US3745778A

US3745778A - Method and system for controlling air temperature in an air conditioned zone

Info

Publication number: US3745778A
Application number: US00087963A
Authority: US
Inventors: R Attridge
Original assignee: Ranco Inc
Current assignee: Ranco Inc of Delaware; Robertshaw US Holding Corp
Priority date: 1970-11-09
Filing date: 1970-11-09
Publication date: 1973-07-17
Anticipated expiration: 1990-07-17

Abstract

An air conditioning system for air circulating through an air conditioned zone is disclosed. A control system is provided which maintains a predetermined temperature in the zone by operating air heating and air chilling units of the system in either an onoff or a modulating mode. The system responds to the temperature of zone air and air flowing into the zone. Two air cooling units are disclosed, the first one of which can be modulated in its cooling effect and a second unit, having a constant capacity, is operated only if the cooling load exceeds the capacity of the modulatable unit. When the cooling load is less than the full capacity of both units, the first unit is modulated while the second unit operates at capacity. The air heating units are operated in stages. The sensed zone air temperature is primarily responsible for initiating cycles of the heating stages while the sensed duct air temperatures terminate operation of the heating stages. The heating stages have low thermal inertias so that they can be rapidly cycled.

Description

United States Patent 1 Attridge, Jr.

[ METHOD AND SYSTEM FOR CONTROLLING AIR TEMPERATURE IN AN AIR CONDITIONED ZONE [75] Inventor: Russell G. Attridge, Jr., Columbus,

Ohio

[73] Assignee: Ranco Incorporated, Columbus,

Ohio

221 Filed: Nov. 9, 1970 [21] Appl. No.: 87,963

[451 July 17, 1973 Primary Examiner-Meyer Perlin Assistant ExaminerRonald C. Capossela Attorney-Watts, Hoffmann, Fisher & Heinke [57] ABSTRACT An air conditioning system for air circulating through an air conditioned zone is disclosed. A control system is provided which maintains a predetermined temperature in the zone by operating air heating and air chilling units of the system in either an on-off or a modulating mode. The system responds to the temperature of zone air and air flowing into the zone.

Two air cooling units are disclosed, the first one of which can be modulated in its cooling effect and a second unit, having a constant capacity, is operated only if the cooling load exceeds the capacity of the modulatable unit. When the cooling load is less than the full capacity of both units, the first unit is modulated while the second unit operates at capacity. The air heating units are operated in stages. The sensed zone air temperature is primarily responsible for initiating cycles of the heating stages while the sensed duct air temperatures terminate operation of the heating stages. The heating stages have low thermal inertias so that they can be rapidly cycled.

12 Claims, 5 Drawing Figures METHOD AND SYSTEM FUR QQNTIRUILLKNG AER TEMPERATURE lN AN AIR CQNDHTIIGNED ZUNE CROSS REFERENCED US. PATENT U. S. Pat. No. 3,498,074 issued Mar. 3, l970 to Solomon S. Fineblum, entitled CONTROL SYSTEM FOR REFlllGERATING APPARATUS.

BACKGROUND OF THE lNVENTlON Field of the Invention This invention relates to comfort control systems and more particularly relates to a method and system for controlling the operation of a comfort control system to maintain a given temperature in an air conditioned zone.

The Prior Art The provision of a single large capacity refrigeration unit for cooling air introduced into the air conditioned zone has long been recognized as undesirable because cycling of such a unit, parh'cularly when the cooling load on the zone was not large, was too frequent. Such systems were inefficient because relatively large amounts of electrical power consumed by frequent compressor cycling. Frequent cycling also subjected the equipment to deleterious stresses and reduced their life. Furthermore, the temperature of chilled air entering the zone was frequently undesirably low causing unduly large temperature ranges in the zone to be commonplace.

According, it was proposed that multistage refrigeration units be employed to cool air conditioned zones. The cooling capacity of the refrigeration unit forming each stage was relatively small compared to the total cooling capacity of the system. Comfort conditioning the air in the zone was then accomplished by, for example, operating one or more refrigeration stages continuously and cycling additional stages, or by cycling a single stage, depending on the cooling load in the zone. Such systems did not obviate cycling refrigeration units but did reduce the inefficiencies incident to frequently cycling relatively large refrigeration units.

Even when relatively small refrigeration units were employed in stages, the cooling capacity of such units increased substantially when atmospheric air temperatures dropped below the atmospheric air temperature at which the units were designed to operate. This was due to increased heat rejection by condensers of the units. Standards for design of air conditioning systems required refrigeration units to operate at their rated capacity at an outside atmospheric dry bulb air temperature of 95 F". However, some manufacturers used an outside design temperature of 105 F" to insure adequate cooling capacity at high ambient temperatures. Thus when atmospheric air temperatures dropped below 95, the cooling capacities of refrigeration units increased with the largest increases exhibited by units designed for rated capacity at l F. in some circumstances, the evaporator temperatures became sufficiently cool to subcool the zone air. This caused wide temperature excursions in the air discharged into the zone and resulted in frequent cycling, excessive zone temperature fluctuations and uncomfortably cold duct discharge air streams.

The prior art recognized that if the capacity of the individual refrigeration stages could be controlled, cycling frequency of individual units could be reduced. lit

was proposed to provide multiple unloader valves in the refrigerant compressors of these units. Unloader valves operated to reduce the pressure of refrigerant at the compressor head, to decrease the unit's cooling capacity in proportion to the extent of refrigerant head pressure reduction.

When zone temperatures rose above a set point temperature, one refrigeration stage was energized with a minimum refrigerant head pressure. As the zone continued to require more cooling, successive unloader valves of the stage were operated so that the capacity of the first refrigeration stage was increased step-wise to full capacity.

if the zone required additional cooling, a second stage was energized, first at minimum cooling capacity and then step-wise to full capacity like the first stage. The cooling load of the zone was balanced by cycling operation of one or more unloader valves rather than an entire refrigeration stage.

There were a number of disadvantages to this last mentioned proposal. One disadvantage was that although small sensible cooling loads on the zone could be dealt with by operation of one stage at low capacity, the latent heat load on the zone was not adequately controlled. That is, the moisture content of the zone air was not reduced sufficiently to product comfort even though the zone air temperature was reduced. Another disadvantage was that, even using unloader valves, the cooling system could not be matched with the cooling load on the zone because cooling capacity was changed step-wise. Thirdly, because the capacity of each refrigeration unit was individually altered in the sequence outlined above, the control systems and refrigeration equipment required to perform these functions were relatively complex, and expensive to purchase and install.

Another shortcoming of some prior art control systems was their inability to automatically govern both heating and cooling of the air conditioned zone. in some control systems, a mode switch had to be manually operated from the zone. Separate thermostats for heating and cooling were also required in some systems.

SUMMARY OF THE INVENTHON The present invention provides a new method and system for automatically controlling air temperature in an air conditioned zone. The new control system is readily installable without excessive labor costs. The new system is particularly suited for inclusion in combined heating and cooling air conditioning systems thereby insuring optimum cooperation between the control system and the air conditioning unit with which it is installed.

Additionally, the new control system enables novel staging of refrigeration stages whereby the capacity of the refrigeration units is modulatable so that the cooling'load can be substantially matched by the cooling capacity of the refrigeration stages. in the preferred system, modulation of the cooling capacity is accomplished by controlling the cooling capacity of a single refrigeration stage whether or not more than one stage is operating; however modulation of the cooling capacity of additional stages is within the scope of this invention.

Modulation of refrigeration stages avoids short cycling of any refrigeration stage due to subcooling of air directed to a zone. Furthermore, modulation is accomplished by reducing the cooling capacity of the controlled unit from full capacity to a desired capacity. This assures that the latent heat load in the zone is maintained under control.

In a preferred embodiment of the invention, an electrical temperature signal is amplified to produce an output signal for operating heating or cooling units as required to maintain the air in the zone at a temperature set point level. The input temperature signal is produced by a sensor unit in the zone and a sensor unit in a zone inlet air duct.

The temperature signal is a composite signal formed by combining the zone and duct sensor signals with a reference signal determined by the temperature set point in the zone. The zone sensor signal has a substantially larger authority" than the duct sensor signal; that is, the ratio of the zone sensor signal level produced as a result of a sensed temperature change of, say 1 F, to the duct sensor signal produced by the same sensed temperature change is relatively large. This authority ratio may, on the average, be about :1.

The zone temperature sensor is principally responsible for initiating operation of the refrigeration stages. When the refrigeration stages are operating, the duct sensor produces changes in the temperature signal which result in the cooling capacity of the refrigeration stages being modulated. Because of the relatively low authority of the duct sensor and the limited ability of refrigeration stages to cool duct air, the effect of this sensor on the temperature signal is preferably not, of itself, sufficiently great to cycle a refrigeration stage. The duct sensor does alter the temperature signal sufficiently to modulate the first refrigeration stage.

The new control system is preferably operated in connection with electrical resistance heaters. When zoneheating is required by the control system, the

heater unit cycling is controlled by the effect of the duct sensor on the temperature signal. The electrical heater units, unlike the refrigeration stages, can be and are rapidly cycled when the zone is heated.

The heating units are constructed and arranged so that their cycling is controlled by the duct sensor so long as the zone temperature is below the set point temperature. The duct sensor thus functions to anticipate the effect on zone air temperature by the heaters.

Because of the relatively low thermal inertia of resistance heating units, the zone temperature approaches the set point temperature as the heating units are cycled but does not tend to overshoot the set point temperature. When the set point temperature is reached, the heating units are maintained off by the zone sensor.

In a preferred embodiment, the system amplifier is associated with an electrical power supply having a reference terminal and relatively positive and negative terminals respectively. When the zone temperature is higher than the set point temperature, the amplifier output signal varies in one sense direction from the reference voltage level and operates the refrigeration stages according to the signal level. When zone temperature is lower than the set point temperature, the amplifier output voltage varies from the reference voltage in the opposite sense direction to control the zone heating equipment.

The refrigeration stages are turned on at two different amplifier output voltage levels and are maintained on as the amplifier output voltage is reduced toward the reference voltage. The refrigeration stages are turned off at different voltage levels. The voltage level at which the second stage is turned off is of lower magnitude than the level at which the first stage is turned on so that the first and second stages can operate simultaneously. Overlapping the first stage turn on signal level and the second stage turn ofi signal level enables operation of the system over a smaller range of signal levels than would be required without overlapping. The first and second stages can also be operated simultaneously at smaller signal levels.

The first stage is modulatable in response to amplifier output levels ranging between the level at which the first stage is turned on and the level at which the second stage turns off. This enables the first stage alone to be modulated from maximum capacity to a lesser level. The first stage is also modulated when the second stage is operating, enabling cooling capacities ranging downwardly from two stages.

Modulation is preferably accomplished by a vortex amplifier connected in the compressor suction line to variably restrict the flow of refrigerant through the vortex amplifier to the compressor. The restriction produced by the vortex amplifier is governed by a pilot flow of condensed refrigerant. The pilot flow rate is changed continuously or in a stepwise fashion in response to changes in amplifier output levels and thereby modulates the cooling capacity of the first stage.

In a preferred embodiment of the invention, ventilation air is supplied to the zone during operation of the first air conditioning stage so that the cooling effect produced by that stage is further modulated. The quantity of ventilation air introduced into the zone is controlled in response to the temperature signal.

A principal object of the invention is the provision of a new and improved air tempering system including a control system for operating air heating and air cooling stages according to sensed air temperatures and wherein the air cooling stages are modulated to minimize cycling of refrigeration units, and reduce inlet air temperature excursions.

Other objects and advantages of the invention will be apparent from the following detailed description made with reference to the accompanying drawings which form a part of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatical illustration of an air conditioning zone, a system for conditioning the air in the zone and a comfort control system according to the invention for controlling the air conditioning system;

FIG. 2 is a diagrammatical illustration of the control system of FIG. 1;

FIG. 3 is a schematic illustration of a portion of the control system of FIG. 2;

FIG. 4 is a schematic illustration of another portion of the control system of FIG. 2; and,

FIG. 5 graphically depicts operation of the control system in relation to the zone air temperature.

DESCRIPTION OF A PREFERRED EMBODIMENT An air conditioning system embodying the present invention is illustrated in FIG. 1. The air conditioning system 10 conditions air circulating through a zone 12 by way of ducting generally indicated at 14. The ducting 14 includes a blower unit 16 for circulating air through the ducting and zone. An air heating system 66 and an air cooling system 26 heat or cool the air circulating through the ducting to the zone 112 as governed by a control system 22.

The ducting 14 includes an air return section 241 through which air passes from the zone into the ducting 14. An outdoor exhaust section 26 communicates with the return section 24 and includes suitable dampers 28 which govern the flow of air from the return section 24 to the atmosphere. Air flowing into the return section 24 may pass to a return air branch 36 and toward the intake of the blower 16. An outdoor air inlet section 32 communicates with the return air branch 36 and the flow of outdoor air into the return branch 36 is controlled by dampers 34 which are moved by a damper actuator 36. A discharge section 36 of the ducting houses the blower 16, air cooling heat exchangers 42, 46 and air heating heat exchangers 41-6, 66. Air which is passed across the heat exchangers is discharged into the zone through grill opening 52.

The Heating System 18 The heating system 18 includes first and second heating stages which are defined by the air

heating heat exchangers

48, 50. These heating stages preferably comprise electrically energized banks of resistance heated elements.

Control switch units

66, 62 are connected in the power circuits for the heat exchangers 46, 56, respectively, for controlling their energization. The

control switch units

60, 62 may be of any suitable construction and are not shown in detail. The switching units are constructed so that when either of the

heat exchangers

48, 50 are energized, energization of individual resistance heated elements is staggered. Hence, excessive electrical power is not consumed when either heat exchanger is initially energized.

Any number of heating stages can be provided in the system 18 depending on the heating requirements of the zone.

The Cooling System 26 The air cooling system 26 includes first and second electrically powered refrigeration stages generally designated at 66, 68. Each of these stages comprises a compressor-condenser-evaporator refrigeration unit including the air cooling heat exchangers 62, d4 comprising the evaporators of the

units

66, 66, respectively.

The first refrigeration stage 66 includes a refrigerant compressor 70 having a high pressure side communicating with a condenser 71 through a high pressure line 72. Condensed refrigerant from the condenser 7H flows through a condenser line 74 to an expansion valve 76. The refrigerant flowing through the expansion valve 76 vaporizes and passes through the evaporator 412 from which it is returned to the compressor 76 through a suction line 76.

The cooling capacity of the refrigeration stage 66 is modulated within limits by operation of a capacity control arrangement generally designated at 66. The capacity control includes a vortex amplifier 62 connected in the suction line 78 between the evaporator 42 and the compressor 70. The vortex amplifier is capable of variably restricting the refrigerant flow through the suction line 78 in accordance with the flow of refrigerant through a pilot flow line 64 The pilot flow through the line 66 is governed by a pilot control valve 66. The pilot valve 66 is adapted to modulate the pilot flow of refrigerant introduced into the vortex amplifier 62 and thereby modulate the capacity of the refrigeration unit 66. When the pilot valve 66 prevents the pilot flow of refrigerant from entering the vortex amplifier, the refrigeration stage 66 operates at full capacity and as the pilot flow increases through the pilot valve 66 the cooling capacity of the refrigeration stage 66 is reduced.

A capacity control arrangement for a refrigeration system similar to the capacity control arrangement 66 is described in detail in the above referenced patent to Fineblum.

in the illustrated embodiment of the invention, the pilot flow is infinitely variable by the pilot valve and infinitely varies the cooling capacity of the stage 66 between limits. Alternately, the pilot valve can vary the pilot flow in a stepwise fashion to modulate the cooling capacity of the stage 66 in a stepwise manner.

The second refrigeration stage 68 is also the compressor-condenser-evaporator type; however, in the illustrated embodiment, this stage is not provided with a capacity control arrangement. The second stage 66 may be of any suitable construction and therefore is shown schematically and not described in detail.

The Control System 22 The control system 22 governs operation of the heating or cooling stages and/or the damper actuator 36, depending upon the duct discharge air temperature and upon zone air temperature. A preferred control system is shown schematically in FIG. 2 and includes a zone sensor unit 96, a duct air sensor unit 92 and a temperature set point adjusting element 94 combined with the zone sensor unit. These units produce electrical temperature responsive signals which are combined to provide a temperature responsive input signal to an amplifier 96. The amplifier 96 and the

units

96, 92 are connected to

terminals

97, 98, 99 of an accurately regulated electrical power supply which is illustrated as a direct current power supply.

The voltage level at the terminal 96 is a reference voltage and in the convention used is sometimes referred to as Zero volts. The levels at the

terminals

97, 99, respectively, are referred to as plus voltage and minus voltage.

The amplifier 96 is capable of producing output signals which vary between the plus and minus voltages depending on the level of the combined temperature input signal to the amplifier. in the preferred embodiment, the amplifier output signals are direct current signals.

Generally speaking, when zone temperatures are higher than the desired or set point level, for example F, the amplifier output voltage is positive with respect to the reference voltage at the terminal 96. When zone temperatures fall below the set point temperature, the amplifier output signals tend to be negative with respect to the reference voltage.

When the zone air temperature is above the set point temperature, the amplifier 96 provides a positive output signal which is transmitted to cooling control circuitry 1166 through an amplifier output lead Mill and a diode H62. The diode 1l62 passes only a positive output signal from the amplifier to the cooling control circuitry 1166.

The circuitry 666 includes a first refrigeration stage triggering circuit 1, a second refrigeration stage triggering circuit 166, a first stage capacity control circuit 1166 and a damper actuator control circuit 1H6.

The triggering

circuits

104, 106 are preferably bistable voltage detecting electronic switches which are responsible for activating their respective refrigeration stages at predetermined amplifier output voltage levels. These triggering circuits may be conventional and therefore are not shown or described in detail. Control relays

l

14, 1 16 are connected to the outputs of the

trigger circuits

104, 106, respectively. These relays control the operation of the associated refrigeration stages 66, 68, respectively by energizing and deenergizing the respective stages. This is accomplished, as is conventional, by controlling energization of the compressors of the refrigeration units. The control relays in the output circuits of each of the

triggers

104, 106 are connected across a suitable power supply via tenninals 104a, 1064.

The capacity control arrangement 108 is shown schematically in FIG. 3. This circuit includes an electronic switch 120 which is connected in series with a heat motor 122 across the full wave rectified unfiltered power supply via a terminal 123. The heat motor 122 includes an electrical heater element 126 and a bimetal actuator 128 for operating the refrigerant pilot flow valve 86. As the heat transferred from the heater 126 to the bimetal actuator 128 is increased, the pilot valve is actuated to throttle the pilot flow to the vortex amplifier thereby causing the cooling capacity of the refrigeration stage 66 to increase towards full capacity. When heat transfer between the heater 126 and actuator 128 is at a relatively low level, the actuator 128 opens the valve 86 to reduce the capacity of the refrigeration stage 66.

The electronic switch 120 governs energization of the heater 126 primarily in response to the command signal from the amplifier 96. As shown in FIG. 3, the switch 120 includes Silicon Controlled Rectifiers [SCRs] 129, 130 which are connected in parallel with each other and in series with the heater element 126. The SCR 130 is connected around the SCR 129 through a voltage dropping resistor 131 so that when the SCR 130 conducts, the power supply voltage falls across the heater 126 and the resistor 131. This prevents operative energization of the heater when the SCR 130 is conductive. When the SCR 129 conducts the resistor 131 and the SCR 130 are shunted and the heater is operatively energized, i.e., nearly the entire power supply voltage falls across it.

The conductive stages of the

SCRs

129, 130 are controlled by an error circuit 132 which includes a resistance bridge and a differential amplifier. The differential amplifier includes

transistors

134, 136 having

emitters

138, 140, respectively, connected to the power supply terminal 123 through an emitter resistor 142, the voltage dropping resistor 131, a diode 146 and the heater element 126. The

collector electrodes

148, 150 of the transistors are each connected to ground through

output resistors

151, 152, respectively.

The base 153 of the transistor 136 is connected to the wiper 154 of a potentiometer 156 connected in one arm of the error bridge. The potentiometer 156 is connected in series with

bridge resistors

158, 160 between the voltage dropping resistor 131 and ground. The

resistors

158, 160 and the potentiometer 154 provide a biasing voltage level at the base 153 of the transistor 136 and determine the voltage level at the emitter 140 which is required to turn on the transistor 136. Once the wiper 154 of the potentiometer 156 is set at a particular location the transistor 136 will be rendered conductive at substantially the same point during each pulsation of the power supply.

The base voltage signal of the transistor 134 is thus controlled by the output of the amplifier 96, or command signal, and the state of the error bridge. The base 162 of the transistor 134 is connected to the output of the amplifier 96 via a junction 164 and conductor 166. The junction 164 is located in a bridge arm defined by a temperature responsive resistor 168 and a resistor 170 connected between the resistor 131 and ground.

The transistor 134 turns on during each power supply pulsation at a time determined by the voltage level at the junction 164. The voltage level at the junction 164 is influenced by both the bridge arm resistance and the level of the amplifier output signal on the conductor 166.

If the voltage level at the junction 164 is less than the voltage level at the base 153 of the transistor 136 during any given power supply pulsation, the transistor 134 will be rendered conductive before the transistor 136 conducts and will conduct more heavily than the transistor 136 at any given instant during a power supply pulsation. On the other hand when the voltage level at the junction 164 tends to be greater than the voltage level at the base 153, the transistor 136 is rendered conductive earlier in the power supply pulsation than the transistor 134 and will conduct more heavily at any given time in the power supply pulsation.

The collector electrode 148 of the transistor 134 is connected to the gate electrode of the SCR 130. The collector 150 of the transistor 136 is connected to the gate electrode of the SCR 129. When conduction through the transistor 136 is sufficiently great that the voltage across the output resistor 152 reaches the triggering level of the SCR 129, the SCR 129 is rendered conductive to establish an energizing circuit from the power supply terminal 123 through the heater 126 and to ground.

Once the SCR 129 is rendered conductive, the diode 146, resistor 131, and the error circuit 132 are shunted by the SCR 129. This prevents turning on of the transistor 134 during any power supply pulsation in which the SCR 129 becomes conductive. When the SCR 129 conducts, the heater element 126 is energized to heat the actuator 128 throughout power supply pulsations during which the SCR 129 conducts.

During power supply pulsations when the transistor 134 is rendered conductive before the transistor 136, the SCR 130 is rendered conductive to establish a circuit from the power supply terminal 123 through the heater 126, diode 146, resistor 131, and to ground. When the SCR 130 is conductive, the small forward voltage drop across it prevents the transistor 136 from triggering the SCR 129 during that power supply pulsation.

When the SCR 130 conducts, the voltage across the heater 126 is relatively small because of the size of the voltage dropping resistor 131. For this reason the heater 126 is substantially unheated throughout power supply pulsations during which the SCR 131 conducts.

The switch renders either of the SCRs conductive substantially throughout each power supply pulsation and is therefore termed a zero" switch, since it conducts at nearly a zero power supply phase angle. When the amplifier output voltage, or command signal, is at or above a predetermined level the SCR 129 is conductive substantially continuously so that the cooling capacity of the refrigeration unit 66 is maximum. As the command signal is reduced in magnitude the SCR 129 is conductive throughout progressively fewer power supply pulsations and the pilot valve 86 is progressively opened thereby gradually reducing the cooling capacity of the unit 66 as the command signal is reduced. The cooling capacity can be reduced to nearly any desired level according to the design of the parts of the capacity control arrangement 108. Suffice it to say that in the preferred embodiment modulation of the first stage cooling capacity reduces the cooling capacity by as much as 50 percent of full capacity as the command signal varies between predetermined limits.

The resistor 168 has a positive temperature coefficient of resistance and provides a temperature feedback arrangement for the switch 120 so that the heater 126 is not excessively heated or permitted to become too cool. The thermistor 168 is located in heat exchange relationship with the heater 126 so that when the heater is heated the thermistor 168 is likewise heated. Heating of the resistor 168 tends to decrease the voltage level'at the base 162 of the transistor 134 and thus tends to turn on the transistor 134 relatively early in each power supply pulsation. This tends to reduce the heating rate of the heater 126.

On the other hand, when the heater 126 is not energized, the resistor 168 cools resulting in a reduction in its resistance. The reduced resistance tends to retard conduction of the transistor 134, thus increasing the heat input to the heater.

The actuator control circuit 1 is operated from the amplifieroutput signal and controls operation of the damper actuator 36. Hence the actuator control circuitry modulates the ventilation air entering the zone.

. erence voltage, heating control circuitry 175 is energized to control heating of the zone. The heating control circuitry is connected to the amplifier output lead 101 through a filter diode 176 which is forwardly biased when the amplifier output voltage is negative. The filter diode connected between the cooling control circuitry and the amplifier 96 is nonconductive during this period so that the cooling control circuitry is deenergized.

The heating control circuitry 175 includes electronic triggering

circuits

177, 178 which operate the heating stage control switches 60, 62, respectively, by way of control relays. The triggering

circuits

177, 178 are preferably bistable voltage detecting switches which may be similar to those referred to above in connection with the cooling control circuitry 100. These triggering circuits operate their respective heating stage control switches at differing negative amplifier output voltage levels to provide staged heating.

As shown in FIG. 4, the amplifier 96 is a differential amplifier having a temperature signal input formed by a temperature input signal lead or conductor 181. The lead 181 connects the amplifier 96 to the duct sensor circuitry 182 and zone sensor circuitry 183 through a junction 184.

The duct sensor circuitry provides a duct temperature signal to the amplifier input lead 181 which is a function of duct air temperature. The duct sensor circuitry 192 includes a thermally responsive resistance string formed by a duct sensor thermistor 166 located in the duct sensor unit 92, a resistor 187 connected in series with the thermistor 186 and a resistor 188 which is connected parallel to the thermistor 166. An output junction 169 of the resistance string is connected between the resistor 187 and the thermistor 166. The output junction 189 provides the duct temperature dependent signal to the amplifier 96.

The magnitude of the duct temperature signal is determined by the setting of a potentiometer 1911. The output junction 189 of the resistance string is connected to the amplifier input lead 161 through a resistor 192 and the potentiometer 196.

The zone temperature circuitry includes a thermally responsive resistance string connected to the input 161 of the amplifier 96. The zone temperature sensor string is connected across the

terminals

97, 99 and includes a thermistor 195, potentiometer 196 and resistor 197 which are connected in series across the power supply terminals. A resistor 198 is connected in parallel with the thermistor 195. The zone temperature resistance string is connected to the amplifier input lead 161 through an output junction 26!) and a lead 2111 connecting the output junction 200 to the junction 1.

The potentiometer 196 includes a wiper 292 which is movable to provide for a set point adjustment of the zone temperature. The wiper 202 is connected to a suitable knob 203 which is moved to adjust the signal level at the junction 2116 and thereby adjust the set point temperature in the zone.

The amplifier 96 further includes a reference voltage input lead 219 which is connected to an output junction 211 of a voltage divider circuit. The voltage divider circuit is formed by series connected fixed

resistors

214, 216 which are connected in series across the

power supply terminals

97, 99.

The reference voltage at the junction 21 1 is provided to the reference input of the difierential amplifier 96 so that the output signal produced by the amplifier 96 has a level which is proportional to the difference between the voltage levels at the reference input lead 216 and the temperature signal input lead 181. The reference and input signals are calibrated during manufacturing. As shown in FIG. 4, a voltage divider 217 is connected across the

power supply terminals

97, 99. This voltage divider consists of a resistor 216, a potentiometer 2241 and a resistor 222. The wiper 224 of the potentiometer 226 is connected to the input reference lead 219 and this wiper is positioned during manufacture to calibrate the reference signal.

Suitable gain adjusting circuitry 239 provides negative feedback for the amplifier 96. The gain adjusting circuitry 239 includes a potentiometer enabling adjustment of the amplifier gain. The amplifier gain is preferably adjusted only during installation of the system.

FIG. 1 schematically shows the zone and duct sensors associated with control system 22. The zone sensor unit and set point adjusting element 94 are disposed in a housing located in the zone 12. The duct sensor unit 92 is a separate housing in the air discharge section of the ducting 14.

It is apparent, particularly from inspection of the FIG. 1, that whenever one or both refrigeration stages 66, 68 operates, the duct sensor unit 92 is subjected to substantially greater air temperature variations than is the zone sensor unit 90. This is because the air passing through the air cooling heat exchanger impinges on the duct sensor. The zone sensor is remote from the air cooling heat exchangers and is, if properly positioned in the zone, not directly impinged on by air discharged into the zone. To avoid rapid cycling of one or more refrigeration stages, the temperature signal produced by the duct sensor has a smaller authority than the signal produced by the zone sensor in response to an identical sensed air temperature change. That is, the effect of temperature changes on the amplifier input signal due to temperature change sensed by the duct sensor is small relative to the effect on the amplifier input signal caused by the same temperature change detected by the zone sensor 90.

The input signal to the amplifier 96 can vary according to the zone sensor unit signal as well as according to the duct sensor signal. These signals are not necessarily dependent on one another. Hence, the output signal from the amplifier 96 can be influenced primarily by either signal or both, depending on the operating condition of the system 22. The low authority of the duct sensor enables the refrigeration stages to be operated to cool the air in the zone without the stages being cycled in response to reduced duct air temperatures which do not reflect the zone air temperature.

The authority of the duct sensor signal in altering the composite input signal to the amplifier 96 is preferably relatively small as compared to the authority of the zone sensor signal. In a preferred system, experimentally operated, the change in amplifier input signal produced by sensing l of zone air temperature change was 20 times the change in the input signal produced by sensing a 1 duct air temperature change. Expressed as a ratio, the authority of the zone sensor to the duct sensor is preferably 20:1.

The authority ratio can be changed by altering the impedance of the potentiometer 194 to change the strength of the duct sensor signal relative to the zone sensor signal. This adjustment is preferably only made in connection with the installation or servicing of a system in a given building.

Operation of the System 22 for Cooling the Zone The operation of a preferred system is graphically illustrated in FIG. 5. FIG. 5 shows a graph having an ordinate corresponding to the output voltage of the amplifier 96 expressed as percentages of the voltage difference between the amplifier output voltage and the neutral power supply terminal 98, or zero volts. Thus the amplifier output voltage is shown as ranging between plus and minus 100 percent voltage. The abscissa corresponds to sensed zone temperatures differing from a set point temperature at the origin. The set point temperature may be, for example 75.

An amplifier output signal is indicated by the line 300 which illustrates the amplifier output voltages as a function of zone temperature only. From FIG. 5 it will be seen that when the sensed zone air temperature increases l.5 F above the set point temperature the amplifier output voltage increases linearly to about 100 percent voltage. Likewise when the zone air temperature drops 1.5" below the set point temperature the amplifier output voltage is reduced to about l00 percent.

The duct sensor signal component of the amplifier input signal is substantially independent of the zone sensor signal (within the limitations of the system equipment). Accordingly, at any particular location on the curve 300, a signal from the duct sensor unit indicating a change in temperature of the duct air will alter the magnitude of the amplifier output voltage according to the sensed direction of the detected air tempera- .ture change. The authority ratio of the zone sensor signal to the duct sensor signal is 20:1, when the duct sensor detects a change in duct air temperature reduction of 3 the amplifier output voltage is reduced by about 10 percent voltage.

Referring now to FIG. 5, assume that the zone temperature set point is adjusted to F. Assume further that the sensed zone air temperature is at or close to 75 F and that the cooling load in the zone is increasing and therefore the zone temperature is increasing.

When the zone air temperature has increased to about 756 F the amplifier output signal is at around 40 percent and the damper actuator 36 is operated so that the blower 16 circulates outside air to the zone through the damper 34 tending to stabilize the zone air temperature. If the zone air temperature is reduced to a level just above the set point temperature (about 10 percent of the amplifier output voltage) the dampers reclose.

In the preferred embodiment, the damper actuator operates the dampers in proportion to the magnitude of the amplifier output voltage. Hence, the dampers are fully opened when the ventilation cycle is initiated and progressively close toward a minimum ventilation position as the amplifier output voltage is reduced toward the set point level. In the preferred embodiment, the dampers are completely closed when the blower unit 16 is off and whenever the blower unit operates the dampers are moved to a minimum ventilation position. The dampers are operated between the minimum ventilation position and the fully open position during the ventilation cycle.

If the cooling load on the zone continues to increase during the ventilation cycle, the sensed temperature in the zone increases still further above the set point temperature unu'l the zone sensor detects an increase in zone air temperature of about 1.2F above the set point level. This causes the output voltage of the amplifier 96 to increase to about percent. This voltage level is sufficient to operate the triggering circuit 104 of the first refrigeration stage 66 whereupon the first stage is energized and operated at full capacity resulting in chilled air being directed through the duct and into the zone.

The chilled air flowing into the zone impinges on the duct sensor unit 92 and causes a reduction of the duct sensor output signal. This in turn reduces the amplifier output voltage at a rate of 10 percent for every 3 of air temperature reduction sensed by the duct sensor.

The amplifier output voltage therefore drops along the broken line curve 302 in response to the reduction in the output signal from the duct sensor. The curve 302 is shown offset from the amplifier output voltage level at which the stage 66 is operated since the first refrigeration stage 66, even operating at full capacity, does not immediately stop the increasing zone temperatures.

When the duct sensor output signal has reduced the output voltage of the amplifier 96 to between 60 and 70 percent of its maximum positive output voltage, the capacity control circuit 108 begins reducing the cooling capacity of the stage 66. This reduction in cooling capacity tends to stabilize the amplifier output voltage by stabilizing the duct air temperature.

If the duct sensor temperature is reduced sufficiently to drop the amplifier output voltage below the lower level of the capacity control modulation range, the stage 66 is operated at minimum capacity while the zone continues to be cooled.

If the cooling load on the zone is balanced by the first stage 66 operating in the capacity modulating range, or below that range at minimum capacity, the unit 66 continues to operate indefinitely. If the first unit 66, operating at minimum capacity, reduces the zone air temperature to about 75.5 F the unit 66 cycles. The compressor 70 is preferably turned off when the combined signal levels of the duct sensor and the zone sensor produce an amplifier output voltage of about 30 percent of maximum voltage.

If the first stage 66, operating at maximum capacity, is not capable of carrying the cooling load on the zone, the zone air temperature continues to increase. The zone temperature increases until the combined duct sensor and zone sensor signals produce an amplifier output voltage of around 100 percent. At this juncture the second stage 68 is operated.

.Sirnultaneous operation of the first and

second stages

66, 68 causes a marked reduction in the duct air temperature. Accordingly, the duct sensor output signal is substantially reduced and the amplifier output voltage falls as is shown by the broken line curve 304 in FIG. 5.

The reduction in the amplifier output voltage may be sufficiently large that the amplifier output voltage level is reduced into the capacity control modulation range. When this occurs, the capacity of the first refrigeration stage is again modulated to stabilize the amplifier output voltage level in the modulation range. In the illustrated embodiment, the second stage operates at full capacity at all times.

If the cooling load in the zone is balanced when the first unit cooling capacity is modulated with the second unit operating at full capacity, both stages are continuously operated and the amplifier output signal is stabilized in the modulation range. This prevents the air discharged into the zone from being undesirably subcooled while avoiding cycling of the second stage.

If the zone temperature is reduced by simultaneous operation of both stages (with the first stage operating at minimum capacity) the second stage is turned off when the amplifier output voltage is reduced below about 50 percent of maximum voltage. Thereafter the zone continues to be cooled by the first stage.

Because the first stage cooling capacity is modulated while the second stage is operating at full capacity, the cooling capacity of the combined stages is thereby "modulated. This minimizes cycling of the second stage.

Furthermore, because of the relatively low authority of the duct sensor, cycling the refrigeration stages occurs in response to sensed 'zone air temperature changes relative to the set point temperature and not as a result of changes in sensedduct air temperatures. The cooling capacity modulation, along with the relatively small authority of the duct sensor further reduce cycling of the refrigeration stages.

-From the preceding description it should be appreciated that by overlapping the signal ranges in which the individual cooling stages are operated and by providing cooling capacity modulation throughout a signal range within the overlapped cooling stage signal ranges, the cooling capacity afforded by multiple refrigeration units can be modulated by modulating only the first stage refrigeration unit. This minimizes cycling of stages, reduces the complexity of the control system required to operate the stages and enables the use of relatively unsophisticated refrigeration units.

While only two refrigeration units are provided in the illustrated comfort control system, it should be apparent that additional refrigeration stages can be provided. Operation of the System 33 for Heating the Zone When the zone temperature drops below the set point level, the amplifier output signal becomes negative with respect to the voltage of the power supply terminal 98 and increases in magnitude negatively as a function of the reduction of sensed zone air temperature below the set point temperature. The air heating stages are cycled as a function of the amplifier output voltage levels as shown in FIG. 5. The heating stages each have a relatively small thermal inertia and thus heat rapidly when energized and cool quickly when they are deenergized.

The control system 22 functions to cycle the heating stages by deenergizing the heater elements in response to the duct sensor signal while reenergizing them principally in response to the zone temperature signal. The amplifier output levels at which the heating stages are individually energized and deenergized are relatively closely spaced. The first stage is energized at a negative output voltage level of 40 percent and is deenergized at minus 10 percent. The second heater stage is energized at minus percent and deenergized at minus 30 percent.

When the sensed zone air temperature is reduced about 0.5 F below the set point temperature, the amplifier output voltage level increases negatively to about 40 percent of its maximum and the first heating stage is energized. The heating unit rapidly produces a flow of hot air through the duct discharge and into the zone. The temperature of the discharge air is sensed by the duct sensor unit. The duct air temperature is increased sufficiently by the heating stage that the temperature signal level produced by the duct sensor alone causes the heating stage to be deenergized.

The deenergized heating unit rapidly cools resulting in the duct sensor signal being quickly restored to a level corresponding to the zone air temperature. When this occurs the zone sensor signal again causes the amplifier to reenergize the first heating stage. The first stage heater is thus cycled at a frequency sufficient to maintain the zone temperature within about lF of the set point temperature.

If the heating load on the zone increases, the zone temperature is reduced further. When the amplifier output voltage level is at about minus 60 percent of maximum, the second heating stage is energized so that both the first and second heating stages are simultaneously energized.

When both stages are energized, the discharge air temperature sensed by the duct sensor causes the second heating stage to be deenergized relatively quickly. The first stage may be maintained energized due to the effect of the sensed zone air temperature on the combined temperature input signal to the amplifier. The second heating stage is thus cycled relatively frequently as a result of operation of the duct sensor.

Due to the low thermal inertia of the heating stages and cooperation between the duct sensor unit and the zone sensor unit in cycling the heating stages the zone temperature is stabilized without any substantial overshooting of the zone temperature above the set point level.

While a single embodiment of the invention has been illustrated and described in considerable detail, the present invention is not to be considered to be limited to the precise construction shown. Other adaptations, modifications and uses of the invention may occur to those skilled in the art and it is intended to cover hereby all such adaptations, modifications and uses which fall within the scope of the appended claims.

What is claimed is:

l. A method of controlling air temperature in an air conditioned zone comprising:

a. providing at least first and second refrigeration units;

b. directing air across air cooling heat exchangers of said units and into said zone through a zone air inlet;

c. producing a zone air temperature signal which varies in magnitude relative to a reference level in accordance with fluctuations in sensed air temperature in said zone;

d. producing an air temperature signal varying in magnitude relative to said reference level in accordance with fluctuations in sensed temperature of air supplied to said zone;

. combining said zone and supply air temperature signals to produce a composite cooling temperature signal varying in magnitude relative to said reference level;

f. initiating operation of said first refrigeration unit in response to production of a first composite cooling temperature signal having a predetermined magnitude relative to said reference level and terminating operation of said first unit in response to production of a second composite temperature signal having a second predetermined magnitude less than the magnitude of said first signal;

g. initiating operation of said second refrigeration unit in response to a third composite temperature signal level having a greater magnitude than said first signal level and terminating operation of said second unit at a fourth composite temperature signal level having a greater magnitude than said second composite signal level and a lesser magnitude than said first composite signal level; and,

h. modulating the cooling capacity of said first unit between full capacity and a lesser capacity in accordance with composite temperature signals ranging between fifth and sixth levels, said range of modulating composite signals between said fifth and sixth levels being of lesser magnitude than said first signal level and greater magnitude than said fourth signal level whereby the capacity of said first refrigeration unit is fully modulatable when said first unit is operating and when said first and second units are operating.

2. A method as claimed in claim 1 wherein modulating said cooling capacity includes variably restricting refrigerant flow in said first refrigeration unit.

3. A method as claimed in claim 1 and further including providing ventilating air to said zone in proportion to changes in composite temperature signals between said reference level and a seventh composite signal level less than said second signal level.

4. A method as claimed in claim 1 wherein said first refrigeration unit is initially operated at full capacity and is modulated from full capacity as said composite signal level is reduced from said fifth toward said sixth level.

5. A method as claimed in claim 1 wherein said zone supply air temperature signal and said zone temperature signal are algebraically combined to produce said composite signal, and wherein a predetermined change in zone supply air temperature produces a small change in magnitude of said supply air temperature signal as compared to the change in magnitude of said zone temperature signal produced by the zone air temperature undergoing said predetermined change.

6. A method as claimed in claim 5 wherein operation of said refrigeration units is initiated primarily in response to zone air temperature signal levels and the cooling capacity of said units is modulated primarily according to zone supply air temperature signal levels.

7. A method as claimed in claim 5 wherein the authority ratio of said zone air temperature signal to said supply air temperature signal is about 20:1 when said zone air temperature and a zone set point temperature are the same.

8. In a system for cooling air in a zone having at least first and second refrigeration units and capacity control means for variably controlling the cooling capacity of at least said first refrigeration unit:

a. first air temperature sensing means exposed to air in said zone;

b. second air temperature sensing means exposed to air supplied to said zone;

c. first and second signal responsive means for operating said first and second signal refrigeration units, respectively;

d. a third signal responsive means for operating said capacity control means;

signal generating means having an input connected to said first and second air temperature sensing means and an output connected to said first, second and third signal responsive means;

f. said first and second air temperature sensing means producing air temperature responsive signals and said signal generating means responding to air temperature signals from said first and second air temperature sensing means to produce a temperature responsive output signal applied to said signal responsive means;

. said first signal responsive means effective to initiate operation of said first refrigeration unit at a first output signal level and to terminate operation of said first unit at a second output signal level of lesser magnitude than said first signal level;

h. said third signal responsive means operating said capacity control means to modulate the capacity of said first refrigeration unit between third and fourth output signal levels of lesser magnitude than said first output signal level and greater magnitude than said second output signal level; and,

i. said second signal responsive means effective to initiate operation of said second unit at a fifth output signal level having a greater magnitude than said first output signal level and terminating operation of said second unit at a sixth output signal level of lesser magnitude than said third and fourth output signal levels;

j. said third signal responsive means modulating the capacity of said first unit between said third and fourth output signal levels during operation of said first unit alone and when both of said first and second units are operating.

9. A system as claimed in claim 8 further including summing circuitry connecting said first and second air temperature sensing means to said signal generating means, said summing circuitry applying the algebraic sum of temperature signals from said first and second air temperature sensing means to said signal generating means, said second air temperature sensing means having a relatively small authority as compared to said first air temperature sensing means. a

10. A system as claimed in claim 8 wherein said signal generating means produces a variable voltage output signal, said first and second signal responsive means comprise voltage responsive bistable electronic switches for energizing and deenergizing respective electrically powered refrigeration units, and said capacity control means comprises a refrigerant flow controlling unit associated with said first refrigeration unit for variably controlling the flow of refrigerant in said first refrigeration unit to thereby control the cooling capacity of said unit in response to operation of said third signal responsive means.

11. A system as claimed in claim 10 and further including zone set point adjusting circuitry for establishing an adjustable zone set point temperature, said zone set point adjusting circuitry comprising an adjustable impedance element for shifting the level of said air temperature responsive signals relative to a reference voltage to thereby change the zone set point temperature, and wherein the levels of said first through sixth output signals vary from said reference voltage level in one sense direction.

12. A method of controlling air temperature in an air conditioned zone comprising:

a. providing first and second refrigeration units;

b. directing air across air cooling heat exchangers of said units and into said zone;

c. producing an electric air temperature responsive signal which varies in magnitude relative to a reference level in accordance with fluctuations in sensed air temperature, said air temperature responsive signal increasing in magnitude relative to the reference level in response to increases in sensed air temperature and decreasing in magnitude relative to the reference level in response to decreases in sensed air temperature;

d. initiating operation of said first refrigeration unit in response to production of a first air temperature signal having a first magnitude relative to said reference level and terminating operation of said first unit in response to production of a second air temperature signal having a second magnitude less than the magnitude of said first signal;

e. initiating operation of said second refrigeration unit in response to a third air temperature signal level having a greater magnitude than said first air temperature signal level and terminating operation of said second unit at a fourth air temperature signal level having a greater magnitude than said second air temperature signal level and a lesser magnitude than said first air temperature signal level; and,

f. modulating the cooling capacity of said first unit between full capacity and a lesser capacity in accordance with air temperature signals ranging between fifth and sixth levels, said range of modulating signals between said fifth and sixth levels being of lesser magnitude than said first air temperature signal level and greater magnitude than said fourth air temperature signal level whereby the capacity of said first refrigeration unit is fully modulatable when said first unit is operating and when said first and second units are operating.

Claims

1. A method of controlling air temperature in an air conditioned zone comprising: a. providing at least first and second refrigeration units; b. directing air across air cooling heat exchangers of said units and into said zone through a zone air inlet; c. producing a zone air temperature signal which varies in magnitude relative to a reference level in accordance with fluctuations in sensed air temperature in said zone; d. producing an air temperature signal varying in magnitude relative to said reference level in accordance with fluctuations in sensed temperature of air supplied to said zone; e. combining said zone and supply air temperature signals to produce a composite cooling temperature signal varying in magnitude relative to said reference level; f. initiating operation of said first refrigeration unit in response to production of a first composite cooling temperature signal having a predetermined magnitude relative to said reference level and terminating operation of said first unit in response to production of a second composite temperature signal having a second predetermined magnitude less than the magnitude of said first signal; g. initiating operation of said second refrigeration unit in response to a third composite temperature signal level having a greateR magnitude than said first signal level and terminating operation of said second unit at a fourth composite temperature signal level having a greater magnitude than said second composite signal level and a lesser magnitude than said first composite signal level; and, h. modulating the cooling capacity of said first unit between full capacity and a lesser capacity in accordance with composite temperature signals ranging between fifth and sixth levels, said range of modulating composite signals between said fifth and sixth levels being of lesser magnitude than said first signal level and greater magnitude than said fourth signal level whereby the capacity of said first refrigeration unit is fully modulatable when said first unit is operating and when said first and second units are operating.

8. In a system for cooling air in a zone having at least first and second refrigeration units and capacity control means for variably controlling the cooling capacity of at least said first refrigeration unit: a. first air temperature sensing means exposed to air in said zone; b. second air temperature sensing means exposed to air supplied to said zone; c. first and second signal responsive means for operating said first and second signal refrigeration units, respectively; d. a third signal responsive means for operating said capacity control means; e. signal generating means having an input connected to said first and second air temperature sensing means and an output connected to said first, second and third signal responsive means; f. said first and second air temperature sensing means producing air temperature responsive signals and said signal generating means responding to air temperature signals from said first and second air temperature sensing means to produce a temperature responsive output signal applied to said signal responsive means; g. said first signal responsive means effective to initiate operation of said first refrigeration unit at a first output signal level and to terminate operation of said first unit at a second output signal level of lesser magnitude than said first signal level; h. said third signal responsive means operating said capacity control means to modulate the capacity of said first refrigeration unit between third and fourth output signal levels of lesser magnitude than said first output signal level and greatEr magnitude than said second output signal level; and, i. said second signal responsive means effective to initiate operation of said second unit at a fifth output signal level having a greater magnitude than said first output signal level and terminating operation of said second unit at a sixth output signal level of lesser magnitude than said third and fourth output signal levels; j. said third signal responsive means modulating the capacity of said first unit between said third and fourth output signal levels during operation of said first unit alone and when both of said first and second units are operating.

9. A system as claimed in claim 8 further including summing circuitry connecting said first and second air temperature sensing means to said signal generating means, said summing circuitry applying the algebraic sum of temperature signals from said first and second air temperature sensing means to said signal generating means, said second air temperature sensing means having a relatively small authority as compared to said first air temperature sensing means.

12. A method of controlling air temperature in an air conditioned zone comprising: a. providing first and second refrigeration units; b. directing air across air cooling heat exchangers of said units and into said zone; c. producing an electric air temperature responsive signal which varies in magnitude relative to a reference level in accordance with fluctuations in sensed air temperature, said air temperature responsive signal increasing in magnitude relative to the reference level in response to increases in sensed air temperature and decreasing in magnitude relative to the reference level in response to decreases in sensed air temperature; d. initiating operation of said first refrigeration unit in response to production of a first air temperature signal having a first magnitude relative to said reference level and terminating operation of said first unit in response to production of a second air temperature signal having a second magnitude less than the magnitude of said first signal; e. initiating operation of said second refrigeration unit in response to a third air temperature signal level having a greater magnitude than said first air temperature signal level and terminating operation of said second unit at a fourth air temperature signal level having a greater magnitude than said second air temperature signal level and a lesser magnitude than said first air temperature signal level; and, f. modulating the cooling capacity of said first unit between full capacity and a lesser capacity in accordance with air temperature signals ranging between fifth and sixth levels, said range of modulating signals between said fifth and sixth levels being of lesser magnitude than said first air temperature signal level and greater magnitude than said Fourth air temperature signal level whereby the capacity of said first refrigeration unit is fully modulatable when said first unit is operating and when said first and second units are operating.