Constant pressure vent system

Has a constant pressure vent system been provided Has it been properly installed Inerts should be injected downstream of the condenser, not upstream ... 

A constant-pressure vent system is normally provided to prevent this pressure variation. Inert gases are provided or removed as required to ensure that the system pressure drop is reflected only by the friction drop and not by changes in vapor pressure. Figure 31-1 indicates such a system. The inert gases should be introduced downstream of the condenser. Introducing inerts upstream of the condenser will reduce the rate of heat transfer requiring more heat transfer surface. This action, however, is sometimes used to control a condenser. 

Figure 31 -1 Constant pressure vent system for condensers.

Most small Hquid helium containers are unpressurized heat leak slowly bods away the Hquid, and the vapor is vented to the atmosphere. To prevent plugging of the vent lines with solidified air, check valves of some sort are included in the vent system. Containers used for air transportation are equipped with automatic venting valves that maintain a constant absolute pressure with the helium container in order to prevent Hquid flash losses at the lower pressures of flight altitudes and to prevent the inhalation of air as the pressure increases during the aircraft s descent. Improved super insulation has removed the need for Hquid nitrogen shielding from almost all small containers. 

Some results of the constant-value pricing system are as foUow generation in a central unit at relatively low pressure, <4.24 MPa (600 psig) tremendous economic pressure to use turbines rather than motors for drives lack of incentive for high efficiency turbines excessively high temperature differentials in steam users tremendous incentive to recover waste heat as low pressure steam and a large plume of excess low pressure steam vented to the atmosphere. 

The gauge has proved equally accurate for constant pressure and blowdown systems, and can also be adapted to vented systems. A typical curve for normalized krypton concn vs the amt of proplnt remaining in the tank is shown in Fig 1. Also shown is the analytical relationship between tracer concn and proplnt remaining in the tank. Statistical error analyses showed the typical average gauging error thruout the entire range of proplnt expulsion to be less than . 3% with a one sigma deviation of less than . 4%. This illustrates the consistency and reproducibility of this measurement technique... 

Consider a container of boiling liquid hydrogen maintained at constant pressure by a suitable vapor-venting system. ITtien, it m denotes the amount of liquid present at time, t, we may write the following differential equations to describe system behavior in time.t... 

Assume that the liquid volume. in the reactor remains constant at 4,000 dm and that the vapor space. Yh, above the reactor occupies 3,000 dm. Any gas. Hi (i.e., D), that is formed by reactions (1) and (2) immediately appears as an input stream to the head space volume. The dissolved H2 and the vapor pressures for the liquid components in the reactor can be neglected. The initial absolute pressure within the reactor is 4.4 atm (50 psig). During norma) operation. Hi generated obeys the ideal gas law. The pressure control system on the Hi vent stream maintains the pressure. P, at 4.40 atm up to a flow of 11,400 mol/hr. The reactor vessel will fail when the pressure exceeds 45 atm or the temperature exceeds 600 K. 

For partial condenser systems, the pressure can be controlled by manipulating vapor product or a noncondensible vent stream. This gives excellent pressure control. To have a constant top vapor product composition, the condenser outlet temperature also needs to be controlled. For a total condenser system, a butterfly valve in the column overhead vapor line to the condenser has been used. Varying the condenser cooling by various means such as manipulation of coolant flow is also common. 

The average dT/dt is typically an arithmetic average between the value at set pressure and the value at peak allowed pressure. The properties Cp, hfg, i, either can be evaluated at the set conditions or can be taken as the average values between the set condition and the peak allowed pressure condition. Alternatively, the term h/g/t)/g in Eq. (23-95) can be replaced by T(dP/dT)tat via the Clapeyron relation. This holds reasonably well for a multicomponent system of near constant volatility. Such an application permits direct use of the experimental pressure-temperature data obtained from a closed-system runaway VSP2 test. This form of Eq. (23-95) has been used to demonstrate the advantageous reduction in both vent rate and vent area with allowable overpressure (Leung, 1986a). 

In addition, the practice of adding acid to water with constant stirring should be observed. When acid mixtures are prepared, only the quantity to be used should be prepared, as these may not be safe to store. Finally, pressure relief valves should be provided to any sealed container in which a digestion is to take place. One should be aware, however, that some analyte can be lost as droplets when these valves vent. This is one of the advantages of the pressure-monitored microwave digestion system. In this, the pressure is controlled by modulating the input power, so venting is avoided. 

The major method of vent sizing for gassy system is two-phase venting to keep the pressure constant. This method was employed before DIERS with an appropriate safety factor [34], The vent area is expressed by ... 

The basic principle of a diffusion pump can be explained with a simple single-stage mercury diffusion pump (see Fig. 7.21). On the system side of the pump (at about 10 2 to 10 3 torr, or better), gas molecules wander around, limited by their mean free path and collisions with other molecules. The lowest section of this diffusion pump is an electric heater that brings the diffusion pump liquid up to its vapor pressure temperature. The vapors of the diffusion pump liquid are vented up a central chimney where, at the top, they are expelled out of vapor jets at supersonic speeds (up to 1000 ft/sec). Below these jets is a constant rain of the pumping fluid (mercury or low vapor-pressure oil) on the gases within the vacuum system. Using momentum transfer/ gas molecules are physically knocked to the bottom of the pump, where they are trapped by the vapor jets from above. Finally, they are collected in a sufficient quantity to be drawn out by the auxiliary (mechanical) pump. 

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