1. Refrigeration and Cooling 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: water vapor in the air
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
Heat Conduction, not Air Movement
Warm
Air
Cool
Air

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

7. Use Ventilation Air to Control the Storage Environment

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. 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

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

11. Need Fresh Air Every Day to Control Carbon Dioxide

12. 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

13. Understanding Moist Air Properties

14. 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

15. 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

16. 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

17. 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.

18. Moist Air and Cooling

19. 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

20. 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

21. 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

22. Refrigeration Systems for Potato Storage

23. 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

24. 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

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

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

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

28. 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

29. 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

30. Weight loss vs. Relative Humidity12 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