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FREEZING Keeps
Page 84
Page 85
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Food Fresh
,n Transit
. C . LIM ATES hotter and more extreme than thia. of this country have emphasized. the value of refrigeration for preserving foodstuffs front deterioratiori. The tendency for fresh commodities such as fish, fowl, fruit and meat, . to be available in quantity far frotn.centres of population has in turn stimulated the development of specialized means for transport, and to-day. refrigerated vehicles are widely used to narrow the gap between producer and consumer..
At the same time there has appeared a long-overdue recognition of the 'fact that hygienic Conditions are essential to maintain food in an edible state. Refrigerated vehicles are, therefore, a step towards the ideal of providing food. in fresh condition in places where. it cannot be produced. and *awards a higher standard of food.conveyancC.
Insulated and refligerated bodies are
nowadays constructed by various British bodybuilders. One of the pioneers in this sphere was Morrisons Engineering, Ltd., Rochester ; which started 22 yetirs ago to make light-alloy bodies for
fooir
'According to Mr. D. M. Proet6r; a diriactor of the concern and an authority on the subject, the construction of refrigerated bodies is an exacting Science. It is essential to create a body, the insulation of which will remain unimpaired over aperiod of years.
Secondly, the interior must be easy to keep sweet, clean and sterile. No opportunity must be given for the entry of ambient air and dust, for which purpose the double rear door normally used on van bodies is unsuitable. Bodies built by Morrisons Engineering, Ltd., are provided with a single rear door 5 fthigh and 2 ft, ,6 ins. wide This is of double-skinned construction, based on exterior and interior framing of extruded channel section, housing the sealing gaskets. These are formed of soft expanded rubber 1 in. square. The framing is spaced 6 ins, apart by laminated densified wood spacers. The door opening, built on similar, principles, includes abutments for the door seals designed to reduce the tendency for the door to freeze.
Hygienic interiors are made possible by employing Smooth,polished. impermeable panelling. Metal panels, if well polished, reflect 95 Per cent, of radiant heat, and temperature-variations can be reduced to a minimum. Low temperatures suspend the activity of bacteria, which, however, will resume as soon as conditions become more
propitious. .
' The materials used in the conSyatction of the body must have a 'lOw heat
absorption capacity, so that change in temperature of the vehicle during unloading, for example, is rapid, and cooling can be equally quickly effected. The strength, light weight and corrosion reiistance of light alloys, plustheir easily cleaned and sterile surfaces, make them a most ecomonical material for the construction. of insulated and
refrigerated bodies.
These considerations are of particular importance in building an insulated body, in which a.low temperature level is maintained by constructing the vehicle with double-skinning throughout, and filling the space between the walls with an insulating material.
Whilst this practice is general, othet
methods-have to be used, in conjunction with insulation, if a body must be maintained at any particular temperature. In such circumstances.it is. usual to employ refrigerated containers, filled with refrigerant materials, or Winget-Dole eutectic plates or mechanically or electrically driven refrigerating plant fitted in the body. The first type is a simple and efficient means for keeping a Commodity cold. Containers in this category are made with varying thicknesses of insulation, but it is difficult to control evaporation of the' solid CO, tipon which refrigeration depends.
Where definite routes and time-tables can be worked out for a vehicle, the Winget-Dole system is particularly useful, but its limitations are expressed in the full name of the product, which is known as the Winget-Dole Hold-over plate. In the circumstances described the system is extremely economical.
The plates are permanently installed in an insulated vehicle and consist of a pair of flat steel sheets welded together at the edges to form a tank. Between them is coiled a refrigerant steel tube. The ends of the tubes are arranged to emerge from the plate to facilitate coupling with, the source of the refrigerant or refrigerating unit. In the tank thus formed from the plates a vacuum is created. This prevents distortion of the plate by exerting a vacuum atmospheric pressure of almrist I ton per sq. ft.
Additionally, a hold-over eutectic solution is introduced between the coils and the sheets, havine a freezing point appropriate to the goods being carried. Once frozen, this maintains the required temperature in the same way as ice and solid CO, (dry ice). The vacuum also prevents strain in the welded edges of the plate; which the expansion of the hold-over solution tends to Create on freezing.
Various sizes of plate ranging from in. to '2 ins, are prochiced by the manufacturer, Winget, Ltd., and the size and the number in the body determine the temperature attained and the hold-over time possible. The use of vehicles employing these plates depends upon the necessity of connecting the coils to a static refrigerating apparatus to charge them up for operation on the following day. The process is usually carried out overnight and the limit of hold-over is about 14 hours,
Self-contained refrigerated vehicles. thermostatically controlled, present an ideal' Ichethod of maintaining the teritberature required over any period of time. Capital cost is high, but the vehicle is independent of outside refrigerating plants. T. COleborn and Sons, Ltd.,Liverpool. has recently introduced a new body for this and other
similar types of vehicle. . , Known as the Holospar 'body, it consists of a patented light-alloy structure. in which interlocking panels are used and rivets are almost eliminated. Alternative arrangements have been made for the refrigerating unit, which can be either driven by the vehicle engine or by an independent unit. For the transport of liquid rubber products a high temperature can be maintained in the body by a different process.
A mechanically refrigerated Frigmobile, based on the Leyland Hippo chassis, with 4-6 ins. of insulating material and an Interior volume of 700 cubic ft., carries a load of 12 tons. By using light-alloy extruded sections for the framing, Jabroc floor bearers e .reinforced by light-alloy T-sections, and alloy panelling, the body weight is reduced to 1 ton 18 cwt.
Great care must be exercised in the construction of the bodies to seal all joints to make the interior absolutely waterproof. Where the inlet and return pipes to the evaporator coils or to eutectic plates pass through the inner skin of the insulated walls, , the holes must be thoroughly sealed w ith non-hardening cement. Thus water will not pass into the insulating space when the vehicle is being washed down. The ease with which light-alloy panelling can be washed is one of its particular advantages for this type of work, coupled with its resistance to Corrosion, Equally important is the necessity of preventing voids, and crevices in the completed body. All joints must be overlapping and corners in which particles of food could be trapped must be eliminated. Such attention to detail is the only way of attaining permanently efficient insulation.
A refrigerated body must embrace the highest technique in body construction if the development of 'faulty joints is to he avoided.