Document zd1047oV0nn88pyZdEVpYjdLR

634 CHAPTER 28 1954 Guide suit in heat transfer much greater than that shown in Tables 1 and 2 for transfer from bare pipe to air. The saving due to use of insulation on piping is illustrated in Example 4- Example 4". If the steam line given in Examples 1 and $ is covered with 1 in. thick 85 percent magnesia, determine the resulting total annual loss through the insulation. Also compute the monetary value of the annual saving and the percentage of saving over the heat loss from the bare pipe. Solution: By referring to Fig. 1, the coefficient for 1 in. magnesia on a 2-in. pipe is found to be 0.300 Btu per (hr) (linear ft of pipe) (deg temperature difference) at a temperature difference,of 169.4 F. The total hourly loss per linear foot of pipe will then be 0.300 X 169.4 = 50.8 Btu. The total annual loss through the insulation = 50.8 X 165 (linear ft) X 4000 (hr) = 33,500 Mb. The annual bare pipe loss as deter mined in the solution of Example t was found to be 181,600 Mb. The saving due to insulation is then 181,600 -- 33,500 = 148,100 Mb per year. From the solution of Example 3, it was found that the heat supplied to the system cost $0,804 per thousand Mb. Therefore, the monetary value of the saving = 0.804 (dollars) X 148.1 (thousand Mb) = $119.07, or 81.5 percent of.the cost when using uninsulated pipe. LOW TEMPERATURE PIPE INSULATION Surfaces maintained at temperatures lower than the surrounding air are insulated to reduce the flow of heat and to prevent condensation. The insulating material should absorb a minimum amount of moisture, because the absorption of moisture substantially increases the conduc tivity of the material. This property is particularly important in the insulation of surfaces that are below the dew-point of the surrounding air. In such cases, due to vapor pressure difference, it is necessary to seal the . surface of the insulating material against the penetration of water vapor which would condense within the material, causing a serious increase in heat flow, possible breakdown of the material, and. corrosion of metal surfaces. An insulating material with a high degree of moisture absorp tion might pick up moisture before application and then, when the seal is in place and the temperature of the insulated surface reduced, release that moisture to the cold surface. There are a number of methods of pro ducing vapor seals, some of which have been worked out by insulation manufacturers to suit their products, and others by applicators and users. Unless time-proven methods are known, specifications of insulation man ufacturers should be obtained and followed carefully. Piping must be carefully protected against corrosion caused by condensa tion of water vapor. All metallic surfaces should be coated with a vaporimpervious barrier (some types of which have an asphaltic or tar base) without any breaks or openings in order to prevent corrosion. The thickness of insulation required to prevent condensation on the outer surface is that thickness which will raise the temperature of the outer surface of the insulation to a point slightly higher than the dew-, point of the surrounding vapor. The dew-point for various humidities can be readily ascertained from a psychrometric chart. The external vapor barrier must be made as nearly perfect as possible in order to prevent leakage of vapor into the insulation. The approximate required thickness of insulation to prevent condensa tion on pipes and flat metallic surfaces may be obtained from Fig. 5 m which a surface resistance of 0.606, corresponding to a film conductance of 1.65, was used in calculating the curves. This value provides a slight factor of safety and its use is known to give satisfactory field results. In using the chart it is advisable to specify the next thicker, rather than Pipe Insulation -I I. * Solve problems as indicated by dotted line, entering chart at lower leit-handle .1 .1 Fio. 5. Thickness op Pipe Insulation to Prevent Condensation on Outer Surface* the next thinner, commercial insulation in cases where an intermediate thickness is indicated. Heat gains fpr pipes insulated with a material having an installed con ductivity of 0.30 Btu per (sq ft) (hr) (F deg per in.) are given in Table 8. This table may be used for any of the commercial insulations offered for this purpose since they have conductivities very near the 0.3 value used. INSULATION OF PIPES TO PREVENT FREEZING If the surrounding air temperature remains sufficiently low for an ample period- of time, insulation cannot prevent the freezing of still water, or of Water flowing at such a velocity that the quantity of heat carried in the water is not sufficient to take care of the heat losses which will result and cause the temperature of the water to be lowered to the freezing point. . Insulation can materially prolong the time required for the water to give UP its heat, and if the velocity of the water flowing in the pipe is main tained at a sufficiently high rate, freezing may be prevented. Table 9 may be used for making estimates of the thickness of insula tion necessary to take care of still water in pipes at various water and surrounding air temperature conditions. Because of the damage and