310 Meat refrigeration example the evaporator fans are left running continuously and therefore the load is the same under peak cooling conditions as when the room is used as a store A contingency or safety factor is often added to the above calculations, to allow for errors In the table of loads this has been added to the average figures, but left out when calculating the peak load. This is because the peak load occurs for only short times and errors in it have less effect than at other Some designers add on an allowance for 18h running. This needs some planation. In many cooling applications the loads on the refrigeration plant are fairly constant over a day. If the refrigeration plant were installed to meet these loads then it would be running for almost 24 h a day in order to meet them. This is felt to be bad for the refrigeration plant and it is considered desirable that the plant should have rest periods in between run times and therefore an allowance is added to ensure this Such an allowance is irrelevant in a batch chilling process where a peak load rapidly decline It is also often only used as a euphemism for a( quite large)contingency When a room is defrosted, cooling stops and some heat is added. When the refrigeration plant reverts to cooling there is an additional heat load on the room. It is a wise precaution to ensure that defrosts do not occur at the same time that peak cooling is required and therefore no further allowance needs to be added to the peak heat loads. However, defrosts may be needed during cooling and when the room is used as a store and therefore the extra cooling has been shown in Table 15.1, but not added into the totals. In this example of an'imaginary'chiller the total heat load is over twice the calculated average heat load. Thus if the refrigeration plant was sized o meet only the average load it would only have half the required capac ity. Another point to notice is that the load on the room, when used as a store, even when the outside temperatures are very high, is very small com pared to both the peak and average loads and is for the most part due to the evaporator fans running continuously. These points will be considered in more detail below The load profile can now be plotted and, for the hypothetical example already discussed, is shown in Fig. 15. 2. At the start of the plot the room is running with no product load and with the doors closed. The load then increases when the doors are opened and the room is washed out or pos- sibly unloaded. Warm carcasses are then loaded into the room and the load rapidly reaches the peak product load that occurs at the end of the loading period. Thereafter, the doors are closed and the load rapidly declines. At the end of the chilling cycle, the doors are again opened to remove the car casses and the infiltration load so caused increases. During the chilling period additional peaks occur after defrosts. Note that the plant only runs at its full or peak load for less than 4% of the time. It runs at or above half load for 25% of the time and for over 70% of the time runs at less than a quarter of its design load.example the evaporator fans are left running continuously and therefore the load is the same under peak cooling conditions as when the room is used as a store. A contingency or safety factor is often added to the above calculations, to allow for errors. In the table of loads this has been added to the average figures, but left out when calculating the peak load. This is because the peak load occurs for only short times and errors in it have less effect than at other times. Some designers add on an ‘allowance for 18 h running’. This needs some explanation. In many cooling applications the loads on the refrigeration plant are fairly constant over a day. If the refrigeration plant were installed to meet these loads then it would be running for almost 24 h a day in order to meet them. This is felt to be ‘bad’ for the refrigeration plant and it is considered desirable that the plant should have rest periods in between run times and therefore an allowance is added to ensure this. Such an allowance is irrelevant in a batch chilling process where a peak load rapidly declines. It is also often only used as a euphemism for a (quite large) contingency. When a room is defrosted, cooling stops and some heat is added. When the refrigeration plant reverts to cooling there is an additional heat load on the room. It is a wise precaution to ensure that defrosts do not occur at the same time that peak cooling is required and therefore no further allowance needs to be added to the peak heat loads. However, defrosts may be needed during cooling and when the room is used as a store and therefore the extra cooling has been shown in Table 15.1, but not added into the totals. In this example of an ‘imaginary’ chiller the total heat load is over twice the calculated average heat load. Thus if the refrigeration plant was sized to meet only the average load it would only have half the required capac￾ity. Another point to notice is that the load on the room, when used as a store, even when the outside temperatures are very high, is very small com￾pared to both the peak and average loads and is for the most part due to the evaporator fans running continuously. These points will be considered in more detail below. The load profile can now be plotted and, for the hypothetical example already discussed, is shown in Fig. 15.2. At the start of the plot the room is running with no product load and with the doors closed. The load then increases when the doors are opened and the room is washed out or pos￾sibly unloaded. Warm carcasses are then loaded into the room and the load rapidly reaches the peak product load that occurs at the end of the loading period. Thereafter, the doors are closed and the load rapidly declines. At the end of the chilling cycle, the doors are again opened to remove the car￾casses and the infiltration load so caused increases. During the chilling period additional peaks occur after defrosts. Note that the plant only runs at its full or peak load for less than 4% of the time. It runs at or above half load for 25% of the time and for over 70% of the time runs at less than a quarter of its design load. 310 Meat refrigeration