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Refrigeration and Cooling Principles for Potato Storages

Refrigeration and Cooling Principles for Potato Storages

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Refrigeration and Cooling Principles for Potato Storages

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  1. Refrigeration andCooling Principles for Potato Storages Roger Brook Professor and Extension Engineer Agricultural Engineering Department Michigan State University

  2. Goal of Storage Management • To maintain near harvest quality potato throughout the storage season • Use ventilation to control the potato storage environment • temperature: potato and air • humidity • oxygen and CO2

  3. Factors Affecting Potato Storage Environment

  4. Respiration • Energy stored in sugars is released for use in maintenance of the tuber. 6CO2 + 6H2O + Energy 6O2 + C6H1206 (85% is heat) Carbon Dioxide Oxygen Glucose Water

  5. Energy Moves due to Temperature Difference Warm Air Cool Air

  6. Wall / Ceiling Cross-Section vapor barrier steel cladding Plywood 1" extruded polystyrene house wrap Insulation and structural TYPICAL WALL SECTION

  7. Ventilation Uniformity • openings too small - size for 1000 ft/min • underpile ducts too small • duct openings too large Sprinkler hose - too many holes / too large

  8. Ventilation System Builds Pressure for Distribution Distribution Ducts Loading and Work Area 8ft. c/c Plenum Fan Room Check distribution with food grade smoke

  9. Understanding Moist Air Properties

  10. Relative Humidity • The actual amount of moisture in the air as a percentage of maximum amount of moisture the air could hold at that temperature. 50 oF 95% RH 55oF 78% RH 60 oF 66% RH

  11. Weight loss vs. Relative Humidity 12 10 75% 8 85% % Weight Loss 6 95% 99.50% 4 2 0 0 1 2 3 4 5 6 7 8 9 10 Months in storage

  12. Moist Air & Cooling Potatoes • Air warms, but also gains moisture from the potatoes, exiting close to 100% relative humidity 50 oF 95% RH 55oF 99% RH 60 oF 99% RH

  13. Dew Point Temperature • The temperature at which the air can no longer hold the amount of water which is contained in it and below which the water starts to condense. Condensed Water 50 oF 95% RH 48 oF 100% RH 45 oF 100% RH

  14. Surface temperature & Condensation • condensation occurs below dewpoint temperature • potatoes or ceiling may be cooler than surrounding air; result can be “wet” surface. • Remedy: circulate air above bin or add insulation to ceiling.

  15. Heat of Vaporization • Energy is neither created or destroyed, just transferred • Energy is required to change water from a liquid to a vapor • Tuber water is essentially liquid • Air water is essentially vapor • The energy needed to evaporate water from the tuber to the air results in a temperature decrease • Evaporation energy provides a significant percent of the cooling in a potato storage

  16. Humidification Systems • High humidity • critical for curing process • minimal weight loss • maximum quality out of storage • Maximize water surface area for rapid evaporation • Allow time for water evaporation

  17. Adding Water to Air • Amount of energy constant • Air conditions • increase relative humidity • decrease air temperature • constant wet-bulb temperature • Evaporative cooling • may be up to 10 F during dry weather • Control on wet-bulb temperature

  18. Refrigeration Systems for Potato Storage

  19. Refrigeration System Components • Compressor - compress refrigerant to high pressure vapor • Condenser - exchange heat with outside to condense high pressure vapor to liquid • Expansion valve - allow high pressure liquid to expand to low pressure liquid • Evaporator - exchange heat with storage to change low pressure liquid to vapor • Misc. control components - pressure based

  20. Refrigeration Specs Decisions • Refrigeration capacity • tons of refrigeration • energy to melt one ton of ice • Fall cool down vs. Temperature maintenance • Evaporator size • Evaporator location

  21. Refrigeration Capacity • Respiration energy • Energy through walls / ceiling • Air exchange (infiltration) • Electrical components • Field heat (harvest temperature) • Rate of cooling

  22. Evaporator Size • Bigger evaporators result in less air temperature change • Higher temperature difference • removes more water • may result in icing

  23. Evaporator Location • Ceiling mounted - above bulk pile or storage boxes • close to recirculation opening for bulk • directed over top of boxes • In plenum between fan and humidifier • allow space for air expansion • Outside plenum (using portable unit) with air diverted through evaporator

  24. Thank You Roger Brook 210 Farrall Hall Michigan State University E. Lansing, MI 48840 brook@msu.edu