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Resumen del manual
Throughout this publication, the chilled liquid will be water. Subjects covered are chilled water temperature control, condenser water temperature control, system design for steam and hot water machines, and general information. CHILLED WATER TEMPERATURE CONTROL The absorption machine is basically a water chiller that can be connected to any conventional open or closed system. However, circulation of chilled water must be continuous during operation of the machine and during the shutdown dilution cycle. Chilled water flow may be restricted at partial load. For fine chilled water temperature control within narrow limits, such as required in precision control of industrial processes or maintenance of laboratory conditions, the chilled water system may require additional storage volume to allow the machine to adjust slowly to changes in load. Normal air-conditioning applications are not subject to such requirements. Systems having large storage volumes of chilled water transmit load changes to the machine slowly, allowing accurate chilled water temperature control. Small storage systems transmit load changes rapidly, making temperature control more difficult. For fine temperature control, the chilled water system volume should be at least ten times the gpm flow through the cooler. If a tank is added to the system for extra storage volume, it should be located in the line from the load to the cooler.» Two-Pipe Cooling-Heating Systems — When machines are used in conjunction with a two-pipe cooling-heating system, certain precautionary steps should be taken during changeover from heating to cooling. Maximum water temperature permitted thru the evaporator is 130 F because of the possibility of tube stress. If system water temperature is above 80 F but less than 130F at changeover time, evaporator flow should be throttled to prevent machine overload. It is recommended that hot water temperatures be reset, based on outside air temperature. If a reset-type control is used, the entering hot water temperature at changeover will normally be lower than 130 F. STEAM MACHINES Boilers — Generally, any boiler capable of modulating its input to maintain design operating steam pressure within 1 psi under varying loads, is suitable for application with the absorption machine. This generally includes all gas- and oil-fired boilers. Some oil-fired boilers are conversions from coal-fired to oil-fired and may have control systems which are too sluggish to give proper response to machine load changes. Direct control of oil feed rate normally ensures proper response. Coal-fired boilers, due to slow buildup and shutdown characteristics, should be used only when the absorption machine represents less than 15% of boiler operating load. This generally limits coal-fired boiler applications to large industrial jobs where process steam is generated in large quantities year-round. BOILER CAPACITY — Minimum boiler capacity for use with the absorption machine is equal to full load steam consumption, plus sufficient capacity to offset piping radiation losses. In the absence of a detailed study of radiation and vent losses, a minimum 10% safety factor should be used. Pressure Reducing Valves — Maximum unit ratings are based on 14 psig steam pressure at the generator inlet. Operation at higher inlet pressures or with more than 100 F superheat is not permissible. Higher inlet pressures may lead to overconcentration. Where steam supply pressures are above 15 psig (14 psig + 1 psig for control) and below 20 psig, the steam control valve can be used to reduce the pressure. If steam supply is above 20 psig, a pressure reducing valve must be provided between the steam supply and the control valve inlet. A safety relief valve should be provided between the steam control valve and the generator inlet. This valve must be set in accordance with paragraph UG-133 (f) of the ASME code to relieve at a pressure not exceeding 17 to 18 psig or the setting determined by applicable local codes. Further specific details relative to pressure reducing stations should follow accepted standards, such as the ASHRAE Guide and manufacturer’s recommendation. For applications on high-pressure district heating, the steam utility should be consulted for local codes or standards. Steam Piping should be sized to avoid excessive pressure drop or excessive velocities. Recommendations and pipe sizing tables are given in the Carrier System Design Manual. It is recommended that lines be sized on the basis of design system flow for the machine plus a 10 to 20% safety factor to allow for normal radiation losses. © Carrier Corporation 1971 Form 16JB-2XA Start-Up Demand — Steam demand by the absorption machine is greatest at start-up (see Table 1 for values). Table 1 — Maximum Condensate Flow (lb/hr) VALVE VALVE INLET STEAM PRESSURE SIZE (in.) 20 psig 14 psig 12 psig 10 psig _____ ‘ 2 ' . 2450 " - 2025 Î 880” 2 'A 4825 4000 3710 3430 3 8175 6760 6285 5810 4 1...