Pressure Ft Per Minute 340 1597 200 1384 100 892 80 804 60 700 40 574 a

PRESSURE FT. PER MINUTE 340 15.97 200 13.84 100 8.92 80 8.04 60 7.00 40 5.74 A full account of some interesting tests can be found in a paper entitled Protection of Steam-Heating Surfaces, by C. L. Norton, Vol. XIX, Proceedings of the American Society of Mechanical Engineers, 1898, from which t hese 't ables have been taken.

Pipe Coverings.To make this loss from radiation as small as possible, it is customary to cover the pipe or boiler with some material which will prevent loss of heat and which will not burn. There is considerable difference in the value of various substances as preventatives of heat radiation. Their value varies nearly in an inverse ratio to their conducting power; but due allowance must be made for the possible deterioration of the pipe covering. The following table gives the relative value of various substances with reference to their ability to prevent radiation of heat. For purposes of comparison, the value of wool is taken as the standard: Relative Values of Various Preventatives of Radiation of Heat Felt, Hair, or Wool 100 Asbestos Sponge 98 Air-Cell Asbestos 89 Mineral Wool 68 — 83 Carbonate of Magnesia 67 — 76 Charcoal 63 Sawdust 61 — 68 Asbestos Paper 47 Wood 40 — 55 Asbestos, Fibrous 36 Plaster of Paris 34 Air Space (Undivided) 22 There are many patented coverings which are very efficient, but they are too numerous even to mention. The above-mentioned article from the Proceedings of the American Society of Mechanical Engineers gives the results of tests of several of these coverings. A good protection is afforded by air confined in minute cells, such as is to be had in the air-cell asbestos board; this is made by cementing together several layers of asbestos paper which have been corrugated or indented by machinery so as to form minute air-cells. The more minute the subdivision of these cells, the better the protection is likely to be. Hair felt is one of the most efficient non-conductors, because it is very porous and contains a large number of air-cells. It is not one of the best coverings, however, because it is liable to deteriorate, and its life on high-pressure pipes is not likely to be more than four or five years. On low-pressure work it may last for a considerably longer

time.

Mineral wool, a fibrous material made from blast-furnace slag, is an efficient and noncombustible covering, but is brittle and liable to fall off.

The coverings most easily applied to pipes are those applied in sectional form, which clasp around the pipe and are fastened by brass bands at convenient intervals. Such coverings are made both of asbestos and of magnesia, and are usually of about 1 inch in thickness.

A good, cheap covering can be made by wrapping several layers of asbestos paper around the pipe, and then covering these layers with a layer of hair felt perhaps inch thick, the whole being wrapped in canvas. On low-pressure steam pipes this covering will last ten to fifteen years.

Cork is perhaps one of the most satisfactory coverings from the point of radiation loss, but is rather more expensive than asbestos or magnesia.

It has generally been the impression that it is not economical to cover a pipe to more than one inch in thickness. This will depend upon the cost of the covering and the length of time it is likely to last. If it does not last more than five years, one inch is probably the most economical thickness; but if the life of the covering is likely to be ten years or more, a second inch in thickness can be applied to advantage. For instance, in the above-mentioned tests, in the case of "Nonpareil cork," increasing the thickness from one to two inches raised the cost from $25 to $30 per 100 square feet, and increased the net saving in five years by $10, and by $30 in ten years. A third inch of covering did not produce saving enough to pay for its cost. In each case with the asbestos fire-board, a second inch in thickness showed a saving of $20 in ten years, while the third inch in thickness showed an actual loss from the dollars-and-cents point of view. It would be well to remark that it is of great importance that the pipe covering should be kept in repair, for a loose-fitting covering is of little value.