Control Circuit for Air Conditioning Systems

This invention relates to industrial and comfort air conditioning systems and, more particularly, to air conditioning systems having controls for automatically operating the dampers in the outside-air duct and the return-air duct.

Typically, air conditioning systems used in manufacturing facilities have the size and capacity to maintain the inside air at a dry bulb temperature of about 75.degree. F. and a relative humidity of about 60%. As shown in standard ASHRAE psychrometric charts, such inside air conditions may be attained by using supply air having a temperature of about 61.5.degree. F. and a relative humidity of typically 95%. Such supply air conditions can be achieved by mixing outside air and return air when the outside conditions or heat content (enthalpy) is below the required supply air. On the other hand, when the outside air conditions or heat content rises above the required supply air conditions or heat content then mechanical refrigeration, or mechanical energy, is required for the cooling medium.

In air conditioning systems, so equipped, dampers control the flow of inside air and outside air to the cooling coil or cooling medium. As between the use of outside air and the return air, it is clearly more energy efficient, during mechanical refrigeration periods, to pass over the cooling coil or cooling medium the air (outside or return) which has the lowest heat content. Control systems have thus been developed to make maximum use of the outside air when its heat content is low and to make maximum use of the return air when its heat content is lower than that of the outside air. One such system is disclosed in U.S. Pat. No. 2,206,445; another is referred to in the background section of U.S. Pat. No. 4,347,712.

As in the prior art air conditioning control systems, it is an object of the present invention to make maximum use of the source of air which has the lowest heat content to achieve a desired inside temperature during mechanical refrigeration periods.

It is another object of the present invention to make use of the temperature differential between the return air and outside air in an air conditioning system and condition the one which has the lowest heat content.

These and other objects of the present invention are achieved by periodically detecting the temperature of the supply air during refrigeration, changing the damper positions in the outside air and return air ducts, detecting a short time thereafter the temperature of the supply air, comparing the temperatures detected both before and after the change in damper position and controlling the positions of the dampers in accordance with the lowest temperature detected.

In the drawing there is shown a schematic diagram of an air conditioning system with a control circuit for positioning the dampers in accordance with the heat content of the outside air and the return air.

In the air conditioning system arranged according to the present invention shown in the drawing, outside air and return air ducts 10 and 12, respectively, are joined to an air conditioning housing 14. The ducts contain dampers 16 and 18 which are operated simultaneously to permit the flow of either outside air or return air, but not both, to the housing 14. In the housing 14, the outside air or the return air, as the case may be, passes through a refrigeration produced cooling medium 20 which may be, for example, a water coil, refrigeration coil, chilled water spray, etc. After the cooling medium 20, the cooled air is blown by a fan 22 into an air supply duct 24.

In order to minimize the heat exchange which occurs during refrigeration of the cooling medium, a control system is provided for controlling the operation of the dampers 16 and 18 and thereby provide for the cooling of the air which has the lowest heat content. The control system includes a pair of relays 26 and 28 coupled to the dampers 16 and 18, respectively, which are alternately energized to open one set of dampers and close the other set of dampers. A temperature sensitive transducer 30, such as a thermocouple or thermistor, is mounted in the housing 10 on the cooling side of the coil 20 and develops an electrical signal having a magnitude which is commensurate with the temperature of the refrigerated supply air.

The control system is enabled by a voltage source 32 that supplies a 120 volt alternating current signal to a timer 34 and a transformer 36. In the transformer 36, the supply voltage is rectified and reduced to about 12 volts. The 12 volt d-c signal is then employed to activate an analog-to-digital converter 38, a comparator 40, an AND gate 42 and a relay 44.

The timer 34 is adjusted to emit periodically, e.g. once an hour, two signals which are separated in time by about two minutes. The first of the emitted signals enables the digital signal stored in the analog-to-digital converter 38 to be stored in the comparator 40. The signal in the converter 38 represents the digital representation of the temperature of the refrigerated air, as sensed by the transducer 30.

With the storage of the digital representation of the temperature of the refrigerated supply air, the comparator 40 generates an enabling signal that is supplied to the AND gate 42. The other input terminals of the AND gate 42 are connected to a thermostat 46 mounted in the outside air duct 10 and a switch that is closed when the refrigeration unit 20 is operating. Ordinarily, the thermostate 46 supplies an enabling signal to the AND gate 42. However, at a predetermined outside air temperature, such as 80.degree. F., the thermostat supplies a signal which disables AND gate 42.

Provided that the thermostat 46 and the refrigeration unit 28 supply enabling signals to the AND gate 42, the gate supplies a d-c voltage signal which changes the state of the relay 44. With the change in state of the relay 44, the states of the relays 26 and 28, respectively, will be changed. The positions of the dampers 16 and 18 change with the change in states of the relays 26 and 28.

The timing signal emitted by the timer 34 about two minutes after the first timing signal is generated permits the gating of a second temperature representative digital signal to the comparator 40. This second digital signal represents the temperature of the refrigerated air after the change in the positions of the dampers 16 and 18. In the comparator 40, the digital signal representative of the temperature of the refrigerated air prior to the change in positions of the dampers 16 and 18 is compared with the digital signal which is representative of the temperature of the refrigerated air after the change in damper positions. If the temperature of the refrigerated air after the change in damper positions is lower than the temperature before the change in damper positions, no output is generated by the comparator 40. If, however, the temperature sensed after the change in positions is greater than the temperature sensed before the change, the comparator generates a further enabling signal that, through the AND gate 42, changes the state of the relay 44. The change in state in the relay 44 causes a change in state in the relays 26 and 28, and, accordingly, the dampers 16 and 18 are returned to their original positions.

In operation, thus, when the outside air damper 16 is open, the return air damper 18 is closed. The positions will be changed periodically under the indirect control of the timer 34. If an increase in temperature is sensed by the transducer 30 after the change, the dampers will be returned immediately to their original positions. If a decrease or no change in the temperature of the conditioned air occurs, the dampers will remain in their switched positions.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.