Sub-ambient Operation

A variety of cooling systems are available for most instruments and these are now mostly automatic in operation. Issues of filling liquid nitrogen systems and of availability mean that refrigerated coolers (intracoolers) are often preferred and some systems can operate at below — 100°C. Intracoolers are preferred for use with autosamplers since there is no risk of the coolant running out. 

When measurements are made at temperatures below 40 °C, the sample holder assembly is cooled using a cooling apparatus. The most 

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The temperature operating range must be large enough to serve individual applications. To make use of the whole range of possible applications, it is necessary to have temperatures of -50°(or -75°) to 400°C. Since sub-ambient operation is not required for most analysis, the sub-ambient capability should be optional. However, operation from 5 to 10° above ambient to 400°C should be adequate for the majority of applications. 

Dynamic Mechanical Analysis and Stress Relaxation Behavior. Samples were compression molded into bars of the dimensions 38.xl2.5x0.78 0.007 mm and 65.x9.7xl.7 0.007 mm in a Carver laboratory hot press model C. A TA Instruments 983 DMA, which was operated in the fixed frequency mode, was used to characterize the storage and loss moduli as a function of temperature. Samples were scanned at fi-equencies from 0.05 to 10.0 Hz over a temperature range from -150 C to above the glass transition temperature. The displacement was 0.4 - 0.6 mm. Stress relaxation curves were determined for the same size samples at a constant strain. The sample was displaced for 10.0 minutes and then allowed to recover for 10.0 minutes. The stress data were taken in five degree increments. A microprocessor controlled Liquid Nitrogen Cooling Accessory (LNCA) was used for sub-ambient operations. 

The sample is positioned in a temperature-control chamber which contains a radiant heater and a coolant distribution system. The radiant heater provides precise and accurate control of sample temperature. The coolant distribution system uses cold nitrogen gas for smooth, controlled sub-ambient operation and for quench cooling at the start or end of a run. Both the radiant heater and the coolant accessory are controlled automatically by the 983 system to ensure reproducible temperature programming. An adjustable thermocouple, mounted close to the sample, provides precise feedback information to the temperature controller, as well as a readout of sample temperature. 

The DMA was operated in the fixed frequency mode at 1 Hz and a peak-to-peak amplitude of 0.06 mm. A DuPont Liquid Nitrogen Cooling Apparatus (LNCA-II) was used to achieve sub-ambient temperatures. The temperature profile used was a ramp to — 20°C, soak for 5 min, and then ramp at 5°C/min to 220°C. Three specimens were tested for each blend. 

This technique has been described fully in Chapter 2. It was developed for operation at temperatures close to ambient but in recent years determined attempts have been made to extend this range. It is now routinely operated from sub-ambient temperatures up to —lOOOK and a number of studies at even higher temperatures have been reported [8]. It has the great advantage of being able to cover a very wide pressure range from a few Torr up to many atmospheres but most studies have been carried out in the range 10-760 Torr. 

Use procedures which are as gentle as possible since disruption of cells or tissues leads to the release of proteolytic enzymes and general acidification. Selection of an extraction technique is dependent as much upon the equipment available and scale of operation as on the type of sample. Examples of common extraction processes are shown in Table 8. Extraction should be performed quickly, at sub-ambient temperatures, in the presence of a suitable buffer to maintain pH and ionic strength and stabilise the sample. Samples should be clear and free from particles before beginning a chromatographic separation. 

Safety is clearly a major consideration and research reactors are designed to fail-safe to prevent fission product release. Reactors operate under a triple containment philosophy. The first container is the cladding of the fuel itself, the second is the swimming pool which is made from heavy, 1.5 m thick, concrete lined with stainless steel. Finally the whole reactor is housed inside a reinforced building that is kept at a slightly sub-ambient pressure and is accessed by an air-lock. 

A TG-DSC instrument capable of sub-ambient temperature operation has been available for a number of years, and is especially valuable in the study of systems containing moisture, or other volatiles. In this case, a DSC heat flux plate was adapted to allow it to be suspended from the thermobalance beam, instead of the pair of thermocouples shown in Figure 1. A completely different approach to TG-DSC is taken by SETARAM, in their TG-DSC 111 instrument. The sample and reference... 

A number of inert purge gases are used when operating at sub-ambient temperatures. The choice of purge gas is largely determined by the proposed experimental parameters. Table 4.1 lists the thermal conductivities of the most commonly used sub-ambient purge gases (Ar, He and N2 ) as a function of... 

For this example, we calculate the Knudsen flux using eq. (7.4-13). The Knudsen diffusivity is = 0.837 cmVsec and the Knudsen flux is 4.5 X 10 mole/cmVsec. We see that the viscous flux in this case is very comparable to the Knudsen flux, and they must be accounted for in the calculation of the total flux. The reason for this significant contribution of the viscous flow is that the pressures used in this example are very high. For low pressure systems, especially those operated under sub-ambient pressure, the Knudsen mechanism is always dominating. 

Compared with a heat-flux DSC, higher scanning rates can be used with a power compensation DSC, with a maximum reliable scanning rate of 60 K min. Maintaining the linearity of the instrument baseline can pose problems at high operating temperatures or in the sub-ambient mode. 

With respect to apparatus, the design and operation of a sub-ambient thermal volatilization analysis (TVA) system has been described. A technique, which is essentially TVA, has also been used for the qualitative and quantitative analysis of trace amounts of volatiles produced during polymer degradation. A pyrolytic mass spectrometric method has also been developed which gives information on the yield of volatile degradation products, on their nature and on the kinetics of their formation. 

A typical TPR/TPO apparatus can be assembled following some general principles. The apparatus essentially consists of three parts (i) the gas line for pre-treatments and analysis (ii) the reactor electrically controlled (iii) the detector for quantitative evaluation of gas-consumption. At present, all the experimental apparatus used operate at ambient (in flowing gas) or sub-ambient (under vacuum) pressure. 

Work-over fluids are used routinely to kill wells for remedial operations, wash-out fill, or provide a safe environment for special logging or other well diagnostic procedures. In such operations it is often necessary to store the work-over fluid in tanks at the well site. TKPP solutions are stable even at sub-freezing temperatures, which provides a distinct advantage over solutions of halide salts that sometimes crystallize at ambient conditions encountered in rig operation. The avoidance of a crystallization problem coupled with the noncorrosive nature of TKPP work-over fluids makes them attractive with respect to other clear work-over fluids now popular in the industry. 

Conventional HRTEM operates at ambient temperature in high vacuum and directly images the local structure of a catalyst at the atomic level, in real space. In HRTEM, as-prepared catalyst powders can be used without additional sample preparation. The method does not normally require special treatment of thin catalyst samples. In HRTEM, very thin samples can be treated as WPOs, whereby the image intensity can be correlated with the projected electrostatic potential of the crystal, leading to the atomic structural information characterizing the sample. Furthermore, the detection of electron-stimulated XRE in the EM permits simultaneous determination of the chemical composition of the catalyst. Both the surface and sub-surface regions of catalysts can be investigated. 

The term heavy oil has also been arbitrarily used to describe both the heavy oils that require thermal stimulation of recovery from the reservoir and the bitumen in bituminous sand (tar sand, q.v.) formations from which the heavy bituminous material is recovered by a mining operation. However, the term extra heavy oil is used to define the sub-category of petroleum that occurs in the near-solid state and is incapable of free flow under ambient conditions. Bitumen from tar sand deposits is often termed as extra heavy oil. 

For transformer feeders there are two choices that are normally available. Most power transformers can be fitted with external cooling fans, provided the attachments for these fans are included in the original purchase order. It is common practice to order transformers initially without fans and operate them as ONAN until the demand increases to justify the fan cooling. Thereafter the transformer is operated as ONAF, see sub-section 6.5. Adding fans can increase the capacity of the transformer by 25% to 35%, depending upon the particular design and ambient conditions. The alternative choice is simply to rate the ONAN transformer for the 125% duty, and initially operate it at a lower level. The decision is often a matter of economics and an uncertainty about the future growth. 

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