Adiabatic compression, 3.15

The autoignition temperature (AIT) of a vapor, sometimes called the spontaneous ignition temperature (SIT), is the temperature at which the vapor ignites spontaneously from the energy of the environment. The autoignition temperature is a function of the concentration of vapor, volume of vapor, pressure of the system, presence of catalytic material, and flow conditions. It is essential to experimentally determine AITs at conditions as close as possible to process conditions. 

Composition affects the AIT rich or lean mixtures have higher AITs. Larger system volumes decrease AITs an increase in pressure decreases AITs and increases in oxygen concentration decrease AITs. This strong dependence on conditions illustrates the importance of exercising caution when using AIT data. 

Auto-oxidation is the process of slow oxidation with accompanying evolution of heat, sometimes leading to autoignition if the energy is not removed from the system. Liquids with relatively low volatility are particularly susceptible to this problem. Liquids with high volatility are less susceptible to autoignition because they self-cool as a result of evaporation. 

Many fires are initiated as a result of auto-oxidation, referred to as spontaneous combustion. Some examples of auto-oxidation with a potential for spontaneous combustion include oils on a rag in a warm storage area, insulation on a steam pipe saturated with certain polymers, and filter aid saturated with certain polymers (cases have been recorded where 10-year-old filter aid residues were ignited when the land-filled material was bulldozed, allowing auto-oxidation and eventual autoignition). 

These examples illustrate why special precautions must be taken to prevent fires that can result from auto-oxidation and autoignition. 

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The adjectives isobaric and isothennaT and the corresponding subscripts are frequently omitted, but it is important to distinguish between the isothemial compressibility and the adiabatic compressibility. ... 

A similar derivation leads to the difference between the isothemial and adiabatic compressibilities ... 

The velocity of sound in liquid thiazole was also measured, and the adiabatic compressibility was determined (302) it was concluded that intermolecular interactions result from the electrical forces, originating in the heteratoms, between the molecules. 

In designing faciUties for handling and processing nitromethane, it is recommended that nitromethane not be processed in high pressure equipment. AH vessels for nitromethane service should be protected to prevent adiabatic compression. Detonation traps should be installed at each end of transfer lines and in every 61 m (200 feet) of continuous line. Nitromethane lines should be located underground or in channels wherever possible. Pressure rehef devices (rated - 690 kPa = 100 psig) should be installed between closed valves (81). 

FIG, 6"22 Adiabatic compressible flow in a pipe with a well-rounded entrance. 

For mnltistage compressors of JX, number of stages with adiabatic-compression in each stage, equal division of work between stages, and intercoohng to the irit e temperature, the following formulas ai e helpful ... 

Adiabatic compression test. High pressure is applied r idly to a liquid in a U-shaped metal tube. Bubbles of hot compressea gas are... 

Compression Adiabatic compression results in high temperatures determined by the compression and specific heat ratios, as shown in Eq. (26-46) ... 

Various types of rapid, adiabatic compressions have caused explosions. With propane at an initial temperature of 25°C, To = 432°K (I59°C) for compression and specific heat ratios of 25 and I.I3, respectively. Assume that now air enters a compressor to bring propane into the flammable range at 5 percent by volume. The mixture then will be mostly air with k = 1.47. The same compression ratio of 25 will elevate the final temperature T2 to 834°K (56I°C), i.e., above the published autoignition temperature of 450°C for propane and perhaps high enough to cause an explosion. 

The thermal efficiency of the process (QE) should be compared with a thermodynamically ideal Carnot cycle, which can be done by comparing the respective indicator diagrams. These show the variation of temperamre, volume and pressure in the combustion chamber during the operating cycle. In the Carnot cycle one mole of gas is subjected to alternate isothermal and adiabatic compression or expansion at two temperatures. By die first law of thermodynamics the isothermal work done on (compression) or by the gas (expansion) is accompanied by the absorption or evolution of heat (Figure 2.2). 

From the heat conduction equation in the presence of adiabatic compression [51]... 

For theoretical cycle work performed in an adiabatic compression cycle (nonideal fluid) ... 

The thermodynamic state of a reactive mixture just prior to combustion is determined by adiabatic compression and by turbulent mixing with combustion... 

Figure 12-12A. Illustration of isentropic path on log pressure-enthalpy diagram, showing Mollier chart method of finding final temperature and calculation of H for reversible and adiabatic compression. (Used by permission Edmister, W. C. Applied Hydrocarbon Thermodynamics, 1961. Gulf Publishing Company, Houston, Texas. All rights reserved.)...

In adiabatic compression or expansion, no release or gain of heat by the gas occurs, and no change occurs in entropy. This condition is also known as isentropic and is typical of most compression steps. Actual conditions often cause a realistic deviation, but usually these are not sufficiently great to make the calculations in error. Table 12-4 gives representative average k values for a few common gases and vapors. 

Saving due to Jacketing of Cylinder Discharge Pressure Adiabatic Compression... 

If Perfect Intercooling between Stages,c Intercooler Pressure Suction Temperature after Interceding Saving due to Jacketing bp Cylinder Adiabatic Compression... 

Compression efficiency is the ratio of the work required to adiabatically compress a gas to the work actually done within the compressor cylinder as shown by indicator cards. Figures 12-12 and 12-16. The heat generated during compression adds to the work that must be done in the cylinder. Valves may vary from 50-95% efficient depending on cylinder design and the ratio of compression. Compression efficiency (or sometimes termed volumetric efficiency) is affected by several details of the systems ... 

Horsepower is the work done in a cylinder on the gas by the piston connected to the driver during the complete compression cycle. The theoretical horsepower is that required to isen-tropically (adiabatically) compress a gas through a specified pressure range. The indicated horsepower is the actual work of compression developed in the compressor cylinder(s) as determined from an indicator card. Brake horsepower (bhp) is the actual horsepower input at the crankshaft of the compressor drive. It does not include the losses in the driver itself, but is rather the actual net horsepower that the driver must deliver to the compressor crankshaft. 

Figures 12-37F and 12-37G use adiabatic compression and expansion (zero heat transfer). All heat added to the cycle comes from heating the engine exhaust by Heat rejected from the cycle, Qni> leaves through the aftercooler.

Adiabatic expansion of the air in the engine causes a maximum temperature drop of the exhaust. Adiabatic compression causes a maximum temperature rise of the compressed air. These effects combine to cause the greatest work loss of any compressed-air system, when pressurized air must be cooled back to atmospheric temperature. The energy analysis parallels the one just made for the polytropic system. This shows that net areas on both PV and TS graphs measure the work lost. 

Adiabatic compression (termed adiabatic isentropic or constant entropy) of a gas in a centrifugal machine has the same characteristics as in any other compressor. That is, no heat is transferred to or from the gas during the compression operation. The characteristic equation... 

Relationship between adiabatic compression and polytropic compression ... 

The working substance being initially at the temperature T2 of the refrigerator, we place the cylinder on the non-conducting stand, and compress the working substance reversibly until the temperature rises to Ti. By the conditions imposed, this is an adiabatic compression, and will be represented by a continuous curve on the indicator diagram, say AB (Fig. 8). 

Finally, consider the case of adiabatic compression. as independent variables, we have... 

Ideal Gases.—The state of unit mass of an ideal gas, undergoing adiabatic compression or expansion, is completely defined by the equations... 

Much work has been done on the adiabatic compression ignition of NPN vapor mixed with various gases by both NOL (Ref 2) and Brit Imp Chem Inds (Ref 4). A table of compression ratios and resultant temps from the detons of NPN vapor saturated gas mixts is presented below... 

An adiabatic compression of the fluid returns the volume to V. Since work is added to the system and heat is not allowed to escape, the temperature increases to the initial temperature 92. 

For an irreversible process it may not be possible to express the relation between pressure and volume as a continuous mathematical function though, by choosing a suitable value for the constant k, an equation of the form Pv = constant may be used over a limited range of conditions. Equation 2.73 may then be used for the evaluation of / 2 v dP. It may be noted that, for an irreversible process, k will have different values for compression and expansion under otherwise similar conditions. Thus, for the irreversible adiabatic compression of a gas, k will be greater than y, and for the corresponding expansion k will be less than y. This means that more energy has to be put into an irreversible compression than will be received back when the gas expands to its original condition. 

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