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680 CHAPTER 65 1962 Guide And Data Book when sufficient pressure is applied. There is a relatively large change in enthalpy and the heat released is large, hing equal to the sum of the work done and the decrease in enthalpy of tiie refrigerant for steady-flow operation. In other refrigerants, such as air, there is a slight reduction of enthalpy on compres sion at room temperature and the heat released slightly ex ceeds the work of compression, but liquefaction does not oc cur because the critical temperature of air is far below room temperature. By including in the apparatus a counter-flow, heat exchanger (see Fig. 4) air may, nevertheless, be liquefied. The heat which flows from the air in the warm zone is given by the equation: Qi = h, - hi + Wt (l) where Q heat flow. hi and A* -- enthalpy at positions 1 W work done. 2. The refrigerative effect of the air stream at position 3 in the diagram is given by the relation Q = Ai -- A* there being no work done at the expansion valve. If an external work such as an engine, is substituted for the valve, the refrigera tive effect is increased to the extent of the work done, that is: Qt - Ai -- Ai -f Wt. If the cold zone is well insulated so that no heat enters, the refrigerative effect is applied to the gas itself and the tempera ture,continues to fall.until the liquid state is reached. Returning to the properties of helium it is apparent that at any .temperature above 30 K the enthalpy is actually increased by compression. The simple apparatus (Fig. 4) could not, therefore, liquefy helium which is initially at room temperature. The temperature would rise at the *rpAnWin valve. On the other hand, the replacement of the valve by an engine makes it possible to liquefy helium by the cycle. The liquid is formed within the engine itself. In actual practice the efficiency of this cycle is always low. Effective liquefaction procedures are illustrated in Fig. 5. In diagram A of fig. 5 a simple expansion valve is used at the cold end of heat Exchanger A. The compressed heluim is cooled to about 15 K by contact with liquid hydrogen boiling at reduced pressure. By reference to Fig. 3 the optimum pressure, the pressure of lowest enthalpy at 15 K, is found to be about 30 atm. Fig. 4 .... Simple Refrigeration Cycle with Heat Exchanger Cryogenics 68.1 la jjagraP1 B of Fig. 5 an expansion engine has replaced the hydrogen coolant rather than the valve. It has been found I'pedteut to use a final heat exchanger and an expansion valve jjawhsn an engine is used for extra cooling. When an engine the pressure level is very flexible. If the pressure is u* the temperature at the warm end of Exchanger A will be tow perhaps as low as 6 K. At higher operating pressures the temperature is higher. Io order to obtain higher efficiencies multiple expansion are sometimes used. The net requirement of a liquefier is to cool gaseous helium from room temperature to Fig. 5 .... Flow Diagram of Helium Liquefier its condensing point and then condense it. The heat to be removed from the gas to change its temperature is large com pared to tiie latent heat of condensation--the ratio being about 75:1. Additional expansion machines along the tem perature scale serve to remove'heat at higher temperature levels so that not all of the heat must be pumped from the lowest level. In order to extract one calorie of heat from boiling helium Anri discard it at room temperature, almost 75 calories of work would have to be expended on a perfect nwKtno and gtiii more on an actual machine of superior de sign. At the liquid nitrogen level only about three calories of work are required per calorie extracted: Helium is a]q~> liquefied by the Simon- free expansion method. A strong fhamhrr is filled with gaseous helium at about 100 atmospheres and cooled to the lowest possible temperature by contact with pumped solid hydrogen. A tem perature of 11 K is possible. When cold and thermally iso lated, a valve is opened and a part of the charge of helium Sows out. The temperature and partial liquefaction of the helium occurs. At the end'of the expansion the bomb may be two^thirds full of helium. The work expended in liquefying helium is generally in the range of 3 to 10 kilowatt hours per liter of liquid. LIQUEFACTION OF HYDROGEN Historically, hydrogen has been liquefied in gmall quan tities for the purposes of determining its -properties at low temperatures and for measurement of physical properties of other materials. In recent years research with liquid hydrogen has promoted increased efforts to obtain engineering data on cryogenic construction materials, equipment, and processes. Hydrogen liquefiers currently are being operated by uni versities, industries, and government agencies, in sizes ranging from one or two liters per hour to approximately 300 liters per hour. Problems and Properties Consideration of hydrogen liquefaction should be preceded by mention of some of the properties of hydrogen which in fluence the liquefaction process. Most important of these properties are the inversion temperature (the temperature at which the Joule-Thomson effect changes sign) of hydrogen, its high And variable specific heat, its ortho-para forms and its purification requirements. Hydrogen Inversion Temperature. The inversion tempera ture for hydrogen is --92 F and Joule-Thomson cooling is small for temperatures above --280 F. Thus, hydrogen can not be liquefied by Joule-Thomson expansion unless it is pre cooled to a temperature lower than -- 92 F. Efficient liquefac tion by Joule-Thomson expansion is realised by precooling with Rnhat.mnspharin liquid nitrogen or liquid air in the tem perature range from --333 F to --352 F. Specific HeaL The constant pressure specific heat of hydro gen gas is higher than that of any other gas. Also, in common with other gases near their critical point, the specific heat of hydrogen rises sharply with moderate pressure in the tem perature range of --333 F to --415 F. These factors result in high heat exchange requirements per pound of gas circulated and considerable difficulty in balancing low temperature heat exchangers. Ortho-Para. Hydrogen exists in both high energy ortho and low energy para forms. The equilibrium mixture of normal hydrogen at room temperature is 75 percent ortho and 25 percent para. With, decreasing temperatures the ortho-frac tion decreases to the equilibrium value of approximately 0.2 percent at the saturation temperature of --423 F. Since the large energy release in converting from the normal to the para form results in prohibitive liquid losses, catalytic con version in the liquefier has become a standard practice. Ortho para conversion in the liquefier imposes an additional heat load and the catalyst beds add some structural complexity. . Purification. Purification of hydrogen is important because of its low liquefaction temperature and broad combustion limits with air and oxygen. The first problem requires re*1 moval of nitrogen and other high boiling point materials to a concentration of a few parts per million prior to liquefaction to prevent freeze-up of the system. The hazardous nature of the hydrogen-oxygen mixture requires removal of essentially all traces of oxygen before feed gas is admitted to the liquefac tion system. This is accomplished by commercial palladium catalyst Deoxo units which react the oxygen impurity with hydrogen to form water vapor at room temperature. - Liquefaction Cycles With one exception, conventional liquefaction systems can be used to liquefy hydrogen. The exception is that hydrogen cannot be liquefied by a cascade process since there is no gas to bridge the gap between the triple point of oxygen and the. critical point of hydrogen. Neon has been used to condense hydrogen but neon cannot be cascaded either since its critical point is 18 F deg below the triple point of oxygen. Use of oxygen is hypothetical at best due to the potential danger of miring it with hydrogen. yu