Ventilation – Having forced recirculation – Including automatic control means
Reexamination Certificate
1999-01-14
2001-05-08
Joyce, Harold (Department: 3744)
Ventilation
Having forced recirculation
Including automatic control means
C454S236000
Reexamination Certificate
active
06227961
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to a Heating, Ventilation, and Air Conditioning (HVAC) custom control system and particularly relates to an integrated control system requiring less field wiring and providing an anti-windmilling capability.
BACKGROUND OF THE INVENTION
Building pressure control is an increasingly important aspect of modern HVAC systems. Building pressure reflects the ratio of outside air entering the building envelope to the exhaust air leaving the envelope. Further, modern building designs often require variable air volume (VAV) systems that supply a constant temperature air (e.g., 55 degrees F.) to the building's supply ductwork. The temperature and air flow in individual areas or zones within the building are controlled by positioning dampers or boxes that allow more or less air into the space depending on the load.
Proper operation of a supply VAV system relies on a constant (i.e., static) pressure being maintained in the supply duct. Such a constant pressure ensures a sufficient supply of conditioned air on demand. Thus, VAV systems use static pressure control to sense and maintain supply duct pressure. VAV systems may achieve proper return air control by employing volumetric flow synchronization (volumetric flow control). Volumetric control systems modulate the return fan based off of actual supply and return fan air flow readings.
In the prior art, volumetric fan control was both complex and very costly, resulting in building owners and engineers opting for less costly, less complex, and consequently less effective systems. However, as the concern for indoor air quality has increased in recent years, so too has the concern for effective HVAC control. One area receiving a great deal of attention is the so-called “sick building syndrome.” Further, new guidelines for indoor air quality have forced engineers and building owners to install systems that allow them to accurately control the mix of outside and inside air in compliance with those guidelines. ASHARE-62 IAQ is an example one such guideline.
Prior art volumetric fan control systems do not adequately meet these new building demands in a cost effective manner. First, prior art volumetric fan control systems require separate control panels, drives and probes. Second, these prior art systems require substantial field installation and set-up. Thus, prior art systems result in, inter alia, increased installation and operating costs. Further, due to the nature of integrating these separate systems, it may be difficult or impracticable for the specifying engineer and/or building owner to show compliance with ASHARE-62 IAQ guidelines.
As indicated above, most HVAC systems employ one or more fans in the supply and return ducts. In the interest of efficiency, as well as pressure and volume control, any given fan is not continuously operated. When a fan is idle (not powered), it may, however, continue to spin either from momentum or from persistent air flow within the duct. This unpowered spinning is referred to as “windmilling.” Further, windmilling can occur in both a forward direction (e.g., the normally desired direction of rotation), or a reverse direction (e.g., the normally undesired direction of rotation).
Prior art HVAC systems do not account for this windmilling when powering or depowering fan motors. Applying power to a fan that is spinning in the reverse direction can cause excess torque forces that stress the fan and/or fan motor. Starting a fan motor when it is spinning in the reverse direction may also cause an overload and trip the applied adjustable speed drive.
There is a need, therefore, for an improved, cost effective, and energy efficient HVAC fan control system. In particular, there is a need for an HVAC fan control system the provides a unitary control and drive panel with reduced installation costs.
SUMMARY OF THE INVENTION
It is therefore seen desirable to provide an improved fan control system. Thus, there is a need for an improved, cost effective, and energy efficient HVAC fan control system. There is a need for an HVAC fan control system that provides a unitary control and drive panel with reduced installation costs. There is also a need for an HVAC fan control system that employs a unitary volumetric flow controller. There is another need for a fan control system with reduce installation costs and complexity. There is also a need for a fan control system that provides an anti-windmilling capability to avoid stressing the fan and/or fan motor. There is a need for a fan control system that uses DC injection breaking to accomplish the anti-windmilling capability. There is further a need for a fan control system that ensures a constant supply duct pressure. There is still another need for a unitary fan control system that selectively supplies power to a supply fan and a return fan in response to air flow rates in the supply and return air ducts. There is also a need for a fan control system that provides verifiable compliance with indoor air quality guidelines. There yet another need for a unitary fan control system that accurately controls fan speed based upon actual flow rate requirements.
The invention comprises a fan control system suitable for use in air moving applications. The fan control system has a pressure sensor responsive to an air pressure in an air duct for supplying a static pressure signal representative of the air pressure. A supply air flow sensor is responsive to an air flow in a supply air duct and supplies a supply air flow signal representative of the air flow in the supply air duct. A return air flow sensor is responsive to an air flow in a return air duct and supplies a return air flow signal representative of the air flow in the return air duct. A supply fan moves air in the supply air duct. The supply fan is driven by a supply fan motor. A return fan moves air in the return air duct. The return fan is driven by a return fan motor. A unitary volumetric air flow controller is connected to the supply fan and to the return fan. The air flow controller selectively applies a supply drive power signal to the supply fan motor to energize the supply fan motor, and selectively applies a return drive signal to the return fan motor to energize the return fan motor. The power drive signals are responsive to the static pressure signal, supply air flow signal, and return air flow signal.
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Brown Michael C.
Moore Scott W.
Webb Terry L.
General Electric Company
Joyce Harold
Senniger Powers Leavitt & Roedel
Wasserbauer Damian
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