70 AMERICAN POTATO JOURNAL [Vol. 39

I N S U L A T I O N A N D E Q U I P M E N T F O R C O L D W E A T H E R P O T A T O S T O R A G E S I

F. H. BUELOW AND A. L. RIPPEN 2

] NTRODUCTIO N

Potatoes liberate heat. moisture, and carbon dioxide while stored under normal conditions. The carbon dioxide appears to be of little practical significance. Understanding the effect of the heat and moisture in the storage atmosphere is important. The conditions under which potatoes should be stored depends, in part, upon their ultimate use. For table stock 40 F. and 85 to 90% relative humidity appears to be satis- factory (2). Potatoes for processing into products such as chips, dehydrated niashed, etc. are usually held at sonie higher temperature.

Cooling potatoes after harvest is a relatively simple procedure. Cool autunm air forced through the pile or pallet boxes is usually adequate. The main duct fan should be sized to deliver air at a rate of at least one cfm (cubic feet per minute) for each 150 pounds of potatoes. The fan should deliver this air flow rate at ~ inch of static water pressure.

Low outside air temperatures during winter months have caused dif- ficulty in maintaining proper storage temperature and humidity. Observ- ations made in several storages in Michigan indicate excessive conden- sation occurs frequently on interior building surfaces during cold weather. Careful control of the fresh air intake, addition of heat, and building in- sulation are necessary in preventing serious condensation problems.

CONSIDERATIONS IN THE CONTROL OF TEMPERATURE

The average amount of heat produced by potatoes in storage during the winter months is normally about 1,430 B T U (British thermal unit) per ton per day (1). During the early part of the storage season the heat of respiration is somewhat higher than later on. However, if sprouting begins, usually near the end of storage, the heat produced will increase somewhat. The heat is dissipated primarily by conduction through wall surfaces and air changes in the storage.

Tile extent of heat loss from a storage by conduction may be con- trolled within practical limits by type of construction and amount of in- su!ation in the walls and ceiling. Insulating value may be thought of as a resistance to the flow of heat. This resistance is commonly known as R value (3) .*

1Accepted for publication August 16, 1961. Approved as scientific Journal Article No. 2861 of Michigan Agricultural Experiment Station.

~Department of Agricultural Engineering, and Department of Food Science, respec- tively, Michigan State University, East Lansing, Michigan. The authors gratefully acknowledge the advice and assistance of Dr. D. R. Isleib, formerly Associate Profes- sor, Department of Farm Crops, Michigan State University, in the preparation of this paper.

*When the size and type of construction is known, heat losses through a building may be estimated using the equation Q = AT/R

where: Q = Heat loss in BTU per hour. A = Area in square feet of walls and ceiling. T ~ Difference in degrees F. between inside and outside temperature. R = Resistance of wall to heat flow (ft.2 hr. ~

1962] BUELOW AND R I P P E N : COLD WEATHER STORAGES 71

The amount of insulation required for a potato storage will depend principally upon the average local minimum temperature and the desired inside temperature. Suggested R values for various outside temperatures are shown in Fig. 1 for 40 and 68 F storages.

For example, assume the conditions of a typical potato storage 100 ft x 40 ft x 18 ft which has a wall and ceiling area of 9,040 square feet. With an assumed R value of 10 and inside temperature of 40 F the calculated heat loss when the outside air temperature is 10 F would be 27,120 B T U per hour.

If this storage shoifid contain 1,120 tons of potatoes, 66,733 B T U of heat would be produced per hour, assuming that one ton generates 1,430 B TU per day. This amount is 39,613 B T U per hour greater than that lost by conduction through the building. To maintain a temperature of 40 F under these conditions, cooler air must be drawn into the storage. The amount of 10 F air intake to maintain the temperature at 40 F may be determined by the following equation:

CFM = (Heat of respiration - - heat loss through building) (55.2) 1 (60 rain per hour) (T )

Then substituting the values assumed in the example storage

( 6 6 , 7 3 3 - 27,120) (55.2) = 1,215 CFM required. 60 ( 4 0 - 10)

During extrenlely cold weather, heat losses tlarough most storage huildings will exceed the heat of respiration. For this reason, plus the fact that a storage nmst undergo air changes to maintain the proper humidity, additional heat niust be supplied.

CONSIDERATIONS IN THE CONTROL OF H U M I D I T Y

Control of the moisture content of storage air is nearly as important as temperature control. The atmosphere exists as a mixture of dry air and water vapor. When the vapor pressure reaches the naaximum attain- able at any given temperature, the air is said to be saturated. Relative hmnidity is the ratio of the existing vapor pressure to that which occurs in saturated air at a given temperature.

The amount of moisture emitted from potatoes will depend upon temperature, length of time in storage, condition of the tubers, and other factors. However, for estimating the amount of water vapor pro- duced, a potato weight shrinkage of .I/2 of 1% per mouth attributed to moisture loss appears logical. If the high moisture air is not exhausted from the storage, the humidity becomes very high. Excessive condensation is then likelv to occur on the ceiling, walls, and other surfaces. A relative humidity between 85 and 90% is considered desirable for minimum weight losses and spoilage. At this huniidity level the problem of con- densation can be controlled with proper ventilation and temperature control in most storages.

Moisture produced by 1,120 tons of potatoes in the storage cited in

1One B T U will raise the t empera tu re of 55.2 cubic feet of air 1 F.

72 AMERJCAN POTATO JOURNAL [Vol. 39

14

~ IAGE

r r

40 F STORAGE-~,~,, ,,~._ 6 " ' - - - -

rr" 4 LO

-30 -20 - I 0 0 I0 20

OUTSIDE AIR TEMPERATURE ~ FIG. 1.--Insulation values recommended for 40 and 68 t; storages for various average

minimum design temperatures.

the previous example may be assumed to be at a rate of 15.55 pounds per hour. If we assume inside air to be 40 F and 85~/(. relative hmnidity and outside air I0 F and 70~ , then 96I elm is required to remove the moisture.

Fluctuations in the lmmidity or temperature of outside air will change the amount of intake and exhaust necessary to maintain the relative hmnidity at 85 3 .

When air intake requirements for maintaining the desired inside tem- t)erature exceeds that necessary for proper humidity, moisture should theoretically be added: however such situations are infrequent in the Northern parts of the U. S. Also, if the air exchange necessary to maintain the proper humidity should exceed the requirements for tem- perature control, heat should be supt)lied. Where omside temperatures below 20 F are common, a storage 100 ft x 40 ft x 18 ft requires an exhaust fan of about 2200 to 2600 cfln for winter operation. Control is obtained by running the fan and/or furnace intermittently to maintain proper conditions. A storage with much insulation will require a larger exhaust fan to dispel the heat of respiration than one with a lesser amount of insulation. In a heavily insulated building, for example, one having an R value of 16, the relative humidity will tend to stabilize at a lower level than in one with an R of 8, The reason for this is that the greater amount of coM air intake needed to prevent a rise in temt)erature

1962] RUELOW AND RIPPEN: COLD WEATHER STORAGES 73

in a highly insulated storage usually removes too nmch moisture. Optimum insulating values for a storage appear to be in the R value range of 8 to 10 for many areas of the United States including Michigan. A source of heat for the control of temperature and humidity is necessary in most potato storages in colder climates.

RECOMMENDATIONS FOR POTATO STORAGES

The preceding analysis makes it possible to recommend equipment and building details for maintaining the tenIperature and lmmidity at or near the desired levels. These recommendations may be summarized as follows:

1. The potato storage should be provided with air ducts and fan that will nlove air through all stored potatoes. Fig. 2 shows an arrangement for providing either outside air or air froln above the potatoes to the main duct fan. The main duct fan should deliver at least one cfln at ~ inch static water pressure for each 150 pounds of potatoes.

]=~ ~LOUVER

:.i:

.~ OUTDOOR

FIG. 2.--A potato storage building with ventilating and control systems for maintain- ing temperature and humidity.

.

.

An exhaust fan should be provided which will move air fronl above the potatoes to outside the building. Fresh outside air will then be drawn into the storage. Tile exhaust fan should be capable of delivering about 2..5 cfln of air at ~ inch static water pressure for each ton of potatoes in storage. A furnace slaould be placed in tim storage to warm the air above the potatoes. The capacity of the furnace should be about 120,000 B T U per hour for a 40,000 bushel, 40 F storage in Michigan. To maintain the temperature at 68 F a 200,000 B T U per hour furnace should be used. A lnuch larger heater (1,000,000 to 2,000,000 B T U / h r . ) is required to raise the temperature of the potatoes several degrees in a short time as is often done in conditioning for chips.

74 AMERICAN POTATO JOURNAL [Vol. 39

4. The building should be insulated to give the R value shown in Fig. 1. (See Ref. 3 for insulating details.) It is not desirable to insulate the building more or less than approximately this R value.

5. The walls must have a good vapor barrier at the inside surface. 6. The thermostat and humidistat above the potatoes should be

located where they will not be influenced by hot air from the furnace. A small motorized louver near the furnace should open when the exhaust fan is on (See Fig. 2). The short duct con- nected to this louver should be insulated to prevent frost formation. Note that the furnace intake is near but not directly connected to this duct.

7. The fans and furnace should be controlled automatically during the winter season. A control system for this purpose is discussed in the following section.

A TEMPERATURE AND HUMIDITY CONTROL SYSTEM FOR POTATO STORAGES DURING THE WINTER MONTHS

The design of a control system to maintain the levels of temperature and relative hunlidity in potato storages is based on the following con- siderations :

1. The potatoes give off as much or more moisture than required to maintain the humidity at the desired level. Any excess hmnidity can be reduced by bringing in cool outside air to replace part of the moisture-laden air in the storage. If the storage becomes cooler than desired, a furnace can supply additional heat.

2. The potatoes give off some heat. If, as a result the storage be- comes too warm and the outdoor air is cool, some outdoor air can be brought into the storage to replace warm air.

3. The air in the storage should be recirculated through the potatoes periodically, whether or not outside air and/or heat are required.

4. If the outside air temperature is warmer than the desired inside temperature, no air is brought into the storage, even though temperature and/or humidity inside are above the desired levels.

The control system is designed to control the operation of a furnace, a small exhaust fan, and a large main duct fan. The chart below shows the conditions under which the exhaust fan should operate.

Storage Storage Outdoor temperature humidity temperature Exhaust

too high too high too high fan on

Yes Yes Yes No Yes Yes No Yes Yes No Yes No Yes No No Yes No Yes Yes No No Yes No Yes No No Yes No No No No No

1962] BUELO'~V A N D R I P P E N : COLD W E A T H E R STORAGES 75

S3

I - - i 5 5 . . .

I

1 " " I 1. -J

Fro. 3.--Wiring diagram of environmental control system for potato storage buildings.

The furnace should be on whenever the inside temperature is below the desired level.

The main duct fan should operate whenever 1. The exhaust fan is operating. 2. The furnace is operating. 3. Called for by a time clock. The circuit which will aceolnplish the above objectives is shown

in Fig. 3. Several additional features are inclnded in the circuit, such as switches for lnanual control.

The components shown in Fig. 3 are as follows:

T~--Outdoor thermostat, turns off when temperature is above desired level.

T.,--Thermostat in storage, turns on when temperature is above desired level.

Ta--Thermostat in main air duct, turns off when temperature is above desired level.

H --Hulnidistat in storage, turns on wl3en humidity is above desired level.

S]- -Master switch for the control systeln. S._,--Switch for furnace operation, nmy be set for "off" or "thermo-

static control." S:,--Manual control switch for exhaust fan, may he set for "on" or

"'automatic control."

76 AMERICAN I'OTATO JOURNAl, [Vol. 39

S4--Manual control for main dnct fan, may be set for "'on" or "auto- matic control."

R,, R, , - - l15 volt a.c. SPST, n.o. relays. Ra--Magnet ic relay for main duct fan. R4--Magnet ic relay for exhaust fan motor (may not be necessary

for small exhaust fan operating from 115 volts a.c.). R~--Magnetic relay for furnace (may not be necessary if furnace

can be controlled with 115 volts a.c.). T.C.--Recycl ing timer, 15 minutes on. 45 minutes off. 115 volt a.c.

clock motor. All of the components shown within the dashed area and the switches

may be assembled into a single control panel.]" Then two-wire cables can be extended to the thermostats, humidistat, motors, and furnace.

The control panel should be installed at sonic readily accessible point. The outdoor thermostat ( T , ) may be placed on a post or wall out-

doors, or if it is a type with separate bulb, only the bulb need be outside. Care should be taken that the bulb is always shaded fronl the sun and sky.

The storage thernlostat (T._,) and humidistat ( H ) should be installed above the potatoes, but at least one foot below the ceiling.

The nlain air duct thermostat (Ta) should be a type with separate bulb. The bulb should be placed in the main air duct at least 20 feet downstream from the main duct fan.

When potatoes are placed into the storage in the fall, they can best

SYSTF.M ePIc;RATION

be cooled by setting the dampers so the main duct fan will draw outside air. The main duct fan should then he turned on to run continuously and the furnace switch turned off.

If the storage is to be kept at 40 F and 85% relative lmmidity, the following control settings are recommended:

T1 - - Outdoor thermostat, 40 F. T., - - Storage thermostat. 43 F. T:, - - Duct thermostat. 38 F. H - - Humidistat, 85%.

Other temperatures may be set in a similar manner. After the cooling off period in the fail, the controller should be set

for automatic operation. The furnace then remains off, and the dampers remain set so that the main duct fan will draw outside air. The timer should be set for 5 minutes on and 55 minutes off, if possible. Otherwise, 15 nlinutes on and 45 minutes off may be used. These settings can remain until lhere is a possibility that the outside air temperatures will reach or go below 32 F.

When freezing temperatures are possible, the dampers should be set so the main duct fan will only recirculate air through the potatoes, and all the controls should be set for automatic operation. Tlte large exhaust

"{'A completely preassembled control panel, the three thermosta ts , and the humid is ta t as described in this article are available commercial ly . The mamffac tu re r ' s name and address may be obtained on request to el*her of the au thors of this paper.

1962] BUELOW AND RIPPEN: COLD WEATHER STORAGES 77

louvers or openings should be closed and the timer set for 15 minutes on and 45 minutes off.

Checking the per cent relative lmmidity at regular intervals (about once a week) with a reliable instrument is desirable.

TRIAL RESULTS

An automatic control system similar to the one described in this article was installed by a grower in a 30,000 bushel bulk box potato storage in Central Michigan. The system has been used for one storage season. A recording thermometer-llygrolneter was placed in the storage dnring the season. The records show that both temperature and humidity were held very close to the desired levels at all times. No condensation appeared on the walls or ceiling at any time and the potatoes remained in good condition during the entire period.

SUMMARY

A potato storage should have sufficient but not an excessive amount of insulation for maintaining the desired temperature and humidity. A vapor barrier on the interior surface of the walls and ceiling is essen- tial for keeping the insulation dry and effective. The fans and air dis- tribution system should be designed to meet the requirements during the various stages of the entire storage period.

During cold weather a relatively simple inexpensive system may be used for automatically controlling the temperature and humidity inside a storage. The system can be installed in most existing storages with minor modifications.

IATERATURE CITED

1. American Society of Refrigerating Engineers. 1949. Refrigeration fundamentals. 6th ed. 243 p.

2. Bennett, A. H.,R. L. Sawyer, L. L. Boyd and R. C. Cetas. 1960. Storage of fall- harvested potatoes in the Northeastern late summer crop area. Marketing Research Report No. 370. USDA. Washington, D. C.

3. Kazarian, E.,J. Boyd and R. Maddex. 1960. Insulation for farm buildings. Michi- gan State University, East Lansing, Michigan. Farm Building Circ. 741.