Potato StorageVentilation and Humidification

Roger BrookExtension 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 CO2

• 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

6O6O22 + C + C66HH12120066 6CO6CO22 + 6H + 6H22O + EnergyO + Energy

GlucosGlucosee

CarbonCarbonDioxideDioxide

WaterWater

(85% is heat)(85% is heat)

OxygenOxygen

Respiratory changes over time

0

20

40

60

80

100

SEP NOV JAN MAR MAY

sprouted sprouted tuberstubers

The effect of temperature on respiration

3030 55553535 4040 4545 5050

00

55

00

55

mg

/g S

ucr

ose

mg

/g S

ucr

ose

mg

/g R

edu

cin

g s

ug

ars

mg

/g R

edu

cin

g s

ug

ars

6060 6565

Gas exchange in potato tubers

OO22

HH2200COCO22

COCO22

HH2200

HH2200

HH2200 COCO22

OO22

OO22

OO22

OO22

OO22

HH2200COCO22

Lenticel and epidermis of potato tuberLenticel and epidermis of potato tuber

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)

Wall cross-section

TYPICAL WALL SECTIONTYPICAL WALL SECTION

vapor barriervapor barrier

PlywoodPlywood

1" extruded polystyrene1" extruded polystyrene

Insulation and structuralInsulation and structural

steel claddingsteel cladding

house wraphouse wrap

Temperature profile in wall cross-section

50°F50°F49.1°F49.1°F48.4°F48.4°F

41.9°F41.9°F

3.4F3.4F

1.8°F1.8°F

0.3°F0.3°F

0°F0°F

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

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

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

Effect of warming air on RH

50 50 ooFF95% RH95% RH

5555ooFF78% RH78% RH

60 60 ooFF66% RH66% RH

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.

Where to measure Relative Humidity

100 FT100 FT 30 FT30 FT

LoadingLoadingand and

Work Work AreaArea

PlenumPlenum

Fan RoomFan Room

Distribution DuctsDistribution Ducts

8ft. c/c8ft. c/c

Relative Humidity measuring devices

Dial hygrometers Sling psychrometers Aspirated psychrometer Electronic RH sensors

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

Example of humidification required

System has two 5-HP fans:System has two 5-HP fans: need ~ 1 gal / 800-1,000 cfm airflowneed ~ 1 gal / 800-1,000 cfm airflow To determine humidifier capacity: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

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8 9 10

75%

85%

95%

99.50%

Months in storageMonths in storage

% W

eig

ht

Lo

ss%

We

igh

t L

oss

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

High pressure nozzles

moderate cost moderate maintenance some excess water in plenum moderate operating cost prone to nozzle plugging

High pressure air/water nozzles

high cost high maintenance low excess water in plenum high operating cost noisy excellent humidification

Air washers

high initial cost may require additional humidifier may provide added cooling