Augustus Egg, The Travelling Companions (detail), 1862, oil on canvas.
It’s late, and you’re still awake. Allow us to help with Sleep Aid, a series devoted to curing insomnia with the dullest, most soporific texts available in the public domain. Tonight’s prescription: Cold Storage, Heating, and Ventilating on Board Ship, a 1911 guide by Sydney Ferris Walker.
A cold store is a chamber that is built expressly to prevent heat from passing from outside to the produce inside. It is not possible to construct a chamber that will not allow some heat to pass through the walls, floor and ceiling, and this heat which is constantly leaking through into the chamber has to be removed, just as that of the produce itself is, and transported to the sea or the atmosphere. The quantity of heat that leaks through depends upon the difference of temperature between the inside and outside of the cold chamber, upon the construction of the walls, floor and roof, and upon the extent of the surfaces exposed to the action of the heat. Certain substances are good thermal insulators, just as certain substances are good electrical insulators, and the thermal insulators are used to prevent the ingress of heat into the cold chambers, in the same way that electrical insulators are used to prevent the egress of electricity from the I conductors. This fact is very often not understood, and is sometimes challenged, because the sizes are so different; but if it be borne in mind that the thickness of the walls of the chamber correspond with the thickness of the insulating envelope of a cable, or even of the insulation of the iron core of the armature of a dynamo machine, though they are much greater, while the air inside the chamber corresponds with the copper or the iron, it will probably be appreciated that heat leaks in through the thermal insulator just as electricity leaks out through the electrical insulator. Dry, still air is the best insulator known, and the other substances that are good insulators owe their property very largely to the fact that they contain a large number of very small air cells, across which the heat current has to pass.
Air in motion is a bad insulator. If the air which is confined in the walls of the cold chamber is able to move, and to set up convection currents, it will by their aid transfer heat from outside to inside. Moisture also is a bad insulator, and if present will very considerably lower the degree of insulation. The importance of good insulation can hardly be exaggerated. It is quite possible, if the insulation is sufficiently bad, that the refrigerating machinery may be grinding away uselessly the whole time, because the heat will be passing into the chamber as fast as it is being taken out. This may be seen from the following example: Peclet, an able French savant, gives the rate of transmission of heat through one inch of powdered wood charcoal, one of the substances used in cold storage work, as 0.63 units per square foot per degree F. per hour. Take a chamber cubical in form, 12 feet long on the side, giving a surface exposed to the heat of 864 square feet, and assume a difference of temperature of 50 degrees F. This would give a passage of 27,216 heat units through the walls, roof and floor on Peclet’s figures, per hour, and would require approximately a 2-ton machine to handle comfortably. (The measurement of refrigerating machines, i-ton, 2-tons, etc., will be explained later.) If we have, instead of one inch of the substance, six inches, as is far more common, the rate of transfer of heat through the walls will be approximately one-sixth that with the one-inch, and a machine designed for a half ton should easily deal with it. On the other hand, if the insulation is so reduced that the rate of transmission through it is (say) 100,000 units per hour, in a room of that size, the whole power of a machine designed for six tons would be required. Cold stores for ship work, and for a good many other places, are built up as follows: The space to be occupied by the cold chamber is surrounded by a double wall, which should extend to the decks, both above and below, both walls being formed of wood. Where it can be arranged; and where it is strong enough to stand the strains that are brought against the walls, they are built of matched boarding, securely nailed to uprights, the two sets of uprights being braced together and lined on the insides (the sides facing each other), and covered with waterproof paper. There are several papers made, consisting of tough manilla or other substances, saturated with substances impervious to water and to the rays of the sun, and these are used to line the space in which the insulating material is to be placed. Between the two walls, it will be understood, there is a space, broken up to a certain extent by the supports of the walls themselves, and into this space the insulating substance is poured and tightly packed. It is of great importance that the substance shall be so stowed that it cannot settle, or move in any way. The division of the space by the supporting timbers assists in this. If the insulation settles, leaving a space at the top, convection air currents are set up, which lower the insulation, and moisture may also get in. The best insulating materials are silicate cotton, cork and finely divided charcoal. Silicate cotton, or slag wool, as it is often called, is a substance made from the slag of iron smelting furnaces. It is very largely composed of silicon, and in its preparation the slag is remelted in a small furnace, to which an air blast is attached which blows the molten slag into fine hair-like threads, or wool, something similar to spun glass. In this condition it is full of small air cells, and when packed tight, and moisture excluded, it makes the best insulator known. Finely divided wood charcoal is prepared in the process of wood distillation. The wood that is employed very largely, in the United Kingdom, is the waste cuttings from the willow used in the manufacture of cotton reels. It owes its property of insulation very largely to the air cells before mentioned. It should be packed tight, and dry. Cork in a finely divided state is another good insulating substance. Cork is an external growth on particular kinds of oak, which grow in certain parts of Spain, America, and a few other countries. It contains a number of very fine cells with thin walls, all built up together, very much as a honey comb is built up, but with the walls very much thinner and the spaces much smaller. It is used in a finely divided or broken up state, as the other materials are, and also in the form of cork bricks, which can be worked into any position required. The bricks have the air cells, just as the small pieces of cork have, the latter having in addition the air spaces between the pieces. Cork bricks have the advantage that they enable a much better mechanical job to be made. With a ship knocking about in a sea way, this is a matter of considerable importance, as, if the walls are sprung and air admitted, the insulation may be practically destroyed. There are a number of other substances that are available as insulators. The problem, it will be understood, is very similar to that of insulating steam pipes, the same materials being used for the two purposes, where they can be. In the one case the object is to prevent heat of the steam inside the pipes from leaking out, while in the other the object is to prevent heat from outside from passing inwards. There is one important difference that should be noted, however. With high temperatures, still dry air is the insulator par excellence, especially where it can be applied in small quantities, as in the jackets of heating apparatus; but with cold storage apparatus air has not always such a good name, though it is acknowledged to be the best insulator; the reason being, the writer believes, that it is sometimes difficult to avoid convection currents. Another insulator is sawdust, which has been used a great deal on shore. It is a good and cheap insulator if applied quite dry and well shaken down. These two points are essential, and they are sometimes difficult to insure in sawdust. Other substances are asbestos and magnesia fiber, peat, ashes, fossil meal and finely divided mica. Felt, cow hair, infusorial earth, cotton wool, sheep’s wool—all are used, to a certain extent, but silicate cotton, charcoal and cork are the principal ones. In constructing the cold chambers on board ship, the outside skin of the insulating wall should be kept away from the ship’s side, and from any bulkhead that is in metallic connection with the ship, the engine room, or stoke hold. As far as it is possible, keep the cold chamber with a layer of air around it, but arrange if possible that this air be perfectly still and perfectly dry. The more nearly this can be attained, the lower will be the cost of running the apparatus. The case is exactly similar to that of electricity and to that of steam, though the direction of motion of energy has to be reversed. If steam or electricity are allowed to leak out, more coal has to be consumed to make up the loss, and the accessories are more heavily worked. If heat is allowed to leak into the cold chambers, more coal has to be consumed in carrying it away, and there is also more work on the accessories. It is a good plan, when the mechanical conditions will allow it, to divide the insulating wall into two portions; the outer portion being simply an air jacket, and the inner portion carrying the insulating substance. Both should be lined with waterproof paper. In small cold stores, air jackets are often used alone with a waterproof paper. It was stated above that the requirements as to temperature varied very considerably with the kind of produce carried, and it follows, therefore, that the construction of the cold chambers will vary though the main lines will be the same. For the large quantities of frozen sheep for instance, that are carried from New Zealand to the London docks—cargoes of 100,000 to 120,000 carcasses being carried—the bulk of the fore and after holds are converted into huge cold chambers, much as the holds are in petroleum tank steamers, except that there is no attempt at division of the holds for meat carriage. Probably the carriage of frozen mutton, with the few crates of frozen rabbits which are taken to fill up, represent the simplest case of cold storage transport. Freezing hard is the order, and as long as this is carried out there is no trouble, and freezing hard is comparatively easy. It is only a question of driving. The sheep are frozen near where they are killed; each is enveloped in a loose linen bag, with the owner’s mark on it, and they are stowed in rows and tiers in the holds. No harm can come to them, so long as the temperature is maintained at a certain low figure, and this is easy to accomplish, provided that everything has been properly carried out in the matter of insulation. It is like the case of the ship which has only to drive as hard as its engines and boilers will allow it, without thinking of the cost. If the insulation is good, and even if it is only moderate, low temperatures mean simply more coal and accessories than higher temperatures. It is when we come to the cases where certain definite temperatures have to be maintained that the difficulties of the problem begin to appear. With “chilled” beef, for instance, which must not be frozen, but is held at about 33 to 35 degrees R, the outer layers will freeze if a much lower temperature is reached, and the condition of the meat on arrival may be seriously deteriorated. On the other hand, if the temperature is allowed to rise to an appreciable extent, the processes of decay, which the low temperature holds in check, may commence, and it is then very difficult to arrest them, particularly as in some cases the processes themselves generate heat. Further, if one quarter of beef commences to decay, the others may take it up. When first received, the meat has to be very gradually and carefully “chilled,” or the outer layers may be chilled while the inside is still warm; and the inside meat being protected by the outer layers, the processes of decay may be continued somewhat vigorously. The meat is, therefore, allowed to lose its animal heat, is then very carefully chilled, so that the process goes right in to the bone, and is then maintained at the desired temperature. For this purpose, in one arrangement the cold chamber is divided into bays, or longitudinal divisions, to each of which is assigned its own brine cooling pipe, or two bays may have a stack of pipes fixed vertically between them, the flow in all the pipes being controlled by valves on the outside of the chamber. Thermometers are fixed in the flow and in the return pipes, so that the attendant can learn what is going on in each bay, and can allow the brine to flow in that set of pipes accordingly. In the case of fruit, it is necessary that there shall be a gentle current of dry air all around each fruit throughout the voyage. Fruit ships are, therefore, fitted something on the lines of frozen mutton carriers, but usually with air cooling only.
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