2 Learning ObjectivesDescribe air circulation through the cargo area.List three different forms of short cycling.Explain the operation of the thermostat.Describe the refrigeration unit operation.Explain the reasons for pre-cooling trucks and trailers.Explain loading procedures to ensure proper circulation .
3 Learning Objectives (continued) Describe cargo floor construction.Explain a pre-trip inspection.Describe the advantages of multi-temperature refrigeration units.
5 System Components Engine- Typically a four cylinder diesel. Compressor- Range from 2 to 4 cylinder.Condenser- Dissipates heat from trailer to ambient air.Thermostatic Expansion Valve- Fixed orifice, externally equalized.Evaporator- Absorbs heat in the trailer.Microprocessor- Brain of unit. Controls all aspects of refrigeration and heating process.
7 System Components (continued) Box Temperature- The actual temperature in the storage unit.Set Point- Optimum temperature to maintain product.Thermostat- Sets the temperature that the equipment will maintain.Refrigerant- Chemical used to absorb and dissipate heat in the storage area.
12 Pre Cooling ProductsProduct must be cooled to the holding temperature prior to loading trailer.Trailer refrigeration equipment is designed to maintain temperature of product not cool it down.Pre cool the trailer prior to loading.Shut down refrigeration unit while loading to prevent refrigerated air from escaping.
13 Proper Loading Procedures Pallets must be loaded so skids allow air flow fore and aft in trailer.Product must be kept away from walls to allow air flow around all sides.Ample air space must be allowed between load and ceiling.Air space must be left between cargo door and cargo.Air space must be left between front bulkhead and cargo.
17 Short CyclingCargo is blocking units discharge air flow.Cold air is routed back to air inlet.Thermostat thinks it has reached set point and switches to heat mode.Warm air flow is blocked and routed back to air inlet.Thermostat thinks it has reached set point and switches to cool mode.Process continues over and over, “Short Cycling.”
18 Auto Stop/Start Auto stop/start used by most equipment today. Constant run mode still available.Constant run mode, unit cycles between heat and cool mode.Auto stop/start mode, unit will shut down once set point is reached and programmed parameters are met.Programmed parameters- unit must run long enough to charge battery and warm the engine.Unit stays off until box temperature reaches preset temperature.
19 SummaryMicroprocessors control all aspects of refrigeration and heating cycles.Modern advances in technology allow distribution to market fresh fruit and vegetables any time of year.Loads must be pre cooled to remove residual heat and humidity.The box must be pre cooled to remove residual heat and humidity.
20 Summary (continued)Correct loading is essential to product quality.Discharge air must be able to circulate freely around all side of the product.Rising fuel costs have contributed to the technology for auto stop/start.The auto stop/start feature is a huge financial savings for the company using it , as well as a way of conserving our natural resources.