Document VG5J9emmxLe1RN2wdq4NLQxGj

238 CHAPTER 19 1962 Guide And Dofa Bool higher. These diffusers have found favor in high velocity systems operating at substantially higher static pressures. Perforated ceiling outlets have proven exceptionally satisfactory in preventing drafts, particularly where air changes are high and ceiling heights are lovr. Exhaust and Return Air Outlets Exhaust and return air terminals are usually either regis ters, grilles, louver or mesh screens. Registers have the ad vantage that fr&Janring provisions are built within the ter minal, particularly where ducts are concealed. With mesh screen, commonly found in toilet and shower spaces, a separ ate hwlancmg damper must be located in the exhaust duct work. This offers no disadvantage where ducts are exposed. Exhaust terminals are selected for a face velocity consistent with noise levels acceptable in the space served, and to pro vide reasonable pressure drop when clogged with lint and dirt. Decorative grilles frequently are provided in public spaces. Return air from individual small spaces like staterooms is usually accomplished by either a sight-tight louver mounted in the door or by undercutting the door leading to the passageway, the latter being confined to email air quantities (75 cfm or less). Louvers are most commonly axed for a veloc ity of 400 fpm based on uet area. CONTROLS A ship, on a single voyage, may encounter extremes of climate--sometimes even within an hour. As a result, the air conditioning load on a ship varies over a wide range in a short period of time. Therefore, not only must the refrigeration . plant successfully meet these variations in load, but the auto matic controls must also be designed to readily adjust the system to sudden climatic changes. Accordingly, it is the general practice to equip the plant with the necessary auto matic controls. Inasmuch as body comfort is a matter of in dividual taste, it is necessary to provide room thermostats in living spaces such as staterooms, which can be adjusted by the occupant to suit personal requirements. Controls used can be divided into' four basic types: self contained, pneumatic, electric and manual. On merchant ships pneumatic controls are used most exten sively, particularly where accurate modulation and se quenced operation are essential. The self-contained control has the advantage of simplicity. They are used to control steam flow where accurate control is not particularly essential. Except for special cases, electric controls are used only on naval vessels. In most cases thee applications require only two position operation. over a wide range to compensate for space load changes occupant preference. The room thermostat regulatea precisely the amount of heating medium; usually hot water, delivered to each room reheater. Similar control is used for CSass A systems, except beating medium is usually steam. Where system serves only one bumidistats are used to override room thermostat when humidity exceeds set point. In such cases room thermostat supplies heat when over cooling occurs. 3. Local Cooling and Heating Control (used with the Type g System). Heating-cooling thermostats control flow of water (hot or cold) through induction unit coils to suit demand of room thermostats (pneumatic). Although most design conditions in spaces can be achieved with th'm arrangement, range of temperature control is rather limited during intermediate seasons (about 35 to 50 F). This is due to tbe fact that the heating effect (temperature) of the primary air must be limited to prevent exceeding tbe available cooling effect of tbe secondary water circulating through the induction unit coils. The flexibility of control therefore is reduced. 4. Air Mixture Control (used with Type 0 system). Individual temperature control is accomplished by the positioning of an air valve either automatically utilizing a room thermostat or manually by remote control, winch duxes the hot and cold air to suit space load demands. 5. The control required for zone reheaters of Type C Systems is achieved by regulating the amount of steam to the zone coils by one of two methods. One method utilises a room thermostat in a representative space set and locked for an average zone tempera ture. Obviously, such am average seldom exists, and spaces served by this type control will not always achieve the desired comfort conditions. A second method utilizes a master-eubmaster control which adjusts the rebeater discharge temperature according to a pre determined schedule so that roll beat is applied at the design out side heating temperature and fere as this temperature rises. This control also does not adjust to meet variations in individual room loads. However, it is considered superior to the representa tive room fAcrmosftu method because it is more foolproof. With the first method occupants of the representative room may restrict air flow,'thus disturbing the control of the entire tone. Preheaters are controlled by a duct thermostat which modu lates steam through the valve. Tbe set point usually is a few de grees below tbe design cooling off coil setting to prevent bucking. Except for Type A systems with humidity control and Type S system primary air coils, cooling ooils (water) are controlled by a dewpoint thermostat to give a constant off-coil temperature dur ing tbe entire cooling cycle. No control is provided for coils of Type B systems, because they are booked up in series with tbe flow through induction units and it is essential that maximum dehumidifkatioD be accomplished by tbe primary coil to provide dry coil operation in room units. Outdoor, return, and exhaust dampers are either manually (as a group) or automatically controlled. If automatic, one of two methods are One controls the damper settings by means of thermostats exposed to weather air. A second method uses s duct thermostat which restricts the outdoor air flow only when the design temperature leaving the coil cannot be achieved with full water flow through thecoolingcoil. Manual volume control has been used to regulate tempera tures in cabins and staterooms with varying results. On low REGULATORY AGENCIES velocity air distribution systems manual volume control has Vessels which operate under the United States flag come a tendency to disturb the air balance of the remainder of the under the jurisdiction of tbe United States Coast Guard- spaces served. Manual controls used on high velocity dual duct systems have been successfully applied where a con Accordingly, the installation and components must conform to the Marine Engineering Rules and Marine Standards of stant volume regulator isemployed in the mixing box the United States Coast Guard. The design of the equip The control of design conditions in staterooms and public ment and the installation must also comply with the require spaces can be obtained by any one or combination of: ments of Uie United States Public Health Service. This in 1. Volume control (used in Type C systems). This is the least expensive aodamplest type of control. However, there are many disadvantages such as lack of flexibility in meeting simultaneous heating and eoaling'deat&nds in adjacent spaces, unsatisfactory air distribution (lack of air motion where volume is reduced or excessive air motion in other spaces on the same system due to changes in static pressure), objectionable noise increase and fluctuations in adequate ventilation due to reduction in air delivery. 2. Reheat Control (used with Types D and P systems room units). Room thermostats provide individual temperature control volves principally rat-proofing. Although comfort air con ditioning installations do not ordinarily come under the cognizance of the American Bureau of Shipping, equipment should be manufactured wherever passible to comply <ritb the American Bureau of Shipping Rules and RegulationsASA B59.1-195S, Recommended Practice far Mechanical Refrigeration Installation on Shipboard, provides a guide for the installation of air conditioning and refrigerating equip- menfc. Refrigerated cargo space installation must comply with A.B.S. requirements, if they are to be so certified. CHAPTER 20 CONTROL OF THE INDUSTRIAL ENVIRONMENT Hoof Control of Industrial Work Areas, Thermal Standards, Control of Heat Exposures, Radiation Shielding, Roof Ventilators, Local Relief Methods, Physiological Aspects of System Design, Types and Requirements of Systems, Outlet Types and Design, Control of Contaminants ONTROL of the industrial environment is concerned heat without adding moisture to the air. Tbe beat load on ex C with the design and application of equipment for pro posed workers is thereby increased but the rate of cooling viding the necessary conditions within industrial areas forby evaporation of sweat is not reduced. Heat balance may fPflfrfairnnff the efficiency, health, and safety of workers. This be maintained although this will be at the expense, perhaps, chapter includes a comprehensive list of the requirements of excessive sweating. In the warm-moisture situation, the for heat control and for control of gases and vapors, and dust' wet process gives off principally latent heat. There may be and fumes. General ventilation may be relied upon in some cases to no significant rise in the heat load on the worker but the increase in moisture content of the air will seriously reduce control the industrial environment, as outlined in this chap the heat loss by evaporation of sweat. The wann-moist situa ter. Additional information on methods of ventilation may be tion is potentially more hazardous the hot-dry. found in the following chapters of the 1961 Guide And Data Examples of hot-dry work situations are seen around hot Book: natural ventilation, Chapter 24, mechanical ventila furnaces, forges, metal-extruding and rolling mills, glass- tion, Chapters U, 12, 13, 37, 38, 40, 47, and 48. forming machines, and the like. Typical of warm-moist oper Information on heating and cooling loads and equipment ations are many textile mills, laundries, dye houses, and deep will be found in Chapters 25, 26, 27, 28, 32, 33, 34, and 39 of the 1961 Guide And Data Boos. In this volume, reference should be made to Chapter 21 for design of hoods and exhaust systems for fumes and dusts. mines where water is extensively used for dust control. The industrial heat problem varies in magnitude with local climatic conditions. Solar beat gain and an elevated outdoor temperature will increase the heat load at the work Chapters 23, 24, 25, 26, 27, 28, 29, 30, 33, 34, and 35 include information on process requirements for specific industrial place but these contributions may not be very important compared with the locally generated heat of the process it applications. For further information on estimating radiant heat condi self. The moisture content of the outdoor air, on the other hand, is a most important climatic factor affecting hot-dry tions, see Chapter 4 of the 1961 Guide And Data Book and Chapter 10 in this volume. Information on globe thermometers and other instruments used to measure the industrial environment may be found in Chapter 16 of the 1961 Guide And Data Book. More com plete information will be found in References 1--4. work situations and on a moist summer day will seriously restrict evaporative cooling. For the warm-moist job, in contrast, solar heat gain and elevated outdoor temperature are the more important, since, compared with the moisture release on the job, that contributed by the outdoor air will be of little significance. Specialists in the field of industrial hygiene should be con sulted in case of doubt concerning the presence of airborne Thermal Standards for Industrial Work Areas industrial hasards to health. Chapters 8 and 10 of the 1961 Guide And Data Book will be of help in establishing the The work situations considered here are those in which the attainment of simple thermal comfort is not always prac atmospheric conditions that should be maintained around the ticable. Tbe heat stress must be kept below the level of out worker. Local codes, ordinances, or state labor laws must like right hazard, but how much below? On what criteria should wise be observed, particularly with regard to ventilation re the specifications for control be based? How should the* quirements for hazardous trades. Comfortable conditions, if standards be varied in recognition of the differing demands posable, or tolerable conditions at least, are desirable because and work rates from one kind of job to another? What dif they are likely to increase the efficiency and, hence, the output ferences exist between older and younger workers or between of workers. The majority of air-conditioning and ventilation workers with differing degrees of physical fitness? To what installations in a typical industrial plant are concerned with extent can periods of heat exposure be offset by alternating the control of sensible, latent, or radiant heat. periods of recovery? How is this expressed in terms of design of the control measures? These and other questions arise in heat control in industrial work areas the analysis of an industrial heat problem and must be con sidered by the design engineer and others before specifica In certain industrial work situations there is considerable triease of heat from tbe process equipment to the environ ment. It is not economically feasible to stop the escape of all of this process heat or to offset it completely by the usual methods of comfort ventilation and air conditioning. In the tions are fixed. As there is no single set of thermal standards applicable to all work situations, failure to give proper recog nition to the many facets of the problem can result in inade quate control measures. -- <ksigD and operation of control measures it is frequently nec- *ary to accept some heat exposure in excess of ample com fort requirements. Biophysical Basis for Fixing Thermal Standards Under conditions of thermal comfort, the rate of internal The engineer must dwtingnigh between the control needs heat production (metabolism) is just balanced by the rate for hot-dry industrial areas and warm-moist conditions. In of heat loss to tbe environment. This comfortable balance is ffie first case, the process gives off only sensible and radiant maintained without active sweating, with optimum skin and 239