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654 CHAPTER 61 1962 Guide And Data Book humidity chambers in which operation ranges from high to low humidity conditions. Altitude Chambers Altitude chambers require the strength to resist an external pressure of 14.7 psi (2,120 psf) at all temperatures encoun tered when the stressed shell is the inner liner. It is especially important to take into consideration the embrittlement of ferrous materials at low temperatures. The inner liner mass should be held to a minimum when rapid temperature changes are required, due to its influence on conditioning equipment size. If the exterior is stressed to withstand the vacuum load, the finer must have pressure relief to the insulation. If the chamber is equipped for high humidity, means must be pro vided to drain and dry the insulation. Another possibility is to use a structural material for the insulation and to maintain the inter-wall space at a fairly high vacuum by connecting it permanently to the main, or an auxiliary, vacuum pump. Considerable reduction of heat flow will result if a high vacuum is used. When the chamber is used at sea level pres1 sure, the structural insulation holds the walls apart; when at altitude, approximately equal pressures exist on both sides of the relatively thin liner. The welding of vacuum chamber seams and connections is particularly critical. The welds must be structurally strong enough to protect against the stresses imposed by expansion and contraction, and yet not produce virtual leak conditions. A virtual leak is a condition which traps air in such a way (as between two welds on a slip on flange) that it may leak into the vacuum e.humhp.r without detection from the outside. Good space chamber design call* for continuous polished welds on the inriHft nnH skip welding for reinforcing on the outside of the vacuum structure. checking must be performed on all parts of the structure. Dye checking and halogen l^alr detection with an electronic detector are considered rough checks for high vacuum system. A mass spectrometer using helium must be used for final checking of a system which is to operate in the 1G~* mm Hg or higher altitude range. Gaskets Gasketing of environmental chambers presents special problems. Inadequate gasket seals on a low temperature chamber can produce overloading of the refrigeration system. It may not be possible to maintain the required control tolerance in any system having a small door seal leak. Vac uum systems have particularly critical gasket problems. Gaskets on low temperature chambers can be protected by heaters against low temperature embrittlement. Gasket sealing depends upon resilience to fill the surface ir regularities and resistance to permanent deformation in order to maintain ability to reseal. Natural rubber has these char acteristics in a limited temperature range but suffers from ozone cracking. Neoprene is the most commonly used syn thetic gasket material because of its resistance to aging and ability to remain resilient from --65 to +400 F. The Viton elastomers offer higher temperature performance (up to 600 F) but, due to cost, are used principally for high vacuum systems. The silicons are suitable for --100 F to +250 F but tend to deform and take a set. Teflon and Kel-F are suitable for cryogenic temperature service, but not for elevated tem peratures. They lack the resistance to deformation charac teristic of rubber. Vacuum systems require carefully designed O-ring type seals using butyl, neoprene, Viton, or other suitable elas tomer. O-ring grooves and flange faces should be precisian machined to produce best results. Permeability, outgassing, and temperature resistance of the O-ring material must be carefully evaluated. Insulation Many types of blanket or batt, loose fill, board or reflective insulations have been used singly or in combination. Factors influencing selection are thermal conductivity, specific heat, density, permeability, cost, moisture effects, vibration re sistance, gw. of handling, temperature suitability and strength. The factor thermal diffusivity, a function of con ductivity, density, and specific heat, is quite important when rapid temperature changes are anticipated. Low values com bined with low thermal mass will permit more rapid tem perature changes for the same system capacity. Many inatla, tions used for commercial or industrial applications are suit able for ultra-low temperatures if they are vapor sealed. Glass fiber is in common use due to low cost, relatively low conductivity, and low thermal diffusivity. It must be pro vided with a vapor seaL Its useful range extends up to 1000 F when no binder is present. Mineral wool blankets are not so frequently used because of their high density and slightly highpr conductivity. Silica aerogel is suitable from below --300 F up to 1200 F. It has a thermal conductivity below 0.15. It settles severely which may require later addition of material. It is permanently damaged by moisture, hence tight vapor sealing is required. Foamed polyurethanes have thermal conductivity factors of about0.14 mid0.22depending upon the foaming agent and' density. Refrigerant 11 gives the lower value and CO* the higher one. Densities from less than 1.0 to about 5 lb per cu ft are possible. Lowest conductivities occur in the 2 to 3 lb per cu ft density range. The material is frequently foamed in place. Expanded polystyrene is a material with a conduc tivity factor of 0.23 to 0.27 and a density of 2 lb per cu ft. Its upper operating temperature is 160 to 170 F, whereas the urethanes are suitable up to 240 to 250 F. Expanded rubber has characteristics similar to the styrenes, except for higher density and greater compressive strength. Reflective insulation, in the form of coated steel, aluminum, or sometimes stainless steel foil has been used, particularly in large chambers. For temperatures above 500 to GOO F, the combination of several materials may be best. High density, fine fiber, batt type materials show (1) lower conductivity than low density materials in the higher ranges, (2) resistance temperature shock, and (3) have a lower thermal mass than block type. This type of material can be combined with more standard glas fiber batts (or equal) toward the exterior. Since the temperature differences are high, reflective foils in combina tion should also be considered. The optimum insulation thickness will generally follow the economic and surface condensation practices followed for larger structures. However in enclosures smaller than a walk- in size, the thickness is frequently less. For instance, a 1 cu ft chamber for --100 F may have as little as 4 in. of glas fiber insulation. The insulation thickness may be based somewhat upon the effect of the sleeves which must be provided for the refrig erant lines, wiring, temperature leads and test accessories. These sleeves are attached by some vapor tight means to the shell and liner. For non-altitude chambers, tubes of micarta or similar phenolic materials may be held in place Environmental Test Equipment by vapor sealed bushings on each ride of the walL Thin stain less gteel tubes, welded at the ends to the shell and liner are frequently used, and will stand more abuse. Sleeves for alti tude chambers should be of stainless steel pipe or tube, welded to liner and shell, with capped ends. The effect of sleeves or other through-conductances is surprisingly large. Transfer should be carefully calculated, particularly when walls are thin. Doors Both overlap and plug-type doors are used, tire latter most frequently on high-humidity chambers because of internal condensation, or on high temperature chambers to prevent or rp;n`mim w&rpage. They must be nigged and well fitted particularly for altitude chambers. Vapor sealing with multi ple gaskets is important since defrosting mpnq are infre quently provided, particularly on smaller chambers. Suitable gasket materials are natural or synthetic rubber or silicone materials. Plastics or light gage stainless steel are recom mended for the thermal breaker strip; pressed wood hardboard type materials generally have insufficient resistance to phys ical damage, high temperatures or high humidities. Low wattage heater cables installed under the breaker strip pre vent larger doors from freezing closed. Safety measures re quired for commercial walk-in freezers are a must for low temperature rooms, with doors easily opened from inside. For altitude chambers, the door assembly must register against a gasket winch will easily seal for tire vacuum. Pat ented gaskets used have a thin lip for initiating the seal and a main rectangular section. Vacuum gaskets must be property retained to prevent their being drawn into the chamber. Hardware for altitude chamber doors must allow for double articulation so the door and vacuum gasket will mate per fectly, and so the door may move as the vacuum pulls the door inward on the gasket. Windows For temperatures from --300 to +600 F, special hermet ically sealed multilight window assemblies are manufac tured. They are suitable for wide range humidity tests and, if provided with an inner pane-of sufficient strength, for alti tude tests as well. Tempered plate is required for the innwr panes in order that temperature shocks will not cause failure. Due to the pressures that will build up, sealed assemblies for higher temperatures require careful design. For temperatures up to 1000 F, windows of vicor glass are required and hermetic sealing is difficult, but work along these lines indicates a possi ble range from --300 to +1000 F. The number of panes and design for a given duty are usually specified by the window manufacturer. A typical --100 F to +300 F window contains 6 lights of glass enclosing dry gas spaces. Accessories The following comments apply to miscellaneous accessories used with test chambers. Interior lighting will nearly always be incandescent, in vapor tight and sometimes explosion-proof fixtures. Ample illumination %jU usually be given by 6 to 10 watts per sq ft of floor space, lighting is sometimes placed outside a window to reduce internal heat load. Lamps are not recommended for use inside chambers tixrve GOO F. Power leads may handle normal alternating current or special mrcraft voltages and frequencies. . Thermocouples in almost any number may be called for. Bare iron-constantan preferably should not be used in a high-humidity chamber. For the operator's convenience, thermocouples are fre quently connected to plup and outlets. 655 Tuning shafts, if small manually-operated, may be coupled to adjustable devices on the item being tested. These shafts should be stainless steel tubes. O-rings on the outside of the chamber may be used for pressure piling Power shafts provided for driving rotating equipment, must have vacuum-tight shaft seals for vacuum chambers. Sleeves and plugs are required for general purpose testing. Holes of various sizes may be specified. On altitude chambers, threaded or flanged caps must be provided. Special lines for pneumatically or hydraulically operated equipment may require pressure pipes with special connections at the ends. Protective panels are used to prevent the possibility of damage to the chamber if an internal explosion or sudden release of pressure occurs. Reach-in ports, with or without gauntlets, may be used for minor work within a test space without opening of doors or disturbing internal conditions. MECHANICAL REFRIGERATION SYSTEMS Most test chambers are equipped to produce temperatures lower than --20 F, hence multistage cascade and compoundcompression mechanical systems are employed. With proper refrigerant selection for each circuit, cascade systems require less horsepower and less displacement per ton at evaporating refrigerant temperatures below about -- 60 F. They are essen tially two or more single-stage systems linked together by a' cascade condenser or interstage heat exchanger. Compound systems were used for all test chambers up to about 1950. They now are primarily used for large field assembled systems or for requirements of --60 F evaporating refrigerant tempera ture and above. Figs. 1 and 2 are schematic piping diagrams for cascade and compound systems. Fig. 3 shows approximate relative capacities for systems of the same horsepower. The cost of a cascade system for evaporating refrigerant temperatures be low about --80 F is frequently more per horsepower, but less per ton in m*H and moderate sized systems. Cascade systems are generally smaller in size because of the lower compressor displacements required. Reciprocating compressors are generally used for the high stage of both compound and cascade systems, and for the low stage of cascade and smaller compound systems. Rotary' compressors find widespread use in the lower stages of large compound systems. Centrifugal compressors are most eco nomically employed on quite large compound systems Refrigerants 12 or 22, and have recently found use in large cascade systems. Standard compressors are suitable for cascade systems and Fig. 1 .... Typical Cascade Schematic Piping