1、966 2009 ASHRAEThis paper is based on findings resulting from ASHRAE Research Project RP-1472.ABSTRACTCryosurgery is a technique for destroying undesirable tissue such as cancers using a freezing process. A thermody-namic modeling tool was developed for a two stage, mixed gas Joule-Thomson (MGJT) cr
2、yoprobe used for cryosurgery. A conventional Vapor Compression (VC) cycle using a pure refrigerant pre-cools the MGJT cycle that provides refrigera-tion at the cryoprobe tip. The model is used with an optimiza-tion routine to investigate the optimal mixture composition for different cryoprobe tip te
3、mperatures as well as optimal precooling temperatures. The cryoprobe system performance is reported in terms of cryoprobe size, and compressor size and power consumption. The results for the two stage system are compared with the performance of a single stage MGJT cycle to demonstrate the benefits a
4、nd limitations associated with the addition of the precooling cycle.INTRODUCTIONBrief Overview of Cryosurgery and Cryosurgical ProbesCryosurgery is a technique for destroying undesirable tissue such as cancers using a freezing process. Cryosurgery is used to ablate prostate and liver cancer tumors a
5、nd is also used in a variety of dermatological and gynecological procedures. Cryosurgical procedures may last anywhere from a few minutes to an hour (Rubinsky 2000). Cryosurgery relies on some type of cryosurgical probe that is inserted into the body in order to create the necessary cryogenic temper
6、atures; the cryoprobe tip reaches approximately 150 K (-190 F) for most procedures. The cryolesion that is formed (Fredrickson 2004) is typically on the order of tens of millimeters in diameter and the lethal zone (i.e., the region in which cell death is complete) extends outward into the tissue fro
7、m the cryoprobe tip approx-imately to the location where the tissue is about 240 K (-28 F), although this will vary by 15 K (27 F) depending on the surgical details of the surgical procedure and location (Rubin-sky 2000). The cryosurgical procedure is inherently mini-mally invasive compared to other
8、 treatments as the affected tissue extends beyond the contact point of the instrument. Cryosurgery is therefore an attractive alternative for proce-dures where surgical resection is not possible because of the proximity of the unhealthy tissue and large blood vessels (Zhong 2006). Cryosurgical treat
9、ment of cancers began in the mid-nine-teenth century when James Arnott (Arnott 1851) investigated the use of freezing for the treatment of cancer. Freezing tissues using a mixture of ice and various solutes had been previously used as an anesthetic, but Arnott found that freezing was also an effecti
10、ve treatment option for tumors in the breast and uter-ine cavity (Rubinsky 2000). Advances in cryogenics over the next century led to availability of various cryogens including liquid oxygen and liquid nitrogen as well as solid carbon diox-ide (dry ice). However, instrumentation for medical cryogen
11、application was limited during this time and generally capable of freezing to a depth of only a few millimeters (Rubinsky 2000). Therefore, the use of cryogenics in medicine was primarily limited to treatment of superficial tissues in the fields of dermatology and gynecology.Irving Cooper and Arnold
12、 Lee (Cooper 1961) invented the first cryosurgical probe that was capable of producing sizable cryolesions deep within the body. Liquid nitrogen (LN2) was pumped through thin concentric tubes; liquid nitrogen entered Modeling and Optimization of a Cascaded Mixed Gas Joule-Thompson Cryoprobe SystemHa
13、rrison Skye Greg Nellis Sanford KleinStudent Member ASHRAE Member ASHRAE Fellow ASHRAEHarrison Skye is a doctoral candidate, Greg Nellis is an associate professor, and Sanford Klein is a professor in the Department of Mechanical Engineering, University of WisconsinMadison, Madison, WI.LO-09-092 (RP-
14、1472) 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted withou
15、t ASHRAEs prior written permission.ASHRAE Transactions 967the probe where it was evaporated and nitrogen vapor exited. Liquid nitrogen cryoprobes are still used today, however, the nitrogen vapor is not recovered which leads to ventilation issues and the cryogen storage tanks must be periodically re
16、filled which leads to limits on the duration of the procedure as well as other logistical issues. Additionally, the probes and other equipment involved in transporting the liquid nitrogen to the cryoprobe must be vacuum insulated and therefore the system is bulky and rigid; these are undesirable pro
17、perties for a piece equipment that is meant to be minimally invasive and used in a surgical setting. The next generation of cryosurgical probes use a single component gas (e.g., argon) in a Joule-Thomson (JT) refrig-eration cycle. A high pressure (often 20 MPa or 3000 psig) gas cylinder is used to p
18、rovide high pressure gas, which supplies an open-cycle JT system; the low temperature gas in the tip of the cryoprobe creates the cooling effect. The advantage of this system is that the gas entering the cryoprobe is at room temper-ature and therefore vacuum insulation is not required; these probes
19、are much smaller than their liquid nitrogen counter-parts. However, the pressures required by single component gas in a JT system are too large to be provided by any portable compressor; thus the need for a high pressure gas bottle. The low pressure gas leaving the open system is not recovered and t
20、herefore represents an asphyxiation hazard; the medical facil-ity must be equipped with an auxiliary ventilation system. The system provides a small amount of cooling per unit of gas consumed and therefore the amount of gas consumed in order to complete a procedure is large and the cylinders must be
21、 replaced frequently. JT systems utilizing a mixture of gases, rather than a single component, represent a significant advance. The pres-sure required by a mixed gas Joule-Thomson (MGJT) system are much less than for a single component JT system (typically 1.5 MPa or 200 psi) and therefore it is pos
22、sible to recover the low pressure fluid leaving the probe and recompress it in a small, portable compressor in the operating room. Therefore, MGJT systems are closed systems that offer the considerable advantage of not using a consumable working fluid; this advantage reduces the hardware, floor spac
23、e, logistical and ventilation requirements, and expense associated with a procedure. Brodyansky et. al (Brodyansky, 1971) showed that mixed gas JT system can provide substantially more cooling per unit mass than a single component JT system which leads to a relatively compact and convenient device t
24、hat is more appropriate for a clinical environment. The limitations on the use of cryosurgery are primarily related to the cryoprobe technology; for treatments that cover large regions deep within the body, current cryoprobe technol-ogy will require that multiple probes be inserted and precisely pos
25、itioned in order to ensure complete cell death. Clearly, a single probe with more power in the same geometric envelope is more desirable. The most recent advancement in cryosurgi-cal probe technology addresses this need by improving the underlying thermodynamic cycle. Multi-stage Joule-Thom-son cycl
26、es are used to divide the large temperature range that must be spanned (from room temperature to approximately 150 K (-190 F) into two smaller temperature spans that can each be addressed more using a more compact system. The result is a probe that can provide more refrigeration in a compact configu
27、ration. A two-stage mixed gas JT system is discussed in more detail in the subsequent sections and is the focus of this paper. Two-Stage Mixed-Gas JT CycleThis paper describes a computational model of a two-stage JT cycle for use in a cryosurgical probe. Figure 1 provides a schematic of the cryoprob
28、e configuration and Figure 2a provides a schematic of the primary components in the entire system, including numbered thermodynamic states. A conventional vapor-compression cycle labeled “1ststage” provides precooling for the 2ndstage mixed gas JT cycle. The working fluid for the 1ststage is a singl
29、e component synthetic refrigerant whereas the working fluid for the 2ndstage is a mixture of components. The cryoprobe tip, which provides cooling to the tissue at the surgical sight, is represented by a heat exchanger; the refrigeration capacity of the cryoprobe is and the nominal tip temperature i
30、s T7. The purpose of the thermodynamic model presented here is to investigate cycle design issues; for example, the model will allow the determination of the optimal mixture composi-tions for the 2ndstage JT cycle as well as the appropriate amount of precooling. This work is partially based on a mod
31、el previously developed at the University of Wisconsin at Madi-son, which evaluated optimum gas mixtures for a single stage JT cryosurgical system (Keppler et al., 2004). This initial work has been verified and used to optimize the design of a single-stage system for cryosurgery (Fredrickson 2006).
32、This paper utilizes the same modeling methodology but expands the approach to the two stage cycle shown in Figure 2a; to our knowledge, the theoretical optimization of a two stage MGJT system for cryosurgery has not previously been reported. Therefore, the model is used to identify the merits as wel
33、l as the potential drawbacks associated with using a two stage system as compared to a single stage, mixed gas JT cycle.The refrigeration capacity of a JT cycle is fundamentally limited by the Joule-Thomson effect associated with the work-ing fluid. The Joule-Thomson effect is related to the isother
34、-mal enthalpy difference between the high and low pressure streams in the recuperator. This phenomenon is briefly discussed here; a more complete explanation can be found in Keppler et al. (2004).An energy balance on the cold end of the 2ndstage of the JT cycle that passes through an arbitrary locat
35、ion in the recu-perator is shown in Figure 2b. The energy balance shows that the refrigeration load is equal to the enthalpy difference between the two streams at any-cross section in the heat exchanger:Qload968 ASHRAE Transactions(1)where is the mass flow rate in the 2ndstage, and are the suction a
36、nd discharge pressures associated with the 2ndstage compressor (neglecting pressure loss in the recuperator and precooler), T is the temperature of the low pressure stream, T is the temperature difference between the streams at the cross section, and is a vector of molar concentrations of each compo
37、nent in the 2ndstage fluid mixture. The pressure drops in the recuperator and precooling heat exchangers were neglected in order to simplify the anal-ysis and to make the model more broadly useful in that it does not rely on specific heat exchanger geometry. Additionally, it is currently not possibl
38、e to accurately predict the pressure drop a priori as limited data are available for calculating pressure drop for a multi-phase, multi-component mixture flowing through a complicated heat exchanger. It is expected that the temperature difference driving heat transfer in the precooler and recuperato
39、r of an actual system will be most sensitive to Figure 1 Geometric schematic of a two-stage cryoprobe showing the fluid flow, expansion valves, cryoprobe shaft, and coiled fin tube heat exchangers.Figure 2 (a) Schematic of two-stage refrigeration cycle showing the thermodynamic states associated wit
40、h each stage and (b) control volume around cold end of JT cycle, which passes through an arbitrary cross section in the recuperator.(a)(b)Qloadm2ndenthalpy Plow 2nd,Ty2nd,()= enthalpy Phigh 2nd,T Ty2nd,+,()m2ndPlow 2nd,Phigh 2nd,y2ndASHRAE Transactions 969pressure drop in the low pressure stream, as
41、 the saturation temperatures are more sensitive to pressure changes at low pressure. In the limit that the recuperator conductance is infinitely large (i.e., the recuperator is providing the maximum possible rate of stream-to-stream heat transfer), the temperatures of the fluid streams will coincide
42、 (i.e., T in Equation 1) will approach zero) at a location in the recuperator that is commonly referred to as the pinch point. The maximum possi-ble enthalpy difference between the two streams, which is equal to the maximum achievable refrigeration load per unit mass flow rate, can therefore be calc
43、ulated as the minimum value of the isothermal enthalpy difference over the range of temperature spanned by the recuperator:(2)Keppler et. al (2004) and many others have demonstrated that the isothermal enthalpy difference exhibited by any pure fluid is large only over a small temperature span near t
44、he vapor dome. The vapor dome associated with a mixture of gases tends to extend over a larger temperature range corresponding to a temperature that is near the lowest boiling point of the components to one that is near the highest boiling point component. Therefore, the use of gas mixtures signific
45、antly extends the temperature range over which the isothermal enthalpy difference is large, making the system more practical, as the recuperator temperatures nominally span 290 K (62 F) (warm inlet of recuperator) to 150 K (-190 F) (load temper-ature recuperator cold inlet) for a single stage cryosu
46、rgical system. However, there is a tradeoff between the maximum cool-ing power that can be provided and the temperature range that must be spanned by the recuperator. For example, consider two 7-component mixtures that could be used in the 2ndstage of the two-stage system in Figure 2a where the load
47、 tempera-ture is 140 K (-208 F), and the high and low pressures are 1000 kPa and 100 kPa (130 psig and 0 psig). The composition of mixtures A and B have been optimized to produce the maxi-mum JT effect over two different temperature spans but both mixtures have the same constituents: nitrogen, ethan
48、e, meth-ane, propane, isobutane, isopentane, and argon. The mole frac-tions of these constituents are listed in Table 1. Mixture A is carefully optimized for a temperature span of 285 K to 140 K (53 to -208 F), which would be typical of a single stage JT cycle (i.e. Figure 2a with the 1ststage remov
49、ed). Mixture B is optimized for a temperature span of 238 K to 140 K (-31 to -208 F), which is typical of a JT cycle with some precooling that lowers the recuperator hot inlet temperature to 238 K (-31 F). Figure 3 shows that the maximum cooling effect (i.e., the minimum value of the isothermal enthalpy change) over the temperature span for mixture A is 73 W/(g/s) (31 btu/lbm), whereas the maximum cooling effect for mixture B over its temperature span is 115 W/(g/s) (49 btu/lbm). Therefore, by reducing the temperature range that must be spanned by the mixed gas JT s
