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

Goal of Storage Management 4 To maintain near harvest quality potato throughout the storage season 4 Use ventilation to control the potato storage environment –temperature: potato and air –humidity –oxygen and CO 2

Factors Affecting Potato Storage Environment

4 Energy stored in sugars is released for use in maintenance of the tuber. Respiration 6O 2 + C 6 H CO 2 + 6H 2 O + Energy Glucose CarbonDioxide Water (85% is heat) Oxygen

Energy Moves due to Temperature Difference Warm Air Cool Air

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

Ventilation Uniformity 4 openings too small - size for 1000 ft/min 4 underpile ducts too small 4 duct openings too large Sprinkler hose - too many holes / too large

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

Understanding Moist Air Properties

Relative Humidity 4 The actual amount of moisture in the air as a percentage of maximum amount of moisture the air could hold at that temperature. 50 o F 95% RH 55 o F 78% RH 60 o F 66% RH

Weight loss vs. Relative Humidity % 85% 95% 99.50% Months in storage % Weight Loss

Moist Air & Cooling Potatoes 4 Air warms, but also gains moisture from the potatoes, exiting close to 100% relative humidity 50 o F 95% RH 55 o F 99% RH 60 o F 99% RH

Dew Point Temperature 4 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. 48 o F 100% RH 50 o F 95% RH 45 o F 100% RH Condensed Water

Surface temperature & Condensation 4 condensation occurs below dewpoint temperature 4 potatoes or ceiling may be cooler than surrounding air; result can be “wet” surface. 4 Remedy: circulate air above bin or add insulation to ceiling.

Heat of Vaporization 4 Energy is neither created or destroyed, just transferred 4 Energy is required to change water from a liquid to a vapor –Tuber water is essentially liquid –Air water is essentially vapor 4 The energy needed to evaporate water from the tuber to the air results in a temperature decrease 4 Evaporation energy provides a significant percent of the cooling in a potato storage

Humidification Systems 4 High humidity –critical for curing process –minimal weight loss –maximum quality out of storage 4 Maximize water surface area for rapid evaporation 4 Allow time for water evaporation

Adding Water to Air 4 Amount of energy constant 4 Air conditions –increase relative humidity –decrease air temperature –constant wet-bulb temperature 4 Evaporative cooling –may be up to 10 F during dry weather 4 Control on wet-bulb temperature

Refrigeration Systems for Potato Storage

Refrigeration System Components 4 Compressor - compress refrigerant to high pressure vapor 4 Condenser - exchange heat with outside to condense high pressure vapor to liquid 4 Expansion valve - allow high pressure liquid to expand to low pressure liquid 4 Evaporator - exchange heat with storage to change low pressure liquid to vapor 4 Misc. control components - pressure based

Refrigeration Specs Decisions 4 Refrigeration capacity –tons of refrigeration –energy to melt one ton of ice 4 Fall cool down vs. Temperature maintenance 4 Evaporator size 4 Evaporator location

Refrigeration Capacity 4 Respiration energy 4 Energy through walls / ceiling 4 Air exchange (infiltration) 4 Electrical components 4 Field heat (harvest temperature) 4 Rate of cooling

Evaporator Size 4 Bigger evaporators result in less air temperature change 4 Higher temperature difference –removes more water –may result in icing

Evaporator Location 4 Ceiling mounted - above bulk pile or storage boxes –close to recirculation opening for bulk –directed over top of boxes 4 In plenum between fan and humidifier –allow space for air expansion 4 Outside plenum (using portable unit) with air diverted through evaporator

Thank You Roger Brook 210 Farrall Hall Michigan State University E. Lansing, MI