Document 3N3R335pw94OQKXDabxKDdVY0

1112 CHAPTER 44 19S8 Guide 200 fpm for continuous exposure. With a high work level and intermittent exposure (relief stations), velocities, of 1000 to 2000 fpm have been used successfully. Great care must be exercised in using such high velocities, however, to avoid the undesirable effects of air at high temperature. Ordinarily the temperature of the air supplied to the worker for convec tive cooling should be about 80 F. Evaporative cooling ventilation sys tems will usually provide this. Preferably, the air should be directed to the front and torso of the body. Impingement on the back of the head, neck and shoulders should be minimized. . The supplied air may require temper ing in winter to raise it to an acceptable temperature. However, in areas of high heat load, the temperature of the air supplied during the winter may have to be lower than room temperature to provide adequate relief for the worker. ,. People vary considerably in their tolerance to air motion, temperature and humidity, and this tolerance varies with the season. It is essential, therefore, that the air supply outlets for most local relief ventilation sys tems be adjustable in direction, and permit reduction in outlet velocity. A level of air motion which feels comfortable and refreshing in hot weather may feel disagreeable and drafty in the winter. Types arid Design Requirements of Systems The supply air can be provided by local man-cooling fans; by outdoor air introduced directly or after dehumidification or cooling (evaporative or mechanical); and by combinations of outdoor and recirculated air either direct or after conditioning, as required. Local man-cooling fans should be used with caution other than in light heat-load areas where the ambient temperature is below the skin tempera ture. In particular, where there is an elevated ambient temperature with or without high radiation load, the high velocity may add consider ably to the convective heat load and thus seriously increase the demand for sweating and evaporative cooling. A relief system employing outdoor air is to be preferred over man-coolers and will provide excellent relief in many industrial areas. However, when the outdoor air temperature exceeds the skin temperature, the direct supply of outdoor air is obviously reduced in effectiveness. Such a system is properly used only in geographical areas where the periods of hot weather are of short duration. In geographic areas where outdoor humidities do not exceed 50 percent relative humidity at high daytime temperatures an evaporative cooling system offers greater rehef for the worker in that the discharge air tem perature can be lowered (within five degrees of the wet-bulb temperature) to obtain adequate convective body cooling. The sensible heat gain by the discharge air (through mixing) in a properly designed system will he sufficient to reduce the relative humidity (and vapor pressure) of the supply air and thus permit the necessary evaporative loss from the workers skin. Generally the supply of evaporatively cooled air should not exceed 20 to 50 percent of the total ventilation through the building. So limited, the relative humidity will be kept low enough to avoid distress to the work ers and rusting of equipment under most weather conditions in .the United States. For information on the design of evaporative cooling systems, see References 5 and 6. An air supply, with mechanical refrigeration offers the. greatest rebel. Such systems are expensive in first cost and this has retarded their us?-, However, air supply with mechanical refrigeration is finding greatly in' creased use in precision work and testing areas, in areas requiring con-|> Control of the Industrial Environment 1113 stant ambient conditions for product uniformity or control, and where the increased efficiency of the worker is reflected in a reasonable return on the investment. It is essential that the outdoor air, (with or without cooling) be brought as directly as feasible to the proximity of the work station. Impingement upon hot equipment or mixing with hot ambient air should be minimized. Supply ducts passing through hot areas should be insulated. The use of aluminum ducts will minimize heat gain from radiant sources. For large' -work areas provided with a high rate of ventilation the local relief can best be obtained by locating the outlets as close to the floor as possible, consistent with the work and structural interferences. Design of Outlets for Local Relief In the design of outlets for local relief it is of great importance to con sider the following: location, discharge velocity, discharge volume, and control of direction. The influence of these items will be discussed in the paragraphs which follow. Location. The outlets should be kept as close to the worker as possible in order to minimize mixing with the warmer air in the space. In most areas the outlets can and should be brought down to the seven-foot level. Outlets at floor level can be used with success in many cases. Discharge Velocity. The discharge velocity may be as high as necessary to obtain the desired velocity at the worker, as outlined above. Outlet velocities of 3000 to 4000 fpm may be necessary for remotely located out lets. Velocities of 1000 to 2000 fpm are the most frequently used for low outlets (at the seven-foot level). When the supply air is cooled, the velocity through an outlet directly at or over the worker must be kept low (around 50 fpm). It should be kept in mind that these recommended velocities are for conditions of maximum heat load. For more moderate weather and ambient conditions the workers will desire to reduce the velocities. Outlet dampers for velocity'control (in the direction of the worker) should always be provided. The control must be designed so as not to reduce the ventila tion for the space. Discharge Volume. The outlet volume required will vary widely, de pending upon whether the system is designed to provide highly localized spot cooling or is to provide general ventilation throughout a sizeable work aref' Generally 1500 to 2000 cfm per station will be adequate for mod erate loads, and 3000 cfm per station for higher loads such as at hot metal tumace stations. With remote outlets large air volumes are required to msure adequate relief because of the mixing of the supply air with the warmer surrounding air through which it is projected. The air stream from a large outlet will maintain an appreciable core of air at the original supply a*H lnperature fr a considerable distance from the outlet. Small outlets na slot outlets have small cores which are rapidly dissipated through in- ction. In small enclosures or semi-enclosures (shields against radiant at, for instance) perforated panel supply outlets are very effective for ecnamcally cooled installations, because of their low induction charac- arwf ?or farther design information on outlet design and the throw induction characteristics, see Chapter 30. necesT^ ^ Section. With very few exceptions, directional outlets are whpSfiPr m faer maintain ventilation or make-up air to the space even In th'he wor^ers not wish to have the air directed into their work zones, and uS ^a^', fae ,a*r can he directed down in the summer when it is needed, .p 111 tae winter for ventilation, heating, and make-up purposes. It as neeHpHSS1^e> *'00' (hre<rt the a'r toward one or another work station ^ed. From the production standpoint, directional control may be