Thermal updraft

Receptor hoods, also called canopy hoods, are designed to capture contaminants given off by heated processes. They take advantage of the thermal updraft caused by such processes by placing the hood in the path of the updraft, they receive the exhaust and capture the contaminants. 

The key variable in determining the applicability of a receptor hood to a particular source is the temperature of the heated source, and the resulting updraft. The temperature must be high enough to cause an appreciable updraft, or the hood will be ineffective. An estimate must be made of the total amount of buoyant airflow set in motion by the heated source the airflow through the hood must be greater than this buoyant airflow, in order to ensure complete contaminant capture. This principle is illustrated in Fig. 10.32, which shows the air spill that occurs when a hood s exhaust airflow is less than the thermal updraft airflow. 

FIGURE 10.3 2 Canopy hood with an airflow rate less than the thermal updraft airflow from a hot process. 

Flemeon is the first standard reference book that presents the equations for calculating thermal updrafts. These equations are repeated and expanded in other standard reference books, including Heinsohn, Goodfellow, and the ACGIFl Industrial Ventilation Manual.These equations are derived from the more accurate formulas for heat transfer (Nusselt number) at natural convection (where density differences, due to temperature differences, provide the body force required to move the fluid) and both the detailed and the simplified formulas can be found in handbooks on thermodynamics (e.g., Perry--, and ASHRAE -). 

Thermal updraft The air movement that is created by a thermal plume. 

Exposure protection systems provide protection by the application of water to structures and equipment for the anticipated duration of the exposure fire. Water spray curtains are generally less effective than direct application due to unfavorable conditions such as wind, thermal updrafts, and inadequate drainage. Extinguishing agents such as CO2 or dry chemical agents are not able to provide this type of cooling. 

The system can be operated with fewer emergency responders and foam loss is minimized. Foam chambers cause the foam to flow down the tank shell and onto the liquid surface therefore, thermal updrafts have little effect on foam application. A similar effect is seen with the use of foam chambers to protect the seals on floating roof tanks. 

Heavy loads with large surface areas should not dwell in the freeboard area longer than necessary to allow good drainage. Thermal updrafts from the hot work may draw vapors from below and expel them. 

A survey of commercial thermal gasification in the United States shows that few gasifiers have been installed since 1984 (115). Most units in use are retrofitted to small boilers, dryers, and kilns. The majority of existing units operate at 0.14 to 1.0 t/h of wood wastes on updraft moving grates. The results of this survey are summarized in Table 36. Assuming all 35 of these units are operated continuously, extremely unlikely, the maximum amount of LHV gas that can be produced is about 0.003 to 0.006 EJ/yr (222—445 td /d). 

The solar updraft tower converts solar-based thermal energy into concentrated aerodynamic energy that drives turbine generators. 

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