CONTROLLED TEMPERATURES1 - Plant physiology · The entire air temperature regulating unit and air duct leading from it to the air chamber are covered bya one-inch layer of cork for

EQUIPMENT FOR THE GROWING OF PLANTS AT CONTROLLED

TEMPERATURES1E. MARION BROWN

(WITH SEVEN FIGURES)

Introduction

Equipment designed for growing plants under conditions of controlledtemperature was installed at the Missouri Experiment Station, Columbia,Missouri, in the fall of 1936. Some features of this apparatus are modifica-tions of temperature control equipment devised and used by other workers.Other features, particularly the use made of standard air-conditioning equip-ment, are believed to represent new developments in apparatus for the grow-ing of plants at controlled temperatures. The description which follows ispresented with the idea that other investigators might make use of certainfeatures in the designing of other equipment for the growth of plants incontrolled environments.

Description of apparatus

The temperature control equipment described herein consists of 3 growthchambers, the devices necessary for the regulation of temperature in them,and the greenhouse in which they are housed. The greenhouse, 15 feet wideby 28 feet long, is oriented so that its long axis runs east and west. Thegrowth chambers are located along its south side, with a space of approxi-mately 6 feet left between the end compartments and the end walls of thegreenhouse, an arrangement which provides equal lighting for all compart-ments. All three growth chambers operate simultaneously with air and soiltemperature independently controlled.A general view of the greenhouse and the temperature control equipment

which it contains is shown in figures 1 and 2.

GROWTH CHAMBERS

Each growth chamber consists of a soil temperature tank and an airchamber. The soil temperature tank occupies all of the compartment, excepta 6-inch space between the tank and rear wall, to a height of 3 feet above thefloor. The air chamber is that part of the compartment above the soil tem-perature tank. This space is limited at the top and on the south side by aportion of the roof and south wall of the greenhouse.

The front and side walls of each air chamber, and the doors in the rear,1 Contribution of the Division of Forage Crops and Diseases, Bureau of Plant In-

dustry, United States Department of Agriculture, and the Missouri Agricultural Experi-ment Station, Columbia, Missouri.

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FIG. 1. Greenhouse containing the temperature control units as viewed from north.

are fitted with double panes of glass, separated by a 1-inch air space to reduceheat loss. The roof, however, is covered by a single thickness of standardgreenhouse glass, in order to admit light with a minimum of absorption bythe glass. The partitions and rear walls of the compartments below the airchamber consist of double walls of masonite, the panels of which are separated

......

FIG. 2. Interior of greenhouse showing temperature control units.

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BROWN: EQUIPMENT FOR CONTROLLED TEMPERATURES

by a 2-inch air space. The concrete side wall of the greenhouse forms thefront wall of each compartment below the air chamber.

The foregoing description, as well as that which follows, will be clarifiedby reference to the drawings presented (figs. 3 to 6). Dimensions of thegreenhouse and growth chambers are shown in figures 5 and 6.

FIG. 3. Longitudinal section of greenhouse showing location and arrangement oftemperature control units. A, compressor; B, air cooling unit; C, recording temperaturecontroller; D, air duct; E, rotary pump and motor; F, doors to air chamber; P, air heater;T, immersion heater; U, relays operating heaters; W, electrical conduit.

FIG. 4. Cross-section of greenhouse containing temperature control units. A, com-pressor; B, air cooler; C, recording temperature controller; D, air duct; E, motor operat-ing rotary pump; F, glazed wall of air chamber; W, electrical conduit; X, roof irrigator;Y, gutter.

MAINTAINING CONSTANT AIR TEMPERATURES

The air temperature is held constant by continuously circulating the airthrough the air chamber and the temperature regulator just outside and back

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FiG. 5. Floor plan of greenhouse and temperature control units. A, compressor; B,air cooler; D, air duct; E, G, rotary pumps and motors; H, unit heater (steam); I, waterpipes to cooling tower; J. soil container; K, soil temperature tank; L, baffle over air out-let; M, sleeve cover.

FIG. 6. Cross-section of greenhouse and temperature control unit. D, air duct; F,air chamber; J, soil container; K, soil temperature tank; L, baffle over air outlet; M,sleeve; N, cooling coils; 0, fan; P, electric heater.

of each compartment (B and P, figs. 3 to 5). The course followed by thecirculating air, as well as the essential parts of the temperature regulator, areshown in the cross-section of greenhouse and temperature control unit pre-sented in figure 6. The air, withdrawn from the air chamber (F, fig. 6)

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under the baffle (L, fig. 6), passes between the soil temperature tank andrear wall of the compartment into the lower part of the temperature regula-tor, through a short air duct. As the air passes through the temperatureregulator it is either heated or cooled, depending on its temperature as itleaves the air chamber, relative to that which the controls are set to maintain.The heated or cooled air is returned to the air chamber through the air duct(D, fig. 6), the outlet of which extends nearly the full width of the air cham-ber near the top. This outlet is provided with curved deflectors placed togive a uniform distribution of the inflowing air throughout the air chamber.

The cooling unit of the air temperature regulator is a Frigidaire roomcooler, Model C-100-B. It consists of a cabinet containing refrigerationcoils provided with fins to increase their cooling surface (N, fig. 6) and a fanwhich circulates the air (0, fig. 6). The heating unit is a Chromalox AirBlast Heater, Model CAB-5, mounted in the air duct just above the coolingunit. Each heater is divided into two equal 1500-watt, 220-volt, 3-phase cir-cuits, either of which can be switched on ilndependently of the other, toincrease or decrease the amount of heat supplied to the air. The heatersare equipped with thermo cut-outs to break the circuit in case of overheating.The entire air temperature regulating unit and air duct leading from it tothe air chamber are covered by a one-inch layer of cork for insulation.

In order to reduce the absorption of heat by the air chamber and lightenthe cooling load during warm, clear days, a sheet of water is run over thesouth slope of the greenhouse roof. A --inch copper pipe with small nozzlesinserted at 6-inch intervals extends the full length of the roof just belowthe ridge (X, figs. 4 and 6). By this means three streams of water, eachapproximately 1/20 inch in diameter, are directed against each 16-inch paneof glass at an angle which spreads the water in a thin sheet over the fullwidth of the sloping glass roof. The water is drained away by a gutteralong the south eaves (Y, figs. 4 and 6). Tap water, having a summer tem-perature of 60° to 700 Fahrenheit is used for this purpose and is not recir-culated.

SOIL TEMPERATURE TANK

The soil temperature tank (K, fig. 6) conisists of an iron tank, insulatedby a 2-inch layer of cork which is in turn covered by an outer sheath of lightermaterial. Built into each tank are three iron sleeves (M, fig. 6), each 10inches wide, 40 inches long, and 20 inches deep. These sleeves are weldedto the top of the tank so that only the sleeves open into the air chamber.Over each sleeve there is a detachable cover made of 2-inch cypress in whichfour equally spaced circular openings have been cut to hold the soil containers.

The tank is filled with water or some non-freezing solution, depending onthe temperature which is to be maintained. The tank solution is preventedfrom coming in contact with the soil containers by the metal sleeves in which

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-the containers are suspended. However, the sleeves themselves can be filledwith water or some other solution in order to facilitate the transference ofheat between soil and tank.

The pots holding the soil in which the experimental plants are grown aremade of 24-gauge galvanized iron soldered at the seams to make them water-tight. They are 8 inches in diameter at the top, 7 inches in diameter atthe bottom, and 18 inches deep. Each soil can has a flange made of j-inchangle iron riveted to its upper rim. This flange not only strengthens thepot, but also supports it when it is suspended in the sleeve of the soil tem-perature tank, the flange resting on the edge of the opening in the cover.

Each soil container is also provided with a bail which, when turned down,fits snugly against the upper edge of the flange.

In order to maintain the liquid in the soil temperature tanks at a con-stant temperature, each tank is provided with a cooling coil, a heating unit,and a circulatory system. A perspective view of one of the soil temperaturetanks with cover and sleeves removed to show the temperature regulatingdevices is represented diagrammatically in figure 7.

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FIG. 7. Perspective view of soil temperature tank with cover and sleeves removed.B, air cooler; D, air duct; E, rotary pump; K, tank; Q, cooling coil; RE, outlet pipe fromtank to pump; R2, R%, return pipes from pump to tank; S, solenoid valve; T, immersionheater; V, refrigerant valves; Z, cork insulation.

The cooling coil consists of a 25-foot length of-inch copper tubing

resting on the bottom of the tank (A, fig. 7). The flow of refrigerant throughthe coil is regulated automatically by a solenoid valve (S, fig. 7), or manuallyby opening and closing a gas valve placed in the refrigeration line (V, fig. 7).A uniform temperature throughout the tank is maintained by constantly

circulating the solution. This is accomplished by drawing the liquid from

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the bottom of the tank through a pipe (R1, fig. 7) leading to a motor drivenrotary pump (E, fig. 7), alnd returning it at the top of the tank near eachend through 8 spaced outlets in the return pipes (R3, fig. 7). The pump andreturn pipe (R2, fig. 7), which lie outside of the compartment, are protectedagainst rapid heat loss or absorption by cork insulation.

Heat is supplied to the solution by a Chromalox Immersion Heater, similarto Model M-175. It is inserted in a T-joint at the upper end of a vertical2-inch return pipe (T, fig. 7). This heater, rated at 300 Watts, has a blade9 inches long and is operated by a 110-volt, single phase circuit.

REFRIGERATION

All cooling coils are connected to a Frigidaire Model FW6J compressorlocated inside the greenhouse (A, figs. 2, 4, and 6). The compressor has awater-cooled condenser, through which water is circulated by means of amotor-driven rotary pump (G, fig. 5). The water is cooled by being runithrough a cooling tower outside of the greenhouse, and is then recirculatedthrough the condenser. A 3-horsepower motor operating on a 220-volt,3-phase circuit drives the compressor. Freon is the refrigerant.

The compressor motor is switched on and off by means of a pressure switchoperated by changes in the pressure of the refrigerant and acting through arelay. In this manner, the opening of any one of the six solenoid valves,each of which controls the flow of refrigerant through a cooling coil, willcause the compressor to operate. When all solenoid valves are closed, thecompressor stops. An excessively high pressure caused by the failure ofwater to flow through the condenser also breaks the circuit to the compressormotor.

The same pressure switch and relay which automatically start and stopthe compressor motor also start and stop simultaneously the pump whichcirculates water through the condenser and the fan in the cooling tower. Anadditional circuit is provided, however, by which the water-circulating pumpcan be put into continuous operation independent of the operation of thecompressor. This is done in cold weather to prevent freezing in the waterpipes outside of the greenhouse. The cooling tower fan can also be put intocontinuous operation, or turned off.

CONTROL OF TEMPERATURE

Each temperature control unit is equipped with a Brown two-pen record-ing control thermometer model 6422-830 (C, figs. 3 and 4). These instru-ments simultaneously record and control the temperature of both the airchamber and soil temperature tank. The thermometer bulbs and capillarytubing are of the mercury expansion type. One bulb of each instrument is

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immersed in the solution of the soil temperature tank, -the other is suspendedbelow the baffle (L, fig. 6) in the stream of air flowing out of the air chamber.The temperature measured by each bulb is recorded on the seven-day chartof the recording instrument.

The control point for each bulb is set at the desired temperature, whichmay be the same or different, for the soil temperature tank and the airchamber of the same unit. A three-point mercury switch within the instru-ment is so wired that a neutral contact is made whenever the temperatureis that for which the control is set. When the temperature drops below thecontrol point, a contact is made by the three-point mercury switch which,through a relay, completes the circuit to one of the electric heaters. If thetemperature rises above that for which the instrument is set, the three-pointmercury switch makes a contact which opens a solenoid valve allowing therefrigerant to expand in one of the cooling coils. The differential in tem-perature required to throw the 3-point mercury switch to the high or lowtemperature side of the neutral point is adjustable.

Performance

Extensive tests to determine the performance of this equipment underthe extremes of outside temperatures to which it has been exposed have notbeen made. The equipment, however, has been operated extensively for thepurpose for which it was designated: the growing of plants at controlled tem-peratures. Thus from January 11 to March 10, 1937, the growth chamberswere operated at air and tank temperatures of 400, 500, and 600 F. Outsideair temperatures ranged from a low of - 60 to a high of 680 F. during thisperiod. During mild, clear days, the air temperature in the 400 compart-ment would rise to a maximum of 500 F. at mid-day. Except during thisbrief period, air temperature did not vary from the setting of 400 by morethan ± 20 F., or from settings of 500 and 600 by more than + 2.50 F. Atno time did the temperature of the soil temperature tanks vary from the settemperature by more than 10 F. in any of the compartments.

From March 18 to May 13, 1937, the growth chambers were operated atair and tank temperatures of 600, 700, and 800 F. During this period,outside temperatures ranged from 230 to 860 F. No difficulty was experi-enced in maintaining the temperatures for which the controls were set withina maximum range of ± 2.5° in the case of air temperatures and 10 in thecase of tank temperatures.

From May 31 to July 26, 1937, the growth chambers were maintained atair and tank temperatures of 80°, 900, and 1000 F. During this period out-side air temperatures ranged from 580 to 960 F. Again air temperatureswithin the compartments were held within a range of ± 2.50 from the settemperature, and tank temperatures within a range of ± 10 F.

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The temperature ranges given above represent extreme variations whichdid not occur during the greater part of the time in which the growthchambers were in operation. Furthermore, except in the case of air tem-perature in the 400 compartment on mild clear days, the variations aboveand below the set temperature were equal.

From July 28 to September 20, 1936, all three soil temperature tankswere maintained at 700 F. while air temperatures were held at 700, 850, and1000 respectively. Variations from set temperatures during this periodwere no greater than previously described. Outside air temperatures rangedfrom a minimum of 41° F. to a maximum of 980 F. during this period.

From September 24 to November 19, 1937, the tanks were again main-tained at 700 F. in all three growth chambers, while air temperature wasalso maintained at 700 in one compartment. In another growth chamberan air temperature of 800 was maintained during the day and one of 600during the night. In the third growth chamber a 900 day and a 500 nightair temperature were maintained. Approximately one-half hour was re-quired to change the air temperature from the day to night or night to daysetting and with the exception of this brief period, air and tank temperatureswere held constant within the ranges previously given. The diurnal changesin the setting of the controls had to be made by hand, but this was a simpleoperation. Outside air temperatures ranged from a minimum of 180 to amaximum of 940 F. during this period.

From November 21, 1937, to January 15, 1938, the growth chamberswere again operated at air and tank temperatures of 600, 700, and 800 F.with no greater variations from control settings than reported for previousperiods. The range of outside air temperatures during this period wasfrom 50 to 600 F.

In order to test the cooling capacity of the soil temperature tanks, onewas lowered to a temperature of - 100 F. and held at this level from 4 to 5P.M. The average outside temperature on this date was 460 F.

One growth chamber was maintained at an air temperature of 600 F.from 1 to 3 P.M., May 20, 1937, a clear afternoon, with an outside wet bulbtemperature of 750 and a dry bulb temperature of 880 F. No water wasrun over the roof of the growth chamber during this test period.

Since at no time has it been necessary to use more than one-half of theheating capacity of the air conditioning unit (two heaters controlled byseparate switches are provided in each), much higher temperatures thanhave been applied so far can undoubtedly be attained.

During the first year of operation, mechanical failures of the equipmenthave been infrequent and of a minor nature. Because of the use made ofstandard air-conditioning and refrigeration equipment, repair parts are

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readily obtained, and necessary repairs and adjustments can be made by thelocal refrigerator service department.

BUREAU OF PLANT INDUSTRYWASHINGTON, D. C.

ANDMISSOURI AGRICULTURAL EXPERIMENT STATION

COLUMBIA, MISSOURI

LITERATURE CITED1. CROCKER, W. Organization, equipment, and dedication. Contrib.

Boyce Thompson Inst. Plant Res. 1: 9-58. 1925.2. DAVIS, A. R., and HOAGLAND, D. R. An apparatus for the growth of

plants in a controlled environment. Plant Physiol. 3: 277-292.1928.

3. DICKSON, J. G. Making weather to order for the study of grain diseases.Wisconsin Agr. Exp. Sta. Bull. 379. 1926.

4. HOTTES, C. F. A constant humidity case. (Abst.) Phytopath. 11: 51.1921.

5. JOHNSON, J. Constant temperature and humidity chambers. Phyto-path. 18: 227-238. 1928.

6. JONES, L. R., JOHNSON, J., and DICKSON, J. G. Wisconsin studies uponthe relation of soil temperature to plant disease. Wisconsin Res.Bull. 71. 1926.

7. LJEUKEL, R. W. Equipment and methods for studying the relation ofsoil temperatures to diseases in plants. Phytopath. 14: 384-397.1924.

8. McKINNEY, H. H. Unpublished description of the controlled tempera-ture chambers at the Virus Disease Laboratory of the Division ofCereal Crops and Diseases. Bur. Plant Ind., Arlington ExperimentFarm, Arlington, Virginia.

9. PELTIER, G. L., and Goss, R. W. Control equipment for the study of therelation of environment to disease. Nebraska Agr. Exp. Sta. Res.Bull. 28. 1924.

10. STEINBERG, R.- A. An apparatus for growing plants under controlledenvironmental conditions. Jour. Agr. Res. 43: 1071-1084. 1931.

11. STOUGHTON, R. H. Apparatus for the growing of plants in a controlledenvironment. Ann. Appl. Biol. 17: 90-106. 1930.

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Documents

CONTROLLED TEMPERATURES1 - Plant physiology · The entire air temperature regulating unit and air duct leading from it to the air chamber are covered bya one-inch layer of cork for