1、_SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising theref
2、rom, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised, or cancelled. SAE invites your written comments and suggestions. Copyright 2010 SAE International All rights reserved. No part of this publication ma
3、y be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: +1 724-776-4970 (outside US
4、A) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.orgSAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/ARP987BAEROSPACERECOMMENDEDPRACTICEARP987 REV. B Issued 1970-03 Revised 2010-06Superseding ARP9
5、87A (R) The Control of Excess Humidity in Avionics Cooling RATIONALEThis revision of this document updates it with more recent information including new references to SAE Standards recently published, and additional avionics cooling and moisture control methods. TABLE OF CONTENTS 1. SCOPE 21.1 Purpo
6、se . 21.2 Field of Application 22. REFERENCES 22.1 Applicable Documents 22.2 Related Publications . 33. SUMMARY OF THE PROBLEM . 43.1 The Effects of High Humidity and Moisture 43.2 Sources of Moisture 44. NATURAL AND INDUCED ENVIRONMENTS . 54.1 On-Aircraft Storage . 54.2 Proposed Design Data for the
7、 Natural Environment 54.3 Induced Humid Environments in Service 75. ENVIRONMENT CONTROL IN “OPEN” EQUIPMENT 125.1 Improving Existing Systems 125.2 New Design . 176. NOTES 216.1 Revision Indicator 21FIGURE 1 SEA LEVEL ATMOSPHERIC HUMIDITY (FROM AFCRL-TR-74-0603) 6FIGURE 2 AVIONICS COOLING WITH VAPOR
8、CYCLE REFRIGERATION (WITHOUT REHEAT)- SI 8FIGURE 3 AVIONICS COOLING WITH VAPOR CYCLE REFRIGERATION (WITHOUT REHEAT)- USCS . 9FIGURE 4 AVIONICS COOLING WITH AIR CYCLE REGRIGERATION (WITHOUT REHEAT)- SI 10FIGURE 5 AVIONICS COOLING WITH AIR CYCLE REGRIGERATION (WITHOUT REHEAT)- USCS . 11FIGURE 6 AVIONI
9、CS COOLING WITH AIR CYCLE REFRIGERATION (WITH REHEAT)- SI 13FIGURE 7 AVIONICS COOLING WITH AIR CYCLE REFRIGERATION (WITH REHEAT)- USCS . 14FIGURE 8 AVIONICS COOLING WITH AIR CYCLE REFRIGERATION but after being heated by the thermal loads in the cabin, the air is delivered in a relatively low relativ
10、e humidity condition to the avionics bay. Moisture production due to night and day temperature cycles in aircraft parked out-of-doors remains a problem in these systems if the equipment is not adequately isolated or insulated from the exterior surfaces of the aircraft. The cabin exhaust air also cre
11、ates a more severe dirt and dust problem than would exist in a directly cooled system. This is a result of the presence of aerosols from smoking and of dust and lint from clothing and carpets. 5. ENVIRONMENT CONTROL IN “OPEN” EQUIPMENT Moisture resistant coatings and reduction of the relative humidi
12、ty of the cooling air will eliminate malfunctions of the avionics equipment due to exposure to moisture. For the former, non-flammable sprays that penetrate and seal out moisture are available. In addition, encapsulants can also be used that embed electronic circuits to isolate circuits from the har
13、mful effects of moisture and other contaminants and also from thermal and mechanical stresses. Encapsulants are typically applied in thick layers exceeding 3.2 mm (125 mils). To reduce the relative humidity and presence of free moisture in the cooling air, systems can be designed or corrected by app
14、lying the techniques noted in the following paragraphs: 5.1 Improving Existing Systems The recommendations made in this section are intended as “minimum change” modifications to control humidity in existing systems for improved avionics life and reliability. See 5.2 for new design. 5.1.1 Air Cycle R
15、efrigeration Systems 5.1.1.1 Adding Reheat to System with a Low Pressure Water Separator A reheat system can be installed in an existing air cycle system to maintain the air at a temperature above the maximum dew point specified for the application. This temperature can range from 1.4 C to 32 C (35
16、F to 90 F), depending on where the aircraft is to be operated, the type of avionics and degree of moisture protection, whether or not a water separator is installed, and aircraft system limitations. To conserve engine bleed air, a variable temperature control system which takes advantage of the low
17、humidity at altitude is commonly used. This would employ a high supply temperature setting for low altitude where humidity is high, and a low temperature setting at high altitudes where the ambient air is very dry. The F/A-18C/D, for example, supplies cooling air at 4.4 C (40 F) from sea level to 76
18、20 m (25,000 ft), varies it linearly from 4.4 C (40 F) to -18 C (0 F) between 7620 m (25,000 ft) and 12 950 m (42,500 ft), and remains constant at -18 C (0 F) above 12 950 m (42,500 ft). Commercial electronics are qualified to short term cold supply air temperatures of -40 C (-40 F) and steady state
19、 cold supply air of -15 C (+5 F). But throughout their lifetimes, exposure to extreme cold temperatures is usually limited, hence enabling high reliability. Use of consistently cold supply air temperatures, per military equipment cooling schemes, would result in abbreviated life expectancies due to
20、thermal fatigue, as well as potential humidity issues. Reheat may be furnished by a controlled bypass of engine bleed air (see Figure 6). The increase in total delivered airflow, which may be required to offset the increase in cooling air supply temperature, can be provided by the bypass air itself
21、if its temperature isnt too high. The selection of the point in the system from which to tap the bypass air (e.g., upstream or downstream of a heat exchanger), can be made after determining the flow increase required to satisfy avionics cooling with warmer air. It should also be noted that the great
22、er the quantity of bypass used, the higher will be the absolute humidity of the mixture delivered to the avionics, since the bypass usually has not been subjected to moisture removal. Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permit
23、ted without license from IHS-,-,-SAE ARP987B Page 13 of 2100.0050.010.0150.020.0250.030.0350.040 5 10 15 20 25 30 35 40 45 50 55Dry Bulb Temp- CHumidityRatio-kg/kg DryAir100%90%80%102030Moisture Condensed,0.0090 kg/kgMoisture Removed,0.0072 kg/kgMoisture Added,0.0014 kg/kgNet Moisture Removed,0.0058
24、 kg/kgControlled ReheatHeat Absorbed from Avionics, 1 kW405060708090100110 Specific Enthalpy, kJ/kg120130140Relative Humidity12435(49.5C)39% RHPsychrometric ChartFor Sea Level FIGURE 6 AVIONICS COOLING WITH AIR CYCLE REFRIGERATION (WITH REHEAT)- SI AmbientCopyright SAE International Provided by IHS
25、under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE ARP987B Page 14 of 210.0000.0050.0100.0150.0200.0250.03030 40 50 60 70 80 90 100 110 120 130Dry Bulb Temp- FHumidityRatio- lb/lbDryAir1(121F)Relative Humidity100%290%80%345Moisture Condensed,
26、 0.0090 lb/lbHeat Absorbed from Avionics, (3414 Btu/h)Specific Enthalpy, Btu/lb152520303540455055Psychrometric ChartFor Sea LevelMoisture Removed, 0.0072 lb/lbControlled ReheatMoisture Added, 0.0014 lb/lb 39% RHFIGURE 7 AVIONICS COOLING WITH AIR CYCLE REFRIGERATION (WITH REHEAT)- USCS In the example
27、 shown in Figure 6 and Figure 7, 2.07 kg/min (4.56 lb/min) of air is cooled (as in the example of 4.3), to 4.7 C (40 F) which produces 9.0 g/kg (0.009 lb/lb) of entrained moisture (fog), and the water separator removes 80% or 7.2 g/kg (0.0072 lb/lb) of the fog. Controlled reheat with air at 132 C (2
28、70 F) raises the cooling air temperature to 27 C (80 F) and increases the moisture content by 1.43 g/kg (0.00143 lb/lb), as shown at point (4), and increases the delivered air flow 22%. The relative humidity of the delivered air is 39% at point (4), and the temperature rise required to absorb 1 kW (
29、3414 Btu/h) from the avionics is less than the example of 4.3 as a result of the increased airflow. 5.1.1.2 High Pressure Water Separation The high-pressure water separator (HPWS) system has replaced the low-pressure system for most air cycle environmental control systems designed since the 1980s. H
30、PWS advantages of elimination of periodic maintenance and improved water removal efficiency more than offset the disadvantages of higher initial cost and weight. A HPWS can deliver dry air at, near, or below freezing temperatures since the water condensation occurs at high pressure upstream of the t
31、urbine. The HPWS typically consists of reheater and condenser heat exchangers, a water extractor, water spray nozzle and a temperature control system as shown in Figure 8 with example state points. The warm side of the reheater further cools the high-pressure air from the secondary heat exchanger. W
32、ater vapor in the air from the reheater is condensed on the cold metal walls of the warm side of the condenser. The liquid water is in the form of droplets larger than those that occur with a low-pressure water separator since the liquid condenses on a solid object. Furthermore, since the condensati
33、on occurs at temperatures above freezing, the water extractor can be located further downstream, allowing droplets to coalesce in the connecting duct. The water extractor uses inertial or centrifugal separation to remove 90 to 95% of the liquid water in the air stream. The air and any remaining liqu
34、id water are warmed in the reheater to increase cycle efficiency and evaporate the remaining liquid water prior to the turbine inlet. The cold air from the turbine exit cools the condenser prior to delivery to the cabin or avionics. Temperature or delta P sensors are used to detect icing in the cond
35、enser and control condenser cold side inlet temperature by varying the addition of warm air to the turbine outlet air. Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE ARP987B Page 15 of 21FIGURE
36、8 AVIONICS COOLING WITH AIR CYCLE REFRIGERATION & REHEAT WITH HIGH PRESSURE WATER SEPARATION 5.1.1.3 Adding Reheat to System without a Water Separator If the cooling system does not have a water separator and if there is not sufficient space in the aircraft to install one, a significant improvement
37、is obtainable by installing reheat provisions alone. The liquid moisture can be eliminated during most operations, even though the relative humidity will remain high in hot-humid climates. This modification requires more bypass air because no water is removed and because all of the water droplets mu
38、st be re-evaporated. For example, if the bypass air temperature is 132 C (270 F) and if the cold flow from the turbine in Figure 4 example remains unchanged, an airflow increase of 38% is necessary to evaporate the entrained moisture. Thus the temperature rise to remove the 1 kW (3414 Btu/h) of avio
39、nic heat is less than the previous examples. However, the avionic equipment must be capable of tolerating the higher cooling air temperature and resultant higher operating temperature. Forced air cooled electronics equipment on most commercial airplanes are qualified to operate steady state with air
40、 supplied between -15 C (+5 F) and 55 C (131 F). Predicted reliability, however, is based upon nominal supply air temperatures between 30 C (86 F) and 40 C (104 F). Hence, to provide this reliability, the supply air temperature should be regulated to between 30 C (86 F) and 40 C (104 F). 5.1.1.4 Des
41、iccant Wheel Desiccants have not been widely used for moisture removal in aircraft, except for very select applications. One drawback is the requirement to restore the bed after it reaches saturation. Desiccant wheels have solved that issue by alternately passing air being dehumidified and air used
42、to dry out (regenerate) the bed (Figure 9). This results in a constant regeneration of the bed and eliminates the need for a maintainer to restore the bed. Desiccant wheels are typically applied to drying cabin air but could be considered for drying avionics cooling air. The desiccant not only remov
43、es moisture, but it also heats the air due to the activation energy released. This increase in temperature will further raise the cooling air temperature above the dew point, providing additional protection against potential moisture issues. Since this temperature increase will reduce the cooling ef
44、fect on the equipment, it must be taken into account. As shown in Figure 9, a small regenerative flow is required to restore the desiccant. Since the regenerative flow must be warm, the exhaust from the avionics could potentially be used. If the avionics exhaust is insufficient or impractical to rec
45、over, a separate heated air source will be required. The vendors of the desiccant wheel should be consulted for the regenerative flow requirement, since it will vary with the configuration and the type of desiccant used. Copyright SAE International Provided by IHS under license with SAENot for Resal
46、eNo reproduction or networking permitted without license from IHS-,-,-SAE ARP987B Page 16 of 21Dried, Slightly Heated Air to Avionics Cool, Moist Inlet AirWarm, Regenerative AirMoist, Warm Air Exhausted FIGURE 9 DESICCANT WHEEL The reliability requirement for a system on a commercial airplane is det
47、ermined both by the direct operating cost goals and by the affects on safety of its failure. 14 CFR/CS 25.1309 and associated advisory material provide the procedures to determine safety related cooling system and component reliability requirements. If an electronics equipment cooling system is an e
48、ssential system, then the failure rate should be on the order of 1E-7 per flight hour. If the failure effect of adesiccant wheel results in loss of effective cooling, then its frequency of occurrence must be less than the 1E-7 per flight hour. To meet the required reliability, a bypass might be required. Installation issues with additional fans, valve, and ducting would need to be considered. 5.1.1.5 Vapor Cycle Refrigeration Systems Reheating the cooling air to a temperature above the am
