Abstract:
A variable speed power flue ventilator with a thermostatically controlled motor cooling system. The thermostatically controlled cooling system employs an auxiliary motor cooling fan separate from the fan used by the power ventilator to extract exhaust gases. A thermostatic sensor switch actuates the motor cooling fan whenever the temperature in the exhaust fan motor housing rises to a preset value. The cooling fan then draws cool ambient air through the motor housing until the enclosed housing area reaches a second lower, preset temperature at which point the cooling fan is shut off by the thermostat.

Description:
RELATED APPLICATION  
       [0001]    The present application claims the benefit of U.S. Provisional Application No. 60/223,380 filed Aug. 7, 2000, which is incorporated herein in its entirety by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to power draft systems for exhausting hot flue gases. More particularly, the invention relates to a power draft system with a thermostatically controlled fan for cooling the motor of the ventilator.  
         BACKGROUND OF THE INVENTION  
         [0003]    Chimneys first became common in Europe in the 16 th  century. Despite improvements in design since then, most chimneys still operate on a natural draft system. A natural draft chimney operates by force of gravity. That is, the hot flue gases in the chimney are lighter than the surrounding ambient air. Being lighter, flue gases are displaced by cooler, heavier air and rise buoyantly through the chimney flue creating a natural draft.  
           [0004]    The efficiency of natural draft chimneys is affected by a host of environmental factors. Ambient air temperature and atmospheric pressure affect the density of the ambient air mass. If the density of the ambient air mass is reduced, the draft efficiency of the chimney is reduced as well.  
           [0005]    Wind can either increase draft by blowing across the mouth of the chimney creating a venturi effect or reduce draft if turbulent and can even cause a back draft, a reverse flow through the chimney, causing flue gases to be vented within the building.  
           [0006]    Factors related to fuel burning appliances also affect the efficiency of natural draft chimneys. Efforts to increase the energy efficiency of heating appliances have resulted in those appliances extracting as much heat as possible from the exhaust gases thereby reducing the exhaust gas temperature. Reduced exhaust gas temperatures increase exhaust gas density and lessen draft.  
           [0007]    Modern boiler systems are designed to operate in modular or modulated fashion. Modular boilers operate in such a way that a number of small boilers may be used individually, in groups or all at one time dependent upon heating demand. A modulated boiler may burn at variable rates in response to heating demand. Typically, modular and modulated boiler systems are vented through a single flue. Other fuel burning appliances such as water heaters may also vent through the common flue. The chimney flue must be sized based on the maximum firing rate of all the units combined. When all of the units are not in use the flue becomes oversized for the task and cannot provide a proper draft.  
           [0008]    These factors create the potential for insufficient draft which may cause condensation within the flue, back drafts, or flue gas spillage. Condensation is a particular concern since flue gases may contain substances such as sulfur oxides that, when combined with water, form acids. Acids can lead to corrosive destruction of the flue itself as well as damage to heating equipment. Corrosion damage along with back drafts and flue gas spillage can lead to health and safety concerns for occupants of the building if flue gases escape into living areas.  
           [0009]    All of these factors have lead to the increasing popularity of power venting systems to ensure the proper venting of hot flue gases. Power draft systems fall into two basic classes. The traditional mechanical draft system is a so called constant volume system in which a fan provides a constant volume gas flow through the flue to carry exhaust gases to the exterior of the structure. The constant flow of air through these continuously operating systems is inefficient and costly. Three to five thousand cubic feet per minute of air may be expelled by these systems causing loss of heat in the winter and loss of cooled air in the summer.  
           [0010]    More recently, constant pressure systems have been introduced. Constant pressure systems include a fan located at the chimney termination as well as a control system that maintains appropriate draft by adjusting the airflow to maintain a constant negative pressure within the flue. In order to maintain a constant relatively reduced pressure within the flue the airflow is continuously adjusted. One way to accomplish this is by operating the exhaust blower at a variable speed. A variable speed motor is called upon to increase airflow when a greater draft is needed and to reduce airflow when a lesser draft is required.  
           [0011]    The application of power draft systems also allows the use of smaller ducts to carry exhaust gases and to provide combustion air. This can present a large cost savings. Due to corrosion concerns, exhaust ducts are more often being constructed from special corrosion-resistant steels such as Allegheny Ludlum™ AL29-4C. Ductwork made of specialty steels of this type can be very expensive.  
           [0012]    The use of smaller ductwork also makes for easier installation since ductwork may pass through smaller chases and smaller openings in partitions are required. Smaller openings require less structural reinforcement than large ones.  
           [0013]    In normal operation, electric motors produce waste heat because of friction and electrical resistance. Generally, this heat is dissipated by a constant airflow through the motor housing produced by a fan attached to the motor shaft, which draws cooling air over the bearings and windings of the motor. In a variable speed blower, such airflow is of course reduced when the motor is operating at lower speed. If the motor were operating in a normal ambient air environment, it would not necessarily be subject to overheating at lower speeds because the motor windings and bearings produce less waste heat at lower operating speeds. A power ventilator motor, however, necessarily operates in a high temperature environment due to its proximity to high temperature flue gas.  
           [0014]    One approach to mitigating the excess heat problem, caused when a power ventilator is operated at low speeds, is to employ a motor with insulated windings. A, so-called, H-class motor has specially insulated windings to protect the windings from damage due to excess heat exposure. However, the motor bearings in an H-class motor are not protected, and may fail prematurely due to excess heat buildup. Additionally, heavy duty insulated motors may be prohibitively expensive.  
           [0015]    Power flue ventilators may also be constructed with massive heat conductive housings to provide a heat sink and to radiate excess heat. Massive housings are expensive and excess weight may require strengthening of flue installations.  
           [0016]    It would be desirable to have a variable speed power flue ventilator which can utilize a relatively inexpensive motor, operate at variable speed while proximate to high temperature flue gases, and yet still maintain long motor life.  
         SUMMARY OF THE INVENTION  
         [0017]    The present invention in large part solves the problems referred to above, by providing a variable speed power flue ventilator with a thermostatically controlled motor cooling system.  
           [0018]    The thermostatically controlled cooling system employs an auxiliary motor cooling fan separate from the blower used by the power ventilator to extract exhaust gases. A thermostatic sensor switch actuates the motor cooling fan whenever the temperature in the exhaust fan motor housing rises to a preset value. The cooling fan then draws cool ambient air through the motor housing until the enclosed housing area reaches a second, lower, preset temperature at which point the cooling fan is shut off by the thermostat.  
           [0019]    In addition, the power ventilator of the present invention includes a thermostatic safety shut off switch. If the interior of the motor housing reaches a preset temperature high enough to threaten immediate damage to the motor, the safety shut off then shuts off the fuel burning appliance system and keeps it off until appropriate cooling has occurred. During the time that the fuel burning appliance is shut off, the auxiliary cooling fan continues to operate to dissipate heat from the motor and motor housing until the temperature reaches a safe level.  
           [0020]    It is notable that the cooling air intakes for the motor cooling system are located below and outside of the flue gas exhaust ports. This assures that air drawn in to cool the motor will be cool ambient air, not hot exhaust gas.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a perspective view of a power flue draft system in accordance with the present invention;  
         [0022]    [0022]FIG. 2 is a perspective view of the power flue draft system depicted with the fan housing opened to reveal the exhaust fan impeller;  
         [0023]    [0023]FIG. 3 is a perspective view of the power flue draft system with the motor cover removed depicting the motor cooling system;  
         [0024]    [0024]FIG. 4 is a perspective view of the power flue draft system with the cooling assembly removed to expose the motor;  
         [0025]    [0025]FIG. 5 is a cross-sectional view of the power flue draft system sectioned along a plane dropped from line A-A in FIG. 1; and  
         [0026]    [0026]FIG. 6 is a cross-sectional view of the power flue draft system sectioned along a plane dropped from line B-B in FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    Referring in particular to FIGS. 1, 3, and  5 , a power flue ventilator  10  for extracting flue gases from a flue  11 , in accordance with the present invention, generally includes an enclosure  12 , a motor  14 , an exhaust fan  16 , and a motor cooling system  18 .  
         [0028]    The enclosure  12  includes motor housing  20  and exhaust fan housing  22  separated from but connected to motor housing  20 . The motor housing  20  includes motor cover  24 , motor pan  26 , insulation  28 , and tilt sensor switches  30 . Motor pan  26  separates motor housing  20  from exhaust fan housing  22 . Insulation  28  covers the surface of motor pan  26 . Tilt sensor switches  30  are enclosed within motor cover  24 .  
         [0029]    Referring particularly to FIG. 2, exhaust fan housing  22  includes an upper shell  32  and a lower shell  34 . Upper shell  32  and lower shell  34  are movably coupled to one another by hinge  36  and secured by opposed latch  38 .  
         [0030]    Upper shell  32  includes flue gas exhausts  40  which are covered by grills  42 . The bottom  44  of lower shell  34  defines flue gas inlet  46 .  
         [0031]    Referring to FIGS. 4, 5 and  6 , motor  14  is enclosed within motor housing  20 . Motor  14  is secured to motor pan  26  above insulation  28 . A space separates motor body  50  from insulation  28 . Motor  14  is supported by motor supports  48 . Motor  14  includes shaft  52 . The motor  14  is oriented within the motor housing  20  such that shaft  52  passes through motor pan  26  into exhaust fan housing  22 . Motor shaft  52  is preferably keyed.  
         [0032]    Motor  14  may be of a conventional three phase, single speed type converted to operate at variable speed by use of a single phase and a variable frequency drive (VFD)  54 . Motor  14  may be connected to a remotely located controller  56 .  
         [0033]    As depicted in FIGS. 2, 5, and  6 , exhaust fan  16  is enclosed within exhaust fan housing  22 . Exhaust fan  16  includes an impeller  58 . Impeller  58  is preferably constructed of type  304  stainless steel, backward inclined in design and computer balanced. Impeller  58  includes a back plate  60 , rim  62 , blades  64 , and hub  66 . Hub  66  is preferably of the keyed-type and is mounted on shaft  52 . Exhaust fan  16  may comprise any type of blower without departing from the spirit and scope of the invention. Other fan designs include other types of centrifugal fans or axial fans. Impeller  58  is located within exhaust fan housing  22  such that rim  62  is proximate to flue gas inlet  46 .  
         [0034]    Referring particularly to FIGS. 3, 5 and  6 , motor cooling system  18  includes radial impeller  68 , auxiliary cooling fan  70 , and shroud  72 . Radial impeller  68  is secured to back plate  60  on the side opposite blades  64 . Auxiliary cooling fan  70  may be electrically powered and located on top of shroud  72 . Auxiliary cooling fan  70  is preferably of permanently lubricated, all ball bearing construction. Shroud  72  encloses motor body  50  and is positioned within and spaced from motor cover  24 .  
         [0035]    Shroud  72 , depicted in FIG. 3, includes air intakes  74  and deflectors  76 . Louvers  78  are located within the mouth  80  of air intakes  74 . Cooling air exhaust  82  surrounds shaft  52  and passes through motor pan  26 . Air intakes  74  are located and directed away from flue gas exhausts  40 .  
         [0036]    Auxiliary cooling fan  70  is actuated by thermostatic switches  84 . Thermostatic switches  84  are preferably located proximal to shaft  52  and shaft bearing  86 . Thermostatic switches  84  are preferably configured to actuate auxiliary cooling fan  70  at a temperature of about 150° F. and to switch it off at a temperature of about 120° F.  
         [0037]    Thermostatic safety control  87  includes shut-off switch  88  located proximate motor cooling system  18  and electrically connected to remotely located controller  56 . Thermostatic safety shut-off switch  88  is preferably configured to actuate at about 190° F.  
         [0038]    While this application discusses cooling with air as a coolant, it is contemplated that the disclosed coolant circulating device may operate with liquid coolant circulated about portions of the motor requiring cooling, with the liquid coolant being passed, for instance, through a radiator to dissipate heat outside the unit housing.  
         [0039]    Portions of the flue exhaust systems, such as the flue gas intake and flue gas exhaust, may be treated with a corrosion resistant coating such as Ryton brand coating available from the Phillips 66 Company.  
         [0040]    In operation, the power flue ventilator  10  is located at the exhaust end of a flue  11  and secured to the flue  11  via exhaust fan housing  22 . The power flue ventilator  10  may be installed at the end of a vertical flue  11  or a horizontal flue  11 . It is notable that when the power flue ventilator  10  is placed at the end of a horizontal flue  11  the power flue ventilator  10  may be oriented so that hinge  36  is at the bottom of the installation. This allows the exhaust fan housing  22  to be opened to provide access for cleaning or maintenance while preventing the housing from accidentally closing and potentially injuring a worker working on the power flue ventilator  10 .  
         [0041]    When required, power flue ventilator  10  draws flue gas from flue  11  and ejects it into the ambient atmosphere. Impeller  58  draws flue gas in through flue gas inlet  46  and expels it from exhaust fan housing  22  via flue gas exhausts  40 .  
         [0042]    Controller  56  may vary the speed at which motor  14  rotates in response to the draft demands of the fuel burning appliances. When power ventilator  10  exhausts flue gas, impeller  58  and exhaust fan housing  22  are of course exposed to high temperature flue gases that are extracted by power flue ventilator  10 . This may cause motor  14 , particularly in the area of shaft bearing  86 , to be exposed to temperatures high enough to damage or at least accelerate the deterioration of motor  14 .  
         [0043]    When the power flue ventilator  10  is operating at a high speed, impeller  58  is turning rapidly carrying with it radial impeller  68 . During high speed operation cooling air is drawn in through air intakes  74 , deflected upward by deflectors  76 , and travels through the space between motor housing  20  and shroud  72 . Cooling air then passes through auxiliary cooling fan  70  to the interior of shroud  72  where it flows over motor  14 , passes between motor  14  and insulation  28 , flows around shaft  52  and particularly the region of shaft bearing  86 , and passes through cooling air exhaust  82 . Radial impeller  68  draws cooling air out into the interior of exhaust fan housing  22 . Cooling air then exits exhaust fan  22  through flue gas exhaust  40  along with hot flue gases. It will be noted that air intakes  74  are located below and exterior to flue gas exhaust  40  assuring that cool ambient air will be drawn into air intakes  74 .  
         [0044]    Insulation  28  serves to reduce heat transfer from exhaust fan housing  22  into motor housing  20 .  
         [0045]    When motor  14  is operating at low speed, radial impeller  68  may not generate enough air movement around motor  14  to sufficiently cool it. Under these conditions, thermostatic switches  84  sense the rise in temperature. When the temperature reaches a predetermined value thermostatic switches  84  actuate auxiliary cooling fan  70  which draws cool air into the interior of shroud  72  and forces it over motor  14  where it is exhausted through cooling air exhaust  82  and thence outward through flue gas exhaust  40 .  
         [0046]    When the temperature inside shroud  72  has reached a sufficiently cool predetermined value, thermostatic switches  84  shut off auxiliary cooling fan  70 . Under extreme heat conditions such as very high ambient temperatures or exposure to bright sunlight, the temperature inside shroud  72  may reach a very high value despite the operation of auxiliary cooling fan  70 . Thermostatic safety shut-off switch  88  is actuated at a predetermined high temperature and signals controller  56  to shut off the heating appliance that is being exhausted. Controller  56  keeps the heating appliance shut off until the temperature within shroud  72  has cooled to an appropriate predetermined value.  
         [0047]    Preferably, thermostatic switches  84  turn auxiliary cooling fan  70  on at a temperature of about 150° F. and turn it off again at a temperature of about 120° F. Thermostatic safety shut-off switch  88  shuts off the vented heating appliance when the temperature inside shroud  72  reaches about 190° F. Auxiliary cooling fan  70  continues to run while the heating appliance is off until the temperature within shroud  72  returns to an acceptable level.  
         [0048]    Tilt sensor switches  30  are configured so as to sense when exhaust fan housing  22  is opened and interrupts all power to power flue ventilator  10  in order to prevent possible injury to workers working on power flue vent  10  should they fail to shut off the power supply before doing so.  
         [0049]    The present invention may be embodied in other specific forms without departing from the essential attributes thereof, therefore, the illustrated embodiment should be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.