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 is, in a way, the opposite of photosynthesis. Energy stored in sugars is now released for use in maintenance of the tuber.” Respiration 6O 2 + C 6 H CO 2 + 6H 2 O + Energy GlucoseCarbonDioxideWater (85% is heat) Oxygen

Respiratory changes over time SEPNOVJANMARMAY sproutedtubers

The effect of temperature on respiration mg/g Sucrose mg/g Reducing sugars 6065

Gas exchange in potato tubers O2O2O2O2 H20H20H20H20 CO 2 H20H20H20H20 H20H20H20H20 H20H20H20H20 O2O2O2O2 O2O2O2O2 O2O2O2O2 O2O2O2O2 O2O2O2O2 H20H20H20H20 Lenticel and epidermis of potato tuber

The effect of stress on respiration l In general, any type of stress causes respiration to increase. l 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 TYPICAL WALL SECTION vapor barrier Plywood 1" extruded polystyrene Insulation and structural steel cladding house wrap

Temperature profile in wall cross-section

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 condensation occurs at dewpoint temperature l potatoes or ceiling may be cooler than surrounding air; result can be “wet” surface. l Remedy: circulate air above bin or add insulation to ceiling.

Where to measure Relative Humidity 100 FT 30 FT LoadingandWorkArea Plenum Fan Room Distribution Ducts 8ft. c/c

Relative Humidity measuring devices l Dial hygrometers l Sling psychrometers l Aspirated psychrometer l 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 cfm of design air flow

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

Weight loss vs. Relative Humidity % 85% 95% 99.50% Months in storage % Weight Loss

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

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

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

Air washers l high initial cost l may require additional humidifier l may provide added cooling