Specifying, designing and optimising refrigeration systems 313 when the load on the refrigeration plant is small, relative humidities greater than 90%can be specified. The engineering specification should specify the relative humidity under full sensible heat load, 1. e. the lowest hypothetica relative humidity that can be obtained, and also under part load conditio when advantage can be taken of the reduction in load to raise evaporation temperatures in the space. 15.2.6 Ambient design conditions The conditions in the air outside the chiller/freezer must also be defined in the engineering specification. Both the infiltration and fabric loads are effect on the capacity of the refrigeration plant. Ambient temperature al dependent on the outside temperature which therefore has an import affects the capacity of the refrigeration plant because heat must be rejected above this temperature via a cooling tower or condenser. If it is intended that the room should function under all possible ambient conditions, very high ambient wet and dry bulb temperatures must be specified. However these normally occur only during exceptional circumstances and, only briefiy at or soon after midday For design purposes, temperatures that are not exceeded for more than 2.5% of the total time in the year are normally acceptable and often a figure of 5% is used Both wet and dry bulb tem- peratures should be specified, giving the option of using an evaporative- pe of condenser or cooling tower for heat rejection to the atmosphere, which leads to a more efficient and smaller cooling plant 15. 2.7 Defrosts The occurrence of defrosts should also be specified to avoid peak periods while still ensuring that during these peak periods the evaporator is clear of ice. It is normal in abattoirs to defrost the evaporators and the chillers at 6 hourly intervals. Although this is often desirable immediately after the peak latent heat load has been removed from the room, it is unnecessary during the later stages of cooling and when the room is used as a store. Limiting defrosts can reduce both energy consumption and weight loss nt use and therefore should be included in the engineerin 15.2.8 Engineering design summary The engineering specification should therefore include each of the iter shown below. chiller, freezer air temperature, air speed and relative humidity for each product specification(covering complete range)and the time that each of these periods will be operatingwhen the load on the refrigeration plant is small, relative humidities greater than 90% can be specified. The engineering specification should specify the relative humidity under full sensible heat load, i.e. the lowest hypothetical relative humidity that can be obtained, and also under part load conditions, when advantage can be taken of the reduction in load to raise evaporation temperatures in the space. 15.2.6 Ambient design conditions The conditions in the air outside the chiller/freezer must also be defined in the engineering specification. Both the infiltration and fabric loads are dependent on the outside temperature which therefore has an important effect on the capacity of the refrigeration plant. Ambient temperature also affects the capacity of the refrigeration plant because heat must be rejected above this temperature via a cooling tower or condenser. If it is intended that the room should function under all possible ambient conditions, very high ambient wet and dry bulb temperatures must be specified. However, these normally occur only during exceptional circumstances and, only briefly at or soon after midday. For design purposes, temperatures that are not exceeded for more than 2.5% of the total time in the year are normally acceptable and often a figure of 5% is used. Both wet and dry bulb tem￾peratures should be specified, giving the option of using an evaporative￾type of condenser or cooling tower for heat rejection to the atmosphere, which leads to a more efficient and smaller cooling plant. 15.2.7 Defrosts The occurrence of defrosts should also be specified to avoid peak periods while still ensuring that during these peak periods the evaporator is clear of ice. It is normal in abattoirs to defrost the evaporators and the chillers at 6 hourly intervals. Although this is often desirable immediately after the peak latent heat load has been removed from the room, it is unnecessary during the later stages of cooling and when the room is used as a store. Limiting defrosts can reduce both energy consumption and weight loss during subsequent use and therefore should be included in the engineering specification. 15.2.8 Engineering design summary The engineering specification should therefore include each of the items shown below: • chiller, freezer air temperature, air speed and relative humidity for each product specification (covering complete range) and the time that each of these periods will be operating, Specifying, designing and optimising refrigeration systems 313