Document 4mG6aJk8rG3Dn6gwVyG1KDJV

180 CHAPTER 13 1965 Guide And Data Book Typical Hot Water Heating'Coil Another very important factor to consider when using hot water as the heating medium, is that the heat output of a coil is not linearly related to flow. As the flow is reduced, a greater amount of heat is transferred from gallon of water, and this tends to counteract the reduction in flow. Fig. 11 shows the heat emission versus flow characteristics far a typical coil with 220 F entering water and a 20-deg design .temperature drop, and shows how the .temperature drop increases as the flow is reduced. . To improve the coil characteristics for hot water, it.is possible to design for higher water. temperature drops. Selecting a temperature, drop of 60 deg would require a slightly larger coil, but would improve control performance and substantially lower the water requirements. This -re duction 'in gpm also results in smaller pipe sin**, wmaller control valves, and reduced pumping costs. A properly, sized control. valve with equal percentage characteristics should be used to control the flow. of hot water to coils for modulating service; CONTROL OF AIR FLOW .The flow characteristics of a damper vary with the number of blades, the manner in which the blades rotate with respect to each other, and the length to width ratio of blades.1 Dampers commonly have blades with either parallel or, oppoped operation. As explained in a preceding section; Control of Flow, the performance of a damper is expressed by the - control industry in terms of its flow characteristics. . . It is common practice to size a damper to fit the duct into which it is installed. This usually results in a very low design pressure drop across the damper and poor control perform ance. This is particularly true of dampers.used to control the entire flow into a large duct, such as a static pressure damper. With a low pressure drop across the damper, fan horsepower requirements are as low as possible; however, a properly sized damper taking a larger drop will not objectionably Md to the fad horsepower requirements. '* * . When the application is of such a nature that the pressure drop across the damper increases substantially as it closes/ it is preferable to use opposed blade dampers. As explained in a preceding section, Control of Flow, the changing pressure Will cause a deviation from the design flow characteristic. This will change the performance from nearly equal percentage to the desired nearly linear characteristic,' as shown in' Fig; -12, if damper size is properly selected. Fig. 12 .... Effect of Variable Pressure Drops on Opposed Blade Damper Performance Face-and-bypass and mixing damper applications differ from the throttling applications explained, in that the pres; sure drop across the - damper remains relatively .constant! Since a constant total flow is desired, and since the design flow characteristics are not changed by varying pressure drops, a damper with a more nearly linear characteristic should be chosen. Fig. 12 indicates that, to produce good performance with an opposed blade damper under the conditions tested, a Vtiunpar of the proper size must be selected. An example emphasising the need for proper sizing is a static pressure damper in a high velocity system. Such a damper should not be placed in the low velocity coil section (400 to 600 fpm) where itwould have a design pressure drop of less than 0.01 in. water, if it is ex pected to control,with 3- to 4-in. pressure drop during periods of low flow. It should be placed in a high velocity section where the design pressure drop will be approximately 0.15. in. .water or more, careful consideration being given to avoid excessive turbulence on either the entering or. leaving side of the damper. Care should also be exercised to avoid taking too large a drop through the damper, or the desired- near-linear characteristic will not be obtained. ^ ZONE CONTROL ' Zone control for any heating, ! vpntila.ting, -nr'' air-onn- ditioning system is employed where it is desired to control, by one set of controls, the heating or cooling effect in a number of rooms or areas having similar orientation or occupancy. For zone control' to be successful, the require ments' must be approximately consistent' throughout the extent of the zone. Whether zoning a building for beating or cooling, the designer should consider the following factors in determining the number and arrangement of the zones: 1 Exposure. Solar effect, prevailing winds, 'and' toe shelter afforded by surrounding structures affect the heat gain or h. - 2. Occupancy: Indoor temperature requirements for the various activities carried on in different portions of'the building and the hours.of occupancy of the various spaces impose definite control problems. 3. Building Structure. The. physical characteristics of the budding will influence the subdivisions of the system into zones, since satisfactory temperature conditions throughout a single AutomoticControl 181 ,. zone of of the buadu nay not be equally obtainable m buildings ^ construction. Other factors are the height horizontal extent and form, several floors have the same wall have parts that must be kept level. There remains, then, only thos* controls that have positive contact action and employ m^hanirtal or magnetic detents on their contacts. 4 Floor*. E adv^able to have separate zones on the lowest expnre, *" sod , ju* to variations of basement or ground effect of roof exposure. It Valso Explosive Atmospheres Sealed-in-gUm contacts are seldom considered explosion- J^SaSetTbave a separate zone or zones for toe intermediate proof, and therefore other means must be. provided to elimi floors. The initial investment will often influence the decision on the final number of zones to be employed. In larger buildings, "Luted practice is to have at. least one zone .for each ex posure For higher structures, each exposure may require subdivisioii vertically into two or more zones. The presence of two or more wings haying the same'general exprcure may easssst the desirability of more restrictive zoning. When the street floor or "y other P0^011 of ^ building is used for public occupancy or activities which differ from those carried nate any possible spark within the atmosphere. When employing electric control systems, the designer can use an explosion-proof case to surround the control case and contacts, .permitting only the capsule and the capillary tubing to extend within the.conditioned space. It is also frequently pnwnMi* to use a long capillary tube and mount the instru ment case in an explosion-free atmosphere. The latter method may be duplicated in an electronic control system by placing an electronic sensor in the conditioned space and by feeding its gigna.1 to an electronic transducer placed in .the explosiop- on in the remainder of the building, it is desirable to provide separate thermostats and controls for each individual area. ' Zone controls alone may not provide satisfactory tem free atmosphere. Because a pneumatic control system utilizes-compressed air as its source of energy, safety is inherent in what other perature, conditions in all rooms or areas within the zone, wise would be hazardous locations. occupancy, lighting load, space arrangements, and mnftf factors cannot always be predicted with accuracy. A Limit Controls Mimhination of zone controls and individual room control A further and most important factor to consider when selecting automatic controls is the type of operation needed for control or high-limit (safety)- In the former, since quite INDIVIDUAL ROOM CONTROL frequent operation'of the control mechanism occurs,'any The ideal temperature control system for any building is one that promotes maintenance of the desired temperature in every room at all times, regardless of location and occu pancy. Individual room temperature control is'desirable for securing proper thermal environment in schools, hospitals, and offices. failure of the control is likely to be noticed before damage is done. In the tatter, the control may be inoperative for days, months, or possibly years and then be required to operate immediately, to prevent serious damage - to equipment or property. Therefore, separate operating and limit controls are always recommended, even when they perfonn:the same func- Control of the temperature in each room, or possibly of adjacent rooms having the orientation (as in an apart-"' ment), overcomes many of the problems encountered.when PRECAUTIONS attempting to regulate the temperature of a building as'a whole or of large areas or zones. Fach individual thermostat Since numerous specific precautions are discussed in the section on Control Applications, this section will deal with controls the heat input to its'particular space regardless of typical general precautions that should be considered when occupancy or exposure to sun and wind. The advantage of designing a heating or cooling system. individual room control is in fuel economy and comfort for the-occupants. Each room should have a thermostat that Cooling and Dehumidification .> controls valves or dampers on all the controllable sources Valve control oftcoils to meet sensible cooling requirements of heating and.cooling. One such uncontrolled source may varies coil temperature and often results in'coil temperatures prevent proper operation of the control system. highpr than the dew-point temperature, and no;dehumidifi- tion is accomplished. Bypassing some return air- around the CONTROL APPLICATION LIMITATIONS. ' , coil to control sensible cooling requirements ^permits the coil In order to use automatic controls most effectively, some to remain at the lowest possible temperature, thereby ac complishing a mn-rimnm amount of-jdehumidification with knowledge of their application, limitations and operating char highw leaving temperatures. A satisfactory'relative humidity acteristics is essential. opnHit.inn may thus be reached'without excessively low dry- bulb temperatures, and less sensible' heat will have to-be Controls for Mobile Units added.- ' '' Any control that relies on pressure to operate a switch or valve is subject to variations in operating point as .the atmos pheric pressure changes. Normal variations in'atmospheric pressure do not noticeably change the operating point, but a change in altitude will affect the control point to an extent governed by the change in absolute pressure. This characteris tic is especially important when controls are being selected for use in planes, buses, trucks, and trains that, in normal use, are subjected to wide variations in altitude. In mobile applications vibration is alwayB present'and will affect the operation of some types ofcontrols. The types that are wholly unsatisfactory for mobile applications .are those employing mercury tubes,' slow-moving contacts,' or. any that Refrigeration Compressor Operation ' On direct-expansion systems, the compressor should not be allowed to operate nnlww the fan of the conditioning unit is in operation. likewise, in a chilled water coil installation, the compressor should not operate unless the water circulating pump is operating. Proper wiring between fan or pump starter and compressor starter will provide this feature. Outdoor Air Dampers It is desirable to prevent outdoor air from entering the conditioning .unit when-the .Tan is off, .particularly during seasons when freeze-up might, occur;-/This may be accom-