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

Energy stored in sugars is released for use in maintenance of the tuber.

Respiration

6O6O22 + C + C66HH12120066 6CO6CO22 + 6H + 6H22O + EnergyO + Energy

GlucoseGlucose CarbonCarbonDioxideDioxide

WaterWater

(85% is heat)(85% is heat)

OxygenOxygen

Energy Moves due to Temperature Difference

WarmAir

Cool

Air

Heat Conduction, not Air Movement

Wall / Ceiling Cross-Section

TYPICAL WALL SECTIONTYPICAL WALL SECTION

vapor barriervapor barrier

PlywoodPlywood

1" extruded polystyrene1" extruded polystyrene

Insulation and structuralInsulation and structural

steel claddingsteel cladding

house wraphouse wrap

Use Ventilation Air to Control the Storage

Environment

Ventilation System Builds Pressure for Distribution

LoadingLoadingand and

Work Work AreaArea

PlenumPlenum

Fan RoomFan Room

Distribution DuctsDistribution Ducts

8ft. c/c8ft. c/c

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 50 ooFF95% RH95% RH

5555ooFF78% RH78% RH

60 60 ooFF66% RH66% RH

Moist Air & Cooling Potatoes

Air warms, but also gains moisture from the potatoes, exiting close to 100% relative humidity

50 50 ooFF95% RH95% RH

5555ooFF99% RH99% RH

60 60 ooFF99% RH99% 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.

48 48 ooFF100% RH100% RH

50 50 ooFF95% RH95% RH

45 45 ooFF100% RH100% RH

Condensed WaterCondensed Water

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

Thank You

Roger Brook

210 Farrall Hall

Michigan State University

E. Lansing, MI 48840

brook@msu.edu

End Cap

Thermometer,hanging onstring

PVC Solid Tubing4 - 8 ft. long

Monitoring TuberTemperature

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

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8 9 10

75%

85%

95%

99.50%

Months in storageMonths in storage

% W

eig

ht

Lo

ss%

We

igh

t L

oss