cross-flow finned tube heat exchanger with serrated fins. The tube passes are arranged in horizontal serpentines inside a rectangular vertical gas duct, approaching a counter flow configuration. The WHRU is designed for a 3 kPa pressure drop on the exhaust side. Figure 4. Pressure ratio vs. produced heat

Figure 5. Expander efficiency vs. produced heat

in) before the waste heat recovery unit. This is called the Brayton cycle.

Models and methodology The main purpose of this study is to investigate how the two proposed layouts for a combined heat and power bottoming cycle manage varying load ratio between process heat production and power production. Realistic off-design evaluation is needed, which imposes advanced geometry-based component models (as opposed to performance-based models that cannot provide off design information). Gas turbine model The gas turbine performance was calculated separately with GT Master from Thermoflow Inc.[7] The chosen model is a GE LM2500+G4 with the dry low emission (DLE) setup. Compressor and turbine maps relating corrected inlet air mass flow to compressor pressure ratio and efficiency were utilized. The gas turbine is supposed to run at 100 percent load for all power/heat ratios for the bottoming cycles. Heat exchanger models An in-house framework is used to model the heat exchangers. The models use geometrical input data to calculate parameters such as hydraulic diameters, perimeters and cross sectional areas for each fluid pass. Based on the geometry specification and the fluid inlet conditions, the outlet conditions are found through integration of the fluid passes (with a 4th order 24

Condenser The condenser is modeled as a plate heat exchanger. The high condensing pressure of CO2 (55–60 bar) makes it unsuitable for standard plate-and-frame configurations. However plate-and-shell configurations could be an option. In this work, the performance characteristics of the heat exchanger at off-design conditions are of the most interest, and this will be relatively independent of the type of heat exchanger. The condenser is assumed to be cooled with sea water. At the design point, the cooling water flow rate is set to give a 10°C increase in cooling water temperature, and this flow rate is kept constant also at off-design conditions. Recuperators and process heat generator (d in) for the IHRU The recuperator model is based on stacked layers of multiport tubes with counter-current flow, and is meant to represent a generic compact heat exchanger. Due to the high operating pressure (200 bar), diffusion bonded printed circuit heat exchangers might be the most relevant solution currently available. Pump and turbine for the bottoming cycle The pump is modeled with constant isentropic efficiencies defined as follows:

Equation 1

The efficiencies are set to 80 percent throughout the load range studied. It is assumed that the pump is equipped with variable frequency drive (VFD). The turbines are calculated using the inlet guide vanes (VIGV) model.[15] An efficiency ηDP of 85 percent was assumed at design. Off design performance was evaluated using manufacturer efficiency charts. The efficiency factor as function of volume flow ratio R (to design volume flow) is obtained from Atlas Copco[16]: Equation 2

ASME Power Division Special Section | APRIL 2015

Profile for Energy-Tech Magazine

April 2015  

Heat Exchangers – Retrofit/Rebuild/Equipment Upgrade – Bearings – Turbine Tech: Steam – ASME: Combined-Cycle Plants

April 2015  

Heat Exchangers – Retrofit/Rebuild/Equipment Upgrade – Bearings – Turbine Tech: Steam – ASME: Combined-Cycle Plants