The Computation of Natural Circulation in Large Boilers

Abstract

At present, many large capacity boilers are being built or are in the preliminary stages of design. At some phase of the design stage, the circulation characteristics of the unit must be determined in order to properly apportion riser and downcomer circuits. The simplest method of doing this would be to extrapolate the existing experimental data on similar units. However, boilers are usually built to a customer's specifications and in general no two units are made identical for the same set of operating conditions. Thus, in extrapolating, it is very possible to introduce serious faults in design because of the inapplicability of the extrapolated data. The technical data available in the literature contains many cases of overheating and subsequent failure of boiler tubes due to faulty and inadequate circulation.

The work of Munzinger in 1922 has been the basis of many papers written on the subject of the circulation of steam and water in natural circulation boilers. In order to obtain a workable solution, various assumptions as to the rate of heat absorption, friction coefficients, and the state of the mixture of steam and water were made. Since the appearance of this work, many excellent papers have been published on the mechanism of heat transfer and the mechanics of boiling. While much work been done and is being done on this subject, it is still not fully understood and remains a fertile field for further investigation. In order to fully develop the subject, this work has been divided into the following three sections:

1. A qualitative description of the mechanics of boiling, i.e., the change of water to steam in a vertical tube according to the latest available data.

2. The mathematical development of all the necessary equations needed to compute the circulation characteristics of a natural circulation boiler. This section also includes curves that have been developed to facilitate circulation computations.

3. A large "Twin Furnace" unit on which circulation tests were made is completely analyzed by means of both the equations and curves of Section II. In addition, two topics of interest are discussed in this section.

A comprehensive bibliography is to be found at the end of this work. It is hoped that this bibliography will be of use for further investigation in this field.