1、46.1CHAPTER 46REFRIGERATION IN THE CHEMICAL INDUSTRYFlow Sheets and Specifications 46.1Refrigeration: Service orUtility 46.1Load Characteristics 46.2Safety Requirements. 46.2Equipment Characteristics. 46.3Start-Up and Shutdown 46.4Refrigerants 46.5Refrigeration Systems. 46.5Refrigeration Equipment 4
2、6.6HEMICAL industry refrigeration systems range in capacityCfrom a few kilowatts of refrigeration to thousands of kilowatts.Temperature levels range from those associated with chilled waterthrough the cryogenic range. The degree of sophistication and inter-relation with the chemical process ranges f
3、rom that associated withcomfort air conditioning of laboratories or offices to that where reli-able refrigeration is vital to product quality or to safety.Two significant characteristics identify most chemical industryrefrigeration systems: (1) almost exclusively, they are engineeredone-of-a-kind sy
4、stems, and (2) equipment used for normal commer-cial application may be unacceptable for chemical plant service.This chapter gives guidance to refrigeration engineers workingwith chemical plant designers so they can design an optimum refrig-eration system. Refrigeration engineers must be familiar wi
5、th thechemical process for which the refrigeration facilities are beingdesigned. Understanding the overall process is also desirable. Com-puter programs are also available that can calculate cooling loadsbased on the gas chromatographic analysis of a process fluid. Theseprograms accurately define no
6、t only the thermodynamic perfor-mance of the fluid to be chilled, but also the required heat transfercharacteristics of the chiller.Occasionally, because the process is proprietary, refrigerationengineers may have limited access to process information. In suchcases, chemical plant design engineers m
7、ust be aware of the restric-tions this may place on providing a satisfactory refrigeration system.FLOW SHEETS AND SPECIFICATIONSThestartingpointinattainingasoundknowledgeofthechemicalprocess is the flow sheet. Flow sheets serve as a road map to the unitbeing designed. They include information such a
8、s heat and materialbalances around major system components and pressures, tempera-tures, and composition of the various streams in the system. Flowsheets also include refrigeration loads, the temperature level atwhich refrigeration is to be provided, and the manner in whichrefrigeration is to be pro
9、vided to the process (e.g., by a primaryrefrigerant or a secondary coolant). They indicate the nature of thechemicals and processes to be anticipated in the vicinity in whichthe refrigeration system is to be installed. This information shouldindicate the need for special safety considerations in sys
10、tem designor for construction materials that resist corrosion by process mate-rials or process fumes.Different portions of a process flow sheet may be developed bydifferent process engineers; consequently, the temperature levels atwhich refrigeration is specified may vary by only a few degrees.Study
11、 may reveal that a single temperature is satisfactory for severalor even all users of refrigeration, which could reduce project cost byeliminating multilevel refrigeration facilities.Most process flow sheets indicate the design maximum refriger-ation load required. The refrigeration engineer should
12、also know theminimum design load. Process loads in the chemical industry tendto fluctuate through a wide range, creating potential operationalproblems.Flow sheets also indicate the significance of the refrigerationsystem to the overall process and the desirability of providingredundant systems, inte
13、rlocking systems, and so forth. In somecases, refrigeration is mandatory to ensure safe control of a processchemical reaction or to achieve satisfactory product quality control.In other cases, loss or malfunction of the refrigeration system hasmuch less significance.Other sources of information are
14、also valuable. A properly pre-pared set of specifications and process data expands on flow sheetinformation. These generally cover the proposed process design inmuch more detail than the flow sheets and may also detail the me-chanical systems. Information about the design principles, includingcontin
15、uity of operation, safety hazards, degree of automation, andspecial start-up requirements, is generally found in the specifica-tions. Equipment capacities, design pressures and temperatures, andmaterialsofconstructionmaybeincluded.Specificationsfor piping,insulation, instrumentation, electrical, pre
16、ssure vessels and heat ex-changers, painting, and so forth are normally issued as part of thedesign package available to a refrigeration engineer.It is imperative that refrigeration engineers establish effectivecommunication with chemical process engineers. The refrigerationengineer must know what i
17、nformation to request and what infor-mation to give to the chemical process engineer for design opti-mization. The following sections outline some of the significantcharacteristics of chemical industry refrigeration systems. A fullunderstanding of these peculiarities is of value in achieving effec-t
18、ive communication with chemical plant designers.REFRIGERATION: SERVICE OR UTILITYRefrigeration engineers unfamiliar with the chemical industrymust understand that, unless the chemical process is cryogenic,chemical plant designers probably consider refrigeration merely asa service or utility of the s
19、ame nature as steam, cooling water, com-pressed air, and the like. Chemical engineers expect the reliability ofthe refrigeration to be of the same quality as other services. When asteam valve is opened, chemical engineers expect steam to be avail-able instantly, in whatever quantities demanded. When
20、 steam is nolonger required, the engineer expects to be able to shut off the steamsupply at any time without adversely affecting any other steam useror the source of steam. The same response from the refrigerationsystem will be expected. This high degree of reliability is usually sostrongly implied
21、that no specific mention of it may be made in spec-ifications.Because refrigeration is frequently considered a service, processdesigners spend insufficient time analyzing temperature levels,potentialloadcombinations,energyrecoverypotentials,andthelike.The potential for minimizing the size of the ref
22、rigeration system, thetotal plant investment, or both, by providing refrigeration at a mini-mum number of temperature levels, is frequently not investigated byThe preparation of this chapter is assigned to TC 10.1, Custom-EngineeredRefrigeration Systems.46.2 2010 ASHRAE HandbookRefrigeration (SI)pro
23、cess engineers. Likewise, the potential for power recovery is fre-quently overlooked.Part of the reason for this attitude is that refrigeration facilitiesrepresent only a minor part of the total plant investment. The entireutilities installation for the chemical industry usually falls in therangeof5
24、to15%ofthetotalplantinvestment,withtherefrigerationsystem only a small portion of the utilities investment. Processrequirements may be overruling, but process engineers must recog-nizelegitimateprocessnecessitiesandavoidunnecessaryandcostlyrestrictions on the refrigeration system design.LOAD CHARACT
25、ERISTICSFlow sheet values generally indicate direct process refrigerationrequirements and do not include heat gains from equipment or pip-ing. Flow sheet peaks and average loads generally do not allow forunusual start-up conditions or off-normal process operation thatmay impose unusual refrigeration
26、 loads. This information must begained by a thorough understanding of the process and by discuss-ingthepotentialeffectonrefrigerationsystemdesignforoff-normalprocess conditions with the process engineer.Once the true peak loads are established, the duration and fre-quency of the peaks must be consid
27、ered. For some simple processesthis information is fairly straightforward; however, if the plant isdesigned for both batch operation and multiproduct manufacture,this can be an enormous task. Computer simulation of such a com-bination of processes has led to optimization of not only the refrig-erati
28、on equipment but also the process equipment. Computersimulation ensures that the refrigeration machine, secondary coolantstorage tanks, and circulating pumps are properly sized to handleboth average and peak loads with a minimum investment. Few appli-cations require computer simulation, but a thorou
29、gh understandingof the relationship of peak loads to average loads and their influenceon refrigeration system component sizing is vital to good design.Sometimes unusually light load conditions must be met. If theprocess is cyclic and on/off operation is undesirable, the refrigerationsystem may run f
30、or significant periods under a very light load or evenno load. Light loads often require special design of the system con-trols, such as multistep unloading and hot-gas bypass with recipro-catingcompressors,acombinationofsuctionthrottlingorprerotationvanes and hot-gas bypass with centrifugal compres
31、sors, and slidevalve unloading and hot-gas bypass with screw compressors. A sec-ondary coolant system might require a bypass arrangement.Investment in refrigeration equipment can be minimized whenonly a few levels of refrigeration are required. Checking the speci-fied temperatures to be sure they ar
32、e based on some process require-ment may show that fewer temperature levels than shown on theflow sheet are necessary. If multiple levels of refrigeration arerequired, a compound system should be evaluated. The evaluationmust consider the limited ability of a compound system to providethe precise te
33、mperatures required of some processes.Production PhilosophyThe specifications generally indicate whether a chemical pro-cess is in continuous or batch operation, but further research maybe required to understand the required continuity of service for therefrigeration facilities. The chemical industr
34、y frequently requiresa high degree of continuity; plant production rates are often basedon 8000 h or more per year. In general, the refrigeration equip-ment is worked extremely hard, with no off-peak period becauseof seasonal changes, and any unscheduled interruption of refriger-ation service may cr
35、eate large production losses. In most cases,scheduled maintenance shutdowns are not only infrequent but alsohighly vulnerable to cancellation or delays because of productionrequirements.As a result, reliability is key in the design of chemical industrysystems. Equipment that is satisfactory in comme
36、rcial or lightindustrial service is frequently unfit where high service rates andminimum availability for maintenance are the rule. In some cases,duplicate systems are justified. More often, multiple part-capacityunits are installed so that a refrigeration system breakdown will notcreate a total pro
37、cess production loss. Major equipment and hard-ware items that require minimal maintenance or that allow mainte-nance with the refrigeration system in operation should be selected(e.g., dual lube oil pumps, oil filters, and in many cases dual oilcoolers, compressors, and bypasses around control valv
38、es) to allowoperation during service of control valve and other components;many systems are built to American Petroleum Institute (API) spec-ification. Particular attention should be paid to equipment layout, sothat adequate access, tube pullout space, and laydown space areavailable to minimize refr
39、igeration system maintenance time. Insome cases, overhead steel supports for rigging heavy equipment, orpermanent monorails, are justifiable.Flexibility RequirementsThe chemical industry constantly develops new processes; conse-quently, the usual chemical plant undergoes constant modification.On occ
40、asion, total processes are rendered obsolete and scrappedbefore design production rates are ever reached. Thus, flexibilityshould be designed into the refrigeration system so it may be adapt-able to some process modification. Designing for optimum flexibil-ity is difficult; however, study of the pot
41、entials or probabilities ofprocess modifications and of the expected life of the process facilityhelps in making design decisions.SAFETY REQUIREMENTSMost chemical processes require special design to ensure safeoperation. Many raw materials, intermediates, or finished productsare themselves corrosive
42、 or toxic or are potential fire or explosionhazards. Frequently, chemical reactions in the process generateextremes of pressure or temperature that must be properly containedfor safe operation. Refrigeration engineers must be aware of thesepotential hazards as well as any abnormal hazards that may d
43、evelopduring start-up, unscheduled shutdown, or other upset to the chem-ical process. When designing modifications or expansions for anexisting facility, the possibility that some construction or mainte-nance techniques may be safety hazards must be considered.CorrosionThe shell, tubes, tube sheets,
44、 gaskets, packing, O rings, sealmaterials, and components of instrument or control hardwaremust be properly specified. The potential hazards of leakagebetween the normal process side and the refrigeration side of heatexchange equipment must be investigated, because an undesirablechemical reaction ma
45、y occur between the process material and therefrigerant.An additional corrosive hazard may be leaks, spills, or upsets inthe process area. Safe chemical plant design must anticipate unusualas well as usual hazards. For example, if refrigerant piping runsadjacent to a flanged piping system containing
46、 a highly corrosivematerial, special materials for the piping and insulation systemsmay be justified.ToxicityIf the refrigeration system indirectly contacts a toxic materialthrough heat exchange equipment, flanges and elements such asgaskets, packing, and seals in direct contact with the toxic mater
47、ialmust be designed carefully. The possibility of a leak in equipmentthat might allow refrigerants or secondary coolants to mix with pro-cess chemicals and cause a toxic or otherwise dangerous reactionmust also be considered. In some cases, the potential for toxic leaksmay be so high that a special
48、ventilation system may be required,double-tube sheets are used, or the tubes are welded and rolled intoRefrigeration in the Chemical Industry 46.3the tube sheets of shell-and-tube heat exchangers. Intermediatereboilers may also be used to isolate the process.Even though containment and ventilation c
49、an handle toxic mate-rials under normal process conditions, toxic material may need tobe vented in abnormal situations to avoid the hazards of fire orexplosion. In such cases, the toxic material is frequently vented ordiluted through a flare stack so that ground- or operating-levelconcentrations do not reach toxic limits. Refrigeration engineersshould evaluate the desirability of locating the refrigeration equip-ment itself or its controls outside the operating areas. An alternativeis to ventilate either the refrigeration system or its controls with asystem that has a remote air intake.Toxici
