Abstract:
A ground freezing method and system which circulates refrigerated heat transfer fluid through freeze pipes in the ground at a low temperature of at least −52° C. (−62° F.) to minimize drilling for the freeze pipe installation. The heat transfer fluid is preferably aqua ammonia (ammonium hydroxide) because of its beneficial characteristics in this application. The circulating heat transfer fluid is preferably cooled by a refrigeration system that includes low and high stage cycles arranged in a cascade relationship and using ammonia or another refrigerant in the high stage refrigeration system and carbon dioxide as the refrigerant in the low stage refrigeration system.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to artificial ground freezing and more particularly to an improved process and system that has particular utility in large scale ground freezing applications. 
     BACKGROUND OF THE INVENTION 
     Artificial ground freezing has been used for many years to freeze selected areas of the ground for a number of different purposes. It is often used to provide support for excavations or to cut off ground water seepage, although it can be used for applications such as the confinement of hazardous materials in the ground and creating impermeable zones for hydrocarbon or mineral extraction or processing. 
     When the soil is frozen, the water within the soil freezes and bonds the soil particles together. It has been determined that colder soil temperature significantly increase the strength of the frozen soil, to the point where its compressive strength can equal that of some types of concrete. The combination of high strength and impermeability makes frozen soil useful as a shoring system for deep excavations. By way of example, mine shafts well over 1000 feet deep have been completed using ground freeze shoring techniques. Frozen soil walls for preventing ground water or chemicals in the soil from migrating through the ground have been formed to provide a barrier in cases where there is no need for an open excavation such as a mine or tunnel. 
     In a conventional ground freeze application, drilling is carried out to form spaced apart bores in which freeze pipes are installed around the perimeter of the proposed excavation or along the ground water barrier. Typically, the freeze pipes are steel pipes three to four inches in diameter installed three to six feet apart along the site of the proposed wall of frozen soil. The most commonly used technique involves circulating a refrigerated liquid through the freeze pipes. Salt water brine and ethylene glycol can be used, and they are cooled using a vapor compression cycle refrigeration system that employs a refrigerant such as ammonia, R-22 or other refrigerant. The refrigeration plant is specially designed for ground freezing and may be either mobile or stationary. After the circulating fluid has been chilled, it is pumped through the freeze pipes and is returned to be cooled again by the refrigeration plant. The entire system is closed to the atmosphere. 
     As the cold liquid circulates through the freeze pipes, the soil around each individual pipe freezes. As more time passes and more circulating liquid is pumped through the freeze pipes, the frozen zone of soil around each pipe is enlarged until the adjacent zones eventually merge to form a barrier that is impermeable. As the freezing process continues and additional freezing occurs, the frozen barrier increases in thickness and the temperature decreases. The result is that a continuous barrier is created so that excavation can take place or, in the case of a ground water barrier, a containment wall is formed. 
     Another ground freezing technique that has been used is known as a direct expansion process in which a cryogenic fluid such as liquid nitrogen or liquid carbon dioxide is applied to the freeze pipes. The fluid boils to a vapor to extract heat from the soil and then discharges to the atmosphere. In an open system of this type, the fluid is not recirculated but is essentially lost to the atmosphere. The advantage of the direct expansion system is that it freezes the ground much faster than a brine circulation system. However, the cryogenic fluids are so costly that it is not practical to use them in many applications and particularly in large scale projects. 
     Each ground freezing project requires an evaluation to determine the appropriate spacing between the freeze pipes. Increasing the spacing between pipes results in a longer time required for the ground to be frozen to form the barrier. Three to six weeks of freeze time is typical for the freeze zone to be completed with the necessary permeability. The time can be reduced by either using a colder circulating fluid or by reducing the pipe spacing. However, if the pipe spacing is reduced, more drilling is required. Because drilling is the single most costly aspect of a ground freezing project, it is highly undesirable to space the pipes close together. Conversely, the overall cost can be reduced significantly by increasing the pipe spacing to decrease the drilling requirements. With increased distance between pipes, the only way for an effective frozen barrier to be formed in a reasonable time period is to decrease the temperature of the coolant that is circulated through the freeze pipes. 
     On some projects, coolant temperatures must be about −52° C. (−62° F.) or less to allow a pipe spacing that is consistent with a reasonably low drilling cost. However, conventional circulating fluids such as calcium chloride brine or ethylene glycol cannot attain such a low temperature. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a method and apparatus for ground freezing that makes use of cooling techniques resulting in circulating fluid temperature of −50° C. (−58° F.) or less. This has the great advantage of allowing the freeze pipes to be spaced relatively far apart while still creating an impermeable frozen earth barrier in a reasonable period of time. The saving in drilling cost that results from the need for fewer freeze pipe bores creates a major economic benefit making ground freezing practical for very large projects. 
     In accordance with the invention, a refrigeration system is used to cool a circulating heat transfer fluid to a temperature of −52° C. (−62° F.) or less. The heat transfer circulating fluid is preferably aqua ammonia (ammonium hydroxide) with 27-30% ammonia, which has the advantage of being readily available at a low cost and the ability to serve as an efficient heat transfer fluid. Equally important, aqua ammonia (ammonium hydroxide) has a very low viscosity (actually less than water at −52° C.) so that it can be easily pumped through the freeze pipes to minimize pumping costs and difficulties. 
     The refrigeration plant used to cool the circulating fluid may advantageously employ low and high stage vapor compression refrigeration systems arranged in a cascade relationship with one another. The low stage system may use carbon dioxide as its refrigerant with its condenser arranged to discharge its heat to the evaporator of the high stage system. Ammonia is preferably the refrigerant in the high temperature system. However, R-22 or other refrigerant may be employed. In this way, the low temperature system can cool the circulated fluid to the requisite temperature −52° C. (−62° F.) or less and thus allow the freeze pipes to be spaced relatively far apart far so that the drilling costs are low enough to make ground freezing practical in large scale projects. 
     Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     In the accompanying drawing which forms a part of the specification and is to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views: 
     FIG. 1 is a schematic diagram of a ground freezing system constructed and arranged according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawing in more detail, the present invention is directed to a ground freezing system in which a plurality of freeze pipes  10  are installed in the ground in bores  11  that are drilled at spaced apart locations along an impermeable barrier that is to be formed by freezing the ground along the barrier. The drilling of the bores  11  and installation of the freeze pipes  10  in them are accomplished by techniques that are well known in the art. As also well known in the art, a refrigerated heat transfer liquid can be circulated through the pipes  10  in order to freeze the ground around the pipes and eventually form an impermeable barrier extending between the pipes when the frozen areas around the pipes become large enough to merge into a unitary barrier. 
     In accordance with a preferred embodiment of the present invention, a refrigeration plant for cooling the circulating liquid may include a high stage refrigeration system generally identified by numeral  12  and a low stage refrigeration system generally identified by numeral  14 . The refrigeration systems  12  and  14  may incorporate conventional vapor compression refrigeration cycles. The two systems  12  and  14  are arranged in a cascade relationship with one another. 
     The high stage refrigeration system  12  preferably uses ammonia as the refrigerant. However, other refrigerants may also be employed. The ammonia in gas form is compressed by a conventional compressor  16  driven by a motor  18 . The compressed ammonia is discharged from the compressor  16  along a vapor line  20 . Line  20  leads to a condenser  22  in which the gaseous refrigerant is condensed to provide a liquid which is discharged from the condenser  22  along a liquid line  24 . The liquid ammonia in line  24  may have a temperature of approximately 95° F. (35° C.). The liquid line  24  leads through an expansion valve  26  to an evaporator  27  contained in a heat exchanger  28 . The ammonia gas is directed from the heat exchanger  28  along line  30  to the compressor  16  which compresses it again. The temperature in line  30  may be approximately (−15° F.) (−26° C.). 
     The condenser of the low stage refrigeration system  14  is part of the heat exchanger  28  and discharges its heat to the evaporator  27  of the high stage system  12 . The refrigerant used in the low stage system  14  may be carbon dioxide. The liquid refrigerant from the high sage condenser flows through line  32 . Line  32  extends through an expansion valve  34  to another heat exchanger  36  which contains the evaporator  37  of the low stage system  14 . The carbon dioxide vapor is directed from the evaporator  37  along line  38  which leads to a compressor  40  driven by a motor  42 . The compressed vapor is discharged from the compressor  40  along line  44  to the condenser in the heat exchanger  28 . The refrigerant temperature in line  32  may be approximately −5° F. (−20° C.). 
     A circulation path generally identified by numeral  46  is provided for the heat transfer fluid that is pumped through the freeze pipes  10 . The cold heat transfer fluid which is circulated through the circulation path  46  is preferably aqua ammonia (ammonium hydroxide) which may contain 27%-30% ammonia dissolved in water. This fluid is particularly advantageous because it is readily available at a low cost and functions as an effective and efficient heat transfer fluid. It also has a relatively low viscosity so that it can be pumped easily through the circulation path  46 . 
     The circulation path  46  passes through the heat exchanger  36  such that the evaporator  37  of the low stage refrigeration system  14  extracts heat from the aqua ammonia (ammonium hydroxide) that is circulated through the circulation path  46 . The cooled liquid discharged from the heat exchanger  36  is directed through line  48  to a cold section  49  of a two compartment tank  50 . The tank  50  and the entire circulation path  46  are maintained at a positive pressure so that the ammonia in the heat transfer fluid is kept at a positive pressure. The temperature of the heat transfer fluid in line  48  is approximately −62° F. (about −52° C.). A pump  52  pumps the liquid from the cold section on the tank  50  along a line  54  leading to the freeze pipes  10 . After passing through the freeze pipes  10 , the circulating liquid is directed along line  56  to a warm section  57  of tank  50  which is likewise maintained at a positive pressure. A pump  60  pumps the circulating fluid from the warm section of tank  50  along a line  62  leading to the heat exchanger  36 . The temperature of the fluid in line  62  may be approximately −50° F. (−48° C.). 
     In operation, the low stage system  14  discharges its heat to the evaporator  27  of the high stage refrigeration system  12 . The evaporator  37  of the low temperature refrigeration system  14  similarly extracts heat from the heat transfer fluid in the circulation path  46 , thus cooling the heat transfer fluid in path  46  to a low temperature at or below −52° C. (−62° F.). Consequently, the temperature of the fluid applied to the freeze pipes  10  is at or below −50° C. (−58° F.), and the pipes  10  can be spaced relatively far apart so that the number of drilled bores  11  that is required for the freeze pipes is reduced, along with the drilling costs. The cascade arrangement of the refrigeration systems  12  and  14  and the use of ammonia in the high stage system and carbon dioxide in the low stage system as the refrigerants is advantageous because it results in the heat transfer fluid in path  46  being cooled to the desired low temperature of −52° C. (−62° F.) or less. Aqua ammonia (ammonium hydroxide) is preferred for the heat transfer fluid because of the advantages previously indicated. The cold section  49  and warm section  57  of the tank  50  allow for accumulation of the circulating fluid and are maintained at positive pressures in order to prevent heat transfer fluid from being subjected to a vacuum. The cold and warm sections can be constructed as separate tanks if desired. 
     From the foregoing it will be seen that this invention is one well adapted to attain all ends and objects hereinabove set forth together with the other advantages which are obvious and which are inherent to the structure. 
     It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. 
     Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative, and not in a limiting sense.