Document dn6EN2JKb5j7M6z128V9Re80G

388 CHAPTER 22 1965 Guide And Data Book (2) the necessity of controlling temperature within a struc ture, cabinet, pipe, vessel, duct, etc., to a specified value, and (3) building structure limitations. Extensive studies of all factors affecting the true economic thickness are warranted only for large installations. For par ticular applications, considerations other than thickness alone may be important. Temperature limit, specific heat, moisture absorption, rnpr.hanirAl strength, health, safety, and flame resistance should all be considered when choosing an insulation for installation. The manufacturer should be consulted for information regarding these properties. The economic thickness of insulation may be determined from Fig. 4 as follows: From the number of hour* of operation per year at the bottom of the chart proceed upward to the line showing value olhcatm dollars per mWiTM Btu using the appropriate scale A or B. Pro ceed right to temperature difference line using other scale A or B Move upward to the line representing conductivity of the ' hring considered, then left to discount line, upward to the percent fixed charge line. Move right to the pipe aseor Oatsurface line and read the economic thickness at top on the scale corresponding to the scales selected for value of heat and tern- perature difference. For example, with Scale B heat value and A temperature difference, or vice versa, use scale A-d economic thickness. If the intersection with the flat surface ta on the flot-eurface-contimied curve, the economic thickness is read on the prime scales A'-A', A'-B' or B'--B'. Cost of insulation refers to incremental cost in place; i.e., average incremental cost per unit thickness after the first unit. The first inch of insulation will usually cost more than each additional inch. This is due to cost of finish, contractor's setup, and operating costs, which do not increase in proportion to thickness. The first inch of insulation, except in very unusual cases, provides enough saving in heat cost over bare surface cost to more than cover these extra first costs. To obtain discount from insulation list price, first obtain applied prices for the desired type of insulation in one-inch, two-inch, etc., thickness. Then determine the average incremental cost per inch above one-inch thickness. Incremental cost in percent discount from list is 100 (>-f) where X -- average incremental cost in place per inch above one inch. Y a list price for l in. thickness. For pipe insulation, use the list price for 1 in. thickness for a pipe rigp consistent with the average incremental cost per inch. For example, if the average incremental cost per inch is based on applied prices for 1,2, and 3 in., the incremental cost of the second and third inches would be averaged and the pipe sise for list price would be that nearest the average pipe rixe of the second and third inch. , ,. Reference 16 provides a proven and precise method lor determination of'economical thickness for heating applica tions and is especially suitable where large installations are to Table 2 .... Typical Thickness of Insulation for Refrigerating Equipment (For Fht Surface* attd Average Operating Condition) F -15 0 Oto 15 25 25 to 35 35 to 50 50 to 60 0.20 7 5 5 4 3 2 2 Thermal Condecfmfy k at 70 F 0.25 0.30 0.35 0.40 8 10 12 13 6 8 9 10 6789 566 4456 334 2233 0.45 15 12 10 8 7 5 3 THICKNESS TO PREVENT SURFACE CONDENSATION OmHcncarinn occurs when the water vapor in air comes in contact with a surface whose temperature is lower than the dew-point of the air. In order to prevent condensation from occurring on the warm ride of insulated rooms, pipes, ducts, equipment, etc., a sufficient thickness of insulation should be used to insure that the insulation surface temperature always exceeds the dew-point temperature. It is impossible to provide insulation of sufficient thickness to prevent condensation at 100 percent relative humidity conditions, because the heat grin would have to be reduced to zero by ig an infinite thickness of insulation. Pig. 5 illustrates the steady-state heat transfer for which the follow ing relationship may be used to calculate the data given in Pigs. 6 and 7. (A - Dp) (D, - U) q " ft, R< where Q _ heat flow rate, Btu per (hour) (square foot). - ambient still air drv-bulb temperature, Fahrenheit. Df - dew point, Fahrenheit. . ft, * surface"thermal resistance ** aes F deg per Btu/(hr) L -- thickness, inch (as read from Pig. 7; flat surface or equivalent thickness). ,,., . k - rr.e*n th-rmal conductivity, Btu per (hr) (sq-ft) (F deg /in-)- SRECTION OF THICKNESS Thermal ingntotinn is used in a variety of thicknesses for many reasons. Refrigerating equipment is normally designed for minimum heat gain, and heating equipment for min imum heat loss, within economical limits. . Table 2 lists insulation thicknesses in accord with general practice for refrigeration requirements for average conditions in northern climate tones. Local climatic or unusual conditions of service would determine the insolation thickness to be used. Thermal Insulation and Water Vapor Barriers Equivalent thickness is the thickness of insulation on a flat surface which would be required to give the same rate of heat trtusniission square foot of outer surface of insulation as on a cylinder or pipe. L = r, log, r,/n where r a outer radius of insulation, inches. r`. m inner radius of insulation, inches. The approximate thickness of insulation required to prevent condensation on pipes and flat metallic surfaces may be ob tained Figs. 6 and 7. In Pig. 6, U is the mean temperature (F deg) for k of a gp^rifin inmtoting material, and is taken as the arithmetic average of A and t: I. - A + (C/2) Actual thickness of pipe insulation is determined from Pig. 7 of actual thtoimrea versus equivalent thicknes. Enter Pig. 7 with L as equivalent thickness determined from Fig. 6 for conditions. Always select next higher nominal thick ness. Example 1: A nominal 4 in. pipe is operating at 10 F in ambient gtiU air conditions of 90 F ana 90 percent relative humidity. De termine the thickness of fibrous glass insulation necessary to prevent condensation. Solution: Assume a nominal'4 in. pipe operating at 10 F in ambient still air conditions of 90 F and 90 percent relative hu midity. In Fig. 6, read up from C " 10 (arrow) to 90 percent RH group, intersecting midcue line, representing 90 F; then read left to Insulation Resistance (ft,- 14.8. Read down from C to the 4, (mean temperature) eerie marked (at the right) 90 F. The 389 value at that point is SO. Assume that the insulation specified hasa i of 0.28 at Is of 50 F, according to manufacturer's tables! Since ft< L, the thickness required for a flat surface or the equivalent thick-M^a in inches () for any pipe rise is: L - Rik =* 14.8 X 0.28 -- 4-144 (for flat surface, increase to 4} in.). To find the actual thickness of a 4 in. pipe, read the Equivalent Thickness scale in Fig. 7, from L of 4.14 to 4 in. pipe sizeline, and find actual thickness of 2} in. on left ordinate. Select next higher standard thirkrw of 3 in. PART IV: GENERAL PRACTICE FOR BUILDING INSULATION Building insulation products should be used for the purpose and in the manner recommended by the manufacturer, who should be consulted for detailed application instructions and also regarding specific problems or unit-anal conditions in volving tiie use of ingutotion. It is necessary that all materials be handled carefully and applied correctly in order to assure (hat the calculated heat loss or heat gain through the insulated sections not be exceeded. Vapor barriers are necessary to re- F*9- 6 .... Thermal Resistance of Insulation to Prevent Surface Condensation fig. 7 .... Conversion of Equivalent Thickness to Actual Thickness for Pipe Insulation