Pressure ratios

The vapor pressure ratio measures the intrinsic tendency of component 1 to enter the vapor phase relative to component 2. Likewise, ri measures the tendency of Mi to add to Mi - relative to M2 adding to Mi-. In this sense there is a certain parallel, but it is based on Mi - as a reference radical and hence appears to be less general than the vapor pressure ratio. Note, however, that ri = l/r2 means kn/ki2 = k2i/k22- In this case the ratio of rate constants for monomer 1 relative to monomer 2 is the same regardless of the reference radical examined. This shows the parallelism to be exact. 

The pressure developed by decomposition of acetylene in a closed container depends not only on the initial pressure (or more precisely, density), but also on whether the flame propagates as a deflagration or a detonation, and on the length of the container. For acetylene at room temperature and pressure, the calculated explosion pressure ratio, / initial > deflagration and ca 20 for detonation (at the Chapman-Jouguet plane). At 800 kPa (7.93... 

The measured explosion pressure ratio for deflagration in a container only a few diameters in length approaches the theoretical value often it is about 10. However, in a pipe hundreds or thousands of diameters in length, deflagration may cause very tittle pressure rise because only a small fraction of the contents is hot at any time. 

The relationship between the gas composition, austenite carbon content, and temperature is known. For example, a gas in equiUbrium with austenite containing 0.8% C must have the partial pressure ratio / Pco ) comprised of only CO and CO2, then the sum of the partial... 

Fig. 13. Combustion turbine engine combined cycle (a) schematic of plant and (b) thermodynamics, where the vertical lines correspond to the pressure ratio, given and the horizontal lines to the combustor temperature, Tp in °C as indicated.

A 165-MW-class gas turbine/generator has been introduced by another manufacturer. This machine, also developed by scaling up a proven design, features a simple-cycle efficiency of 37.5% a turbine inlet temperature of 1235°C a pressure ratio of 30 1, up from 16 1 on the previous generation and an output of 165 MW for gas fuel firing under International Standards Organization (ISO) conditions (101 kPa, 15°C (14.7 psia, 59°F)). A combined-cycle facihty based around this machine could achieve efficiencies up to 58% or a heat rate of about 6209 kj/kWh (5885 Btu/kWh). 

The 212-MW unit features a turbine inlet temperature of 1260°C and a pressure ratio of 13.5 1. The manufacturer has subsequently installed a number of larger, more powerful versions of this unit, which produce up to 226.5 MW. Turbine inlet temperature is 1288°C the pressure ratio is 15 1. Five of these high output machines anchor a 1675-MW facihty in the Netherlands. These machines were developed by geometric scaling from a 168-MW,... 

The measuring of temperature rise permits monitoring efficiency for a fixed pressure ratio and suction temperature. Efficiencies should always exceed 0.6, and 1.00 is approachable in reciprocating devices. Their better efficiency needs to be balanced against their greater cost, greater maintenance, and lower capacity. 

By contrast, the single-screw and double-hehcal screw compressors (Fig. 6) can operate at high or low pressure ratios. They can be appHed at pressure ratios below 2 1 and above 20 1 in a single stage. These compressors are suitable for use with higher pressure refrigerants such as CFC-12, HCFC-22, or ammonia. 

Fig. 1. Vapor-pressure ratios of the alkanes, alkenes, and dienes with respect to / -butane A, isobutane B, isobutylene C, 1-butene D, 1,3-butadiene E,...

This quantity is minimised when the stage is operated at a pressure ratio across the barrier corresponding to r = 0.285. Furthermore, if power were the only economic consideration, the stage would be operated at this pressure ratio. However, as the value of ris decreased from this optimum, although the cost of power is increased, the number of stages required and hence the capital cost of the plant is decreased. Thus, ia practice a compromise between these factors is made. 

While the deflagration pressure ratio for ethylene oxide vapor is about 11 or less, Hquid mist decomposition can give much greater pressures and very fast rates of pressure rise (190). 

Note that under choked conditions, the exit velocity is V = V = c = V/cKTVM not V/cKT(/M, . Sonic velocity must be evaluated at the exit temperature. For air, with k = 1.4, the critical pressure ratio p /vo is 0.5285 and the critical temperature ratio T /Tq = 0.8333. Thus, for air discharging from 300 K, the temperature drops by 50 K (90 R). This large temperature decrease results from the conversion of internal energy into kinetic energy and is reversible. As the discharged jet decelerates in the external stagant gas, it recovers its initial enthalpy. 

Critical Flow Nozzle For a given set of upstream conditions, the rate of discharge of a gas from a nozzle will increase for a decrease in the absolute pressure ratio po/pi until the linear velocity in the throat reaches that of sound in the gas at that location. The value of po/pi for which the acoustic velocity is just attained is called the critical pressure ratio r. The actual pressure in the throat will not fall below even if a much lower pressure exists downstream. 

The critical pressure ratio r can be obtained from the following theoretical equation, which assumes a perfect gas and a frictionless nozzle ... 

Polytropic efficiency is the hmited value of the isentropic efficiency as the pressure ratio approaches 1.0, and the value of the polytropic efficiency is higher than the corresponding adiabatic efficiency as seen in Fig. 10-66. 

---