Refrigeration and Cooling Principles for Potato Storages

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 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
Glucose
Water
Oxygen
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
1" extruded polystyrene
house wrap
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
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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 gt 15 F, then 10 fresh air for 15 minutes
• otherwise, 5 fresh air for 30 minutes
• Artificial cooling - dont 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
48 oF 100 RH
50 oF 95 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
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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
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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
Thank You

• Roger Brook
• 210 Farrall Hall
• Michigan State University
• E. Lansing, MI 48840
• brook_at_msu.edu

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

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

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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

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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 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