Temperature monitoring and measurement 10 Door opening for 30 seconds Fig. 5. 4 Effect of 'Damping'air sensor Temperature controlled vehicles An independent refrigeration unit usually powered by diesel, often with an auxiliary electric motor, is used to circulate cold air around the vehicle compartment from an evaporator unit at the front of the vehicle. A trend in many multiple distribution depots is to use vehicles with movable bulkheads so that vehicle can carry frozen and chilled foods at different temperatures in the same load. Each compartment will have its own evaporator, which can control temperatures independentl The cold air is distributed in different ways within the different vehicles, but the majority have cold air leaving from the top of the cooling unit near the roof, and returning via the base to the front of the vehicle and the return air intake (Fig 5.5). Correct loading and spacing of the load within the vehicle is crucial to ensure adequate cold air distribution within the compartment. If the load is not spaced correctly, circulation can be restricted and"hot spots can occur. The maximum length and width of vehicles is set by regulation, and hence the free space available to loads within an insulated chamber, place further restrictions on achieving correct loading. Some vehicles are cooled by direct evaporation of liquid nitrogen from a reservoir tank on the vehicle. These vehicles have the advantage of being much quieter than mechanically refrigerated vehicles, and temperature control can be better. However, an adequate supply of liquid nitrogen is required for the journey, which can limit their range and number of
Temperature controlled vehicles An independent refrigeration unit usually powered by diesel, often with an auxiliary electric motor, is used to circulate cold air around the vehicle compartment from an evaporator unit at the front of the vehicle. A trend in many multiple distribution depots is to use vehicles with movable bulkheads so that one vehicle can carry frozen and chilled foods at different temperatures in the same load. Each compartment will have its own evaporator, which can control temperatures independently. The cold air is distributed in different ways within the different vehicles, but the majority have cold air leaving from the top of the cooling unit near the roof, and returning via the base to the front of the vehicle and the return air intake (Fig. 5.5). Correct loading and spacing of the load within the vehicle is crucial to ensure adequate cold air distribution within the compartment. If the load is not spaced correctly, circulation can be restricted and ‘hot spots’ can occur. The maximum length and width of vehicles is set by regulation, and hence the free space available to loads within an insulated chamber, place further restrictions on achieving correct loading. Some vehicles are cooled by direct evaporation of liquid nitrogen from a reservoir tank on the vehicle. These vehicles have the advantage of being much quieter than mechanically refrigerated vehicles, and temperature control can be better. However, an adequate supply of liquid nitrogen is required for the journey, which can limit their range and number of stops. Fig. 5.4 Effect of ‘Damping’ air sensor. Temperature monitoring and measurement 109
10 Chilled foods behind bulkhead N return to evaporator Fig 5.5 Air temperature monitoring of temperature controlled vehicle. usea emperature read-out and single-channel chart records, which have been T ed for many years on refrigerated vehicles, placed the sensor so as to measure the air return temperature. This returning air should indicate the mean mperature in the load, provided that there is good distribution to all parts of the load. Short circuiting of air may result in colder return air temperatures Long vehicles, especially those without air duct distribution of cold air in the ceiling of the compartment, are advised to fit a second sensor placed nearer the rear of the vehicle(Fig. 5.5). The addition of a second sensor is not sufficient to give a full and accurate picture of temperature distribution within the chamber, but it will measure the cold air leaving the evaporator, and may give a better picture of cold air circulation inside the compartment. This second sensor will serve as a check on the functioning of the measuring system, and makes tampering more difficult. It should demonstrate that the evaporator and fan unit are functioning properly and that cold air is reaching the back of the vehicle. It will give a temperature baseline with which to measure the return air, and indicate more easily when the cooling unit has been switched off, or a load added which has been insufficiently cooled. Prevention of freezing of part of the load can also be more easily avoided. Comparison of the normal differential temperatures between the rear sensor and the return air sensor may also indicate air distribution within the compartment The frequency of recording for electronic loggers will depend on the length of the journey. A maximum interval of 15 minutes is recommended for journeys of up to 8 hours. Longer journeys may use longer intervals between recordings Other information such as defrost cycles, door openings and load identification may be required. It is important that a driver be aware of any problem occurring with the temperature of the load. The temperature read-out is often visible to the driver in the vehicles wing mirror, and in some cases the read-out is presented as a mirror image. Obviously, the driver should have complete concentration on the road, and it is better that an alarm system be fitted which warns the driver when something is wrong
Temperature read-out and single-channel chart records, which have been used for many years on refrigerated vehicles, placed the sensor so as to measure the air return temperature. This returning air should indicate the mean temperature in the load, provided that there is good distribution to all parts of the load. Short circuiting of air may result in colder return air temperatures. Long vehicles, especially those without air duct distribution of cold air in the ceiling of the compartment, are advised to fit a second sensor placed nearer the rear of the vehicle (Fig. 5.5). The addition of a second sensor is not sufficient to give a full and accurate picture of temperature distribution within the chamber, but it will measure the cold air leaving the evaporator, and may give a better picture of cold air circulation inside the compartment. This second sensor will serve as a check on the functioning of the measuring system, and makes tampering more difficult. It should demonstrate that the evaporator and fan unit are functioning properly and that cold air is reaching the back of the vehicle. It will give a temperature baseline with which to measure the return air, and indicate more easily when the cooling unit has been switched off, or a load added which has been insufficiently cooled. Prevention of freezing of part of the load can also be more easily avoided. Comparison of the normal differential temperatures between the rear sensor and the return air sensor may also indicate poor air distribution within the compartment. The frequency of recording for electronic loggers will depend on the length of the journey. A maximum interval of 15 minutes is recommended for journeys of up to 8 hours. Longer journeys may use longer intervals between recordings. Other information such as defrost cycles, door openings and load identification may be required. It is important that a driver be aware of any problem occurring with the temperature of the load. The temperature read-out is often visible to the driver in the vehicle’s wing mirror, and in some cases the read-out is presented as a mirror image. Obviously, the driver should have complete concentration on the road, and it is better that an alarm system be fitted which warns the driver when something is wrong. Fig. 5.5 Air temperature monitoring of temperature controlled vehicle. 110 Chilled foods