BOILER ACCESSORIES: LAGGING When steam pipes are exposed to the air, a considerable amount of condensation will collect in them, depending on the condition of the surface of the pipe, on the difference in temperature between the steam and the surrounding air, and on the velocity of the steam through the pipe. This condensation will cause a large amount of heat to be lost to useful work, and will make the dangers of waterhammer possible unless carefully drained. Tests have shown that about 2 B.T.U. are lost per square foot of pipe per hour per degree of difference in temperature. While the loss for a few hours is not likely to be great, yet, if taken for an entire year throughout a considerable length of pipe, the sum total will be very large indeed. The following table gives some idea of the loss of heat through bare pipe at 200 lbs. pressure: Heat Losses in Bare Pipes B.T.U. Loss PER SQ. FT.