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Tham khảo tài liệu 'refrigeration and air conditioning 3e episode 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 174 Refrigeration and Air-Conditioning trailers. Access turning docking and parking space is needed for such vehicles and the loading dock should be at the tailboard height with adjustable ramps to allow for small differences in this. The loading platform usually runs across the full side or end of the store with doors opening onto it. The absolute minimum width is 3 m and many docks are as wide as 12 m. The check-in office will be on the dock and may have a weighbridge or rail scale for carcases. The refrigeration machine room should have separate access. 15.2 Insulation The purpose of insulation is to reduce heat transfer from the warmer ambient to the store interior. Many different materials have been used for this purpose but most construction is now with the following 1. Cork a natural material - the bark of the Mediterranean cork oak tree. It is largely air cells and the fibrous cell walls have a high resin content. When baked the resin softens and welds the pieces of bark into a comparatively homogeneous mass which is sliced into blocks commonly 50 75 and 100 mm thick. 2. Expanded polystyrene. The plastic is formed into beads containing an expanding agent. When placed in a mould and heated they swell and stick together. The blocks are then cut into thicknesses as required. 3. Foamed polyurethane. The basic chemicals are mixed in the liquid state with foaming agents and swell into a low-density foam which sets by polymerization into a rigid mass. As the swelling material will expand into any shape required it is ideal for the core of sandwich panels and the sheet material skins may be flat or profiled. When the panels are manufactured the mixture is injected between the inner and outer skins and expands to the thickness required adhering to the lining materials. The value of an insulant to reduce heat flow is expressed as resistivity or its reciprocal conductivity. The units of the latter are watts per metre kelvin W m K . Values for these materials used are
