Document QJXR5XerV0nGnRx8bKX23B1Ko

IT 632 CHAPTER 43 1960 Guide switches in sequence by means of & proportional electric or pneumatic operator. They are commonly used for controlling a number of steps of refrigeration capacity and may be ar ranged to prevent simultaneous starting of compreaora and to alternate the sequence to equalize wear. They may also be used for sequence operation of electric heaters and other equipment in response to the demands of a proportional con troller. 2. Clocks or timers for turning apparatus on and off at pre determined times, for switching control systems from day to night operation, and for other time sequence functions. PART II--DESIGN COORDINATION A heating or cooling system is designed to meet maximum conditions but functions at partial capacity much of the time. The ability to properly control the system at all times is one of the most important factors having to do with the Hpwign of the system and for this reason the proper use and application of controls should be considered at the time the heating or cooling system is being designed. Wherever possible the system should be designed to facilitate control operation. This section includes factors which should be considered in the derign of a new heating or' cooling system, or the modernization of an existing one, to enable the con trol system to produce the best possible results. The design of the system and its controls should take into consideration the nature and type of the building in which it is to be used and the results it is desired to accomplish. Quality of the control equipment should be consistent with the accuracy of control required. Where results more pre cise than those for normal comfort applications are re quired, such as in certain manufacturing areas, constant temperature rooms, etc., both the air-conditioning system and the control system should be given special consideration so that the required results can be produced. Controls can be made to produce almost any degree of accuracy required, but it is useless to provide such controls unless the airconditioning system is capable of properly responding to the demands of the controllers. For example, it is virtually impossible to maintain close control of temperature and humidity by starting and stopping a refrigeration com pressor or opening and closing a refrigerant valve. It is necessary instead to employ refrigeration equipment that will permit proportional control by one of the methods discussed in Chapter 38, or to use a chilled-water system with a coding coil or air washer that will permit proportional control. Neither is it good practice or economical to select equipment capable of producing far more precise control than the application requires, or to complicate the system to obtain special sequences or cycles of operation when not necessary. It is well to remember that the system must be adjusted and maintained in operation for many years, and that the simplest system which will produce the neces sary results is usually the best. SIZE OF CONTROLLED AREA No individually controlled area should be excessively large because the difficulties of obtaining good distribution and of finding a representative location for the space con trols become greater as the area increases. Each individually controlled area must have similar load characteristics throughout and otherwise should conform to the recom mendations given in the section on Zone Control. For uni form conditions throughout the area equitable distribution must be provided by competent engineering design, careful sizing of equipment, and proper balancing of the system. The control can measure conditions only at the point where it is located, and it cannot compensate for variable conditions throughout the area* caused by improper distribution or in adequate design. Areas or rooms having dissimilar load characteristics, or having different conditions to be main tained, should be individually controlled. The smaller the controlled area the better will be the control obtained and the more nearly will the system approach the optimum in performance and flexibility. Even when individual room control is employed it often is well to consider some form of overall zone control of the whole system in accordance with the major factor influencing system load. Thus the water temperature in a hot water heating system, the steam temperature or pressure in a steam heating system, or the delivered air temperature in a central fan system may be varied inversely as the out door temperature varies. This obtains a fair share of the necessary control on the system as a whole, relieves the individual space controls of a part of their burden, and malffw more accurate space control possible. Furthermore, modifying the basic rate of heating or cooling input to the system in accordance with system load reduces losses in the distribution system. The cost of the additional controls is often justified by the reduction in operating costs. Where such overall system controls are employed, accurate soning, while desirable, is not necessary. The system must always be able to satisfy the area or room having the greatest Hpmftnrt The individual controls take care of the variations in demand within the area served by the system. The more accurately the system is zoned the larger will be the share of the control accomplished by the overall system controls, the smaller will be the system distribution losses, and the more accurately will the individual space controls be able to maintain the space conditions. EQUIPMENT SELECTION AND LAYOUT Heating and cooling coils should be carefully selected to avoid oversizing. A coil that is too large is not only more difficult to control but will increase air temperature strati fication. When temperature measuring elements are to be placed between coils, ample space between the coils must be provided. The measuring element should be located as near as possible to the coil it is controlling. For example, an idle water coil interposed between a thermostat and the coil it is controlling introduces enough thermal inertia to make it difficult to produce stable control. The elimination of stratification in an air-handling system is essential not only in producing good control but in ob-.. taining equitable distribution of heating or cooling. Outdoorair and returo-air dampers should be arranged in such a way as to help blend the outdoor air and return air. Face dampers should be placed far enough from coils to allow for equalization of air flow over the coil surface. The con troller for face and bypass dampers must be located far enough downstream so that the air has become thoroughly mixed before reaching the controller. Other sources of strati fication are coils which heat or cool unevenly and coils ar ranged in parallel and controlled in sequence. Whenever stratification cannot be eliminated by proper design or ar rangement of equipment, special baffles, mixing chambers, or other mechanical devices should be used for this purpose. .If stratification still exists at a point where the measuring element of a thermostat is to be located, the use of a long averaging element, or one made up of a number of small elements, may make a suitable temperature measurement possible. This does not solve the other problems relating Automatic Control 633 to stratification which can cause discomfort or, in extreme cases, freezing of equipment. Good air distribution and properly spaced diffusers and grilles will greatly assist in maintaining even temperatures throughout an area. A space temperature controller cannot prevent drafts. This can be accomplished only by proper at tention to such factors as balancing the system, by intro ducing air in the correct manner, and by providing a suitable low-limit control on the delivered-air temperature. A system that employs constant air volume with variable air temperature results in more uniform distribution than one which requires variations in air volume to achieve tem perature control. Varying the volume of air affects both ventilation and distribution, two factors which usually have little relation to thermal requirements. Return-air grilles usually can and should be located so as to aid both the distribution and control of the system. A further aid to both distribution and control is the pro vision of adequate facilities for exhausting air from the building. The larger the percentage of outdoor air introduced the more important becomes the provirion for adequate exhaust. To prevent drafts due to infiltration, it is quite common to provide for positive exhaust of less air than is brought in. On systems where the quantity of outdoor air is large but constant, this may be accomplished by use of a properly sized exhaust fan. Systems having large inyternal heat gains, or large solar gains, may use a variable amount of outdoor air up to 100 percent when cooling is required during cool weather. In such cases the outdoorair quantity may be controlled from space conditions and the quantity of exhaust air may be controlled by a staticpressure controller set to maintain the space under a slight positive pressure. LOCATION OF SPACE CONTROLLERS ,, - Space controllers such as room thermostats must be placed where they will measure the variables they are to control and where the condition is representative of the whole area served by the controller. If the return-air duct is used to represent space conditions, the controller should be located as near the space as possible to eliminate the effect of any heat gain or loss in the duct. Room thermostats should usually be placed in occupied areas on inside.walls or columns. They should never be placed in spaces such as corridors, lobbies, and foyers unless they are used for the control of these areas only. In a typical home which is not zoned a satisfactory loca tion for a room thermostat can usually be found on an inride wall of the living room or dining room. It should always be located toward the center of a relatively open room situated on the coolest rather than the warmest ride of the building. It should not be mounted on an outside wall or other cold surface where it is exposed to cold drafts from an outride door. Neither should it be mounted where it will be affected by direct rays of the sun, by heat from a nearby warm sur face such as a chimney, pipes or ducts in the wall, or radiators, or by direct air currents from a register. The loca tion should provide ample air circulation unimpeded by furniture or other obstructions, and should afford protection from mechanical injury. CONTROL OF FLOW The successful operation of practically any air-condition ing system is dependent on the proper control of flow in one form or another. The flow being controlled may be steam, water, air, or some combination thereof. In each case a controlled device is used to regulate the flow at the command of a controller. The proper design and rising of these controlled devices, valves for steam or water, and dampers for air, are essential to obtain the desired results from the air-conditioning system. The performance of a valve or damper is expressed by the control industry in terms of its flow characteristics as it operates through its stroke, based on a constant pressure drop. Three common characteristics are shown in Fig. 6 and may be defined as follows: Qxuck Opening: Maximum flow is approached rapidly as the device begins to open. Linear: Opening and flow are related in direct proportion. Equal Percentage: Each equal increment of opening in creases the flow by an equal percentage over the previous value. Fig. 6.... Typical How Characteristics Since the pressure drop across a valve or damper seldom remains constant as its opening changes, actual performance usually deviates from the published characteristic curve. The magnitude of the deviation is determined by the over all system design. For example, in a system arranged so that control valves or dampers ran shut off all flow, the pressure drop across a controlled device increases from a minimum at design conditions to the total system pressure X drop at no flow. Fig. 7 shows the extent of the resulting deviations for a valve or damper designed with a linear characteristic, when the selection is based on various per centages of the total system pressure drop. In order to approximate the designed characteristic of the valve or damper, the design pressure drop should be a reasonably large percentage of the total system pressure drop or the system should be designed and controlled in such a manner that this pressure drop remains relatively constant. Higher pressure drops for controlled devices are obtained by using smaller sizes in conjunction with a possible in crease in size of other equipment in the system, . Since