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

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: water vapor in the air oxygen and CO2

Factors Affecting Potato Storage Environment

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

Energy Moves due to Temperature Difference Heat Conduction, not Air Movement Warm Air Cool Air

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

Use Ventilation Air to Control the Storage Environment

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

Ventilation Uniformity openings too small - size for 1000 ft/min size plenum so that air velocity is no more than 85% of the underpile duct velocity underpile ducts too small - size ducts so that air velocity is 85% of the air outlet velocity duct openings too large - size outlet for 1000 ft/min

Sprinkler hose without endcap Sprinkler hose - too many holes / too large Sprinkler hose - correct holes / uniform distribution

Need Fresh Air Every Day to Control Carbon Dioxide

How Do We Get Fresh Air New storages are better insulated and better sealed - helps control temperature, but … CO2 flush - at least once per day if > 15 F, then 10% fresh air for 15 minutes otherwise, 5% fresh air for 30 minutes Artificial cooling - don’t forget fresh air

Understanding Moist Air Properties

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

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

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

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.

Moist Air and Cooling

Heat of Vaporization Energy is neither created or destroyed, just transferred Energy needed to change water from liquid to vapor Tuber water is essentially liquid Air water is essentially vapor 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

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

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

Refrigeration Systems for Potato Storage

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

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

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

Monitoring Tuber Temperature PVC Solid Tubing End Cap 4 - 8 ft. long Thermometer, hanging on string

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

Evaporator Size Bigger evaporators result in less air temperature change Higher temperature difference removes more water may result in icing Make sure you leave enough space for air movement

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

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