Heat exchange and control

JAEA conducted an improvement of the RELAP5 MOD3 code (US NRC, 1995), the system analysis code originally developed for LWR systems, to extend its applicability to VHTR systems (Takamatsu, 2004). Also, a chemistry model for the IS process was incorporated into the code to evaluate the dynamic characteristics of process heat exchangers in the IS process (Sato, 2007). The code covers reactor power behaviour, thermal-hydraulics of helium gases, thermal-hydraulics of the two-phase steam-water mixture, chemical reactions in the process heat exchangers and control system characteristics. Field equations consist of mass continuity, momentum conservation and energy conservation with a two-fluid model and reactor power is calculated by point reactor kinetics equations. The code was validated by the experimental data obtained by the HTTR operations and mock-up test facility (Takamatsu, 2004 Ohashi, 2006). 

Before any work inside the column commences, it is essential to take steps for preventing small parts such as nuts eind bolts from finding their way into downstream equipment, such as pumps, heat exchangers, and control valves. In one case (145), a column bottom pump frequently lost suction because a leftover piece of rope ladder reached its inlet and lodged there. Temporary plugs in the column base and some drawoffs can effectively prevent such incidents. It is important to positively ensure that these plugs are removed prior to startup. 

The ability to run a complete performance test requires experience and preparation. For a first performance test, you should simply obtain a complete unit pressure profile. This is done with a single pressure gauge, which you move downstream from point to point. Obtain pressure drops across heat exchangers and control valves. Then, obtain a complete temperature survey. Where no thermowells exist, use a glass thermometer inserted under the pipe insulation. Next, record all flow rates. You will probably find many flow inconsistencies. Finally, obtain a complete set of samples. Learn how and where the samples are taken (see Chapter 27). 

COUNTER FLOW HEAT EXCHANGERS AND CONTROL VOLUME CODES... 

The size of the leakage was determined as follows. Radiation detectors were mounted on the inlet of the first side, and the outlet of the second side of the heat exchanger and at additional locations for control. A short pulse of Kr-85 tracer (<0.1 sec) was injected into the feed stream, which gave rise to detector responses shown in figure 3. 

In open fibers the fiber wall may be a permselective membrane, and uses include dialysis, ultrafiltration, reverse osmosis, Dorman exchange (dialysis), osmotic pumping, pervaporation, gaseous separation, and stream filtration. Alternatively, the fiber wall may act as a catalytic reactor and immobilization of catalyst and enzyme in the wall entity may occur. Loaded fibers are used as sorbents, and in ion exchange and controlled release. Special uses of hoUow fibers include tissue-culture growth, heat exchangers, and others. 

The simplest unit employing vacuum fractionation is that designed by Canadian Badger for Dominion Tar and Chemical Company (now Rttgers VFT Inc.) at Hamilton, Ontario (13). In this plant, the tar is dehydrated in the usual manner by heat exchange and injection into a dehydrator. The dry tar is then heated under pressure in an oil-fired hehcal-tube heater and injected directly into the vacuum fractionating column from which a benzole fraction, overhead fraction, various oil fractions as side streams, and a pitch base product are taken. Some alterations were made to the plant in 1991, which allows some pitch properties to be controlled because pitch is the only product the distillate oils are used as fuel. 

NO -laden fumes are preheated by effluent from the catalyst vessel in the feed/effluent heat exchanger and then heated by a gas- or oil-fired heater to over 600° F. A controlled quantity of ammonia is injected into the gas stream before it is passed through a metal oxide, zeolite, or promoted zeolite catalyst bed. The NO is reduced to nitrogen and water in the presence or ammonia in accordance with the following exothermic reactions ... 

Some toll processes lend themselves to test runs in the pre-startup phase. Actual materials for the toll may be used in the test or substitute materials, typically with low hazard potential, are often used to simulate the charging, reaction, and physical changes to be accomplished in the toll. Flow control, temperature control, pressure control, mixing and transferring efficiency can be measured. Mechanical integrity can be verified in regard to pumps, seals, vessels, heat exchangers, and safety devices. 

Process flow diagrams are more complex and show all main flow streams including valves to enhance the understanding of the process as well as pressures and temperatures on all feed and product lines within all major vessels and in and out of headers and heat exchangers, and points of pressure and temperature control. Also, information on construction materials, pump capacities and pressure heads, compressor horsepower, and vessel design pressures and temperatures are shown when necessary for clarity. In addition, process flow diagrams usually show major components of control loops along with key utilities. 

Buildup of condensate in a heat exchanger can cause operating problems as well as water hammer. If the steam supply is controlled by a motor valve and the valve is not fully open, the steam pressure may be too low to expel the condensate, and its level will rise. This will reduce heat transfer, and ultimately the steam supply valve will open fully and expel the condensate. The cycle will then start again. This temperature cycling is bad for the heat exchanger and the plant and may be accompa-... 

The discharge pressure developed hy the compressor must he equal to the process gas s total system resistance, of control valves, hand valves, orifices, heat exchangers, and any other process-related devices through which the discharge gas from the compressor must flow. As this resistance changes, the gas flow through the compressor will automatically adjust itself to equal the new resistance. ... 

The mold or die should also be recognized as one of the most important pieces of production equipment in the plant. These controllable, complex devices must be an efficient heat exchanger and provide the product s shape. The mold or die designer thus has to have the experience or training and knowledge of how to produce the tooling needed for the product and to meet required tolerances with the plastic to be processed. 

Fresh feed that has the proper hydrogen carbon monoxide ratio (i.e., slightly above the 3 1 minimum stoichiometric ratio) is mixed with recycle gas and sent through a heat exchanger and into the first catalyst bed, Stage I. The space velocity in Stage I is controlled so that all of the carbon monoxide fed to it is completely converted. The inlet gases are... 

In adiabatic operation, there is no attempt to cool or heat the contents of the reactor (that is, there is no heat exchanger). As a result, T rises in an exothermic reaction and falls in an endothermic reaction. This case may be used as a limiting case for nonisothermal behavior, to determine if T changes sufficiently to require the additional expense of a heat exchanger and T controller. 

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