Thermal radiation shields

The liquid helium evaporates in the heat exchanger and thus cools dovm the cryopanel. The waste gas which is generated (He) is used in a second heat exchanger to cool the baffle of a thermal radiation shield vi/hich protects the system from thermal radiation coming from the outside. The cold helium exhaust gas ejected by the helium pump is supplied to a helium recovery unit. The temperature at the cryopanels can be controlled by controlling the helium flow. 

Fig. 2.68 shows the design of a cryopump. It is cooled by a two-stage cold head. The thermal radiation shield (5) with the baffle (6) is closely linked thermally to the first stage (9) of the cold head. For pressures below 10 2 mbar the thermal load is caused mostly by thermal radiation. For this reason the second stage (7) with the condensation and cryosorption panels (8) is surrounded by the thermal radiation shield (5) which is black on the... 

High vacuum flange Pump casing Forevacuum flange Safety valve for gas dixharge Thermal radiation shield Baffle... 

The API 521 also states that "it is essential that persormel within the restricted area have immediate access to thermal radiation shielding or protective apparel suitable for escape to a safe location." Radiation shields are structures sometimes located near the base of the flare sfack designed wifh a galvanized or reflective steel roof. Some planfs design a roofed corridor that leads from fhe base of fhe sfack to a safe location away from high radiation levels. 

It is essential that personnel within the restricted area have immediate access to thermal radiation shielding or protective apparel suitable for escape to a safe location. ... 

Such transparent heat mirrors have important application, for example, in combination with cold light mirrors as thermal radiation shields in projection and illumination techniques and potential applications also in solar energy collection, window insulation, etc. 

Cryogenic tanks (h) are built up to a volume of 100 m. Their insulation loss amounts to about 1% of the design liquid inventory per day. These low heat losses at storage temperatures of about 4 K are possible due to superinsulation, thermal radiation shields and deep vacuum in the clearances of the double-walled container. The heat radiation shields transport the heat to the pipes in which either nitrogen or helium itself evaporates. A typical layout is shown in Fig. 4.4 [4.2]. 

The main use of lead metaborate is in glazes on pottery, porcelain, and chinaware, as weU as in enamels for cast iron. Other appHcations include as radiation-shielding plastics, as a gelatinous thermal insulator containing asbestos fibers for neutron shielding, and as an additive to improve the properties of semiconducting materials used in thermistors (137). 

The high cross-section for thermal neutrons results in the use of boron and boron compounds for radiation shielding (14). The ease of detecting the a-particle produced when boron absorbs thermal neutrons results in the use of boron for neutron counters as weU. 

There are a number of papers in the open literature explicitly reporting on the properties of boron cluster compounds for potential neutron capture applications.1 Such applications make full use of the 10B isotope and its relatively high thermal neutron capture cross section of 3.840 X 10 28 m2 (barns). Composites of natural rubber incorporating 10B-enriched boron carbide filler have been investigated by Gwaily et al. as thermal neutron radiation shields.29 Their studies show that thermal neutron attenuation properties increased with boron carbide content to a critical concentration, after which there was no further change. 

From eq. (5.2) we see that the total power emitted by 1 cm2 with e = 1 at 300 K is 45 mW corresponding to an evaporation of 70cm3/h of 4He. At 77 K, a surface of 1 cm2 emits about 0.2 mW, with a 4He consumption of 0.3 cm3/h. Hence the part of the dewar cooled at helium temperature is surrounded by radiation shields or baffles at intermediate temperatures. The latter are either gas cooled or thermally anchored to a LN2 reservoir. 

To reduce the radiation power input, several solutions are possible inserting n thermally floating shields between the cold and hot surfaces, the transmitted power is reduced by a factor (n + 1). The practical realization is the so-called superinsulation used in the dewar of Fig. 5.1 a few layers of a thin metallized insulating foil about 4 xm thick is used. To prevent thermal contact between adjacent layers, the material is often corrugated or a thin layer of fibreglass cloth is inserted between layers. 

Both types of insulation act to suppress thermal radiation by the intermediate shield principle. The insulation also acts to reduce the effective cell size for any residual gas in the vacuum space, thereby suppressing the thermal conductivity of the gas. In a typical commercial superinsulated dewar, there are about 50 layers of superinsulation, corresponding to a thickness of about one inch. The first few layers are the most effective in the attenuation of thermal radiation however the subsequent layers are important for the suppression of thermal conductivity of any residual gas. One can define an effective thermal conductivity for these insulations, which in the case of superinsulation is about 10 6 W/(cmK) between 300 and 4K. 

Enclosed ground flares are most commonly used as a supplement to an elevated) flare on the same relief system. The primary reason for an enclosed ground flare is to reduce the visual impact of flared gas combustion on a nearby community. They are often used when it is desirable that all or part of a flare load be disposed of in a way that causes the minimum of disturbance to the immediate locality. They offer many advantages in comparison to elevated flares there is no smoke, no visible flame, no odor, no objectionable noise, and no thermal radiation (heat shield) problems. Enclosed ground flares are typically used for normal process flow (continuous) flaring, but with recent technical advances they are now also used for emergency flaring (AIChE-CCPS, 1998). 

In many applications, it is often necessary to attach pumps to systems via elbows or valves, or to shield devices to protect them from thermal radiation or X-rays. Some examples are given below. 

The circular atom microwave spectroscopy experimental set-up is sketched on Fig. 1-a. A thermal beam of Li atoms crosses three sections of the apparatus the excitation, the microwave interaction region and the detection zone. The whole set-up is protected from room temperature thermal radiation by a liquid nitrogen cooled shield (which can be replaced in a later stage of the experiment by a liquid helium cooled one). 

Silicon carbide, widely employed as an abrasive (carborundum), is finding increasing use as a refractory. It has a better thermal conductivity at high temperatures than any other ceramic and is very resistant to abrasion and corrosion especially when bonded with silicon nitride. Hot-pressed, self-bonded SiC may be suitable as a container for the fuel elements in high-temperature gas-cooled reactors and also for the structural parts of the reactors. Boron carbide, which is even harder than silicon carbide, is now readily available commercially because of its value as a radiation shield, and is being increasingly used as an abrasive. 

In addition, SCTA offers a unique possibility for preparative scale thermal analysis whilst maintaining high levels of sensitivity and resolution. Such an SCTA system has recently been developed.The focus of the system is a microbalance and a high-temperature water-cooled furnace, located above the balance and allowing operation to 1000°C. The sample is contained in a silica or platinum crucible and sits on top of a rise rod connected directly to the balance pan. Samples in excess of 1 g can be investigated, but routinely, samples of 500 mg are usually involved. Grinding of powdered samples to mesh size 150-250 pm is advantageous. The sample temperature is monitored by a thermocouple located centrally within the furnace tube. Radiation shields attached to the thermocouple... 

The space between the two tanks is filled with layers of thin aluminized plastic film separated by a lightweight coarse plastic screen. These serve as a shield against the passage of thermal radiation from the outer to the inner tank. The air between the tanks and around the insulation is removed with a vacuum pump. The high vacuum serves to stop heat flow by conduction. The liquid fill and gas withdrawal lines are coaxial that is, one inside the other. They are made from materials with low thermal conductivity and are coiled inside the insulation to minimize heat flow down the length of the pipe from the outside into the inner tank. 

In addition to this major use to improve mechanical properties, high aspect ratio flake-type fillers have been added to polymers for a variety of other purposes. They include improved thermal stability (3), high voltage resistance (4), electrical conductivity, radiation shielding (5) and optical and aesthetic effects (6). 

Radiation shields [90] can be used to isolate a probe from a distant medium so that there will be relatively little radiation heat transfer to it at the same time, they do not interfere with good thermal contact between the probe and the surrounding fluid. Designs of thermometer probes for gas temperature measurement are described in Refs. 91 and 92. Analyses to account for some uncertainties in probe measurement can be found in Ref. 93. 

Several DTA instruments have been described by Barrall et al. (94, 95). A DTA calorimeter cell in which the AT-sensing thermocouples are attached to the sample container is shown in Figure 6.35. In this apparatus, copper-constantan thermocouples are soldered to a 4-mm-OD copper cup fitted with a copper lid. The thermocouples and sample cups are supported on ceramic insulator tubes which are attached to a metal base. All the cups were heated by thermal radiation received from the blackened copper radiation shield this prevented radiation hot spots due to furnace windings. The entire DTA cell was enclosed by a glass bell jar which provided a controlled atmosphere from reduced pressures to about 2 atm. 

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