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Running hot by design
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. . . and keeping it that way
MAINTAINING a relatively high body temperature is necessary in the case of a wide variety of loads. It is an exercise that becomes more complex as delivery and standing times are extended.
The range of critical temperatures in carrying refrigerated products is small compared with that for substances needing higher temperatures to be kept. In certain cases there is less tolerance in the rated temperature of the loads.
Overall it may be necessary to cater for product temperatures of below 40 deg F to more than 600 deg F, with considerable variations in loading temperature of a given type of load and in the time taken to load and unload. And in the higher ranges preheating of the body is generally required.
Moreover, localized cooling—for example at the outlet and in the domes of a tanker— has to be avoided when carrying certain types of load. Changes in ambient temperature, too, represent an all-important variable with every kind of load when considering the most appropriate type of bodywork and equipment to employ.
As with a vehicle used for low-temperature deliveries, the thermal property of the insulation compared with delivery time may determine whether it is necessary to employ temperature-control equipment and if so whether built-in equipment or depot-based equipment is the more suitable.
Because of the relatively short delivery times of "hot product" vehicles operating in this country, most are of the plain insulated type. But the probable growth of Continental traffic involving the carriage of what are termed "complex" substances may well foster the use of more sophisticated bodies and equipment.
The possibility of providing insulation that obviates the need for temperature-con
trol equipment should obviously be the first consideration of the bodybuilder. He should therefore co-operate closely with the operator on vehicle design with due regard to variations in running conditions and in the length of route.
Apart from its insulating properties, the suitability of an insulant with regard to heat resistance and deterioration in service when hot loads are being carried should be known. A working knowledge on the part of the operator of the basic factors that determine the efficiency of an insulant is highly desirable for him to co-operate with the bodybuilder.
Neglecting heat paths that may exist between the inner and outer skins of an insulated body in the form of bridging members, the efficiency of an insulant is denoted by its K factor. This is a measure of the heat transferred through the material in a specific type of test in an ambient temperature of 68 deg F.
The K factor is unity if the heat transferred in an hour through a square foot of lin thick material is just sufficient to raise lib of water ldeg F, the amount of heat transferred being lBtu. This is measured in a laboratory test, the smaller the K factor the higher the insulating effect.
Given that there are no bridging losses, heat flow is inversely proportional to the thickness of the material, the K factor being reduced to half its original value if the thickness is doubled.
In the case of man-made glass-fibre materials, the K factor varies between 0.23 and 0.27. Insulating properties compare favourable with cork and unfavourably with plastics foams, the K factor of which may be as low as 0.12—about half that of glassfibre. Glass-fibre materials, can, however, withstand very high temperatures without deterioration. They have a density of about l+/21b/cult. and can readily be formed to any shape.
Compression of glass-fibre does not materially increase its insulating properties. There is, therefore, no alternative to using thick material if effective insulation is required.
Take Rocksil, for example. Produced by Cape Insulation and Asbestos Products Ltd., 114 Park Street, London, WI, it comprises long rock-wool resin-bonded fibres.
Rocksil is manufactured in a variety of grades and forms. The usual type for vehicle applications is MDS.
Supplied in bonded slabs MDS has a density of 51b/cu.ft. The limiting temperature is 450deg. F, materials for special applications catering for temperatures up to 1,400deg. F.
With a hot-face temperature of 100deg. F, MDS slabs have a thermal conductivity of 0.26Btu in/sq.ft.h.deg. F. At the other end of the scale the conductivity is 0.43Btu in/sq.ft.h.deg. F when the hot-face temperature is 450deg. F.
Polyurethane foams, in addition to their highly favourable K factor, have an intrinsic strength that can be exploited to advantage by the bodybuilder in the production of lightweight structures. They have the disadvantage for some applications that a temperature of more than, say, 150deg. F may alter their physical and insulating properties.
Most foams are unstable at much higher temperatures, the limit being probably about 200/220deg. F.
Moreover, according to some authorities on insulation, difficulties in the foaming operation can result in serious inconsistencies in service and the effective K factor may be increased to 0.18/0.2.
I give these examples to draw attention to the unpredictability associated with employing types of insulating material requiring highly skilled workmanship to exploit it to the best advantage. Glass-fibre may be regarded as possibly the "safest" for hightemperature applications.
Tankers designed for carrying loads at high temperatures may expand several inches during the loading operation. If the structure has not been designed for this with leak-proof joints, leak paths will be created that allow air and moisture to reach the insulant. There may alqo be a risk of moisture penetration if th■; vehicle is washed down with a high-pressure jet.
When considering the most suitable insulant and whether it will be necessary to install heating equipment, note should be taken of the specific heat of the material to be carried. This is a measure of the heat "contained" by the material.
Water is the reference liquid in denoting specific heat—the ratio of the quantity of heat required to raise the temperature of a substance ldeg. F to that necessary to raise the same mass of water an equal amount. In several cases operators have purchased tankers fitted with heating equipment to maintain the fluidity of a hot load, only to find that in the coldest weather on the longest run the equipment is not needed. A simple sum that took account of the specific heat of the material might well have obviated this expenditure and saved the extra weight.
If the substance carried has a low specific heat, a given heat loss will cause a relatively large drop in temperature: insulating material and/or heating equipment should be selected accordingly.
For example, bitumen and many heavy oils have a specific heat of around O.4— about half the specific heat of a range of chemicals commonly carried in road tankers. The drop in temperature for a heat loss of 1 Btu from a load of bitumen is about double that for a similar heat loss from, say, caustic soda.
The temperature at which a substance becomes viscous may be important in two ways: firstly as to the temperature at which it can be unloaded and, secondly, because the heat transfer characteristics of a viscous material are poor compared with a fluid. The heat needed to raise the temperature of a viscous substance a given amount is considerably higher than that producing an equal increase in the temperature of a fluid of the same specific heat.
Heating the load by using steam coils fed from a site installation at the end of a run is commonly necessary; if the load is viscous the time taken may be excessive and the process costly in heat wastage. If the substance carried solidifies when the vehicle or equipment breaks down, or the insulant fails, irreparable damage may be caused.
Before considering specific applications of body heating it is appropriate to note that the inherent insulating properties of plastics bodies are sufficient in certain applications to maintain the required temperature of the product for the duration of the run. Also a mechanical type of refrigerating plant can be operated on a reverse cycle to heat the load space.
As mentioned by Petters Ltd., of Hamble, Hants, makers of Thermo-King refrigerating systems, heating by this means would not be economic for vehicles regularly employed to transport loads at higher temperatures. But it is economic if refrigeration is a requirement on the outward run and the type of load carried on the return trip has to be heated.
In an example of the latter quoted by Mickleover Transport Ltd. makers of allplastics bodies and containers, an insulated vehicle equipped with a Thermo-King reverse-cycle refrigerating system is employed to transport meat to the Continent and Latex on the return run.
Of the relatively simple types of vehicle employed to carry hot loads, insulated tippers have been used for years to transport tarmacadam and bitumen for roadmaking. Such loads are temperature-critical in two ways.
Firstly, it is essential that the material is above a minimum temperature for user handling; secondly, solidification can be costly to the operator because of the great difficulty in removing the material without causing damage.
According to Foster Yeoman Ltd., of Dulcote Quarries, Wells, Somerset, "mixed" types of tarmacadam and bitumen macadam are now being produced for roadmaking, the site working temperatures of which are considerably higher than more conventional materials. Its transport manager is highly satisfied with three Leyland-based insulated tippers equipped with special aluminium double-skinned bodies by Bonallack and Sons Ltd., Basildon, Essex, to carry the new materials.
The vehicles are used for deliveries from Wells to the London area and throughout Southern England, apart from Kent. The material is mixed for loading from a batchweighing machine in 3-ton batches at 3am and the vehicles proceed direct to the working sites. Larger vehicles with similar Bonallack bodies are used by contractors in the area. They are loaded from a 4-1-ton batchweighing machine.
Foster Yeoman tippers have a 7 cu.yd. capacity, interior measurements of 11ft 9-in by 7ft 2in by 211 4in, and Edbro U38C underfloor unit tipping gear. Insulation comprises 2in of Rocksil material and all joints are sealed and welded; the covering comprises three sheets, one of quilted canvas with glass-fibre insulation, one of hessian and one of black plastics. They are permanently secured to the bulkhead.
Bodies of 15 cu.yd. capacity have been built for mounting on Atkinson eightwheeled chassis.
Built by I. McKinney and Son Ltd., Scratton Industrial Estate, Portsmouth, for Hampshire County Council, two 8 cu.yd. insulated roadmaking-material bodies are based on AEC Mercury chassis and are insulated with glass-fibre. The inner skins of the sides are of 12g mild steel, the floor of lOg mild steel and the insulant of the sides and floor have a thickness of 2in and 4in respectively.
Tailboard and headboard are also insulated with 2in material. Galvanized 18g mild-steel sheet is used as floor cladding, and the sides, tailboard and headboard are clad in Stucco aluminium.
Body elevation is provided by an Edbro 4LNX twin-ram front-of-body gear.
A considerable number of hot-load bodies insulated with glass-fibre have been produced by McKinney who considers that this is preferable to polystyrene foam which tends to coagulate. And glass-fibre insulated bodies are easier to repair.
Two t pes
In recent years two types of insulated tipper body have been built by Duramin Engineering (Lydney) Ltd., Harbour Road, Lydney, Glos. One has a capacity of 10 cu.yd. and a lin insulating sandwich of aluminium honeycomb; the other has an 11 cu.yd. body insulated with 3in of Rocksil.
The sides, front and rear of the first body are lined with 14g mild-steel sheet and panelled in 20g duralumin, the floor is Kerning fin hardwood covered with 14g mild-steel sheet, two cross-layers of aluminium foil being sandwiched between the timber and the steel.
Duralumin framing and panelling of the same material are features of the I lcu.ycL body which has channel-section top rails and side pillars and is fitted with the Duramin special tailboard locking gear which automatically takes up wear and prevents rattle. Interior panels are of lOg material, the exterior panels and floor lining being of 18g Duralumin.
Metal-to-metal contact between the outer panelling and main frame is eliminated by the use of fibre strips.
Continuous heating of the material is provided in the hot asphalt-carrying tipper bodies that have been produced for a number of operators by Sparshatts (Metal Bodies) Ltd., The Causeway, Redbridge. Southampton. The bodies have double-skinned allsteel construction which are heated if required by feeding the exhaust gas from the engine into an underfloor radiator through a change-over valve. Insulated with glass fibre, the bodies cater for temperatures up to 350 deg F.
More sophisticated types of bitumen tanker may be exemplified by the articulated trailer tank built by Yorkshire Engineering and Welding Co. (Bradford) Ltd., Bradford Road, Idle, Bradford, recently supplied to the Esso company. It has a capacity of 4,500gal.
Constructed of mild steel and insulated with 3in of Rocksil, the tank is designed to transport its load to within a few degrees of the loading temperature of 450 deg F. Steam-heating coils under the tank are designed for charging with steam from site equipment.
In contrast to the YEWCO bitumen tank with regard to the temperature of the product carried, the company has recently produced a mild-steel tanker, based on a rigid eight-wheeler, to carry sulphur trioxide at 85/95 deg F. This vehicle features electric heating blankets, supplied with current from a generator powered by an auxiliary diesel engine behind the cab. .
The insulant is 3in of Rocksil and the tank is clad with 18g aluminium sheet. Payload is 15 tons.
The blankets are fitted to the underside of the tank shell below the insulant. Operation of the generator is controlled by thermostats automatically switching the diesel engine on or off at pre-set minimum and maximum temperatures of the cargo. Manual control can be employed if required.
As supplied to Kraft Foods Ltd., of Manchester, to carry "soft" shortening fats as well as edible oils, a Bonallack 3,740ga1 stainless-steel tipping tanker is insulated with 4in of foamed polyurethane and is fitted with a Telesig hydrostatically-driven cartridge pump for discharge of the fat through a Wynn Sealatex pivoted valve and manifold assembly. A second 6in outlet discharges edible oils, a pressure of 30 p.s.i. being provided by a Reavell compressor driven from the power take-off. The tank is mounted on an ERF eight-wheeled chassis.
A similar type of tanker has been supplied to Procter and Gamble Ltd., of Newcastle upon Tyne, for carrying "hard" shortening fats in addition to edible oil. It has electrical heating blankets with thermostats automatically controlling the temperature to 80 deg. F. The blankets are fitted to removable light-alloy panels insulated with glassfibre..
Other Bonallack bodies designed for carrying heated loads include a light-alloy thy-old-chick box van and a van produced for a research institute to carry sick animals.
The first van takes 16,500 chicks in 350 boxes on removable racks and is insulated with polystyrene, fresh air being circulated by a Clayton-Dewandre fan through an electric heater to supply conditioned or heated air controlled from the cab. Fans in the roof and at the rear are used to extract the air.
The livestock transporter is equipped with a Tembon cooler /heater unit in the front compartment, a fan in the bulkhead feeding air into the remaining compartments. An extractor fan is in the centre of the three rear doors. Livestock is carried in the two rear compartments which have wire-mesh partitions.
Produced for the Shell-Mex and BP Co. by Gloster Saro Ltd., of Hucclecote, Gloucester, to carry heavier fuel oils up to a temperature of 200 deg F, a 5,300gal articulated aluminium tanker is insulated with polyurethane foam in medium-density slab form which is fixed to the tank with contact adhesive to obviate the need for support members.
The tank is constructed of AA5454 alloy which has a higher resistance to heat than the more conventional NS5 alloy. When clad with continuous alloy sheeting the external surface is sprayed with polyester resin; as an alternative the tank may be covered with glass-reinforced moulded sheet, the joints of which are sealed with epoxy resin.
The importance of heated-load longdistance traffic is exemplified by the fact that about two-thirds of the fleet of 230 tankers operated by Thomas Allen Ltd., Stanford-le-Hope, Essex, carry this type of load and that the proportion in the case of Harold Wood and Sons Ltd., Heckmondwike, Yorkshire is only marginally lower.
Thomas Allen use the same type of insulated vehicle to carry edible oils, glycol, fats, liquid rubber and various chemicals.
Steam-heated tankers are used for molten resins, bitumen and so on. A steam plant at the Stanford-le Hope depot is provided to heat bitumen tankers in multiples, the vehicles
This vehicle, seen feeding high-temperature asphalt to a road-laying machine, has an insulated body constructed from Alcan aluminium. It is one of 40 such vehicles, with bodies by Truck Engineering Ltd., Oldbury, being supplied to Tarmac Roadstones Ltd. The low weight of the aluminium, supplied by Alcan Industries Ltd., was a major contribution to achieving tare of less than 7 tons on a vehicle with a gross weight of 22 tons.
The high -payload is considered to be something of a breakthrough for vehicles
being backed up to couplings arranged at 10ft intervals along a 50yd supply pipe.
Over 50 per cent of the 700/800 tankers operated by Harold Wood are of the insulated type fitted with heating equipment to maintain the temperature of the load.
Ten per cent of the remainder are insulated tankers without heaters designed to reduce the cooling rate of the loads. In some cases the temperature of the load has to be maintained at over 200 deg C.
As several operators said, new types of "highly complex" materials are constantly being developed. Evolution of the most suitable equipment in conjunction with tanker manufacturers is an exercise requiring expert know-how and ingenuity. Details of many interesting types of heated-load vehicles are a closely guarded "sec ret"—and are likely to remain so for some time.
of this type. Other design factors have also contributed to the low tare. The body is rigid, which makes it possible to use an Edbro 5LNc single-ram tipping gear instead of a twin-ram gear.
With the introduction of the new plating regulations, allowing a set legal gross weight, reduction in vehicle weight will be the only way to increase payload. This fact has resulted in many enquiries about aluminium bodies from the more forwardthinking tipping vehicle operators.