RUNNING COSTS FOR COLD STORAGE
The refrigeration load will comprise some or all of the following elements:
• Sensible: i. e. the cooling of fluids or solids
• Latent: i. e. freezing
• Heat of respiration: from vegetables, fruits, etc.
• Insulation: i. e. heat gain through the walls, ceiling and floor
• Infiltration: air ingress through cold store doors and hatches
• Auxiliaries: evaporator fans, pumps, lighting
184 • Other: people and trucks in cold stores, etc.
If the product is at storage temperature on arrival then the first two will be negligible. Insulation and infiltration will be the major source of heat load, and both are dependent on the storage temperature. The efficiency or COP of the system is dependent on the temperature lift, and the lower the storage temperature, the greater the lift must be. Whilst low temperatures are beneficial for the product, there is a trade-off against running costs. The graph in Figure 15.1 illustrates typical power consumption characteristics for the same amount of cooling at different store temperatures in medium ambient conditions. But at lower store temperatures the amount of cooling is greater because heat infiltration tends to be greater. The power costs could be 50% greater at —30°C compared to — 15°C. Additionally, the compressor capacity is lower at the lower evaporating temperature and larger plant will incur additional capital costs.
Storage temperature (°C)
Figure 15.1 Typical specific power consumption variation with storage temperature
Posted in Refrigeration and Air Conditioning