Heat Recovery Steam Generators (HRSGs)



Heat Recovery Steam Generators (HRSGs) are used to recover waste heat from the exhaust of combustion gas turbines (CGTs) in the useful form of steam. The efficiency of the CT alone (simple cycle) can be in the 40% range. Recovery of the waste heat in a HRSG can boost cycle efficiencies to nearly 60% (some claim higher!). The steam generated can be used for process heating (cogeneration), the production of power in a steam turbine (combined cycle), or both (combined heat and power or CHP).

N/E’s “typical” HRSG is in general terms a horizontal turbine exhaust gas (TEG) flow, vertical tube, top supported, natural circulation design with all welded pressure part construction and a cold external TEG pressure casing. With many hundreds of HRSGs in operation around the world, we have led the way in bringing this technology to the forefront of major HRSG supply globally. Horizontal TEG flow and top support allow the heat transfer tubes to hang in tension where they can grow freely up and down with varying temperature gradients and avoid the additional stress from the compressive loads of their own weight and the weight of additional equipment on top of the HRSG. Employing natural circulation eliminates the first cost, parasitic power consumption, and high maintenance costs associated with the evaporator circulation pumps necessary with forced circulation (sometimes euphemistically referred to as assisted circulation) systems. N/E’s natural circulation design instead use the natural buoyancy of the steam bubbles and the density differential between an all liquid external downcomer leg and the two-phase flow in the heat transfer tubes and external risers to drive the evaporator circulation. This leads to higher circulation ratios and a safer more robust design and operation. N/E's standard internally insulated casing design utilizes a cold, gas tight outer pressure casing to virtually eliminate thermal expansion of the casing and prevent rapid thermal transients from overstressing and cracking the casing. The casing will be internally insulated and lined with a special floating liner that is free to move to accommodate thermal growth without distortion or warping. In high turbulence areas, each liner plate will be supported on its perimeter with a rigid structural system, and floating batten channels will be added to the perimeter of each liner plate for additional support.