Introduction
Gas turbines are used in many applications including power generating stations, cogeneration plants, and in the oil and gas industry to provide power in remote offshore and onshore production facilities locations.
A wide range of air temperature and humidity variations occur in most locations on the globe. Even locations that typically have a warm climate can experience sub-zero cold with high air humidity. In cold environments where unfavorable ambient air temperature and humidity occurs, cold start-up and operation of gas turbines can be a challenge. One of the unique problems of operating gas turbines in cold climates, is ice formation on the inlet side of the systems.
There are many factors that can lead to ice formation. When ambient temperatures are below 4.4 °C (40 °F) and the relative humidity is above 64% ice formation and ice build-up in the turbine intake system can occur. Depending on the combination of ambient air temperature and relative humidity, metal surface temperature and air velocity, the ice formation process can develop rapidly and take on different forms. The most obvious icing formation is precipitation icing, where either wet snow or freezing rain is drawn into the inlet filter house.
Another unique ice formation condition can also occur when humid ambient air cools below the freezing point. As the air cools, a fog of sub-cooled liquid droplets forms. These droplets freeze on contact with a cold metal surface, causing a build-up of hoar frost which can potentially damage the internal components of the inlet filter house, inlet ducting and gas turbine.
Ice formation is also possible even when the ambient conditions are above the freezing temperature of water and if the humidity of the ambient air is sufficiently high. As the air enters the filter house and flows through the inlet ducting into the bell-mouth of the turbine, it accelerates in a nearly isentropic process. As the specific energy of the air stream is constant, the gain in kinetic energy comes at the expense of static pressure. Thus, as the flow accelerates entering the gas turbine, the air flow pressure and temperature decrease. This localized temperature depression can potentially cause ice formation on the internal surfaces of the gas turbine inlet system. Build-up of ice is a concern since a piece of ice can potentially disengage and brake off from the metal surface, and be ingested by the engine, causing impact damage to the inlet guide vanes and other internal components of the gas turbine resulting in expensive repairs and power plant shutdown.
Anti-Icing Systems are designed to prevent ice formation on the gas turbine inlet components in order to protect the gas turbine from damage due to icing and allow it to operate reliably in a wide range of cold climates with temperature and humidity ranges where ice formation can occur.
Anti-Icing System
The waste heat recovery Anti-Icing System uses the gas turbine exhaust flow to preheat the ambient air as it enters the filter house. The Anti-Icing System contains waste heat recovery shell-and-tube heat exchanger as shown below, ambient air blower, back draft and control/isolation dampers, hot inlet air supply duct, inlet air manifold and hot air distribution sparger pipes.
The heat exchanger is mounted between the gas turbine exhaust diffuser and the inlet exhaust stack flange. The ambient air flows through this shell-and-tube heat exchanger, warming as it travels. A fan blower, installed upstream of the heat exchanger inlet manifold, forces ambient air through the manifold into multiple circular arrays of tubes. The size and the location of the inner tubes is strategically designed. These tubes are positioned around the inner shell of the heat exchanger, to effectively maximize the heat transfer, causing negligible back pressure on the turbine. The heated air is then transported through air supply ducting to distribution spargers installed at the filter house inlet. This hot air is uniformly injected upstream from the inlet filter elements, raising the overall temperature of combustion and ventilation airflow out of the ice formation region.
Uniform air flow mixing and energy exchange over the face of the filter house elements is achieved by the complex turbulent air flow motion. The mixing action of air turbulence causes small air masses to be swept back and forth across the mean air flow direction. As a small mass of fluid is swept from a low-velocity zone into a relatively high-velocity zone through an exchange of momentum, an exchange of energy takes place.
Due to the relative simplicity of this Anti-Icing System design and its low maintenance, this system is proven to deliver a cost effective solution to the unique icing formation problems of gas turbine operation in cold climates.
Anti-Icing System Process Flow Diagram
A typical Process Flow Diagram for an Anti-Icing System is comprised of a Heat Exchanger; Centrifugal Fan; Control/Backdraft/Isolation Dampers; Flexible Expansion joints; Metal Bellows and Supplemental Electric Heaters is shown below in Figure 1 and 2.
Although most of the Anti-Icing System designs are similar, each configurational layout is unique, hence, the Anti-Icing System design and its process flow diagram can vary from project to project.
Figure 1. Anti-Icing System Process Flow Diagram
Figure 2. Anti-Icing System Process Flow Diagram