1、4715 (RP-1042) Double-Walled Heat Exchanger Risk Analysis for Group BI and Group B2 Refrigerants Aaron E. Berger Donald L. Fenton, Ph.D., P.E. Member ASHRAE Prasad V. Kaitay ABSTRACT In compliance with ANSIIASHRAE Standard 15-200 1, Safety Standard for Refrigeration Systems, a large quantity of Grou
2、p BI or Group B2 refrigerant cannot enter the occupied space ofa building. Instead, a secondarypuid must be used to circulate through the refrigeration system?s components located within the occupied space. In typical systems, a single- walled liquid-to-liquid heat exchanger is used outdoors with an
3、 air coil in the occupied space. In this study, a double- walled heat exchanger is considered to replace these two components, where the double-walled conjiguration prevents leaking refrigerant from entering the occupied space of a building. Consequentlv, the double-walled heat exchanger located in
4、the occupied space serves as a direct expansion coil. While any Group BI or Group B2 refrigerant may be consid- ered, ammonia, which is a Group B2 refrigerant, became the focus ofthisstudy due to theavailabilityofcommercialabsorp- tion units. Four refrigeration system concepts are developed. One sys
5、tem-the reference system-represents current system design, incorporating a single-walled heat exchanger and an air coil. Three otherproposedsystems are also described, each containing a double-walled heat exchanger. Twelve design criteria accompany these proposed systems, which must meet the safety
6、requirements provided. From reliability data gathered and the calculations made, the three proposed conceptual systems are found to be at least as, ifnot more, reliable than the reference conceptual system in two comparisons. Considering the systems in their entirety, the reference system has a reli
7、ability value of 0,639, and the three proposed conceptual systems have reliability values of 0.654 or greater Similar results in comparison are attained Shuting Lei, Ph.D. when considering theportion of the system from which refrig- erant would escape to the occupied space. Since the proposed system
8、 reliabilities are equivalent to the reference system using a secondary fluid, the proposed systems are determined to be no more of a risk to safety than current systems. Thus, the application of the double-walled heat exchangel; which permits the refrigerant to enter the occu- pied space, appears a
9、cceptable on the basis ofthis risk anal- ysis. INTRODUCTION An absorption refrigeration cycle using ammonia as the working fluid typically has an expansion valve to regulate the flow of refrigerant through the evaporator. All refrigerants, including ammonia, which is a Group B2 refrigerant, are allo
10、wed by ANSI/ASHRAE Standard 15-2001, Safety Code for Mechanical Refrigeration, to circulate inside equipment in contact with air in the occupied space. However, in addition to allowing all refrigerants to enter the occupied space, Standard 15 also limits the quantity of these refrigerants. If the qu
11、antity limit of the refhgerant in the system is exceeded, the refriger- ant is not permitted to enter the occupied space. Instead, a secondary coolant, normally a water-glycol mixture, circu- lates through a finned coil and cools the air in the occupied space. This configuration prevents the refrige
12、rant from enter- ing the occupied space. The quantity limits imposed by Stan- dard 15 apply to all Group B 1 and B2 refrigerants and depend on the particular refrigerant?s toxicity and the refrigeration cycle. This paper is directed at exploring the conditions that allow a Group B1 or B2 refrigerant
13、 to circulate through the occupied space of a building. While the refrigerants under Aaron E. Berger is a graduate student and Donald L. Fenton is aprofessor in the Department of Mechanical andNuclear Engineering, Kansas State University, Manhattan, Kans. Prasad V. Kaitay is a design engineer at M/s
14、 Vima Engineers, Mumbia, India. Shuting Lei is an assistant professor in the Department of Industrial and Manufacturing Systems Engineering, Kansas State University, Manhattan, Kans. 02004 ASHRAE. 235 consideration have safety group classifications of B1 and B2, the refngerant of particular concern
15、is ammonia (R-7 17), which is a B2 refigerant. With ammonia, several advantages, such as potentially reduced first cost, would result due to the elimination of an intermediate heat exchanger. This paper is focused on comfort cooling for residential and small commercial buildings. The concept is to u
16、se a double-walled heat exchanger performing as an evaporator, where the inner and outer walls serve as two barriers separat- ing the refrigerant from the air in the occupied space and where the annular space between the inner and outer walls is piped to the outdoor air for accidental discharge ofth
17、e refrigerant. This configuration prohibits refngerant from entering the air inside the building and is, therefore, a means by which the refriger- ation systems classification may be converted from high- probability to low-probability. This, in turn, has favorable implications on the refrigerant qua
18、ntities allowed by Standard 15. The approach taken in this paper in considering the appli- cation of double-walled heat exchangers is to first define a reference system. This system represents the currently avail- able ammonia-water absorption systems and consists of a conventional ammonia-water abs
19、orption refrigeration system using a secondary coolant and a single-walled, finned air coil in the occupied space. The reliability of the reference system is computed, yielding a single value based on the best infor- mation available from an accumulation of literature providing reliability and failu
20、re rate data. Next, proposed conceptual systems using a double-walled heat exchanger are analyzed for their overall reliability. The proposed systems differ from one another in the way failure of the double-walled heat exchanger is monitored and in the way pressure relief is provided to the annular
21、space. Modifications to the conceptual systems are made in order to align the overall system reliabil- ity with that of the reference system, which provides insight to design criteria for that proposed system. To the extent possi- ble, the overall system reliability of each proposed system concept i
22、s made equivalent to the overall reliability of the conventional system. Guided by reliability analyses, a set of design criteria is developed for the use of a double-walled heat exchanger in this application. In conjunction with these delib- erations, modifications to Standard 15 are considered, if
23、 needed, that would allow application of the double-walled heat exchanger as an evaporator in a direct expansion system. STANDARD 15 APPLICATION The pertinent paragraphs of Standard 15 concerning the application of Group B 1 and B2 refrigerants to double-walled evaporators in occupied spaces are tra
24、ced to identify the current situation. Quotes from Standard 15 are included for clarity and are differentiated by a different type font and bold lettering. Proceeding through Standard 15, therefore, yields the following. Paragraph 5.1.1 Definition: A direct system is one in which the evaporator or c
25、ondenser of the refrigerat- ing system is in direct contact with the air or other substances to be cooled or heated (Standard 15-2001). Direct contact means that the evaporator or condenser consists of single-walled construction. With a direct sys- tem, a leak in the evaporator, condenser, or other
26、refrig- erant-containing component allows refrigerant to enter the air in the occupied space. Paragraph 5.1.2 Definition: An indirect system is one in which a secondary coolant cooled or heated by the refrigerating system is circulated to the air or other substance to be cooled or heated. Indirect s
27、ystems are distinguished by the method of application given below (Standard 15-2001). (Standard 15 then defines four indirect system types: indirect open spray system, double indirect open spray system, indirect closed sys- tem, and indirect vented closed system.) With an indi- rect system, the seco
28、ndary coolant has two functions: the first is that it is cooled by the refiigerant in the sys- tem, and, second, it is circulated to the air being cooled in the occupied space. With double-walled construction, the refrigerant is inside the inner tube and the air being cooled is outside the outer tub
29、e. This causes the refriger- ation system to be classified as an indirect system. The other refigerant-containing components, which include piping and expansion devices, must also be double- walled. Paragraph 5.1.2.3 Definition: An indirect closed sys- tem is one in which a secondary coolant passes
30、through a closed circuit in the air or other substance to be cooled or heated (Standard 15-2001). A second- ary coolant circulates within a closed circuit that is divided and separate from the air being cooled. Double- walled configurations comply with this definition. Paragraph 5.2.2 Definition: A
31、low-probability system is one in which the basic design-or location of the components-is such that leakage of refrigerant from a failed connection, seal, or component cannot enter the occupied space. Typical low-probability systems are (a) indirect closed systems or (b) double indirect systems and (
32、e) indirect open spray sys- tems . (Standard 15-2001). A properly designed and installed double-walled heat exchanger causes the sys- tem to comply with this definition of a low-probability system. With the annular space of the double-walled heat exchanger having pressure relief to the atmosphere, t
33、he potential leak path of the refrigerant to the air in the occupied space is broken. Engineered safeguards are required in order to minimize the likelihood of this to occur. Paragraph 7.4: Location in a Machinery Room or Outdoors. All components containing refrigerant shall be located either in a m
34、achinery room or out- doors, where (a) the quantity of refrigerant needed 236 ASHRAE Transactions: Research exceeds the limits in 7.2 (Standard 15 Table 1) or (b) direct-red absorption equipment, other than sealed absorption systems not exceeding the refrigerant quantity limits indicated in Table 2,
35、 is used (Standard 15-2001). A sealed absorption system is one where all of the components containing refrigerant are assembled using only welding or brazing. Standard 15, therefore, allows single-walled evaporators and condensers in occupied spaces operating as direct expansion units as long as the
36、 specified refrigerant quantities are not exceeded for the given occupancies. No distinction is made in this paragraph between low- and high-probabil- ity systems. Only residential, commercial, and mercan- tile occupancies are allowed. All direct-fired absorption systems are required to be located i
37、n a machinery room or outdoors without exception. Paragraph 7.5.2.1: Applications for Human Com- fort. Group B1, B2, and B3 refrigerants shall not be used in high-probability systems for human comfort except in sealed absorption and unit systems having refrigerant quantities less than or equal to th
38、ose indi- cated in Table 2 (edited for clarity) (Standard 15- 2001). As a practical matter, this provision applies only to ammonia for the reason that ammonia is the only refrigerant listed in Table 1. This provision is helpful because the allowable refrigerant charge may be suffi- cient for practic
39、al ammonia-water absorption units con- figured as high-probability systems. However, if the refrigerant quantity is greater than the Table 1 value, then the unit must be placed outdoors and any compo- nents that are in the occupied space must be double- walled. Paragraph 9.4.1: Refrigerating systems
40、 shall be pro- tected by a pressure-relief device or other approved means to safely relieve pressure due to fire or other abnormal conditions (Standard 15-2001). This provi- sion also applies to the annular space between the inner and outer walls, which may contain refrigerant if the inner wall fail
41、s due to an abnormal event. Standard 15 identifies requirements for the appropriate pressure- relief device depending on the inside dimension and the design pressure inside the annular space. Type of Refrigeration System Sealed Ammonia/Water Absorption System In summary, sealed ammonia-water absorpt
42、ion and unit systems are restricted to the quantity of refrigerant charge by the values given in Table 1. The system may be classified as either high- or low-probability. When the refrigerant charge exceeds the Table 1 values, the refrigeration unit must be located outdoors and be provided with appr
43、opriate pressure relief. With the application of the double-walled configura- tion, our interpretation of Standard 15 produces a low-proba- bility system classification that allows the refiigerant to circulate inside the occupied space. Pressure relief to the outside air for all of the double-walled
44、 components must also be provided. Verification of the low-probability classification is needed from ASHRAEs Standard 15 Committee. Maximum Pounds (kg) for Various Occupancies Institutional PublicLarge Mercantile Residential Commercial RELIABILITY DATA LITERATURE SEARCH In public hallways or lobbies
45、 In adjacent outdoor locations To assess the proposed refrigeration system conceptual designs for safety, the heat exchanger and other system components must first have their individual reliabilities deter- mined. A combination of the component reliabilities that compose the system gives the overall
46、 system reliability. Since determination of the component reliability is needed before the unit is constructed, analysis using available computer programs, such as Monte Carlo and pc-PRAISE (Khaleel and Simonen 1999) and other inspection devices are ineffective. Therefore, the most useful course of
47、action is to gather reli- ability data of used components that are similar to those in the reference design and the proposed conceptual designs and that operate under conditions comparable to those of the proposed designs. Several sources were obtained, supplying useful reliabil- ity data for situat
48、ions relative to those under consideration. The data attained from these sources were combined and are provided in Table 2. With all of the reliability data collected, comparisons and contrasts with the equipment comprising the reference system design and the proposed conceptual designs could be mad
49、e with greater accuracy. This comparison of data between systems is the purpose of the reliability data search and system reliability calculations. Since the reliability values 0 (0) 0 (0) 3.3 (1.5) 3.3 (1.5) 0 (0) 0 (0) 22 (10) 22 (10) Table 1. Special Quantity Limits for Sealed Ammonia-Water Absorption and Self-Contained Systems (ANSIIASHRAE Standard 15-2001, Table 2) Unit Systems In other than public hallways or lobbies - 22 (10) 0 (0) 0 (0) 6.6 (3) In other than uublic hallways or lobbies I O(0) I 6.6 (3) I 6.60) 1 2210) I ASHRAE Transactions: Research 237 Table 2. Compilation
