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A simple temperature control for phytotrons
J. agric. Engng Res. (1967) 12 (3) 233-237
A Simple Temperature Control for Phytotrons s. M. HENDERSON* ; F. P. ZSCHEILE, JR. t 1. Introduction Phytotrons or plant gro wth rooms frequently operate at a temperature that is above the external temperature part of the day and below for the remainder of the day. Such an operating condition is most characteristic of outdoor rooms where solar heat during the day and cooling by negative radiation at night are operating parameters. Occasion ally, small rooms located as part of or within a building experience the same temperature pattern. In these cases, both heating and refrigeration equipment are required. Room temperature control may be somewhat complicated under these conditions. Frequently, in the case of small rooms, heat may be applied continuously and refrigeration on control used to bring the temperature down to that desired; or a reverse procedure may be used, with continuous refrigeration and controlled heating. Either system is effective but the power costs in each case are high. Sophisticated control systems are avail able that will hold the room temperature at the desired level and will operate the heating and cooling facilities independently and never at the same time or in rapid succession. Such controls may have a high initial cost and may require expert service to keep them in good operating condition. Control mechanisms are discussed in many books. Two good references are by Fribance' and Miles." For much work in plant growth rooms, temperatures can experience a significant variation (up to ± 2° F) without ill effect on the experiments. Consequently, control equipment can generally be less complicated than controls for certain more critical types of research. This paper describes a simple but effective control that was used on a large sunlit phytotron 3 • 4 unit ( Fig. 1) which may require heating and /or refrigeration at some time each day.
Fig. 1. A phytotron unit" designed to oscillate daily so that it faces the sup at all times during the day
2. Equipment Two simple single-pole double-throw thermostat switches were wired into the heating and cooling power circuits (Fig. 2). Actually, the thermostats were in the low-amperage pilot circuits that controlled the magnetic switches for the respecti ve circuits but this fact does not change the schematic operation. The thermostat sensing elements were located close together at a point in • Agricultural Engineering De par tment. University of California, Davis, Ca liforn ia t Agronomy Department. University of California, Da vis, California
233
234
A SIMPLE TEMPERATURE CONTROL FOR PHYTOTRONS
the air mass leaving the room (growth chamber) where the air velocity was high (1100 ft/min) and where they were shielded from radiant heat sources. Three refrigeration compressors of different sizes and three separate series of heating elements could be switched into use as needed. The control circuit was switched from the upper to the lower pair by a time clock. Operation was as follows: The tolerance of thermostat A was ±2° F, that of B ±lo F. Each was adjusted to control at the same average air temperature. Assume that a room temperature of 70° F is required and that solar heat is significant. Thermostat switch A is in the c position and the refrigeration system is operating. When the air is cooled one degree below 70°F, switch B snaps to the h position and the refrigeration unit is turned off (Fig. 2). The air in the room then warms up (from outside
c
---I_~ h
4_H.I\el\a'Jf\inl'g/\u/\nri_'s_ _
Fig. 2. Schematic control wiring for refrigeration and heatingunits (c) Switch positionfor cooling ; (h) Switch positionfor heating
sources) to 71° F and switch B snaps back to the c position which starts the refrigeration system. This process continues as long as external conditions are such that heat enters the room by radiation, by conduction and perhaps by ventilating air. When cool (night) conditions develop, the room temperature goes down and when the air reaches 69° F, thermostat B snaps to the h position, but the air continues to cool and thermostat A snaps to the h position when the temperature reaches 68° F. The heater is then controlled by thermostat B and the room is again held between 69 and 71° F. When external (or internal) heat sources increase and the room air temperature reaches 72° F, thermostat A snaps to the c position and refrigeration is again available for cooling. 3. Operation of temperature controls The performance of elements for temperature controls can be characterized by (i) tolerance, that is, range within which they operate, and (ii) rate of response to temperature changes. Small sensing bulbs, well exposed to the air, will tend to follow the temperature of the air closely. Larger bulbs or sensing elements, perhaps insulated or shielded in some manner from the moving air, will lag in response to air temperature change. In the latter case, the room air temperature range will be greater than the tolerance of the control. In either case, response time will be shortened if the sensing element is located in air of high velocity. Satisfactory performance of a control system of this type will depend upon the following conditions: (a) One control should have slightly greater tolerance than the other. If each has the same tolerance, they can be insulated from the air so that a lag in response is introduced which will have the same general effect. Fig. 3 shows that one of the two bulbs is fitted with metal fins to
S. M. HENDERSON; F. P. ZSCHEILE, JR.
235
increase the rate of response to a temperature change (B) while the other is shielded by a sheet metal cylinder to decrease the response rate (A). If abrupt changes in temperature of short duration occasionally take place, for example, due to outside doors being opened, clouds covering sun, adding lag to thermostat A may be advisable to prevent equipment from responding to such momentary changes. (b) The control sensors should be located close together so there will be no difference in the temperatures which they sense. (c) The air velocity past the sensors should be the same and as high as practicable. (d) Refrigeration and heating equipment should be designed so that the capacity can be varied by switching components of various capacity into use. When the capacity of the heating or refrigerating system matches or is only slightly greater than the demand, overshoot will be minimized and a more nearly uniform temperature will result.
Fig. 3. Two identical matched sets of thermostats
4. Performance records and discussion A thermograph record of the phytotron unit shown in Fig. 1 for a sunny day in November (1965) is shown in Fig. 4, top. The night temperature of 63° F was held by heating until 4 a.m. when the time clock changed control to 72° F. (The control setting was raised slightly at 8 a.m.). Heating continued until 10 a.m. when the solar heat plus the sensible heat and heat from the fan tended to increase the average room temperature, at which time switch A brought the refrigeration equipment into operation and discontinued the heating function. Refrigeration was required until 4 p.m., at which time the heat gained nearly balanced that lost until nearly 6 p.m. The change-over by switch A was made from refrigerating to heating at 6 p.m. and the time clock reduced the room temperature (average) to 63° F at 8 p.m, The steepness of the heating record between 8 and 10 a.m. indicates that too many heating units have been switched into the circuit during this period. At 10 a.m. the heating rate was decreased 50 % so that the temperature range would be less when heating was again required later in the day. Less variation in the room air temperature resulted when the heating output nearly matched the demand. In practice a range of 3° F is readily attained in both heating and cooling cycles. The record for refrigeration from 10 a.m. to 6 p.m, indicates that the refrigerating capacity may have been somewhat excessive but the range is only 1° F greater than the control range of switch B, so performance (± 1'5° F) would be considered satisfactory. Fig. 4, bottom, illustrates the action of refrigeration and heating systems when both A and B
236
A SIMPLE TEMPERATURE CONTROL FOR PHYTOTRONS
-f
-r
·H - 7-·· ,:.:L;
- --, . ,~
controls are in use, and when only B control is in the circuit. The controlled temperature was only slightly above the average indoor temperature resulting from the fan heat (8_10° F above ambient). The phytotron unit faced north in cloudy weather, minimizing effects of radiant energy. Note the long periods of inaction of either heating or refrigeration when both controls were functioning. In contrast, when only B was in use, rapid cycling of heating and refrigeration occurred continuously even when conditions of heat flow required neither. In this example, the daily power consumed for heating and refrigeration per degree differential with outdoors was 11 times as much with one control (9'15 kWh/d) as with two controls (0,835 kWh/d). Such large economies are typical of controlled temperature conditions near ambient; as the ambient temperature departs from the controlled temperature, cycles of heating or refrigeration become longer or more of their steps (Fig. 2) are used continuously, resulting in smaller but still significant savings of power. Note that controls for both heating and cooling operate at the same average temperature. Alberda" has employed a system of two controls, one for heating, the other for cooling; however, they must be set at slightly different average temperatures if opposition or rapid succession of heating and cooling are to be avoided. The control system described is recommended for phytotrons or growth chambers if initial cost must be minimized and expert technical service is unavailable. By proper selection of controls and reasonable care in adjustment, room air temperatures that are generally satisfactory for plant growth studies may be provided with a minimum of cost and routine service.
S. M. HENDERSON; F. P. ZSCHEILE, JR.
237
REFERENCES 1 2 3
4
5
Fribance, A. E. Industrial instrumentation fundamentals. McGraw-Hill Bk Company, 1962 Miles, V. C. Thermostatic control. Principles and practice. Chern. Rubb. Company, 1965 Zscheile, F. P.; Henderson, S. M.; Leonard, A. S.; Neubauer, L. W.; Sluka, S. J. A sunlight phytotron unit as a practical research tool. Hilgardia, 1965,36 (14) 493 Neubauer, L. W.; Zscheile, F. P. Rotating solatron receives more sunlight for plant growth. Calif. Agric. 1966,20 (1) 4 Alberda, Th. The phytotron of the Institute for Biological and Chemical Research on Field Crops and Herbage at Wageningen. Acta bot. neerl., 1958,7,265
A Simple Temperature Control for Phytotrons s. M. HENDERSON* ; F. P. ZSCHEILE, JR. t 1. Introduction Phytotrons or plant gro wth rooms frequently operate at a temperature that is above the external temperature part of the day and below for the remainder of the day. Such an operating condition is most characteristic of outdoor rooms where solar heat during the day and cooling by negative radiation at night are operating parameters. Occasion ally, small rooms located as part of or within a building experience the same temperature pattern. In these cases, both heating and refrigeration equipment are required. Room temperature control may be somewhat complicated under these conditions. Frequently, in the case of small rooms, heat may be applied continuously and refrigeration on control used to bring the temperature down to that desired; or a reverse procedure may be used, with continuous refrigeration and controlled heating. Either system is effective but the power costs in each case are high. Sophisticated control systems are avail able that will hold the room temperature at the desired level and will operate the heating and cooling facilities independently and never at the same time or in rapid succession. Such controls may have a high initial cost and may require expert service to keep them in good operating condition. Control mechanisms are discussed in many books. Two good references are by Fribance' and Miles." For much work in plant growth rooms, temperatures can experience a significant variation (up to ± 2° F) without ill effect on the experiments. Consequently, control equipment can generally be less complicated than controls for certain more critical types of research. This paper describes a simple but effective control that was used on a large sunlit phytotron 3 • 4 unit ( Fig. 1) which may require heating and /or refrigeration at some time each day.
Fig. 1. A phytotron unit" designed to oscillate daily so that it faces the sup at all times during the day
2. Equipment Two simple single-pole double-throw thermostat switches were wired into the heating and cooling power circuits (Fig. 2). Actually, the thermostats were in the low-amperage pilot circuits that controlled the magnetic switches for the respecti ve circuits but this fact does not change the schematic operation. The thermostat sensing elements were located close together at a point in • Agricultural Engineering De par tment. University of California, Davis, Ca liforn ia t Agronomy Department. University of California, Da vis, California
233
234
A SIMPLE TEMPERATURE CONTROL FOR PHYTOTRONS
the air mass leaving the room (growth chamber) where the air velocity was high (1100 ft/min) and where they were shielded from radiant heat sources. Three refrigeration compressors of different sizes and three separate series of heating elements could be switched into use as needed. The control circuit was switched from the upper to the lower pair by a time clock. Operation was as follows: The tolerance of thermostat A was ±2° F, that of B ±lo F. Each was adjusted to control at the same average air temperature. Assume that a room temperature of 70° F is required and that solar heat is significant. Thermostat switch A is in the c position and the refrigeration system is operating. When the air is cooled one degree below 70°F, switch B snaps to the h position and the refrigeration unit is turned off (Fig. 2). The air in the room then warms up (from outside
c
---I_~ h
4_H.I\el\a'Jf\inl'g/\u/\nri_'s_ _
Fig. 2. Schematic control wiring for refrigeration and heatingunits (c) Switch positionfor cooling ; (h) Switch positionfor heating
sources) to 71° F and switch B snaps back to the c position which starts the refrigeration system. This process continues as long as external conditions are such that heat enters the room by radiation, by conduction and perhaps by ventilating air. When cool (night) conditions develop, the room temperature goes down and when the air reaches 69° F, thermostat B snaps to the h position, but the air continues to cool and thermostat A snaps to the h position when the temperature reaches 68° F. The heater is then controlled by thermostat B and the room is again held between 69 and 71° F. When external (or internal) heat sources increase and the room air temperature reaches 72° F, thermostat A snaps to the c position and refrigeration is again available for cooling. 3. Operation of temperature controls The performance of elements for temperature controls can be characterized by (i) tolerance, that is, range within which they operate, and (ii) rate of response to temperature changes. Small sensing bulbs, well exposed to the air, will tend to follow the temperature of the air closely. Larger bulbs or sensing elements, perhaps insulated or shielded in some manner from the moving air, will lag in response to air temperature change. In the latter case, the room air temperature range will be greater than the tolerance of the control. In either case, response time will be shortened if the sensing element is located in air of high velocity. Satisfactory performance of a control system of this type will depend upon the following conditions: (a) One control should have slightly greater tolerance than the other. If each has the same tolerance, they can be insulated from the air so that a lag in response is introduced which will have the same general effect. Fig. 3 shows that one of the two bulbs is fitted with metal fins to
S. M. HENDERSON; F. P. ZSCHEILE, JR.
235
increase the rate of response to a temperature change (B) while the other is shielded by a sheet metal cylinder to decrease the response rate (A). If abrupt changes in temperature of short duration occasionally take place, for example, due to outside doors being opened, clouds covering sun, adding lag to thermostat A may be advisable to prevent equipment from responding to such momentary changes. (b) The control sensors should be located close together so there will be no difference in the temperatures which they sense. (c) The air velocity past the sensors should be the same and as high as practicable. (d) Refrigeration and heating equipment should be designed so that the capacity can be varied by switching components of various capacity into use. When the capacity of the heating or refrigerating system matches or is only slightly greater than the demand, overshoot will be minimized and a more nearly uniform temperature will result.
Fig. 3. Two identical matched sets of thermostats
4. Performance records and discussion A thermograph record of the phytotron unit shown in Fig. 1 for a sunny day in November (1965) is shown in Fig. 4, top. The night temperature of 63° F was held by heating until 4 a.m. when the time clock changed control to 72° F. (The control setting was raised slightly at 8 a.m.). Heating continued until 10 a.m. when the solar heat plus the sensible heat and heat from the fan tended to increase the average room temperature, at which time switch A brought the refrigeration equipment into operation and discontinued the heating function. Refrigeration was required until 4 p.m., at which time the heat gained nearly balanced that lost until nearly 6 p.m. The change-over by switch A was made from refrigerating to heating at 6 p.m. and the time clock reduced the room temperature (average) to 63° F at 8 p.m, The steepness of the heating record between 8 and 10 a.m. indicates that too many heating units have been switched into the circuit during this period. At 10 a.m. the heating rate was decreased 50 % so that the temperature range would be less when heating was again required later in the day. Less variation in the room air temperature resulted when the heating output nearly matched the demand. In practice a range of 3° F is readily attained in both heating and cooling cycles. The record for refrigeration from 10 a.m. to 6 p.m, indicates that the refrigerating capacity may have been somewhat excessive but the range is only 1° F greater than the control range of switch B, so performance (± 1'5° F) would be considered satisfactory. Fig. 4, bottom, illustrates the action of refrigeration and heating systems when both A and B
236
A SIMPLE TEMPERATURE CONTROL FOR PHYTOTRONS
-f
-r
·H - 7-·· ,:.:L;
- --, . ,~
controls are in use, and when only B control is in the circuit. The controlled temperature was only slightly above the average indoor temperature resulting from the fan heat (8_10° F above ambient). The phytotron unit faced north in cloudy weather, minimizing effects of radiant energy. Note the long periods of inaction of either heating or refrigeration when both controls were functioning. In contrast, when only B was in use, rapid cycling of heating and refrigeration occurred continuously even when conditions of heat flow required neither. In this example, the daily power consumed for heating and refrigeration per degree differential with outdoors was 11 times as much with one control (9'15 kWh/d) as with two controls (0,835 kWh/d). Such large economies are typical of controlled temperature conditions near ambient; as the ambient temperature departs from the controlled temperature, cycles of heating or refrigeration become longer or more of their steps (Fig. 2) are used continuously, resulting in smaller but still significant savings of power. Note that controls for both heating and cooling operate at the same average temperature. Alberda" has employed a system of two controls, one for heating, the other for cooling; however, they must be set at slightly different average temperatures if opposition or rapid succession of heating and cooling are to be avoided. The control system described is recommended for phytotrons or growth chambers if initial cost must be minimized and expert technical service is unavailable. By proper selection of controls and reasonable care in adjustment, room air temperatures that are generally satisfactory for plant growth studies may be provided with a minimum of cost and routine service.
S. M. HENDERSON; F. P. ZSCHEILE, JR.
237
REFERENCES 1 2 3
4
5
Fribance, A. E. Industrial instrumentation fundamentals. McGraw-Hill Bk Company, 1962 Miles, V. C. Thermostatic control. Principles and practice. Chern. Rubb. Company, 1965 Zscheile, F. P.; Henderson, S. M.; Leonard, A. S.; Neubauer, L. W.; Sluka, S. J. A sunlight phytotron unit as a practical research tool. Hilgardia, 1965,36 (14) 493 Neubauer, L. W.; Zscheile, F. P. Rotating solatron receives more sunlight for plant growth. Calif. Agric. 1966,20 (1) 4 Alberda, Th. The phytotron of the Institute for Biological and Chemical Research on Field Crops and Herbage at Wageningen. Acta bot. neerl., 1958,7,265