Document 82nqGYG3YBLZOq04Q18OYoD9e

of 1936American Society Heating and Ventilating Engineers Guide, found to be 0.622 sq ft. The total annual loss through the insulation = 89.04 X 0.622 X 165 (linear feet) X 4000 (hours) = 36,550,000 Btu. The annual bare pipe loss as determined in the solution of Question 1 was found to be 188,000,000 Btu. The saving due to insulation is then 188,000,000 -- 36,550,000 = 151,350,000 Btu per year. From the solution of Question 2 it was found that the heat supplied to the system cost $0,804 per million Btu; therefore, the monetary value of the saving = 0.804 (dollars) X 151.35 (million Btu) = $121.69, or 81.2 per cent of the cost when using uninsulated pipe. 5 The manufacturer's list price for 85 per cent magnesia insulation is $0.36 per linear foot for 1-in. (standard thick) material to cover a 2-in. pipe. De termine the period of time required for the saving found in Question- 3 to pay for the cost of the insulation if it can be purchased and applied at 80 per cent of list price (20 per cent discount). The applied cost of insulation = 165 (linear feet) X 0.36 (dollars) X 0.80 (net) = 47.52. Since the annual saving as found in Question 3 amounts to $121.69, the in sulation will pay for its cost in 47.52 + 121.69 = 0.3905 years; in other words, the cost will be repaid 2.56 times by the saving obtained in one heating season. 6 The conductivity of magnesia insulation is 0.455 at the mean temperature which will result under the conditions of 'Question 3. Estimate the most economical thickness of magnesia for application on the pipe when operating under the conditions which are given in the foregoing problems and when a 20 per cent return is required on the investment for insulation. Use chart given in Fig. 3. Begin at the left bottom margin and proceed successively as shown by the dotted line example to the following essential data which are collected from the problems previously given: 4000 hours operation per year. $0,804 value of heat, dollars per million Btu. 169.4 deg temperature difference. 0.455 conductivity of insulation. 20 per cent discount from list, cost of insulation. 20 per cent fixed charges, return on investment. 2-in. pipe size. Solution of the problem by use of Fig. 3 results in a required thickness of approximately 1.05 in. The nearest commercial thickness procurable is standard thick (1H2 in.) magnesia. (It is of interest to note that the use of Fig. 3 will generally result in solutions which, for all practical pur poses, agree closely with the specifications for thicknesses given in Table 13.) 7 Determine the minimum thickness of wool felt insulation having a con ductivity of 0.30 necessary to prevent condensation of moisture'on a 4-in. pipe carrying cold water at a temperature of 40 F when the surrounding air reaches maximum conditions of 90 F with a relative humidity of 90 per cent. The difference between the temperature of the pipe and the surrounding air is 90 -- 40 = 50 deg. For quick estimating purposes use the chart given in Fig. 2. Enter this chart at the lower left margin on the 90 per cent relative humidity line and proceed horizontally to the right to intersect the 90 deg air temperature line. Project a line up to the 50 deg temperature difference line, and then horizontally to the right to the intersection with the 4-in. pipe size line. From this point proceed down to intersect the 0.30 line which denotes the conductivity of the insulation. Directly opposite this point of intersection the correct thickness of insulation is read from the scale on the lower right margin. This chart solution denotes that wool felt 2.4-in. thick is sufficient to, prevent condensation. The nearest commercial thickness procurable is 2J in. For prevention of condensation as well as for protection against freezing, if the thickness determined theoretically cannot be had, it is better to apply the next greater thickness procurable rather than to use any lesser thickness because an additional factor of safety is thus obtained. 670 ^ Chapter 37 i DISTRICT HEATING Underground Steam Piping, Selection of Pipe Sises, Provision for. Expansion, Capacity of Returns with Various Grades, Pipe Con duits, Pipe Tunnels, Service Connections, Steam per-Square Foot of Heating Surface, Fluid Meters and Metering, Rates 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 and for the design of heating systems for buildings which are to be supplied with purchased steam. A complete district heating instal, lation should not be attempted without a thorough study of the entire problem by men competent and experienced in that industry. UNDERGROUND STEAM 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 said regarding it. Having established 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. Any unusual requirements such as those for process steam should be individually calculated. The steam requirements for water heating should be taken into account, but in most types of buildings this load will be relatively small compared with the heating load and will seldom occur at the time of the heating 671