The Transmission of Low-pressure Steam for Process Heating

Abstract

In this paper is shown how in the case of steam transmitted entirely for use in process heaters, it is the total heat energy that finally counts; and hence a pipe-line can be designed for a high pressure drop and a high velocity of flow, since the friction work and the kinetic energy will be reconverted into heat which is also available at the outlet of the pipe-line. The size and cost of the piping and lagging can therefore be kept low, and radiation losses will also be reduced.

In a long transmission, however, the very rapid increase of velocity towards the end of the pipe-line may cause the steam velocity to reach the “acoustic” value; and when this occurs a “choking” effect results in the discharge being lower than the expected value.

The method of design proposed is to calculate the size of pipe required so that the given length of pipe-line is an assumed fraction, say, 0.80 to 0.90 of this “choking” length. This ensures that the required discharge will be obtained with the minimum size of pipe, and with no danger of “choking”.

Simple formulae are derived with the help of the approximate Callendar expression for total heat, which is applicable with reasonable accuracy at low pressures; and a case is worked out for a combined power and process steam installation, which shows that the problem may have several solutions of equal heat utilization efficiency.