Potato Storage Ventilation and Humidification Roger Brook Extension Engineer Michigan State University
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 CO 2 • dry wet potatoes • warm up cool potatoes
Respiration “ Respiration is, in a way, the opposite of photosynthesis. Energy stored in sugars is now released for use in maintenance of the tuber. ” 6 O 2 + C 6 H 1206 6 CO 2 + 6 H 2 O + Energy (85% is heat) Oxygen Glucose Carbon Dioxide Water
Respiratory changes over time 100 sprouted tubers 80 60 40 20 0 SEP NOV JAN MAR MAY
The effect of temperature on respiration 5 mg/g Sucrose mg/g Reducing sugars 5 0 0 30 35 40 45 50 55 60 65
Gas exchange in potato tubers O 2 CO 2 O 2 H 2 0 CO 2 H 2 0 O 2 CO 2 H 2 0 Lenticel and epidermis of potato tuber
The effect of stress on respiration l l In general, any type of stress causes respiration to increase. Stresses to watch for: • lack of fresh air (O 2, CO 2) • handling • temperature fluctuations • exhaust gases (CO, C 2 H 4)
Wall cross-section vapor barrier steel cladding Plywood house wrap 1" extruded polystyrene Insulation and structural TYPICAL WALL SECTION
Temperature profile in wall cross-section 50°F 49. 1°F 48. 4°F 41. 9°F 1. 8°F 0. 3°F 0°F 3. 4 F
Psychrometrics in potato storage l 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
Relative humidity l 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
Relative Humidity Terms l Dry bulb temperature • measured by ordinary thermometer l Wet bulb temperature • measured by a thermometer with the bulb covered by a water moistened wick in a moving stream of ambient air l Dewpoint temperature • temperature at which moisture starts to condense from cooled air
Effect of warming air on RH 50 o. F 95% RH 55 o. F 78% RH 60 o. F 66% RH
Surface temperature and condensation l l l 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.
Where to measure Relative Humidity 100 FT 30 FT Distribution Ducts 8 ft. c/c Loading and Work Area Plenum Fan Room
Relative Humidity measuring devices l l Dial hygrometers Sling psychrometers Aspirated psychrometer Electronic RH sensors
Humidification Systems l High humidity • critical for curing process • minimal weight loss • maximum quality out of storage l Need 1 gal. water per 800 1000 cfm of design air flow
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.
Weight loss vs. Relative Humidity % Weight Loss 12 10 75% 85% 99. 50% 8 6 4 2 0 0 1 2 3 4 5 6 7 Months in storage 8 9 10
Centrifugal humidifier l l l also known as “spinning disk” most common; moderate cost low to moderate maintenance low operating cost excess water in plenum prone to mineral buildup
High pressure nozzles l l l moderate cost moderate maintenance some excess water in plenum moderate operating cost prone to nozzle plugging
High pressure air/water nozzles l l l high cost high maintenance low excess water in plenum high operating cost noisy excellent humidification
Air washers l l l high initial cost may require additional humidifier may provide added cooling
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