Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSHIGH TEMPERATURE WATER HEATING SYSTEM CONTROLINTRODUCTIONHIGH TEMPERATURE WATER (HTW) HEATINGHigh temperature water systems operate with supply watertemperatures of 120 to 230°C and pressures from 500 to3000 kPa.HTW is typically generated by boilers; however, experimentalsystems utilizing geothermal or solar energy have been built.First costs are similar for steam and high temperature watersystems, however, maintenance and operating costs aregenerally lower for HTW systems. The use of the same boilerfor both HTW and steam generation is not recommendedbecause feed water requirements for steam eliminate some ofthe advantages of HTW.When relatively small amounts of steam are required, steamcan be produced from HTW at the required location. A steamgenerator using 175°C HTW (900 kPa) will produce 200 kPasteam and using 190 to 210°C HTW (1300 to 1900 kPa) willproduce 800 kPa steam allowing a HTW temperature drop of30 to 35 kelvins.A HTW system offers several advantages over a steam system:— Boiler sizes can be smaller than low pressure boilersbecause of the high heat capacity in a HTW system.— Diameter of distribution piping can be reduced.— The piping system requires no grading for return ofwater to boiler.— Feedwater requirements are minimal, eliminatingtreatment costs and introduction of air which is a sourceof corrosion. HTW systems tend to remain clean.— Steam traps and condensate receivers, sources of heatloss and maintenance costs are eliminated.— Heat not used in terminal heat transfer units is returnedto the HTW generator.Several major design characteristics are typical of HTWsystems:1. The HTW boiler is controlled by pressure rather thantemperature to eliminate flashing if heating load fluctuates.2. Multiple boiler systems must be designed so that loadsare divided between the boilers. Generally it is less costlyto operate two boilers at part load than one at full load.3. HTW systems can be pressurized by steam or air in theexpansion tank but typically an inert gas such as nitrogenis used because it absorbs no heat energy and excludesoxygen.4. All piping is welded except at mechanical equipmentwhich must be maintained. Connections at equipment areflanged, including provision for removal of small threadedcontrol valves.5. Terminal units are rated for the high temperature andpressure.Figure 117 illustrates the elements of a typical HTW system.EXPANSIONTANKPRESSURE CONTROLMANUAL AIR VENTSAT HIGH POINTS OFSUPPLY AND RETURNUNITHEATERSOTHER HEATING COILSBOILERNITROGEN BOTTLESSYSTEMCIRCULATINGPUMPEXPANSIONLOOPSINSTANTANEOUSCONVERTERLOADDOMESTIC HOT WATERFOR BUILDINGCHECKVALVEMAKE-UPFEED PUMPPIPING FOLLOWSCONTOURS OFLAND.ALL DISTRIBUTIONPIPING IS WELDED.STORAGECONVERTERSTEAM FOR PROCESSOR AIR CONDITIONINGSTEAM GENERATORCONDENSATERETURNREVERSEC2581Fig. 117 Typical Nitrogen Pressurized High Temperature Water System.374ENGINEERING MANUAL OF AUTOMATIC CONTROL