1、M IL-HDBK-251 19 January 1978 . l- MILITARY HANDBOOK i- RELIABILITY/DESIGN THERMAL APPLICATIONS FSC-RELI - -rl i Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-DEPARTMENT OF DEFENSE WASHINGTON, D.C. MIL-HDBK-251 Reliability/Design, Thermal Applicati
2、ons l. This handbook was developed by the Department of Defense in accordance with established procedure. 2. This publication was approved on 19 January 1978 for printing and inclusion in he military handbook series. 3. This document provides basic and fundamental information on the thermal design o
3、f military electronic equipment. It will provide information and guidance to personnel concerned with such design. The handbook is not intended to be referenced in purchase specifications except for in- formatlonal purposes, nor shall it supersede any specifi- cation requirements. completeness and c
4、urrency. Beneficial -comments (recomnen- dations, additions, deletions) and any pertinent data which may be of use in improving this document shd be addressed to: Naval Electronic Systems Command (Code !3)43), Depart- ment of the Navy, Washington, D.C. 20360, by using the self-addressed Standardizat
5、ion Document Improvement Proposal (DD Form 1426) appearing at the end of this document, or by letter. 4. This handbook will be reviewed periodically to insure its ii Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-251 RE W Paragraph 1. 1.1 1
6、.2 1.3 2. 2.1 3. 3.1 4. 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 5. 5.1 5.2 5.3 5.4 5.5 6. 6 1 6.2 6.3 6.4 7. c 7.1 7.2 7.3 7.4 7.5 7.6 8. 8.1 8.2 8.3 MIL-HDBK-251 CONTENTS Title SCOPE Purpose. Scope. Need for adequate thermal performance. REFERENCED DOCUMENTS Issues of documents. DEFINITIONS Termin
7、ology of definitions. APPROACHES TO THERMAL DESIGN Fundamental aspects and procedures. Fundamental thermal principles. Design approaches. Basic thermal design procedures. Step-by-step procedure. Stress effects and conservatism. Thermal reliability requirements. Methods of thermally rating parts and
8、equipment. Methodsfor minimizing heat dissipation. Units and conversion factors. DETERMINATION OF THERMAL REQUIREMENTS General considerations. PurDose. Page 1 1 1 2 516 516 7 7 9 9 10 11 11 11 13 13 14 14 15 19 19 21 Relationship between parts stress and reliability. 22 Cost benefits of thermal reli
9、ability. 24 Determination of the maximum parts temperature. 24 THERMAL DESIGN REQUIREMENTS 27 Electrical analogy of heat flow. 27 Determination of required thermal resistances, 29 Tentative assignment of thermal resistance values. 33 Computer assisted thermal analysis. 34 THE SELECTION OF OPTIMUM CO
10、OLING METHODS 37 General. 37 Heat transfer within a unit. 37 Heat transfer to the ultimate sink. 40 Tradeoff techniques. 42 Selection of optimum cooling method. 42 Appl ication of selected cool ing method to electro-thermal analog, 44 I - NATURAL METHODS OF COOLING 47 Theory. 47 Natural methods of c
11、ooling electronic parts. 68 Thermal design of equipment for natural cooling. 84 iii Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-253 RE m q777970 -0033387 2 i MIL-HDBK-251 Paragraph ” Title Page 8.4 Electronic equipment in spacecraft. 112
12、 9. 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 10. 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 11. 11.1 11.2 11.3 11.4 11.6 11.6 11.7 THERMAL DESIGN OF FORCED AIR COOLED ELECTRONIC EQUIPMENT Theory. Fan fundamental s. Pressure losses and their determination. Forced air cooling design. Ducting of
13、 air flow Design of forced air cooled cabinets and enclo- sures. Ventilation and air conditioning. Forced air coo1,ing and air conditioning systems in aircraft. Design considerations for forced air cooled equi pent Air fi 1 ters and cleaners. . Maintenance considerations. THERMAL DESIGN OF LIQUID CO
14、OLED ELECTRONIC EQU I PMENT Theory. Direct liquid cooling. . Indirect Liquid cooling. Equipment design considerations for liquid cool i ng . Heat exchangers. Cooling systems and fluid system components. Cool ants. Design of liquid cooling systems. Design methods for predicting thermal resis- tances
15、using liquid cooling. THERMAL DESIGN OF VAPORIZATION COOLED ELECTRONIC EQU I PMENT. Theory. Direct vaporization-cooling. Indirect vaporization cooling sysem. Expendable vaporization cooling systems Vaporization cooling systems, design cons-i- dera ti ons. Design methods for predicting thermal resist
16、ances using vaporization cooling; application notes for groundbas, shipboard, aircraft, and spacecraft. Design examples. 12. SPECIAL COOLING TECHNIQUE 12.1 General. 12.2 Heat pipes. 115 115 132 145 160 204 208 21 3 228 243 245 246 249 24 9 259 263 268 272 290 304 31 O 31 8 363 363 373 379 383 385 39
17、1 395 405 40 5 405 iv Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-253 RE W 7797770 0033388 LI W Paragraph 12.3 12.4 12.5 12.6 12.7 12.8 13. 13.1 13.2 14 14.1 14.2 14.3 14.4 14.5 15. 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.9 15.10 15.8 16.
18、16.1 16.2 16.3 16.4 16.5 16.6 $ 17. 17.1 17.2 17.3 17.4 17.5 17.6 Title Thermoel ec tri c cool i ng . Absorptive refrigeration. Change of phase cooling . Vortex tubes. Cryogeni c cool 3 ng . Other cooling techniques. fl STANDARD HARDWARE PROGRAM (SHP) THERMAL DESIGN Standard hardware program, genera
19、l. SHP thermal aspects. EQUIPMENT INSTALLATION REQUIREMENTS AND CONSI- DERATIONS General. Anticipated and actual thermal environments. Provisions for the removal of heat. Installation aspects for maintainability Aircraft equi pment and cool i ng sys terns interfaces. THE THERMAL EVALUATION OF ELECTR
20、ONIC EQUIPMENT General. Thermal indices and criteria. Methods of temperature measurement. Thema1 reconnaissance. Thermal eval ua ti on. Thermal testing of avionic equipment. Transient-temperature time history tests. Check list. Thermal survey, Methods of measuring pressure and flow rate. IMPROVING T
21、HE THERMAL PERFORMANCE OF EXISTING ELECTRONIC EQUIPMENT General. Determinati on. of thermal i nadequacies. Tradeoff studies of life cycle costs vs. modification costs. Use of indirect fresh water cooling for im- proving shipboard equipment. THE THERMAL CHARACTERISTICS OF PARTS. General. Thermal char
22、acteristics of semiconductor devi ces. Thermal characteristics of electron tubes. Magnetic core devices. The thermal characteristics of resistors. The thermal characteristics of capacitors. v MIL-HDBK-251 Page 449 454 455 460 461 467 469 469 469 509 509 509 51 O 51 O 51 1 51 3 51 3 51 3 51 3 533 534
23、 534 535 535 539 539 57 1 57 1 571 572 572 573 573 575 575 57 5 584 592 596 596 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-b MIL-HDBK-253 RE .W 7777770 0033387 b I-“ MIL-HDBK-251 Paragraph Title 17.7 Special parts. 18. DESIGN OF ELECTRONIC EQUIP
24、MENT FOR OPERATION AT 18.1 General. 18.2 Theoretical considerations. 18.3 High temperature materials. 18.4 Thermal design of high temperature parts. 18.5 Mounting, housing, connecting. 150 TO 35OOC ENVIRONMENTAL TEMPERATURE Figures 1. 2-4 5 -6- 28 29-81 82- 122 123-132 133- 160 161-183 184-210 * 211
25、-217 fl 218-229 FIGURES Chapter 5 6 7 8 9 10 11 12 13 15 17 18 TABLES Tab1 es I II I I I-VI I VI I I-XXI XXII-XXIX xxx-XXXI c XXXI I XXXI I I XXXIV xxxv-XXXVI XXXVII-XL Chapter 1 7 8 9 10 11 12 13 15 17 18 Page 597 603 603 603 61 4 61 9 625 vi Provided by IHSNot for ResaleNo reproduction or networki
26、ng permitted without license from IHS-,-,-WIL-HDBK-251 RE W 7777770 0033170 2 W MIL-HDBK-251 Appendix . A. B. C. D. E. F. G. H. I. J. APPENDICES Table of Symbols and Nomenclature Table of Conversion Factors Table of Constants Contact Resistance Data Thermal Conductivity Values of Commonly Used Mater
27、ial S Table of Emissivity Values Coolant Effectiveness Fluid Coolant Properties Orifice Calibration Curves Bibliography; List of References .“c. Page 631 638 645 646 655 660 664 665 684 689 vi i Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-5 MIL-H
28、DBK-251 RE W -7777770 0033191 4 W MIL-HDBK-251 1. SCOPE 1.1 Purpose. This handbook has been prepared specifically to guide engineers in the thermal design of electronic equipment with improved reliability. The primary purposes are: to permit engineers and design- ers, who are not heat transfer exper
29、ts, to design electronic equipment with adequate thermal performance with a minimum of effort; to assist heat transfer experts, who are not electronic experts; to aid engineers in better understanding the thermal sections of Department of Defense specifications and standards for equipment; and to as
30、sist Navy personnel in evaluating thermal design during the various stages of equipment pro- curement and development. 1.2 Scope. This handbook recommends and presents electronic parts stress analysis methods which lead to the selection of maximum safe temperatures for parts so that the ensuing ther
31、mal design is consistent with the required equipment reliability. These maximum parts temperature must be properly selected since they are the goals of the thermal design, a fact which is often overlooked. Many thermal designs are inadequate because improper maximum parts temperatures were selected
32、as design goals. Consequently, the necessary parts stress analysis procedures have been -emphasized. Specific step by step thermal design procedures are given in chapter 4. Proper operation at the desired performance and reliability levels can only be achieved if the electronic, thermal, and mechani
33、cal designs are all well executed and carefully integrated. Such a result can be accomplished best by the equipment designers, who must control all pertin- ent factors. It must be emphasized that the thermal design is fully as important as the circuit design. Poor communications, lack of funds, and
34、obsolete, incomplete, and unreliable data have hampered the development of proper thermal design. Methods of predicting the thermal performance of electronic equipment are becoming commonly known, but most organizations that have produced success- ful designs have achieved their goals by techniques
35、peculiar to a specific equipment design. ously published, such as NAVSHIPS 900, 192 (Design Manual of Natural Methods of Cooling Electronic Equipment), NAVSHIPS 900, 194 (Design Manual of Methods of Forced Air Cooling Electronic Equipment), and NAVSHIPS 900, 195 (Design Hanual of Methods of Liquid C
36、ooling Electronic Equipment). This handbook covers the thermal design of shipboard, ground based, airborne (avionic), and space electronics. Simplified methods of design calculation, including nomographs and curves, are included so that engin- eers without heat transfer background can design accepta
37、ble equipment. Alternatively, the proper mathematical expressions are included along with comments on computer analysis methods so that experts in heat transfer can utilize these sophisticated techniques. When specific recommendations are given, care must be taken that the adoption of such recommend
38、ations does not conflict with the requirements of the contracting activity. This handbook supersedes certain Navy thermal design manuals previ- 1 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-I l MIL-HDBK-251 RE m 7779770 0033272 b m. MIL-HDBK-251
39、Throughout this handbook, equations intended specifically for use in We wish to express our appreciation for the productive cooperation we design are labeled “D.,“ (Design Equation). have received during this program from government agencies and industry. Source materials are 1 i sted in the Referen
40、ces together with reference numbers at pertinent locations in the discussion. This handbook is not to be con- strued as an endorsement of any commercial products or techniques mentioned herein. 1.3 Need for adequate thermal performance. High temperature is a particu- larly insidious enemy of most el
41、ectronic parts because it causes slow progres- sive deterioration rather than catastrophic failure. The mean time to failure of each part is a statistical function of its stress level and the entire complex of thermal history and chemical structure. Some authorities consider that inadequate cooling
42、is presently the primary cause of poor re1 iability in mi 1 i tary el ectroni c equipment. It is the failure of individual parts that leads to equipment failure. Electronic parts are prone to premature failure due to overstress (i.e., thermal, electrical or mechanical stress). Electri.ca1 and therma
43、l stresses are closely interrelated and a reduction in electrical power dissipation correspondingly tends to ease the thermal stress. Examinatioh of MIL-HDBK-217 (Re1 iabil i ty Prediction of Electronic Equipment) shows that failure rates of typical parts vary significantly with temperature. The fol
44、lowing table presents a few extreme examples to indicate the effects: TABLE I. Failure Rates - Ab Failures Per Million Hours. Part Base Fail Ure Rate . . Description 1- .“_ PNP Si 1 icon .O63 at 130“ Transistors and 0.3 stress NPN Si 1 i con Transistors ; ,033 at 130“c and 0.3 stress G1 ass Capacito
45、rs .O47 at 120“c and 0.5 stress Transformers I and Coils MIL-T-27 1 .O267 at 85Oc Low Temperature ,0096 at 25“c and 0.3 stress “ .O064 at 25“c and 0.3 stress .O01 at 25“c and 0.5 stress Class Q I Resistors and 0.5 stress Carbon Comp .O065 at 1OO“c 2 ,0008 .at 25“c .O003 at 25“c and 0.5 stress T AT“
46、c 105 ._- 105 95 60 75 tatio of High ;o Low Failure 33: 1 22: 1 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-251 RE W 7797770 0033173 B W MIL-HDBK-251 This information shows that the certain components (capacitors, resistors, coils, and t
47、ransformers) are actually more temperature sensi- tive than transistors. Decreases in failure rates as great as those shown above are not always attainable, but very significant reductions can be and have been achieved by reduction of thermal stress (temperature) Cooling systems must be designed to
48、control parts temperatures to the de- sired levels under all anticipated thermal environments. Significant improvements in reliability and availability have been achieved by modifying the cooling systems in various existing shipboard and airborne electronic equipments. In several instances the relia
49、bility gains were as great as 500 percent in MTBF improvement. Even so the im- proved cooling systems were not of optimum thermal design; rather they could be better descri bed as salvage jobs. Further, these equipments were used equipments composed of parts with a previous history of severe thermal stress. If optimum thermal designs had been applied with new parts, it was estimated th
