Document Rp74NXKk4Xrb09ZRZJmyymgxV

666 CHAPTER 29 1955 Guide are not as high as desired, higher pressures can be obtained by use of a steam compressor.2 It is sometimes desirable to install a separate high-pressure line from the boiler plant for steam supply to laundries or other high-pressure equip ment rather than to operate the entire distribution system at higher pres sures. BOILER PLANTS In the design of a plant for district heating, consideration should be given to the following factors: 1. Annual load factor. The ratio of the average hourly plant send-out (defined as rate of flow to the distribution system) to the maximum hourly send-out, which is a direct measure of the extent to which investment in facilities is used. This factor is often employed to determine savings by equipment offering higher efficiency. The load factor for space heating is usually between 10 and 30 percent. For Bpace heating Table 3. Loan Duration for Typical District-Heating System (iCumulative Hours of Plant Send-out per Year at Various Loads Expressed in Percent of Peak Hourly Load*) Load % of Peak Hours pee Yeah Load % of Peak Hours per Year Load % of Peak Hours per Year 100 95 5 70 750 30 4100 40 65 1050 20 5150 90 85 80 90 150 300 60 55 50 1400 1800 2200 15 10 6 5700 6400 8760 75 500 40 3100 a Based on curve on page 89, District Heating Handbook, Third Edition, 195L Note: A plant having a peak send-out at 100,000 lb per hr will send out 80,000 (80 percent of 100,000) lb per hr or more for 300 hr per yr. Elus additional steam requirements such as water heating, cooking, or laundry, it is etween 15 and 40 percent. Load factors for various types of buildings are given in Table 10, Chapter 18. 2. Duration of load. The cumulative number of hours for the various plant sendouts, is used to determine the number and sizes of boilers in order to effect most efncient loading of individual units and of -the plant as a whole.. The cumulative number of hours a typical district heating plant will send out steam at various loads, with loads expressed in percentage of maximum hourly rate, is given in Table 3. It will be noted that the duration of load for the higher send-outs is very low. 3. Feed-water treatment. This is desirable in plants to prevent corrosion and scale formation. 4. Smoke abatement. This is a modern requirement. The installation of suitable fuel-burning and dust-collecting equipment to limit the emission of smoke and ny ash is usually necessary. 5. Plant location. A boiler plant should be situated where fuel deliveries, water supply, and other utility services are convenient and should be as near as possible the distribution center in order to simplify and shorten the distribution piping. STEAM DISTRIBUTION PIPING The methods used in district heating work for the distribution of steam are applicable to any problem involving the supply of steam to a group 0 buildings. The first step is to establish the route of the pipes which is in fluenced by local conditions and investment. Important points in laying out distribution piping are: 1. The depth of the conduit should be kept at a minimum. Excavation costs are a District Heating 667 large factor in the total cost. On the other hand, institutional lawns will be dam aged if steam conduit is too close to the surface. 2. The use of basement or sub sidewalk space for the distribution piping where feasible may reduce the cost of installation and maintenance. 3. In some industrial and institutional applications, the distribution piping may be installed, entirely or in part, above ground. This method of construction has the advantage of requiring no excavation and permits easy maintenance but it may in troduce freeze-up problems. Thin corrugated aluminum sheets are available for covering the insulation on lines exposed to the weather. 4. Piping routed in loops with suitable valves to provide alternate feeds for major loads, allows service to be maintained despite trouDle in any one feeder. PIPE SIZES After establishing the route of the pipes, the next step is to determine the pipe sizes. In district heating work it is common practice to design the piping system on the basis of pressure drop. The initial pressure and the minimum permissible terminal pressure are specified, and the pipe sizes are so chosen that the required amount of steam, with suitable allowances for future increases, will be transmitted without exceeding this pressure drop. The steam velocity is therefore almost disregarded and may be very high. Velocities of 35,000 fpm are not considered high. By the use of high velocities the pipe sizes are kept to a minimum with consequent savings in investment. Some lines in a loop system may need to be in creased in size in order to handle reverse-direction flow in emergencies. The steam flowing through any section of the piping can be computed from a study of the hourly steam requirements of the several buildings served. Steam requirements for ventilation-air heating, water heating, laundering, cooking, or processing, should be individually calculated and added to the heating.load. Unusual features, such as large heaters for swimming pools, should not be overlooked. When the lengths of pipe, steam quantities, and initial and terminal pressures have been chosen, the pipe sizes can be calculated by . means of Babcock's pressure drop formula given in Table 2 of Chapter 21. The district-heating industry uses Unwin's formula for determining pipe sizes because pressure drops can be determined simply and directly from a chart. This formula has been checked by tests, and found correct within ranges of velocity used in district heating., A convenient chart developed from this formula is available from the National District Heating Association,8 and provides a simple yet accurate means for determining pipe sizes. CONDUITS FOR PIPING Conduits for underground steam pipes should be reasonably waterproof and able to withstand earth and traffic loads. Since it is difficult to make a concrete or masonry conduit absolutely watertight, provision should be made for some seepage. The pipe should ,e protected by a waterproof jacket over the insulation, unless subject w actual submersion, and the seepage should be drained from the inside 0 the conduit. Underdrainage of the conduit is generally obtained by H* ( a tile drain laid in gravel underneath the conduit. The tile nderdrain should discharge to a sewer or some other drainage point. anholes are required at intervals for access to valves, traps, and some types of expansion joints. There are many types of conduits, some of which are prefabricated prod-