Abstract:
A unit ventilator includes a means for selecting a minimum volume of fresh air to be delivered to a room in accordance with governmental ventilation codes. A control means is responsive to the selecting means and adjusts an outside air damper to achieve the minimum volume of fresh air selected. An outside air temperature sensor provides a signal corresponding to outside air temperature. The damper control means is responsive to said signal for reducing the volume of outside air admitted to the ventilator in corresponding relation to the outside air temperature so that the volume of fresh air delivered to the room is maintained substantially constant.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the control of ventilation air in heating, ventilating and air-conditioning systems and more particularly to a system whereby the volume of outside air heated by the system is reduced as the outside temperature drops. 
     2. Description of the Prior Art 
     In heating, ventilating and air-conditioning systems, it has become a normal practice to introduce at least a minimum volume of fresh air into an occupied room for ventilation purposes. This practice assures that in recirculation type systems the air being discharged into a room contains a minimum percentage of fresh air with the remaining air being recirculated air from the room. This practice is so desirable that many governmental agencies have established ventilating codes requiring that a certain percentage of the air being discharged into an occupied room be fresh air. In order to conform to these ventilation codes, prior art heating, ventilating and air-conditioning systems set the outside and recirculation air dampers so that the dampers could not be moved to a position that would allow less than the minimum code requirement of fresh air when the system was in operation. 
     While providing a minimum volume of fresh air is desirable, the cost of operating such a system is rapidly becoming prohibitive during cold winter weather. The outside air must be heated to the discharge air temperature and on cold days, the energy required to heat the outside air to discharge air temperature is a large percentage of the total energy required for heating a room. Thus, any excess outside air that is heated and discharged into the room becomes a major source of wasted energy which, in turn, results in higher operating costs. Thus, the prior art devices have allowed the outside air dampers to close to the minimum position to meet code requirements without any further control over the damper operation. 
     The prior art devices have ignored the important Charles Law relationship between air volume and temperature. By ignoring this relationship a significant waste of energy is experienced since an excess volume of outside air is heated to discharge air temperature. As the outside air is heated from low outdoor air temperatures to the discharge air temperature, the volume is significantly increased and the volume increase gets larger as the outdoor temperature drops. Thus, less outside air is required at low temperatures than at higher temperatures in order to achieve a constant volume or percentage of fresh air being discharged into the room. 
     As an example, consider a heating and ventilating system that provides 1000 cubic feet per minute of discharge air at a temperature of 70° F. and a minimum of 25% fresh air. Each minute 250 cubic feet of outside air is drawn into the system and heated to 70° F. If the outside air temperature is 0° F., the 250 cubic feet of outside air will increase to 300 cubic feet during the temperature rise to 70° F. Thus, 50 extra cubic feet of air is heated from 0° to 70° F. during each minute of operation or an extra 72,000 cubic feet of air is heated per day. Thus, it is clear that a substantial waste of energy is experienced by the prior art devices. 
     SUMMARY OF THE INVENTION 
     The present invention contemplates a heating, ventilating or airconditioning system wherein the minimum code requirements for fresh air are maintained while the outside air admitted to the system is reduced in corresponding relationship to a reduction in outside temperature. By reducing the amount of outside air drawn into the system in corresponding relationship to a reduction in outdoor temperature, only the required minimum amount of fresh air will be provided in the discharge air, thereby saving a substantial amount of energy during cold weather operations. 
     The primary objective of the present invention is to conserve energy in a heating and ventilating system by properly controlling the volume of outside air drawn into the system. 
     Another objective of the present invention is to reduce the amount of outside air drawn into a heating system in accordance with the outside air temperature. 
     Another objective of the present invention is to provide a convenient means for selecting the minimum percentage of outside air as prescribed by local ventilation codes. 
     The previously mentioned objectives and other advantages will become apparent from the specification which follows, taken in conjunction with the drawings referred to therein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows schematically a ventilation system in which the present invention may be used. 
     FIG. 2 is a schematic diagram of a control system constructed in accordance with the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown a heating and ventilating system constructed in accordance with the present invention wherein outside air dampers 2 are controlled by a damper motor 4 for admitting a controlled amount of outside air into a fresh air duct 6 of the system. The system may be of a recirculating type that has recirculation air dampers 8 also operated by motor 4 in a manner complementary to outside air dampers 2. That is to say as dampers 2 close, dampers 8 open. Dampers 8 admit room air into a recirculating air duct 10. The outside air and recirculating air are drawn through a heating means 12 by a blower 14 where the combined air is heated to the desired temperature and thereafter discharged to the room through a duct 15. 
     Thus, outside air is drawn through dampers 2 at ambient temperature of T A  while recirculating air is drawn through dampers 8 at a temperature of T R  and the combined air is discharged through duct 15 at a temperature of T D . 
     Damper motor 4 is adapted to receive a control signal V C  for operating said motor and driving the dampers. 
     Control signal V C  is provided by a control circuit as shown in FIG. 2 where there is shown a means 16 for selecting a minimum percentage or volume of fresh discharge air as required by the local ventilation code. The selecting means 16 is connected to the wiper arm 18 of a potentiometer 20. Potentiometer 20 is connected between positive +V and negative -V potentials. The wiper arm 18 of potentiometer 20 is connected to an inverting input of an operational amplifier 22 through a resistor 24. Operational amplifier 22 is a standard operational amplifier and has a non-inverting input connected to a reference signal which may be zero volts. A resistor 26 is connected from the output of the operational amplifier 22 to the inverting input for providing negative feedback. A resistor 28 is connected from the negative -V potential to the inverting input of amplifier 22. 
     A feedback potentiometer 30 is connected between positive +V and negative -V potentials and has a wiper arm 32 which is connected to the inverting input of operational amplifier 22 through a resistor 33. 
     The output of amplifier 22 is connected to the damper control motor 4 through a resistor 25 for providing a motor control signal V C  thereto. As previously mentioned motor 4 is connected to drive dampers 2 and 8 in a complementary manner. Wiper arm 32 of potentiometer 30 is drivably connected to the motor so that the signal appearing at the wiper corresponds to the damper position and completes the damper motor control loop. 
     Operation of the circuit as thus far described is that of a standard servo loop wherein the required minimum percentage or volume of fresh air is set by adjusting means 16 which positions the wiper arm 18 of potentiometer 20 at a particular position to provide a voltage to the inverting input of amplifier 22. The voltage from potentiometer 20 causes the amplifier 22 to provide a control signal V C  to the damper motor 4 which drives dampers 2 and 8 and the wiper arm 32 of potentiometer 30 to a position in which the voltage on the wiper arm 32 cancels out the voltage on wiper arm 18 and the system is stabilized at the desired setting of adjusting means 16. 
     Reduced outside air capability is provided by the additional control circuit 34 which provides a reset signal to the inverting input of operational amplifier 22 and causes the motor 4 to drive the dampers and wiper arm 32 to a new position in which the reset signal is balanced by a change in the signal from potentiometer 30. 
     Considering now the details of the circuit 34. A temperature sensor 36 is connected in series with a resistor 38 and the series connection is connected between a positive +V potential and an inverting input of an operational amplifier 40. Operational amplifier 40 is a standard operational amplifier and has a non-inverting input connected to a reference signal which may be zero volts. A resistor 42 is connected between a negative -V potential and the inverting input of amplifier 40 and a variable resistor 44 is connected between the output of amplifier 40 and the inverting input for providing an adjustable amount of negative feedback thereto. 
     The temperature sensor 36 is mounted in a location where it senses outdoor ambient temperature and may be a thermistor having a negative temperature coefficient so that the resistance of the thermistor increases as the temperature to which it is exposed decreases. It is to be understood that other types of temperature sensors could readily be used with the present invention, such as other sensors that provide a variable resistance corresponding to the sensed temperature or active solid state temperature sensors that provide a voltage or current output corresponding to temperature. Many thermistors have non-linear resistance characteristics; therefore, resistor 38 is selected to provide a first order of linearization, thereby making the signal at the inverting input of amplifier 40 substantially linear over a range of at least 60° F. It may be desirable that resistors 38, 42 and thermistor 36 be selected so that a zero voltage is seen at the inverting input of amplifier 40 when the outdoor temperature is approximately 60° F. so that a linear output is provided over the approximate range of between 60° F. and 0° F. 
     The adjustable resistor 44 is set to provide a predetermined number of volts per degree change in outside ambient temperature so that a reset signal is provided to inverting input of amplifier 22 through a resistor 46. The characteristics of potentiometer 30, the setting of adjustable resistor 44 and the resistances of resistors 33 and 46 must all be selected so that the change in temperature and the reduction in damper opening will be linear. 
     Thus, during initial installation of the control system, the local code requirement for minimum percentage or volume of fresh air will be set into potentiometer 20 by adjustment means 16. The adjustable resistor 44 is set to provide a corresponding reduction in outside damper opening for a reduction in outside temperature. After these adjustments are made, the system will automatically reduce the amount of outside air drawn into the system in accordance with a reduction in outside temperature in a manner that follows Charles Law which, when reduced to its simplest form, states that volume = Kt.sub. absolute. Since the volume of a pound of denser cold outdoor air is decreased, the system automatically reduces the volume of such air that is drawn into the system so that when the reduced volume of outside air is heated to the desired discharge temperature, the volume of fresh air will be sufficient to satisfy the ventilation code requirements. 
     Thus, it is clear that the present invention reduces energy consumption and improves efficiency by eliminating the wasteful heating of excess outdoor air as resulted in the prior art devices. The present invention also provides a convenient means for selecting the minimum volume or percentage of outside air as required by local codes which vary considerably from municipality to municipality. Thus, a standard heating, ventilating or airconditioning system may be provided that is easily adjusted to meet local code requirements. 
     The invention is clearly applicable to all types of hot air heating systems where outside air is drawn into the system for ventilation purposes. The outside air may be mixed with return air and then heated or the system may be of a type that only heats outside air and is required to discharge a predetermined volume of the heated outside air. In either case the invention saves substantial amounts of energy and thereby reduces heating costs. 
     In a more sophisticated embodiment, it may be desirable to provide a second reset signal to the inverting input of amplifier 22 to adjust for variations in return air temperature from the room; however, it is felt that for most applications, such a refinement would not be necessary. 
     Thus, the present invention clearly conserves energy in a heating and ventilating system by properly controlling the volume of outside air drawn into the system so that local ventilation code requirements are met with heating excess outside air. The invention achieves this result by reducing the volume of outside air drawn into the system in corresponding relation to a reduction in outside air temperature. 
     The invention has been described in terms of air volume control by controlling the damper opening because this relationship is easy to visualize. However, it is to be understood that the invention also applies to the control of the weight of the air admitted to a system. When air is heated the volume increases but the weight remains constant. For a constant weight of fresh outdoor air, the damper 2 must close down to reduce the volume of fresh outdoor air admitted as the outdoor air temperature decreases. Thus, it may be said that an objective of the invention is to deliver a constant weight of outdoor air per minute and the potentiometer 20 is set by adjusting means 16 to provide a signal corresponding to a minimum weight of outdoor air to be delivered. 
     While the principles of this invention have been described above in connection with a specific apparatus, it is to be understood that this description is made only by way of example and not as a limitation on the scope of the invention as set forth in the accompanying claims.