Abstract:
A heated air makeup unit comprising a heating chamber with a modulated furnace one or more non-modulated furnaces air temperature control and efficient furnace operation over a wide heating rate range. The utilization of indirect fired furnaces in the heated air makeup unit is disclosed. A control system is included to provide control of heated air temperature to within +/−1 F.° using continuous control of the heating rate of the modulated furnace and stepwise activation and de-activation of the non-modulated furnaces in response to heating demand. A programmed combustion air flow controller programmed to provide optimal airflow for any gas flow in a flow range of a modulating gas valve is provided to assure combustion efficiencies of at least  83 %.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to a heated makeup air unit for heating outside air and directing the heated outside air into a room or building to replace air exhausted therefrom. 
       BACKGROUND 
       [0002]    Buildings or rooms comprising commercial kitchens, research laboratories, and industrial installations often include one or more exhaust fans that vent smoke, steam, and other air-polluting substances from areas where cooking units, chemical process, or manufacturing operations are located. To replace the exhausted air, heated makeup air devices are used to heat and introduce outside air into the building or room. These heated makeup air devices basically consist of a duct structure open to both the outside atmosphere and the building or room, a fan for blowing air through the duct structure into the building or room, heating units to heat the outside air as needed, and a control system for activating and monitoring the heater makeup air device as needed. 
         [0003]    In some situations, amount of heating needed to heat outside air before introducing the air into the building or room varies widely over relatively short periods of time. For example, in some climates and at some times of the year, outside air temperatures may be very low in the morning as compared to the desired air temperature within the building or room. However, later in the day, the outside air temperature may rise considerably, perhaps up to about the desired air temperature in the building or room. At night, outside temperatures may again drop significantly. This situation requires highly variable heating rates throughout the day, and it is often the case that heated air makeup units are sized based on the highest heating demand. Likewise, venting requirements may vary over relatively short periods of time. For example, in a restaurant the intensity of cooking activities varies as dining customers come and go. While during a meal rush, venting of cooking fumes is done at a high rate and heated makeup air is consequently needed at a high level, during another period there may be little need for venting and the demand for heated makeup air is consequently low. 
         [0004]    In most cases the air in the building or room is conditioned by a building or room heating system that is not interconnected with the heated makeup air unit The degree to which a heated air makeup device functions adequately can have a major bearing on heating load and heating load changes on the building or room air heating system. If heated makeup air is provided at temperatures varying even a little from the control target temperature of the building or room heating system, inefficient cycling of the building or room heating system can occur. However, providing precise control of the temperature of heated make up air using a very high capacity heating unit can be difficult and expensive. Moreover, actively varying the heating rate of a large heated air makeup unit often results inefficient energy conversion in the unit. 
         [0005]    There is a need for heated air makeup devices which provide precise control of makeup air temperature and can operate efficiently over wide heating rate ranges. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention relates to a heated makeup air system comprising a furnace unit and a control system for controlling the furnace unit. The furnace unit includes one or more non-modulated gas furnaces and one modulated gas furnace. The control system is operative to provide course control by actuating or de-actuating one or more of the non-modulated gas furnaces. Fine or Vernier control is exercised by modulating the modulated gas furnace. Therefore, when the heat demand can be met with the capacity of the modulated furnace, the control system simply relies on the modulated furnace. When the heat demand cannot be met with the modulated furnace, then the control system relies on one or more of the non-modulated furnaces. 
         [0007]    In one embodiment, the control system includes a controller. The function of the controller is to modulate the modulated furnace, control the activation and deactivation of the non-modulated furnaces, and, at the same time, maintain the combustion efficiency of the total system at a selected level such as 83% or better. 
         [0008]    In one embodiment, the controller of the control system directs one control signal to a power vent and a modulation gas valve associated with the modulated gas furnace. The control signal effectively controls the flow of combustion air into the modulated furnace and, by controlling the modulating gas valve, controls the flow of gas into the modulated furnace and is therefore effective to vary the heat output of the modulated furnace. 
         [0009]    Other objects and advantages of the present invention will become apparent and obvious from a study of the following description and the accompanying drawings which are merely illustrative of such invention. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  is a pictorial view of a heated air makeup unit. 
           [0011]      FIG. 2  is a schematic representation of a heated air makeup unit, 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0012]    A typical heated air makeup unit generally includes a furnace unit through which outside air is passed and heated before supplying it to a heated space such as a room or building to make up for lost heated air from the room or building. Such a heated air makeup unit is typically configured to be installed on a roof or outer wall of a building and connected by appropriate ducting to the heated space serviced by heated air makeup unit. The furnace unit typically includes a housing enclosing a heating chamber fitted with one or more furnaces. The housing includes an air intake and an air discharge so that unheated outside air can be directed into the heating chamber by way of the air intake, heated by the one or more furnaces in the heating chamber, and discharged into the room or building serviced by the heated air makeup air unit. Applications for heated air make up units include buildings or rooms where there are air exhausting systems operating to exhaust dangerous or objectionable byproducts of processes ongoing in the room or building. One example is a commercial kitchen where one or more large exhaust hoods operate to exhaust cooking smoke and fumes from the spaces over cooktop units. Other examples include industrial and laboratory installations where fume hoods or other air exhausting systems are utilized in connection with various operations which may produce dangerous or objectionable airborne materials. For a more complete and detailed description of heated air makeup units and applications, reference is made to U.S. Pat. No. 5,771,879 the disclosure of which is expressly included herein by reference. 
         [0013]    The heated air make up unit of the present invention, indicated generally by the numeral  10  in the accompanying drawings, includes a housing  12  with a heating chamber  18  enclosed therein. Housing  12  includes a makeup air intake  14 , which may form a part of a makeup air intake assembly  15 . Makeup air intake assembly  15  includes a fan or blower for inducing air flow through heated air makeup unit  10 . Housing  12  further includes a heated air discharge  16  through which air heated by heated air makeup unit  10  is directed to a room or building space requiring heated air makeup. Extending from housing  12  are one or combustion vent rain caps  17  through which fuel combustion byproducts may be exhausted from heated makeup air unit  10 . 
         [0014]    The heated air makeup unit  10  of the present invention also includes a control system configured to enable heating air to a desired temperature while assuring efficient conversion of heating fuel energy to heat for warming the air. Before discussing the control system in detail, heating and air flow components and their interconnections to form the heated air makeup unit will be discussed 
         [0015]    Heating chamber  18  is in one embodiment comprised of an insulated duct with an air inlet  18 A for receiving air to be heated from makeup air intake unit  15  and directing the received incoming air stream into the heating chamber. A modulated furnace  20  is disposed in heating chamber  18 . One or more non-modulated furnaces may also be disposed in heating chamber  18  upstream of a modulated furnace  20 . For purposes of explanation, three non-modulated furnaces  22 A,  22 B, and  22 C are illustrated in  FIG. 2 . The incoming airstream is directed to and about furnaces  20  and  22 A,  22 B, and  22 C where the air may be heated. Heating chamber  18  also includes a heated air outlet or discharge  18 B for directing the heated air stream to heated space  60  of a room or building serviced by the heated air makeup unit  10 . Modulated furnace  20  and, when included, non-modulated furnaces  22 A-C are managed by a control system to be described below so as to provide the required heat to warm the makeup airstream. 
         [0016]    The furnaces, whether modulated  20  or non-modulated  22 A-C, may be of a common design, each having the same rated maximum heating capacity as the others. In one embodiment, furnaces  20 ,  22 A-C are indirect fired gas furnaces, each furnace having a rated maximum heating capacity of 400,000 btu/hr, for example. Each furnace  20 ,  22 A-C includes combustion chamber, a gas inlet  22 , a combustion air inlet  23  permitting only inflow of combustion air, and a combustion exhaust outlet  24 . In one embodiment combustion air inlets  23  direct air for combustion from the incoming air stream into furnaces  20 ,  22 A-C where the combustion air is mixed with gas received through respective gas inlets  22  and burned in respective combustion chambers to produce heat. Combustion byproducts are exhausted through combustion exhaust outlets  24 . 
         [0017]    For each of furnaces  20 ,  22 A-C, gas inlet  22  is connected to and in fluid communication with an ON/OFF gas valve  30 . The gas valves are supplied with gas from a common source (not shown). Gas valves  30  are electrically actuated valves of well known design and operable in a binary manner to provide no gas flow when in the OFF mode and full gas flow when in the ON mode. Valves  30  are selected such that full gas flow will support respective rated maximum heating capacity of each of the non-modulated furnaces  22 A-C. To provide optimal combustion air flow for maximum combustion efficiency, a power vent  32  is connected to and in fluid communication with a combustion exhaust outlet  24  of each of furnaces  20 ,  22 A-C. Each power vent  32  includes a fan or blower powered by an electric motor, and each power vent is covered by one of the rain caps  17 . Power vent  32  is sized so that when the power vent and gas valve  30  are both in the ON mode with full gas flow entering each of the combustion chambers, an appropriate rate of combustion air flow is provided to assure an 83% or greater combustion efficiency in one particular embodiment. Each power vent  32  that is connected to one of the non-modulated furnaces  22 A-C is connected to an electric motor ON/OFF control or relay  34  for activating or deactivating the fan or blower of the power vent. 
         [0018]    As noted above, in one embodiment modulated furnace  20  has the same rated maximum heating capacity as do each of any included non-modulated furnaces  22 A-C. However, modulated furnace  20  is configured to operate differently from non-modulated furnaces  22 A-C. In particular, modulated furnace  20  is configured to operate in a variable heating rate mode where the heating rate may be selectively varied from a minimum fire heat output up to the rated maximum heat capacity of the furnace. To enable varying the heating rate of modulated furnace  20 , provision is made to vary both the gas flow rate and the combustion air flow rate to the combustion chamber of the furnace. Varying the gas flow rate is enabled by including a modulating gas valve  36  in series with the ON/OFF gas valve  30  connected to modulating furnace  20 . In one embodiment, modulating gas valve  36  is interposed between the ON/OFF gas valve  30  and the combustions chamber of modulating furnace  20 , so that gas is enabled to flow through the ON/OFF gas valve and thence through the modulating gas valve. Varying the rate of flow of combustion air into combustion chamber of the modulating furnace  20  is enabled by powering the electric motor of the power vent  32  associated with furnace  20  with a power vent speed control  38 . 
         [0019]    Modulating gas valve  36  is a of a common design having a flow capacity that in one embodiment varies in response to a 0-10 V DC signal applied to electric terminals thereof. For example, when the voltage supplied to modulating gas valve  36  is 0 V DC, a minimum gas flow is permitted through the valve while when the voltage supplied to the valve is 10 V DC, the maximum rated flow of gas is permitted through the valve. At a voltage between 0 and 10 V DC is supplied to the valve, the gas flow permitted will be at a flow rate corresponding to that voltage. Power vent speed control unit  38  is a programmable motor speed control that accepts an input signal in the 0-10 V DC range and produces an AC power output voltage that increases from a minimum AC voltage, when the input signal is 0 V DC along a programmed trajectory of input signal values up to a maximum AC voltage, when the input signal is 10 V DC. The trajectory programmed into power vent speed control unit  38  is determined such that for any level of an input signal supplied simultaneously to the speed control and modulating gas valve  36 , the combustion air and gas flow rates into combustion chamber will be at values to produce combustion efficiency of 83% or better. The trajectory may be determined by experiment. Once determined, the trajectory is programmed into power vent speed control unit  38 . 
         [0020]    Heated makeup air unit  10 , configured as described above has a maximum heating capacity that is the sum of the maximum heating capacities of modulated furnace  20  and the non-modulated furnaces, furnaces  22 A-C in the example of a unit having three non-modulated furnaces. As stated above, the maximum capacities of the individual furnaces comprised in makeup air unit  10  are generally the same. For example, modulated furnace  20  and non-modulated furnaces  22 A-C, might each typically have a maximum capacity of 400,000 btu/hr. In such a case, the maximum heating capacity of heated makeup air unit  10  would be four times that of an individual furnace of 1,600,000 btu/hr. Said another way, heated makeup air unit  10  can be operated to provide a varying heating rate to meet an instantaneous heating demand that can be anywhere between a minimum rate and 1,600,000 btu/hr. It would be said that heated makeup air unit  10  has a 1,600,000 btu/hr range. Modulated furnace  20  can be operated, in this example, to provide a varying heating rate to meet instantaneous heating requirement on that furnace that can be anywhere between a minimum rate and 400,000 btu/hr, or that the furnace provides one fourth of the range of unit  10 . Each of the non-modulated furnaces  20 A-C, can be operated only at full rated capacity, or 400,000 btu/hr. That is, each non-modulated furnace  20 A-C can be operated to either produce no heat or to produce heat at its maximum rated capacity. Each non-modulated furnace  22 A-C, provides one fourth of the range of unit  10 . 
         [0021]    A control system is embodied in heated makeup air unit  10  that enables continuously varying the heating rate of the makeup air unit over its range, a minimum rate to 1,600,000 btu/hr, for example, by continuously varying only the heating rate of furnace  20  and selectively activating or de-activating one or more of non-modulated furnaces  20 A-C based on the instantaneous heating demand from unit  10 . That is, when the heating demand on heated air makeup unit  10  varies between a minimum rate and 400,000 btu/hr, the heating demand is met by varying the heating rate of modulated furnace  20  over its range. When the heating demand is, for example, as much as 400,000 btu/hr and up to 800,000 btu/hr, non-modulated furnace  22 A is activated to supply 400,000 btu/hr while modulated furnace  20  produces a varying heating rate between 0 and 400,000 btu/hr so that the two furnaces together meet any heating demand from 400,000 btu/ht and 800,000 btu/hr. Should the heating demand rise to as much as 800,000 btu/hr, non-modulated furnace  22 B is activated to supply 400,000 btu/hr while non-modulated furnace  22 A continues to supply 400,000 btu/hr and modulated furnace  20  supplies heat at a varying rate of 0 to 400,000 btu/hr. The three furnaces  20 ,  22 A, and  22 B then operate together to supply heat at a rate of 800,000 btu/hr up to 1,200,000 btu/hr. In the same manner non-modulating furnace  22 C is added so that unit  10  operates to meet a heating demand varying between 1,200,000 and 1,600,000 btu/hr. Similarly, as heating demand falls to the lower end of the range in which furnace  20  is supplying heat, one or more of the of non-modulated furnaces  22 A-C is deactivated. Thus, the control system of heated air makeup unit  10  functions to continuously or more finely control total heating rate on a sub-range, that of modulated furnace  20 , and to shift the sub-range as required based on heating demand within the range of the unit by selectively, or in a stepwise fashion, activating or deactivating non-modulated furnaces  20 A-C. 
         [0022]    Various implementations of the above-described control system could be utilized, ranging from alarm-based manual range shifting to a microprocessor that includes memory or other storage for holding computer program instructions, the execution of which configures the microprocessor to carry out the control logic for controlling the furnace unit. In one embodiment, the control system embodied in heated makeup air unit  10 , the control system comprises a command signal generator  50  that produces a DC signal ranging from 0-10V DC, where 0 V is indicative of no heat demand and 10 V is indicative of maximum heat demand. Command signal generator  50  may take any of various forms. 
         [0023]    One embodiment implements discharge control. For this embodiment, the command signal generator  50  comprises a manually settable thermostat with a temperature sensor disposed in discharge  18 B of heated makeup air unit  10  and an output signal range of 0-10 V DC. In this embodiment, the thermostat is manually set for a desired discharge air temperature T D . The thermostat compares the temperature T S  of discharge air sensed by the discharge sensor. When the T S  equals T D , the output signal of the thermostat assumes a value of 0 V DC. If T S  is less than T D , the output signal assumes a value within the 0-10 V DC range. The greater the difference [T D -T S ], the larger the output voltage. The thermostat has a control range that is the maximum difference [T D -T S ] for which the thermostat will function. When [T D -T S ] assumes the control range maximum, the output voltage of the thermostat assumes a value of 10 V DC. 
         [0024]    Another embodiment implements space control in which the air temperature of the room or building is controlled by a manual thermostat of the general type described above for discharge control, but where sensed temperature T S  is the room or building air temperature. The 0-10 V DC command signal is generated as described above for the discharge control embodiment. 
         [0025]    Yet another embodiment is configured to interact with a building automation control system where the system provides a 0-10 V DC signal indicative of heat demand based on air temperature and other variables sensed by the system. In any case, the control system is provided a DC signal that varies between 0 and 10 V DC. The signal assumes a value of 0 V DC to indicate that generally no heating is demanded of heated makeup air unit  10 , and the signal assumes a value of 10 V DC to indicate that heating at the maximum available rate is demanded of the unit. When the signal successively assumes values increasing or decreasing between 0 and 10 V DC, heating rate is demanded successively and correspondingly increased or decreased levels less than the maximum available rate. 
         [0026]    However the 0-10 V DC command signal is generated, the signal is utilized by the control system to control the heating rate of heated makeup air unit  10 . While it is appreciated that heated makeup air unit  10  includes one modulated furnace and possibly a plurality of non-modulated furnaces, the control system for a typical installation with modulated furnace  20  and three non-modulated furnaces  22 A-C embodies and illustrates the same functionality with any number of furnaces and will be use as a basis for describing the control system. 
         [0027]    The control system further includes the gas valves  30  and power vent relays  34  that are associated with non-modulated furnaces  22 A-C and modulated gas valve  36  and power vent blower motor speed control  38  interconnected as here above described. Also included is an electronic modulation step controller  40  as shown in  FIG. 2 . Controller  40  includes a command signal input  41  and a command signal control output  42 . Also included in controller  40  are a series of gas valve control outputs  43 A,  43 B, and  43 C and a series of power vent motor relay control outputs  44 A,  44 B, and  44 C. Gas valve control outputs  43 A-C are electrically connected to ON/OFF gas valves  30  associated with non-modulated furnaces  22 A-C respectively. Relay control outputs  44 A-C are connected to relays  34  associated with non-modulated furnaces  22 A-C respectively. 
         [0028]    Controller  40  includes an electrical conductor that continuously connects command signal input  41  to command signal control output  42 . The output of a latching maximum-minimum detecting circuit of well known design is connected to each of gas valve control outputs  43 A-C and relay control outputs  44 A-C. A sequencing circuit of well known design is selectively connectable between command signal input  41  and the inputs of the latching maximum-minimum detecting circuits. At system startup, the sequencing circuit is connected between command signal input  41  and both gas valve control output  43 A and relay control output  44 A. 
         [0029]    The operation of the control system can be understood and explained by stipulating that at startup, the heat demanded is small but greater than 0. That is, at startup, the command signal is at a value slightly above 0 V DC. When heat demand increases, the command signal increases, increasing both the gas flow and the combustion air flow rates into modulated furnace  20 . The heat demand may be satisfied by increasing the heating rate of modulated furnace  20 , warming the air and thereby keeping the command signal from reaching its maximum of 10 V DC. In the event that the heat demand is so great as to drive the command signal to its maximum of 10 V DC, the modulated furnace is driven up to its maximum rated capacity, otherwise know as “high fire.” The latching maximum-minimum detecting circuit connected between command signal input  41  and both gas valve control output  43 A and relay control output  44 A detects the attainment of 10 V DC by the command signal, latches in a pre-set command voltage to each of gas valve control output  43 A and relay control output  44 A to start non-modulated furnace  22 A and keep the gas valve ON and the blower of the power vent  32  associated with the furnace running at rated speed. The sequencing circuit of controller  40  connects command signal input  41  to the input of the latching max-min detecting circuit connected to both gas valve control output  43 B and relay control output  44 B. The heat supplied by the just-started non-modulating furnace  22 A immediately begins to increase the heating rate of heated makeup air unit  10 , driving T S  upward and thereby driving the command signal downward. Because non-modulated furnace  22 A essentially conies on instantly at rated heating capacity, the command signal is driven downward rapidly to about 0 V DC, rapidly lowering the heating rate of modulated furnace  20  to near 0. If, for example, the temperature intake air drops or is so low that with non-modulated furnace  22 A operating, there is remaining demand for heat to warm the air, the command signal will increase. As the command signal increases upwards towards 10 V DC, modulated furnace  20  is modulated upwardly, increasing the heating rate until the furnace is a full rated capacity and the command signal reaches its maximum again, 10 V DC. At this point the latching max-min detecting circuit connected to both gas valve control output  43 B and relay control output  44 B latches in a pre-set command voltage that is directed to each of the valves to fire non-modulated furnace  22 B, and the process described after firing furnace  22 A repeats. Further heat demand will bring on non-modulating furnace  22 C by the same process. It is to be noted that once one of the non-modulated furnaces  22 A-C is activated or fired, that furnace remains on and producing heat at its maximum rated capacity until such time as heating demand reduces to an extent that cannot be accommodated by modulating down or reducing the heating rate of modulated furnace  20 . As heat demand decreases to a greater extent than can be accommodated by down-modulating modulated furnace  20 , non-modulated furnace  22 C, followed by non-modulated furnace  22 B, and then followed by non-modulated furnace  22 A are sequentially de-activated. For example, in a condition where the modulated furnace  20  is operating at essentially 0 capacity in response to the command signal being at 0 V DC, the latching max-min detecting circuit will detect the attainment of 0 V DC by the command signal, and de-latch the pre-set command voltage from gas valve output  43 C and relay control output  44 C thereby deactivating non-modulated furnace  22 C. The abrupt reduction in heat production due to deactivation of non-modulated furnace  22 C creates heat demand that drives the command signal upwards, modulating modulated furnace  20  upwards. In the event of continuing overall reduction in heat demand, this process repeats to sequentially de-activate non-modulated furnaces  22 B and  22 A, leaving modulated furnace  20  on and being modulated to respond to heat demand changes within the capacity of the furnace. 
         [0030]    The above-described control system provides the capability to maintain T S  within a range of one F.° below T D  to one F.° above T D , based on two features. First, the control system utilizes the full 0-10 V DC control range to vary the heating rate of modulated furnace  20 . This generally prevents greater that 1 F.° departures of T S  from T D  and thereby providing fine control of the heating rate of unit  10 . Second, the system operates to sequentially activate or deactivate non-modulated furnaces depending on the heating rate of the modulated furnace at a particular time. The non-modulated furnaces are activated and deactivated in an ON/OFF fashion that provides coarse control of the heating rate of unit  10 . 
         [0031]    The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and the essential characteristics of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.