Science has found that the food value of young spring grass can be preserved indefinitely by drying, and machinery has been evolved which enables grass to be dried and stored for the winter feeding of livestock
THE CURTIS-HATHEROP MACHINE is arranged to dry grass in two stages. Wet grass is first placed on a perforated tray, through which hot gases rise from the furnace. In the second stage, the grass is dried in an enclosed tray. There are two pairs of trays, the grass being dried in one pair, while the filling, transferring and emptying processes are taking place in the other pair.
ONE of the most significant developments in agriculture of recent years has been the new practice of grass drying. More than seventy patents were taken out in connexion with grass drying equipment in 1936 and 1937, and no previous development in British agriculture during the last fifty years has created such great interest or advanced so rapidly. Grass driers are used to dry young spring grass in such a way that it will preserve its properties for winter feeding of cattle and other livestock. The machines are sometimes used also for haymaking in bad weather.
It has been discovered by scientists that young spring grass has a high protein value, and that a substance known as carotene is also present in it. This substance is of great value for increasing the milk yield of cows. The science of grass drying consists of drying the grass so that the moisture is driven off without destroying these two valuable constituents. Long and patient research has shown that this goal can be achieved if the correct methods are adopted.
Grass has a high content of moisture and is a poor conductor of heat. When heat is applied, the leaves give up their moisture almost immediately, and when the grass is dry it can stand temperatures up to 300 degrees Fahrenheit without suffering chemical change. It is fortunate, also, that layers of grass are pervious to the passage of air, if not subjected to any great pressure.
Thus, grass can be dried by passing heated air through it, and the ideal grass drier will discharge the air at the lowest possible temperature for the whole of the drying process, the heat being used to evaporate the water. The engineer strives to design a plant which will provide a high temperature for the wet grass and at the same time will ensure that a temperature in excess of 300 degrees Fahrenheit is not imposed upon the dry grass.
Grass is composed of a large number of cells, each of which is surrounded by walls of cellulose material. This material varies considerably in composition, some of it being hard and almost impermeable by heat, and some quite soft and easily penetrated. The most valuable food materials are contained in the living part of the cell, where they are dissolved in the water.
At certain times of the year, especially after heavy rain, the young grass may contain as much as 90 per cent of water. On the other hand, in dry weather, or when the grass has been allowed to dry in the open for a time, the water content may only be about 50 per cent, the parts of water and dry material thus being equal. This means that with a water content of 90 per cent, nine tons of water must be got rid of for every ton of dry matter; with a water content of 50 per cent of water the ratio is ton to ton.
In addition to water within the grass, such water having been abstracted from the ground, there is also a considerable amount of water on the surface of the leaves, which may have come direct from rain or dew. It is far easier to remove this surface water than it is to remove the water contained in the structure of the grass. The water in the grass must needs travel through the cell walls and so to the surface before it can be evaporated. Moisture on the surface evaporates as soon as heat is applied.
The skill of the engineer is called upon to dry the grass at such a rate that, the surface moisture will not be driven off too rapidly, leaving the grass in a scorched condition and destroying its value. Thus he must strike a nice balance between the evaporation of the surface water and the rate at which water will come to the surface from the inner structure. In the various types of drier now commonly used the grass may be stationary on trays, moved slowly upon a conveyer, or turned over slowly in a rotating drum. By this means the grass is in contact with the hot air for a considerable time.
All driers have one thing in common — a large chamber in which the grass is heated and dried. The temperature in the usual form of small drier used on the average size of farm is generally about 150 degrees centigrade. The grass must be dried properly and economically and the machine must be so built that it is easily and simply controlled. Grass which has been properly dried comes out of the machine with its colour and feeding value unimpaired, and it can be stored indefinitely before it is consumed.
Cost of fuel is of great importance in grass drying. In ideal conditions 1 lb. of coke should evaporate about 10 lb. of water. This ideal is never achieved, because there are considerable heat losses in the drier by radiation, in addition to the heat carried away by the air which leaves the drier. In practice 1 lb. of coke will evaporate about 6 lb. or 7 lb. of water.
Few Working Parts
To the farmer, the most important point is that of economics. In a drier, the production of dried grass must vary according to the amount of moisture in the wet grass. The design of the apparatus will, therefore, depend upon this factor. In general, it is safe to assume that there will be four parts of water to one part of dry matter, so that if a drier is to deal with 3½ cwt. of dried grass an hour, it must be capable of evaporating 14 cwt. of water an hour.
It has been found in practice that a three-men drier, in which two men handle the grass and one man bales or binds the grass after drying, will have a maximum output of about 6 cwt. of dried grass an hour. With the type of drier in which two men do all the handling and attend to the furnace, the average production will be about 2¾ cwt. an hour and a one-man drier will produce about 1¼ cwt. an hour. Whatever the size of the drier it must be simple to build and easy to operate.
After intensive research, ingenious plants have been devised for this work, and special care has been taken to reduce the working parts to a minimum. The equipments are also of necessity foolproof and extremely robust, to withstand the rough usage to which they will be subjected on the farm. A simple furnace may be used for supplying the heat, but a better and more efficient arrangement is known as a “producer”. In this apparatus, gas is formed by drawing air through a bed of red-hot coke, the gas being burned at jets which come between the coke and the drying chamber containing the grass.
COMBINED GRASS-CUTTER AND ELEVATOR used on large grasslands. Drawn by a motor tractor, the cutter reaps the grass and stacks the cut material so that it falls into the trailer behind.
By this method the heat can be easily controlled and there is no possibility of soot being deposited on the grass. There is still some doubt as to the best method of heating the grass, just as it remains to be settled whether it is better to have a drier in which the grass is taken through the heating chamber on a moving conveyer, or to have it forked over stationary trays. The former method requires machinery, which increases the initial cost and adds to the running cost of the drier, but the latter method is somewhat laborious.
A point to be borne in mind in connexion with the design of grass drying equipment is that the moisture content in grass varies from day to day. Thus the rate at which the grass is fed through the drying chamber must be varied within wide limits, so that with a moving band drier continual adjustment of the working speed is necessary. It is difficult to predict this moisture content, and in this respect the fixed tray type of drier has certain definite advantages. There is no adjustment of a moving band, and all that is necessary is to leave the grass on the fixed trays for as long a time as is necessary to dry the grass thoroughly.
On the Continent driers have been in use for many years, particularly in Germany and the Scandinavian countries, where they are used for drying many different types of crop, in addition to grass. In America driers are used extensively for drying lucerne. In most instances these machines are of large size, and are costly, as they vary in price from £4,000 to £5,000. In Great Britain the types of drier in general use vary from the communal factory plants, producing between twenty and thirty tons of grass a day, to the small farm drier, producing about one ton a day. Large capital cost places the factory drier outside the scope of the small farmer, unless it is possible to organize the whole process of grass growing and cutting so that the factory is kept continually supplied.
Fire Bed Always Alight
The Curtis-Hatherop machine works with air at temperatures of from 280 to 300 degrees Fahrenheit. It is fired by coke or anthracite and can be supplied in two sizes, of 2½ to 3 cwt. and 3¾ to 4½ cwt. an hour capacity. This drier is arranged to dry the grass in two stages. Wet grass is first placed on a perforated tray, through which the hot gases are passed from the furnace and discharged to the atmosphere above the grass. The grass, having been partly dried on this tray, is transferred by hand to a second tray to complete the second drying stage. In this tray the gases are trapped in a cover and drawn back through a pipe to be reheated and recirculated. There are two such pairs of trays, drying operations being carried out in one pair while the filling, transferring and emptying processes are taking place in the other pair. The time required for drying varies according to the moisture in the grass, but the average is about twenty minutes. The amount of moisture on the surface of the grass has little effect on the time required for drying, as this surface moisture dries off rapidly. The air is circulated through the system by an electric fan, driven by a 10 horse-power motor. The furnace is of a special type known as a “semi-producer” furnace. The fire bed is only 2 ft. 6 in. thick, and is always alight at the outlet. There is thus no possibility ol the gases leaving the furnace unignited.
A series of interesting tests was recently carried out on one of these driers, the grass dried being a mixture of rye grass and clover. The duration of the test was 15½ hours, and the proportion of water to grass was in the ratio of 4.45 to 1. Some 3 cwt. an hour of grass were produced, giving an evaporation of 7.18 lb. of water per lb. of coke burnt. This figure compares favourably with that given to the Conference on Mechanized Farming held at Oxford in 1937, when it was stated that the average figure was about 8 cwt. of water per cwt. of coal.
UNLOADING GRASS AT THE DRIER, where it will pass through a heating chamber and be subjected to temperatures up to about 300 degrees Fahrenheit. At and below this temperature grass undergoes no chemical change.
Another stationary tray machine is the Billingham drier, which has been developed by Imperial Chemical Industries, Limited, pioneers in the introduction of grass drying into Great Britain. Driers in which the grass is conveyed through the machine on a moving band are represented by the Ransome and by the Petrie and McNaught driers. Machines which can be used either as mobile or as fixed plants are the Rowell-Sanderson and Kaloroil machines. The Kaloroil operates with a rotating drum and uses oil fuel.
In the moving-band type of drier the general design consists of a steel casing, which may be from 30 to 50 feet long; inside the casing is a floor made of perforated plates, over which the grass is drawn by a conveyer formed of cross-bars connected to an endless chain. The casing is divided into two chambers, and two fans are provided for supplying hot air to the respective chambers at the right temperature for the particular drying stage.
Air supplied to the first chamber has a temperature of 150° centigrade, and air to the second chamber is at a temperature of 104°. The speed of the conveyer can be altered by a simple adjustment, and the furnace can be fed by hand with coke or anthracite coal.
The Petrie and McNaught drier also operates on the conveyer principle, but here the casing is divided into four sections, and the hot air passes alternately upwards and downwards through the grass. The object of this is to ensure even drying of the grass, and also to prevent the grass from drying from the bottom upwards. The whole of the casing is formed of asbestos sheeting, so that the structure will not suffer any corrosion during idle periods.
One of the most important features of this drier is that it can be worked in the open, except for small sheds at either end to cover the loading and unloading bays. It burns coal in an automatic stoker, and it is stated that it can evaporate one ton of moisture for a cost of between 5s. and 6s. The output amounts to 5 cwt. an hour of dried grass, with a water ratio of 3½ to 1.
Radiant Heat
The Rowell-Sanderson drier in its mobile form consists of two separate units mounted on pneumatic tyres. It can be drawn by a tractor to the working site. In one unit are the fan, furnace and drying gear, and in the other there are six superimposed endless conveyers. An elevator raises the grass to the top conveyer, after which it traverses the drying chamber, falling from one conveyer to the other and being automatically ejected. In this drier the grass is subjected to radiant heat. The output of grass is about 5 cwt. an hour.
These examples of the grass driers available are quoted to indicate the general trend of design, and to emphasize the fact that the technique of grass and crop drying are dependent ultimately upon the work of the engineer. As the practice of grass drying increases many new ideas will doubtless be forthcoming, but the technique is only in its infancy and there is still a great deal to be learned about it.
The cost of grass drying is of great importance to the farmer, and this was reviewed extensively during the Conference on Mechanized Farming held at Oxford in January 1937. The costs quoted were those for 1936, and it is perhaps unfortunate that it was such a wet year, which made the costs unduly high. It was, however, advantageous to have obtained costs of grass drying in the worst conditions.
As it was a new process, lack of experience was another cause of comparatively high cost. The organization of the grass cutting, minor adjustments to machinery, and similar matters can be learned only in the hard school of experience. The costs given at the conference averaged about £6 a ton throughout the whole period of 1936. For the whole season, the average water content of the grass was about 5 to 1, which compares with about 3½ to 1 for 1934 and 1935, when rainfall was much less. This increase in the water ratio is stated to have been responsible for an increased cost of no less than 18s. a ton over the two years mentioned.
The aim of the farmer is to have the grass as dry as possible before it reaches the drier, and it is quite possible that engineering ingenuity will devise some method of squeezing the moisture from the grass when it is collected in the field. It has been found, for example, that by passing the grass between rollers it is possible to remove as much as from 1 to 1½ tons of water from every ton of dried grass. Another method which may be applied in the future is to dry the grass by centrifugal action, the wet grass being placed in a container which is rotated at high speed, thus throwing off the water which lies on the surface.
General organization of the whole process of growing and drying is vitally important. It is essential to reduce any idle period, when the fuel is burning but not drying grass, to the absolute minimum. For example, on several farms where comparatively high costs of operation were noted during the period under review, it was found that the fires were burning for weeks at a time, but grass drying was carried out only during eight hours a day and four on Saturday. The waste of fuel was considerable, as the driers cooled when they were shut down, and it required some considerable time to bring them back to operating temperature. Taken by and large, it is reasonable to assume that the cost of grass drying, taken over a normal season, will amount to about £4 or £5 a ton of grass dried.
Application to Other Crops
Another important aspect of the technique is the efficient management of the grassland, as the production of grass for drying requires the same attention as is devoted to hay and similar crops. An authority on the subject has stated that a yield of 50 cwt. of dried grass an acre during the season can be obtained in three or four cuts — thus forty acres must be cut for 100 tons of dried grass, eighty acres for 200 tons and 120 acres for 300 tons.
Grass cannot be cut in this manner without providing it with sufficient fertilizers to replace the substances extracted from the ground. This is where the chemical engineer comes into his own with fertilizers that give to the soil the richness which is so essential to healthy growth. Such fertilizers as nitro-chalk are absolutely essential for speeding up growth and increasing yield, and the grass produced from well-fertilized soil has a high content of protein and mineral substances. Recent experiments have shown that the percentage increase in the weight of a dry crop, when nitrogenous fertilizers are used, is as much as 79 per cent, and that the increase in the weight of nitrogen is 95 per cent.
Grass-drying plants can. be used also for drying many other types of crops. For example, the drying of potatoes has been practised in Germany for many years. In recent years lucerne has been dried in Norfolk. Corn crops, when young and green, provide excellent material for drying and winter rye and oats are also good.
Threshed grain has also been dried successfully, and grain drying is much easier than grass drying. There is only a small amount of water to be removed, but care must be taken that the temperature is carefully controlled if it is desired to use the grain for seed or malting. Too high a temperature will kill the germ in the grain.
BALING MACHINE, used for baling dried grass after its emergence from a drying chamber. The bale illustrated contains 85 lb. of dried grass.
