104 Chilled foods relate the air temperatures to product temperature, it is necessary to carry out a load test. The load test involves examining the differential of air temperatures and comparing it to product temperature over a sufficient period of time to ensure the system is working under normal conditions With closed systems such as chill stores and vehicles, where the only perturbation derives from defrost cycles, door opening and changing loads, deter- mination of the relationship between air and product temperature is simpler. The warmest locations in the system have to be determined, and product temperatures ollowed over a period of time in order to relate them to air temperatures with open systems such as display cabinets, their operation is more sensitive to environmental conditions and location. Room temperature and humidity variations, perturbation of the air curtain by draughts or customer movement can hange the temperature distribution Under these circumstances, load testing can be more difficult Cabinet manufacturers perform a load test to check cabinet's performance (BS EN 441-5: 1996), using a set loading of standardised blocks of a gel (Tylose)(BS EN 441-4: 1995)under controlled environmental conditions of temperature with a constant air flow across the front of the cabinet. Whether the manufacturer's load test will deviate from that in situ will depend on how close the conditions and load follow the actual working operation of the cabinet. The siting and environmental effects of draughts and lighting should be checked with a range of foods on display 5.3.4 Alternatives to air temperature monitoring There are some circumstances where air temperature monitoring is not appropriate or needs modification. In closed cabinet systems, such as chill storage cabinets using gravity cooling from an ice-box or backplate, the air temperature inside requires significant time to recover after door opening Thus, periodic readings of air temperatures would have little meaning and bear no relationship to the temperatures of the food being stored. In this case it would be better to monitor either a food sample or a 'simulated food sample. The hermal mass of the sample would make it less sensitive to rapid air temperature hanges. Also it is possible to match the food simulant to have a similar cooling factor or similar thermal diffusivity to the food being monitored. The use of such monitoring would be essential for example where cooling is by conduction such as cold plate(dole plate) serving units in catering, or where air flows are low velocity(gravity-fed serve-over cabinets) Even where the system is cooled by forced air but the variations in air temperatures are large, e.g. small delivery vehicles and cabinet refrigerators, air temperature monitoring is still difficult to interpret. By increasing the response time or damping the sensor or measuring system, the trends in air temperature can be followed, whilst removing the short-term variations. Damping' can be achieved physically by increasing the thermal mass of the sensor or electronically by alteration of the read-out circuitry
relate the air temperatures to product temperature, it is necessary to carry out a load test. The load test involves examining the differential of air temperatures and comparing it to product temperature over a sufficient period of time to ensure the system is working under normal conditions. With closed systems such as chill stores and vehicles, where the only perturbation derives from defrost cycles, door opening and changing loads, determination of the relationship between air and product temperature is simpler. The warmest locations in the system have to be determined, and product temperatures followed over a period of time in order to relate them to air temperatures. With open systems such as display cabinets, their operation is more sensitive to environmental conditions and location. Room temperature and humidity variations, perturbation of the air curtain by draughts or customer movement can change the temperature distribution. Under these circumstances, load testing can be more difficult. Cabinet manufacturers perform a load test to check cabinet’s performance (BS EN 441-5: 199617), using a set loading of standardised blocks of a gel (Tylose) (BS EN 441-4: 199518) under controlled environmental conditions of temperature with a constant air flow across the front of the cabinet. Whether the manufacturer’s load test will deviate from that in situ will depend on how close the conditions and load follow the actual working operation of the cabinet. The siting and environmental effects of draughts and lighting should be checked with a range of foods on display. 5.3.4 Alternatives to air temperature monitoring There are some circumstances where air temperature monitoring is not appropriate or needs modification. In closed cabinet systems, such as chill storage cabinets using gravity cooling from an ice-box or backplate, the air temperature inside requires significant time to recover after door openings.19 Thus, periodic readings of air temperatures would have little meaning and bear no relationship to the temperatures of the food being stored. In this case it would be better to monitor either a food sample or a ‘simulated’ food sample. The thermal mass of the sample would make it less sensitive to rapid air temperature changes. Also it is possible to match the ‘food simulant’ to have a similar cooling factor or similar thermal diffusivity to the food being monitored.20 The use of such monitoring would be essential for example where cooling is by conduction such as cold plate (dole plate) serving units in catering, or where air flows are low velocity (gravity-fed serve-over cabinets). Even where the system is cooled by forced air but the variations in air temperatures are large, e.g. small delivery vehicles and cabinet refrigerators, air temperature monitoring is still difficult to interpret. By increasing the response time or ‘damping’ the sensor or measuring system, the trends in air temperature can be followed, whilst removing the short-term variations. ‘Damping’ can be achieved physically by increasing the thermal mass of the sensor or electronically by alteration of the read-out circuitry. 104 Chilled foods
Temperature monitoring and measurement 105 5.4 Temperature monitoring in practice 5.4.1 Chill storage Walk-in chill stores Walk-in stores consist of an insulated store chamber cooled by one or more fan- assisted air cooling units, depending on their size. The position of cooling units round the chamber varies, but is usually at ceiling level(Fig. 5.1).Air circulation should be designed to give proper distribution throughout the chamber, and to eliminate any hot spots'or stratification of air layers. In nearly all cases, air temperature recovery after door openings or defrost is rapid, permitting air temperature to be used as the most convenient means of monitoring. Retention of cold air can be further improved with the use of strip plastic curtains, or an air curtain above the door, minimising the ingress of warm The number of sensors to be used to monitor air temperatures in a chill store will depend on its size and the number of cooling units. Table 5. 1 gives an indication of the minimum number of sensors related to volume of the store ith stores less than 500 m being able to monitor air temperatures using one sensor. The positioning of the sensor is such that it gives an indication of the warmest air temperature and hence the warmest food in the store. This warmest location depends on the design of the store, especially the position of the air Doling unit in the store Figure 5.2 gives an example of air temperatures during the 24-hour operation of a large chill store. The graph shows temperature variations during peak activities of movement of chilled foods in the afternoon and evening compared Air return Air-off Possible sensor Fig, 5.1 Air circulation in a chill store
5.4 Temperature monitoring in practice 5.4.1 Chill storage Walk-in chill stores Walk-in stores consist of an insulated store chamber cooled by one or more fanassisted air cooling units, depending on their size. The position of cooling units around the chamber varies, but is usually at ceiling level (Fig. 5.1). Air circulation should be designed to give proper distribution throughout the chamber, and to eliminate any ‘hot spots’ or stratification of air layers. In nearly all cases, air temperature recovery after door openings or defrost is rapid, permitting air temperature to be used as the most convenient means of monitoring. Retention of cold air can be further improved with the use of strip plastic curtains, or an air curtain above the door, minimising the ingress of warm air on door openings. The number of sensors to be used to monitor air temperatures in a chill store will depend on its size and the number of cooling units. Table 5.1 gives an indication of the minimum number of sensors related to volume of the store, with stores less than 500 m3 being able to monitor air temperatures using one sensor. The positioning of the sensor is such that it gives an indication of the warmest air temperature and hence the warmest food in the store. This warmest location depends on the design of the store, especially the position of the air cooling unit in the store. Figure 5.2 gives an example of air temperatures during the 24-hour operation of a large chill store. The graph shows temperature variations during peak activities of movement of chilled foods in the afternoon and evening compared Fig. 5.1 Air circulation in a chill store. Temperature monitoring and measurement 105
106 Chilled foods Table 5.1 Number of sensors recommended in chill stores Chambers volume above(m) Number of sensors 5000 85000 6 to quieter loading activity in the morning. Differences between wall sensors and air return temperatures are very small in this case, and can be affected by their positioning in the store. For chill stores less than 500m, the single sensor could be placed in the air return of the cooling unit. In a closed system such as a store with adequate air distribution, the temperature reading of the air return approximates to the mean temperature of the food load. If there is not good air distribution, then it may be better to put the one sensor in a position more representative of the warmest air temperature. This may be located at the following po the maximum height of the food load, furthest away from the cooling unit at approximately two-thirds the height of the chamber, away from the door and the direct path of the cooling unit two metres above floor level, directly opposite the cooler unit. If the cooling unit is placed above the door, the negative pressure produced by the fan can increase the amount of air drawn into the chamber during door openings. Thus, air return temperature monitoring is not often appropriate in this case. For larger stores, different sensors can be used to indicate the temperatures in different parts of the store. In addition, placing extra sensors in the air outlet nd air intakes of one or more of the cooling units gives further information on the performance of the refrigeration system erators Cabinet refrigerators are free-standing, small-sized units with single or double doors. They can be cooled by fan-assisted cold air or by gravity-circulated air from an integral icebox or backplate(Figs. 5.3, (a),(b)and(c). As indicated earlier, air temperature monitoring is not as appropriate to these types of refrigerated systems as it is to walk-in chill stores Fan-assisted refrigerators will recover relatively quickly after door openings, but a large number of door openings, especially at most active periods of us will make any temperature readings difficult to interpret. Air temperature monitoring can be more meaningful if a damped sensor, with a response time of around 15 minutes, in the air return position is used( Fig. 5.3(a)). Damping can be achieved by using a metal or plastic sheath over the sensor or suspending the sensor in water, oil or glycerol. Figure 5.4 shows the effect of 'damping
to quieter loading activity in the morning. Differences between wall sensors and air return temperatures are very small in this case, and can be affected by their positioning in the store. For chill stores less than 500m3 , the single sensor could be placed in the air return of the cooling unit. In a closed system such as a store with adequate air distribution, the temperature reading of the air return approximates to the mean temperature of the food load. If there is not good air distribution, then it may be better to put the one sensor in a position more representative of the warmest air temperature. This may be located at the following positions: • the maximum height of the food load, furthest away from the cooling unit • at approximately two-thirds the height of the chamber, away from the door and the direct path of the cooling unit • two metres above floor level, directly opposite the cooler unit. If the cooling unit is placed above the door, the negative pressure produced by the fan can increase the amount of air drawn into the chamber during door openings. Thus, air return temperature monitoring is not often appropriate in this case. For larger stores, different sensors can be used to indicate the temperatures in different parts of the store. In addition, placing extra sensors in the air outlet and air intakes of one or more of the cooling units gives further information on the performance of the refrigeration system. Cabinet refrigerators Cabinet refrigerators are free-standing, small-sized units with single or double doors. They can be cooled by fan-assisted cold air or by gravity-circulated air from an integral icebox or backplate (Figs. 5.3, (a), (b) and (c)). As indicated earlier, air temperature monitoring is not as appropriate to these types of refrigerated systems as it is to walk-in chill stores. Fan-assisted refrigerators will recover relatively quickly after door openings, but a large number of door openings, especially at most active periods of use, will make any temperature readings difficult to interpret. Air temperature monitoring can be more meaningful if a ‘damped’ sensor, with a response time of around 15 minutes, in the air return position is used (Fig. 5.3(a)). Damping can be achieved by using a metal or plastic sheath over the sensor or suspending the sensor in water, oil or glycerol. Figure 5.4 shows the effect of ‘damping’ Table 5.1 Number of sensors recommended in chill stores Chambers volume above (m3 ) Number of sensors 500 2 5 000 3 20 000 4 50 000 5 85 000 6 106 Chilled foods
Temperature monitoring and measurement 10 Door ports 6×乏 8 Door ports Sensor 8 W AND Aaa 20.002400 10.00 1500 A Sensor 9 Fig 5.2 Air temperature monitoring record of large chill store(40 000m) when the sensor is set at the centre of a plastic tub of water, and compared to air temperatures after door openings Since cabinets cooled by a backplate or icebox have weak air circulation and ong recovery times after door openings, it is more appropriate to monitor their temperatures using food temperatures or, even better, a simulated food
when the sensor is set at the centre of a plastic tub of water, and readings are compared to air temperatures after door openings. Since cabinets cooled by a backplate or icebox have weak air circulation and long recovery times after door openings, it is more appropriate to monitor their temperatures using food temperatures or, even better, a simulated food Fig. 5.2 Air temperature monitoring record of large chill store (40 000 m3 ). Temperature monitoring and measurement 107
108 Chilled foods Forced air A: Air off. B: Air return(air-on). C: Load limit or warmest point. Fig. 5.3 Cabinet refrigerators. (a) Forced air refrigerator.(b) Icebox refrigerato (c) Backplate refrigerator. temperature. As foods are microbiologically unstable, food temperature monitoring would require using different foods each day, and might lead to wastage. Permanent positioning of a sensor requires a stable food simulant. It is important when choosing a food simulant that it behaves similarly to the food being monitored, and it is robust to different working conditions. It is recommended to determine the cooling factor of the specific package or piece of food and match this with a particular food simulant, or match the thermal diffusivity of the food to that of the simulant. Values for cooling factors of different foods, package sizes are also published, as well as thermal diffusivities for a range of plastic materials. Regular checks should be made with a food simulant system to ensure that the sensor embedded in it is accurate and functioning properly, and that the simulant is performing as it should 5.4.2 Chilled transport Distribution of chilled foods ut in many different types of vehicle ranging from large 40-foot hea ehicles with independent cooling units to light goods vehicles relying on containers to maintain temperature of pre-chilled foods. Pre-chilling to the correct temperature is essential given that most refrigeration units are designed to maintain temperature not cool the load
temperature. As foods are microbiologically unstable, food temperature monitoring would require using different foods each day, and might lead to wastage. Permanent positioning of a sensor requires a stable food simulant. It is important when choosing a food simulant that it behaves similarly to the food being monitored, and it is robust to different working conditions. It is recommended to determine the cooling factor of the specific package or piece of food and match this with a particular food simulant, or match the thermal diffusivity of the food to that of the simulant.20 Values for cooling factors of different foods, package sizes are also published,20 as well as thermal diffusivities for a range of plastic materials. Regular checks should be made with a food simulant system to ensure that the sensor embedded in it is accurate and functioning properly, and that the simulant is performing as it should. 5.4.2 Chilled transport Distribution of chilled foods is carried out in many different types of vehicle ranging from large 40-foot heavy goods vehicles with independent cooling units, to light goods vehicles relying on insulated containers to maintain temperature of pre-chilled foods. Pre-chilling to the correct temperature is essential given that most refrigeration units are designed to maintain temperature not cool the load down. A: Air off. B: Air return (air-on). C: Load limit or warmest point. Fig. 5.3 Cabinet refrigerators. (a) Forced air refrigerator. (b) Icebox refrigerator. (c) Backplate refrigerator. 108 Chilled foods