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.
Chimneys first became common in Europe in the 16th 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.
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.
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.
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.
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.
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.
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.
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.
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(trademark) AL29-4C. Ductwork made of specialty steels of this type can be very expensive.
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.
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.
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.
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.
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.
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.
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.
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.
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.