Document gb37eEYnBK1qZZ6ObM9x3wmdq

HEATING VENTILATINC AIR CONDITIONING CUIDE 1942 milinches. For the third set of risers, a resistance of 120 milinches is required and an 0.60-in. orifice in a 1-in. union will provide sufficient resistance. EXPANSION TANKS Expansion tanks may be either of the open or of the closed type. In the open type, (see Fig. 9) the water is subject to atmospheric pressure only, but in the closed tank (see Fig. 10) the system is under pressure and, therefore, a relief valve should be placed on the tank. Water expands about 4 per cent when being heated from 40 F to 200 F, and the expansion tank should have a volume about twice the actual expansion or about 8 per cent of the total volume of water in the entire system including boiler, radiators, pipes, etc. Open expansion tanks should be at least 3 ft above the highest radiator and be protected against freezing. Closed tanks are generally placed in the cellar over the boiler. A relief valve installed on a closed tank will not operate often provided the tank is of adequate size. It is essential that the relief valve be kept in good condition to eliminate any possible failure when operation is necessary. INSTALLATION DETAILS Attention should be paid to the following: All piping must be so pitched that all air in the system can be vented either through an open expansion tank, radiators or automatic relief valves. All piping must be arranged so that the entire system can be drained. Sections of piping individually valved shall have corresponding drain valves. In large buildings, the piping may be zoned according to exposure of building, usage of building, or method of control. All piping must be installed so that it is free to expand and contract with changes of temperature without producing undue stresses in the pipes or connections. For this purpose it is generally sufficient to allow for a variation in length of 1 in. for 100 ft of pipe. The pipe system should be designed so that each circuit has its correct friction head for balanced water distribution. This may be done by change of pipe size or change in piping detail. The connections from the boiler to the mains should be short and direct, to reduce the friction head and allow for expansion. It is frequently possible to avoid an elbow and to reduce the length of the pipe by running the pipe in a diagonal direction, either in a horizontal or in a vertical plane. The mains and branches should pitch up and away from the heater, generally not less than 1 in. in 10 ft. The connections from mains to branches and to risers should be such that circulation through the risers will start in the right direction. Hence, in a one-pipe system the flow connection must be nearer the heater than the return connection. In a correctlydesigned two-pipe system, the pressure in the flow main is higher than that in the return main, and a slight variation in the distances of the flow and return connections from the heater is not material; but it is generally best to have the two connections about equally distant from the heater. Generally connections to risers or radiators are taken out of the top of mains either 45 or 90 deg. In some cases it may be advisable to take the flow connection off the top of the main and the return connection from the side. With forced circulation and high velocities, it is advisable to let the water enter at the top of the radiator and leave at the bottom of the opposite end. With gravity circulation the flow connection may be either at the top or at the bottom of the radiator. With short radiators both flow and return may be at same end, but top and bottom. Unless used as heating surface, all piping, both flow and return, should be insulated. 324 Chapter 17 DISTRICT HEATING Steam Distribution Piping, Selection of Pipe Sizes, Provision for Expansion, Capacity of Returns with Various Grades, Conduits for Piping, Pipe Tunnels, Inside Piping, Steam Re quirements, Fluid Meters and Metering, Rates, Utilization, Automatic Temperature Control THOSE phases of district heating which frequently fall within the province of the heating engineer are outlined here with data and information for solving incidental problems in connection with institutions and factories. Some data are included to cover the piping peculiar to heating systems which are to be supplied with purchased steam. A com plete district heating installation should not be attempted without a thorough study of the entire problem by men competent and experienced in that industry. 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 of buildings. The first step is to establish the route of the pipes, and in this matter the local conditions so fully control the layout that little can be saiHdarveignagrdeisntgabitli.shed the route of the pipes, the next step is to calculate 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 reach a very high figure. Velocities of 35,000 fpm are not con sidered high. By the use of this method the pipe sizes are kept to a minimum with consequent savings in investment. The steam flowing through any section of the piping can be computed from a study of the requirements of the several buildings served. In general a condensation rate of 0.25 lb per hour per square foot of equiva lent heating surface is a safe figure. This allows for line condensation which, however, is a small part of the total at times of maximum load. Miscellaneous steam requirements such as laundry, cooking, or process should be individually calculated. The steam requirements for water heating should be taken into account,