Vapor shielding

Figure 1. Insulation design for pressure vessel. The figure shows a vacuum space for obtaining good performance from the multilayer insulation, instramentation for pressure, temperature and level, and a vapor shield for reducing hydrogen evaporative losses.

Liquid and vapor shields Very low-temperature, valuable, or dangerous liquids such as helium or fluorine are often shielded by an intermediate cryogenic liquid or vapor container that must in turn be insulated by one of the methods described below. 

Vapor Shields. Unlike liquid shields that utilize an inexpensive cooling fluid like nitrogen, vapor shields exploit the cold vent gas liberated directly from the product liquid. This escaping vent gas is routed past an intermediate shield and sensibly absorbs some of the heat that would otherwise warm the contained liquid as shown in Fig. 7.15. This technique is mainly used for storing lower boiling liquids such as helium and hydrogen rather than nitrogen. 

The thermal value of a vapor shield can best be illustrated by means of a simple energy balance. The heat transfer rate to or from the vapor shield is a combination of the heat received from the ambient, 02 the heat given directly to the inner container, Qi, and the heat transferred to the vapor shielding gas, or... 

It is necessary to describe these three rates of heat transfer in terms of measurable quantities. The heat transferred to the vapor shield from the ambient theoretically can be written as... 

Now it is possible to quantify the value of a vapor shield. By assuming the mass flow rate in Eqs. (7.27) and (7.28) to be zero and making the same substitutions as before, an expression for 9 can be derived which represents a system with no vapor shield, namely,... 

The ratio of heat inleak with vapor shielding to that without a vapor shield can now be expressed as the ratio of the 6 parameters, or... 

A plot of this heat-inleak ratio for various values of the tc, is presented in Fig. 7.16. As anticipated, the heat inleak is large when the fluid is helium and small when it is nitrogen. Thus, a vapor shield provides the largest benefit with liquid helium systems Fig. 7.16 shows that as much as 80% of the possible heat inleak to liquid helium can be intercepted by the vapor shield. Conversely, for a liquid-nitrogen-contained vapor-shielded system, less than... 

A liquid helium storage dewar utilizes a vapor cooled shield to intercept some of the heat leak into the dewar. The overall heat transfer coefficient between the warm 300 K outer container and the vapor shield is 6 mW/m K and the overall heat transfer coefficient between the vapor shield and the cold 4.2 K surface of the inner container is 8 mW/m K. The mean specific heat of the cold helium gas flowing in contact with the vapor shield is 5.39 kJ/kg K. If the heat of vaporization of the helium in the container is 19.69 kJ/kg, determine (a) the temperature of the shield, (b) the heat transfer rate to the vapor shield, and (c) the heat transfer rate to the inner container. 

Section 7.17 of the text develops a mathematical description of the heat transfer into a vapor-shielded cryogenic container. This results in an expression, Eq. (7.31), that relates the temperature difference between the vapor shield and the inner container to the temperature difference between the ambient and the inner container in terms of two dimensionless ratios, namely, the ratio of the sensible to the latent heat of the gas and the ratio of the inside overall heat transfer coefficient to that of the outside overall heat transfer coefficient. If the mass flow rate through the shield is reduced to zero, the expression reduces to the form shown in Eq. (7.32). Provide the missing steps to show that this is true. 

Gas shielding from vaporized flux core (and shielding gas)... 

Because of Hquid helium s uniquely low temperature and small heat of vaporization, containers for its storage and transportation must be exceedingly weU insulated. Some containers are insulated with only a fairly thick layer of very efficient insulation, but containers with the least heat leak use an inexpensive sacrificial cryogenic Hquid, usually Hquid nitrogen, to shield thermally the Hquid helium contents. 

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. 

Alkylamiaes are toxic. Both the Hquids and vapors can cause severe irritations to mucous membranes, eyes, and skin. Protective butyl mbber gloves, aprons, chemical face shields, and self-contained breathing apparatus should be used by aH personnel handling alkylamiaes. Amines are flammable and the lower mol wt alkylamiaes with high vapor pressures at ordiaary temperatures have low flash poiats. Amines should be handled ia weH-veatilated areas only after eliminating potential sources of ignition. 

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