Potato Storage Ventilation and Humidification

1. Potato StorageVentilation and Humidification Roger Brook Extension Engineer Michigan State University

2. Potato Storage Ventilation • main function is to control temperature of potatoes in storage • remove field heat and heat of respiration • supply and/or control humidity • provide oxygen and remove CO2 • dry wet potatoes • warm up cool potatoes

3. Respiration “ Respiration is, in a way, the opposite of photosynthesis. Energy stored in sugars is now released for use in maintenance of the tuber.” 6CO2 + 6H2O + Energy 6O2 + C6H1206 (85% is heat) Oxygen Glucose Carbon Dioxide Water

4. Respiratory changes over time 100 sprouted tubers 80 60 40 20 0 SEP NOV JAN MAR MAY

5. The effect of temperature on respiration 5 5 mg/g Sucrose mg/g Reducing sugars 0 0 30 35 40 45 50 55 60 65

6. Gas exchange in potato tubers O2 H20 O2 CO2 O2 H20 H20 O2 CO2 H20 CO2 O2 H20 CO2 O2 Lenticel and epidermis of potato tuber

7. The effect of stress on respiration • In general, any type of stress causes respiration to increase. • Stresses to watch for: • lack of fresh air (O2, CO2) • handling • temperature fluctuations • exhaust gases (CO, C2H4)

8. Wall cross-section vapor barrier steel cladding Plywood 1" extruded polystyrene house wrap Insulation and structural TYPICAL WALL SECTION

9. 50°F 49.1°F 48.4°F 41.9°F 1.8°F 0.3°F 3.4F 0°F Temperature profile in wall cross-section

10. Psychrometrics in potato storage • Psychrometrics is the study of dry air-water vapor mixtures (moist air) and how mixtures are affected by varying amounts of water vapor and changing temperatures

11. Relative humidity • The ratio of the actual amount of moisture in the air to the maximum amount of moisture the air could hold at that temperature. Relative humidity (RH) is expressed as a percentage

12. Relative Humidity Terms • Dry bulb temperature • measured by ordinary thermometer • Wet bulb temperature • measured by a thermometer with the bulb covered by a water moistened wick in a moving stream of ambient air • Dewpoint temperature • temperature at which moisture starts to condense from cooled air

13. Effect of warming air on RH 50 oF 95% RH 55oF 78% RH 60 oF 66% RH

14. Surface temperature and condensation • condensation occurs at 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.

15. Where to measure Relative Humidity 100 FT 30 FT Distribution Ducts Loading and Work Area 8ft. c/c Plenum Fan Room

16. Relative Humidity measuring devices • Dial hygrometers • Sling psychrometers • Aspirated psychrometer • Electronic RH sensors

17. Humidification Systems • High humidity • critical for curing process • minimal weight loss • maximum quality out of storage • Need 1 gal. water per 800 - 1000 cfm of design air flow

18. Example of humidification required System has two 5-HP fans: need ~ 1 gal / 800-1,000 cfm airflow To determine humidifier capacity: water = 32,000 cfm/1,000 cfm/gal = 32 gal. water = 32,000 cfm/800 cfm/gal = 40 gal.

19. 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

20. Centrifugal humidifier • also known as “spinning disk” • most common; moderate cost • low to moderate maintenance • low operating cost • excess water in plenum • prone to mineral buildup

21. High pressure nozzles • moderate cost • moderate maintenance • some excess water in plenum • moderate operating cost • prone to nozzle plugging

22. High pressure air/water nozzles • high cost • high maintenance • low excess water in plenum • high operating cost • noisy • excellent humidification

23. Air washers • high initial cost • may require additional humidifier • may provide added cooling