Tower Silo Capacities

Agdex#:	100/732
Publication Date:	02/88
Order#:	88-033
Last Reviewed:	09/96
History:	Replaces Factsheet #82-076, "Tower Silo"
Written by:	J.C. Jofriet - School of Engineering/University of Guelph; T.B. Daynard - Crop Science/University of Guelph

Table of Contents

1. Introduction
2. Concrete Silo Capacities for Forages
3. Steel Silo Capacities for Forages
4. Concrete Silo Capacities for High Moisture Corn

Introduction

The capacity of a tower silo depends on: (a) the compressibility of the stored material; (b) the total volume of the silo; (c) the friction between the wall of the silo and the material; and (d) the ratio of the settled depth of silage to the diameter of the silo. This Factsheet contains estimates of tower silo capacities for alfalfa silage, whole-plant corn silage and various types of high moisture corn based on a series of analyses of silo capacity carried out at the University of Guelph and on research done at research establishments in the Netherlands and Sweden. Values shown are more accurate than those available in previous extension publications. Most previously published capacity tables overestimate capacities for corn silage and underestimate those for high moisture corn.

Tables 1 and 2 show the capacities, in tonnes, of most common silo sizes for alfalfa and whole-plant corn silage. The capacities in Table 1 were obtained using a friction coefficient of 0.6, a reasonable estimate for as-cast concrete walls. Table 2 capacities were calculated using a friction coefficient of 0.4; this coefficient has been found appropriate for glass-lined steel walls. Concrete silos coated inside with a hard glass-like coating will probably have capacities approximating those in Table 2. Badly corroded rough concrete walls, on the other hand, may well reduce the capacity below that shown in Table 1.

Table 1. Estimated concrete silo capacities for forages, in tonnes.^(a)

^(a) 1 tonne 1000kg: the capacity in tons (2000 lb) can be obtained by multiplying the capacities in the table by 1.1
^(b) moisture content in percent (wet basis)  

Table 2. Estimated steel silo capacities for forages, in tonnes.^(a)

^(a) 1 tonne 1000kg: the capacity in tons (2000 lb) can be obtained by multiplying the capacities in the table by 1.1
^(b) moisture content in percent (wet basis)

No tests have been carried out at Guelph for grass. Based on work done in the Netherlands grass consolidates somewhat more than alfalfa. Capacities for grass silage may be estimated from Tables 1 and 2 by increasing the alfalfa silage capacities by 10, 15 and 20% for moisture contents of 50, 60 and 70% respectively.

Table 3 presents silo capacities for high moisture corn. Capacities for whole shelled and ground shelled corn, as well as ground ear corn are shown. The capacity figures in Table 3 were calculated on the basis of a friction coefficient of 0.6 an appropriate estimate for concrete. High moisture corn is far less compressible than forage and, therefore, friction has a much less effect on capacity. It is recommended that for very smooth wall surfaces, such as glass-lined steel or epoxy coated concrete, that the capacities in Table 3 be increased by 3%.

Table 3. Estimated concrete silo capacities for high moisture corn, in tonnes.^(a)

The capacity tables are based on fully settled silage. The silo height may be used if the silo is topped up one to two weeks after initial filling. If this is not done the capacities in Tables 1, 2 and 3 should be determined on the actual settled depth of silage after about two to three weeks rather than the silo height. Silage may settle between 10 and 15% during the first two weeks after filling. Another adjustment should be made if space in the top of silo allows the piling up of material in a cone-shaped heap. To allow for this pile, one-third of the height of the cone can be added to the settled depth of the silage measured at the base of the cone.

There is no significant difference in capacity between topunloading silos and those with sweep-arm bottom-unloading equipment. However, the tables do not account for the large cavity formed with flail-type bottom-unloading equipment and should not be used directly for silos thus equipped.