Hot water pipes and hot water tanks that are uncovered lose by radiation from their surface about 13 B. T. U. per minute per square foot of surface. To prevent this loss of heat and consequent extra consumption of coal, hot water pipes, circulation pipes, and hot water tanks in large installations are usually covered with some non-heat conducting substance.

The relative values of different makes of pipe coverings, as determined by tests conducted by Charles L. Norton, of the Massachusetts Institute of Technology, for the Mutual Boiler Insurance Co., of Boston, can be found in Table LX.

Carbonate of magnesia is a very poor conductor of heat, therefore it is a good material for covering hot water pipes. The name "Magnesia," however, is often applied to pipe coverings made of carbonate of lime, or of plaster of paris. The following table shows the percentage of lime and magnesia found by C. L. Norton in several well-known brands of "Magnesia" coverings:

Table LXI - Lime And Magnesia In Pipe Coverings

Data on pipe covering taken from Circular No. 6 of the Mutual Boiler Insurance Company, of Boston.

Mineral wool, which was always considered a good covering, was not reported upon by the above experimenter, for the reason that mineral wool is of no value as a heat retardant.

"Under vibration it is apt to become more and more massed into a semi-solid, leaving the top of a pipe partially covered, the under side of the covering more and more solid and therefore less effective. It is a dangerous material to handle and to use. The fine dust getting under the nails creates irritation and sometimes bad sores, or, passing into the bronchial tubes and the lungs, sometimes causes hemorrhage."

The conclusions to which we have been led by the tests on which report is now made, are as follows:

There are a sufficient number of safe, suitable and incombustible coverings for steam pipes and boilers to maintain a reasonable and adequate competition, without giving regard to any of the composite pipe coverings which contain combustible material in greater or less quantity, according to the integrity of the makers, and without giving regard to pipe coverings which contain substances like the sulphate of lime, which may cause the dangerous corrosion of the metal against which it is placed. We therefore name as the pipe and boiler coverings which may have the preference in respect to safety from fire and efficiency in service, the following makes:

Hair felt and wool felt when new are good heat retard-ants, but deteriorate with age, and besides furnish a breeding place for house bugs and vermin.

The value of pipe coverings is not proportional to its thickness. Sectional pipe coverings average about 1 3/8 inches in thickness and reduce the loss by radiation about 90 per cent., doubling the thickness of pipe covering only saves about another 5 per cent, of heat loss. In specifying covering for pipes and boilers, therefore, a thickness of 1 3/8 inches will be sufficient.

Covering For Tanks

On account of the objectionable appearance they would present, range boilers are seldom covered to prevent loss of heat by radiation. Hot water tanks, however, are usually located in the basement or cellar where appearance is of less importance than the prevention of loss of heat, therefore they should be covered with about 1 3/8 inches of some good non-heat conducting covering. Tanks are generally covered with plastic asbestos troweled over a band of expanded metal, or asbestos blocks, Fig. 139, held in place by a wire netting.

Fig. 139