Meat refrigeration fit. Therefore another option should be selected and the process repeated If there are no more options available there are only two alternatives: either standards must be lowered, recognising in doing so that cooling specifica tions will not be met, or the factory operation must be altered Having found a temperature/velocity/time option that fits in with the plant operation the next question is, is this the best option? All possible options must be evaluated in order to ensure that the optimum is obtained rom this a final set of times, temperatures, air speeds and relative humid ties will be obtained. in a chiller/freezer intended for several uses with product going to different customers this list can be quite long and if future uses are also included it can be longer still 15.2.2 Room size sing throughput information from the user specification and the chill- ing/freezing times now worked out, the size of the room can be determined To achieve this, the operation of the whole abattoir may have to be changed and also the flow of carcasses to and from the chiller/freezer, the position of doors, and so on. If the size and position of the room has been rigidly fixed before this tage, the cooling times determined above will not be met. 15.2.3 Refrigeration loads Refrigeration load calculations can now be performed, leading to a load profile for the room. Table 15.1 shows a typical load profile for an'imaginary'beef chillerThis hiller is to have two uses: first, to chill beef in a single stage process, and second, to store previously chilled beef sides. The product loads have been worked out for three separate conditions (in practice more may well be needed), the peak load, which will occur at the end of the loading period the average load during chilling and the load on the room when it is used for storing previously cooled sides of beef. The peak product load can be obtained from data provided in other chapters of this book. This load is very high and occurs for only a short period of time. The average product load is then calculated, based on the amount of heat to be extracted over the entire chilling period divided by the time available for chilling. Finally, when the room is used as a chill store there will be no product load in the room The infiltration load is the next most important feature yet there is little published information. The best is possibly in the AshRae guide. When loading a beef chiller the doors are invariably left open for long periods allowing a fully established air flow to take place to and from the room either from gravity through a single door or by a through flow of air if more than one door is open. Designers often decide that the door will only be
fit. Therefore another option should be selected and the process repeated. If there are no more options available there are only two alternatives: either standards must be lowered, recognising in doing so that cooling specifications will not be met, or the factory operation must be altered. Having found a temperature/velocity/time option that fits in with the plant operation the next question is, ‘is this the best option?’ All possible options must be evaluated in order to ensure that the optimum is obtained. From this a final set of times, temperatures, air speeds and relative humidities will be obtained. In a chiller/freezer intended for several uses with product going to different customers this list can be quite long and if future uses are also included it can be longer still. 15.2.2 Room size Using throughput information from the user specification and the chilling/freezing times now worked out, the size of the room can be determined. To achieve this, the operation of the whole abattoir may have to be changed and also the flow of carcasses to and from the chiller/freezer, the position of doors, and so on. If the size and position of the room has been rigidly fixed before this stage, the cooling times determined above will not be met. 15.2.3 Refrigeration loads Refrigeration load calculations can now be performed, leading to a load profile for the room. Table 15.1 shows a typical load profile for an ‘imaginary’ beef chiller.This chiller is to have two uses: first, to chill beef in a single stage process, and second, to store previously chilled beef sides. The product loads have been worked out for three separate conditions (in practice more may well be needed), the peak load, which will occur at the end of the loading period, the average load during chilling and the load on the room when it is used for storing previously cooled sides of beef. The peak product load can be obtained from data provided in other chapters of this book. This load is very high and occurs for only a short period of time. The average product load is then calculated, based on the amount of heat to be extracted over the entire chilling period divided by the time available for chilling. Finally, when the room is used as a chill store there will be no product load in the room. The infiltration load is the next most important feature yet there is little published information. The best is possibly in the ASHRAE guide. When loading a beef chiller the doors are invariably left open for long periods allowing a fully established air flow to take place to and from the room either from gravity through a single door or by a through flow of air if more than one door is open. Designers often decide that the door will only be 308 Meat refrigeration
Specifying, designing and optimising refrigeration systems 309 Table 15.1 Refrigeration loads for a beef chiller Refrigeration load(kw Peak Average Store Product load Infiltration 0.5(<17 Fabric nil Lightin nil Evaporator fans 25(10%) Defrost (<12) 18h running Totals 66.8 278 85(<25) refers to the maximum load at the end of loading, averag load during chilling when the product is at its average value, tore to the loads present when the chiller is used to store lously cooled sides. open for short periods and that fully established airflow will never occur However, most chiller doors stand open for ca &h in every 24 h and they are certainly open while the chiller is being loaded. This infiltration load occurs at the same time as the peak product load, i.e. immediately at the end of the loading period, and it is therefore important that the infiltration load is added to the product load to find the maximum loading on the chiller. Experimental data obtained by FRPERC have shown that the ASHRAE calculations are approximately correct and are therefore rec ommended. They are often of an order of magnitude higher than figures used by designers of chill rooms. The infiltration load during chilling, when the average product load occurs, is much smaller because the door is nor mally closed or only opened for short intervals. It is therefore more accept able to use quite small infiltration loads during this time to allow for air exchange through faulty door seals or for short door openings only When the room is being used as a store, particularly if it has been unloaded, there may be periods when the infiltration load rises to maximum levels again and this has been shown in Table 15.1 The fabric load is shown next in Table 15.1 and is insignificant compared to the previous two loadings. The same applies to the heat load imposed by people in the chiller and by lighting. Unfortunately both these loads are normally concurrent with the peak product and infiltration loads and must therefore be added to these to calculate the total peak load The evaporator fans also produce quite high loads. At this point in the design an approximate figure for evaporator fan power must be used but when the final design is completed and more accurate data are available, this must be substituted and the calculations reworked. In the current
open for short periods and that fully established airflow will never occur. However, most chiller doors stand open for ca. 8 h in every 24h and they are certainly open while the chiller is being loaded. This infiltration load occurs at the same time as the peak product load, i.e. immediately at the end of the loading period, and it is therefore important that the infiltration load is added to the product load to find the maximum loading on the chiller. Experimental data obtained by FRPERC have shown that the ASHRAE calculations are approximately correct and are therefore recommended. They are often of an order of magnitude higher than figures used by designers of chill rooms. The infiltration load during chilling, when the average product load occurs, is much smaller because the door is normally closed or only opened for short intervals. It is therefore more acceptable to use quite small infiltration loads during this time to allow for air exchange through faulty door seals or for short door openings only. When the room is being used as a store, particularly if it has been unloaded, there may be periods when the infiltration load rises to maximum levels again and this has been shown in Table 15.1. The fabric load is shown next in Table 15.1 and is insignificant compared to the previous two loadings. The same applies to the heat load imposed by people in the chiller and by lighting. Unfortunately both these loads are normally concurrent with the peak product and infiltration loads and must therefore be added to these to calculate the total peak load. The evaporator fans also produce quite high loads. At this point in the design an approximate figure for evaporator fan power must be used but when the final design is completed and more accurate data are available, this must be substituted and the calculations reworked. In the current Specifying, designing and optimising refrigeration systems 309 Table 15.1 Refrigeration loads for a beef chiller Refrigeration load (kW) Peak Average Store Product load 40 16.8 nil Infiltration 17 0.5 0.5 (<17) Fabric 2 2 2 People 0.25 nil nil Lighting 1.5 nil nil Evaporator fans 6 6 6 Contingency – 2.5 (10%) – Defrost – (<12) (<12) 18 h running – – Totals 66.8 27.8 8.5 (<25) ‘Peak’ refers to the maximum load at the end of loading, ‘average’ to the load during chilling when the product is at its average value, and ‘store’ to the loads present when the chiller is used to store previously cooled sides