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652 CHAPTER 61 1962 Guide And Data Book wick in order to prevent the evaporation at the wet bulb from raising the internal vapor pressure and relative humidity of the chamber. Multi-range instruments may be desirable in certain applications such as the temperature-humidity chamber which operates from --100 F to +500 F with hu midity control in the +35 to 180 F range. If tire dry-bulb con troller has the full 00 deg range, it will provide relatively poor control during a humidity test. Therefore, multi-range instruments are frequently used so that the range can be ex panded for the humidity section of the test (usually between 0 and 200 F). Many of the more elaborate instruments can be obtained with a variety of control forms to provide whatever action is required for the system. Auxiliary switches, actuated at any point in the range, can be provided to perform auxiliary functions such as draining and filling a humidifier pan near the freezing point, draining and filling a secondary refrigera tion coil at required temperatures, and performing many staging functions. In addition, many controllers can be equipped with programming systems with which predeter mined cycles can be set up for automatic operation without manual readjustment of the controller set point. These are particularly useful for many of the 24 hour humidity cycling tests which may be mo as long as 30 days. An altitude measurement and control system may consist of a simple manometer, for indicating pressure, and a hand throttling or bleed valve to control the level; or it may be a completely automatic system operated by a mechanical or electronic sensing device which operates modulating bleed controls to automatically regulate the vacuum. The Urtube manometer shows relative pressure between the site baromet ric pressure and the internal vacuum of the chamber. Ab solute vacuum measuring manometers are recommended for altitude chambers to avoid the need for correction due to local barometric pressure variation. Opposed-bellows absolute pressure controllers and elec trical instruments using strain gage pressure transducers with bridge circuit electronic controllers are available. These in struments are suitable for altitudes up to 200,000 ft (0.5 mm Hg). They provide on-off control or proportioning output signals and can be used to control various functions of the vacuum system. Electronic vacuum-sensing devices measure the change in interference across an air gap between two electronic devices due to the reduction in gas molecules in the space. Coupled with amplifiers, this effect is used to initiate action in control circuits at a set point. One system uses an alpha emitter and measures the amount of energy reaching the grid to determine the vacuum level. Another uses ionisation of a filament. A thermocouple is used to measure the change in heat exchange rate as a function of vacuum. Most electronic vacuum instruments are available with millivolt output which may be coupled to a standard potentiometer controlling device to provide a record and an output signal for controlling the vacuum system. The most common way of automatically controlling a vacuum system is to bleed air into the chamber through a modulating valve. This merely floods the pump to the de sired capacity so that good control can be maintained with the proper size bleed valve. Frequently two or more valves are required to control the pressure in the chamber over a wide range. Altitude chambers can be controlled by cycling a solenoid valve in the pump suction line or amply turning the pump on and off. The latter system is not recommended because of the excessive strain on the motor and the drive pwniumwTn. These two methods are not practical for diffusion pumping systems. ^ Accessories In order to provide satisfactory operation, a control system must include other accessories betides sensing elements and controllers. A pneumatic system should include a pressure reducing valve, a filter-dryer, and a surge tank which are standard with such systems. Pressure switches, operated by the control air-pressure signal, may be used to switch off banks of beaters as progress is made toward the control point and thereby provide step modulation near the control point. Thus one controller can modulate the flow of brine to a cooling coil and also provide on-off control of electric heaters to decrease the number of heaters in circuits or modulate a vacuum bleed valve, or also switch pumps in and out of tire system. Pressure switches are also used on time-pattero transmitter and pneumatic controller combinations. The transmitted air pressure is proportional to the value of the variable set on the controller the action of these pressure switches can put into operation the equipment required to achieve this value. For pimple, the object may be the selection of either one- or two-stage operation of the refrigeration system depending on how low a temperature is called for by the controller. The use of other accessories will occur to the designer. Safety, and other interlock relays and switches, should be carefully considered not only to provide safe operation but to control the system and reduce the number of switch set tings that are to be made by the operator. Vacuum switches are frequently used to cut out the heaters or switch them to series wiring whenever an altitude chamber is under vacuum, thus preventing burnout due to low convection transfer. Damage of test parts from exposure to too high (or too low) a temperature is an important consideration. Test pack ages that are placed in environmental chambers frequently have a very high monetary value; a malfunction of the control system may represent considerable loss. It is there fore becoming common to require an adjustable independent safety device to prevent over-heating in case of malfunction of tiie basic temperature control system. In addition to the conventional control systems for tem perature, humidity and vacuum, the designer may encounter requirements for measuring or controlling many other vari ables. These may include hydrogen ion concentration (pH); fluid velocity and pressure, dust and smoke density; explosive fuel and air mixtures; ozone, oxygen, and carbon dioxide levels; and others. DESIGN CALCULATIONS Cooling Loads Steady state cooling loads are rAlr.nla.tAH in a manner sim ilar to normal low temperature walk-in freezers. Loads must be accurately determined because refrigeration system capaci ties are relatively low within the temperature ranges of en vironmental test chambers. When internal live loads are specified at several operating temperature levels, the load for each set.of conditions must be calculated to determine which requires, the larger cooling system. Transmission heat gains include transfer through insula tion, framing, windows, sleeves and penetrations, and fan shafts or other through conducting materials. Internal live heat loads include lighting, product electrical loads, flow to and horn the workspace of fluids warmer than the test space Environmental Test Equipment 653 temperature, etc. A safe approach is to include as load the heat equivalent of the total ma-Timnm kilowatt input to eU electrical equipment including lights, motors,' and other electrical apparatus. The heat equivalent of the fan brake horsepower input should be used including addition for in ternal bearings and correction for the higher density of the cold airIf personnel are inside the chamber at low temperatures, the higher metabolism rate and the electrical input to protec tive clothing, when used, should be taken into account. The load due to door openings or other infiltration can be calculated by assuming conservative air changes and adding the difference in specific and latent heat content between ambient and inside air. Systems utilizing pumps or secondary refrigerants intro duce additional load such as the full electrical input to pump motors, piping insulation transmission gains, conduction through shafts, etc. In tire calculation of transient condition cooling loads,.all steady state heat loads are part of the load. For small cham bers, wherein the transmission heat gain is a small portion of the total load, its maximum value may be used as the as sumed load. For larger chambers, the Schmidt method as applied to mechanical refrigeration systems by Threlkeld aiul Kusuda* is recommended. A commonly used approach is to estimate the thermal mass of all items which will change temperature as the cham ber temperature is pulled up or down and predict the rate of temperature change of these items. The product of time rate of temperature change of these items and their thermal mass will give the instantaneous load due to temperature change at the point under consideration. The total load includes these dead loads, the transmission, and the live loads. The items for which the thermal mass must be calculated may include chamber air, interior liner and bracing, insulation and framing evaporators, beat exchangers, heaters, blowers, brackets, shelves, or other internal gear as well as the test specimen. Under transient temperature conditions there will be a gradient between the room ambient and the heat source or heat sink. For smaller chambers, conservative design will re sult if. it is assumed that all of the items in direct contact with the chamber air change at the same rate as the air. For larger chambers changing at a fairly rapid rate, the required capacity of the conditioning equipment may be reduced by taking into account the slower temperature change of those items more thermally remote from the heat source or mnlr. In order to simplify the calculations, it is possible to use some rule-of-thumb values from performance data of typical chambers. Such empirical data may be used to arrive at a pull down or pull up factor which remains reasonably con stant for temperature changes within certain limits and for conditioning systems of a fairly constant capacity. Heating Loads Steady state heating loads are calculated in a manner sim ilar to those for cooling adjusted for the uniform capacity of dectnc heaters. Internal live loads may be neglected or credited in the computation. Transient heating may be ap proached in the sam* manner as transient cooling-but using uniform capacity as a function of temperature. Relative Humidity Equipment selection for relative humidity control is similar to normal air conditioning applications And yet more oom- plex in that widely varied control conditions must be pro duced. A typical control range is 35 to 185 F dry-bulb tem perature and from 20 to 98 percent relative humidity above 35 F dew point. Very low dew points may be required for special test programs. Sensible cooling loads may be calcu lated in a similar manner to low temperature loads, but must also account for additional factors such as the sensible heat of steam when used for humidification. Since dehumidi fication is normally accomplished by mechanically cooling and reheating the air, the most common approach is to take a portion of the total recirculating air, cool it to the required dew point, then reheat as required to control at the desired dry-bulb temperature level. If the total air flow were cooled and then reheated, rather large dehumidification and reheat capacities would result. When long term tests are required at controlled dew points near freezing, heat transfer cooling surfaces must be amply sized to operate without freezing. Generally, finned cooling coils at least ten rows deep and four fins per inch are indicated as well as means to maintain refrigerant evaporating levels to prevent icing. Care should be taken in calculating the amount of dehumidification coil surface required. Where rather large internal heat gains are present, and when it is desirable to control at high humidities, spray contactors producing adiabatic saturation are desirable. By controlling the temperature of the recirculating spray, such systems may be used for simultaneous cooling and humidifica tion. These systems can easily humidity at elevated tempera tures with very little heat added to the water because there is rapid evaporation due to great vapor pressure differences. The sizing of heaters for increasing dry-bulb temperature is the same as for normal chamber heating. For humidification in systems not employing adiabatic saturators, either atmos pheric pressure vapor generators or direct steam injection-is used. The amount of water which must be added to achieve the desired increase in absolute humidity for the known volume of air within the chamber determines the rate of moisture addition. Both of these methods add sensible heat as well as latent heat, which must be added to the cooling load to obtain the total load. CHAMBER CONSTRUCTION Environmental testing chambers for high and low tempera ture and humidity work are most commonly constructed of type 302 or 304 stainless steel inside and of mild steel out side to provide a vapor tight structure for the insulation. Vapor pervasion of the insulation will cause considerable loss of thermal efficiency and also cause corrosion of any mild steel structure. Aluminum and metallized mild steel interior liners are limited, in their high temperature range and have not proved satisfactory for a long-range investment. Aluminum becomes embrittled from the extreme temperature cycling. Sprayed-on coatings provide only temporary protection for the mild steel. The structure must be designed to provide for the con siderable expansion and contraction that takes place during wide range temperature cycling. Doors are a particularly difficult problem. Any fan baffles, coil mountings, or other accessories must be carefully planned so that interference does not occur with moving parts during temperature cycling runs. Shock loads induced by liquid nitrogen and other re frigerants can cause violent stress concentrations. Humidity chambers must be designed so that any condensate formed on the coils, walls, or floor will be rapidly drained from the chamber. This is particularly important in programmed