Heat Recovery Steam Generator – Combined Cycle Power Generation

The Heat Recovery Steam Generator, or HRSG, is configured in various shapes, designs, configurations, arrangements, and so on. To streamline our dialogue herein, we shall 1st say that the kind of HRSG we are now reviewing is what may perhaps be referred to as a water tube(as opposed to a fire tube) sort heat recovery unit. This refers to the process fluid, i.e., the steam or water being on the inside of the tubing with the products of combustion being external of the tube. The products of combustion are typically at or about atmospheric pressure, therefore, the shell side is normally not regarded as to be a pressure vessel.

In the style of any HRSG, the 1st step generally is always to perform a theoretical heat balance which is able to provide us with the correlation of the tube side and shell side process. Before we can calculate this heat balance, one need to determine the water side components which will make up your HRSG unit. Despite the fact that these elements may perhaps consist of other heat exchange components, right now we will primarily take into consideration the 3 major coil varieties that may be present, i.e., Evaporator, Superheater, Economizer. Whenever I allude to an Evaporator Section, this encompasses all the evaporator coils making up the complete evaporator for the Pressure System. A pressure circuit consists of all of the components incorporated within the many streams associated with that pressure level.

Evaporator Module

The most essential component would, certainly, be the Evaporator Section, given that with out this component (or coils), the unit would not be an HRSG. All through the article, we’ll refer to a main heat transfer element as a “section”. When the section is broken into more than one particular component, i.e., such as for a change in tube size, material, fin configuration, location, etc., I will refer to the tube sections as coils. So an evaporator section might consist of 1 or more coils. In these coils, the water/steam flowing through the tubing is heated to near the steam saturation point for the operating pressure it can be at.

Superheater Module

The Superheater Section of the HRSG is utilized to dry the saturated vapor becoming separated within the steam drum. In some units it may perhaps only be heated to small above the saturation point where in other units it may well be superheated to a substantial temperature for additional energy storage. The Superheater Section is commonly situated in the hotter gas stream, in front of the evaporator.

Economizer Section

The Economizer Section, often known as a preheater or preheat coil, is utilised to preheat the feedwater being introduced towards the system to replace the steam(vapor) becoming removed from the method via the superheater or steam outlet and the water loss via blowdown. It can be normally located inside the colder gas downstream in the evaporator. Because the evaporator intake and outlet temperature conditions are both near to the saturated steam temperature for the HRSG pressure, the quantity of heat that could be removed from the flue gas is limited on account of the approach to the evaporator, known as the pinch which is explained later, while the economizer inlet temperature is actually low, allowing the exhaust gas temperature to be reduced lower.

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