1. Field of the Invention
The following invention relates to controls for fans in heating, ventilating, and air-conditioning (HVAC) systems, specifically to controls for converting constant speed fans to variable flow operation while preserving proper zone temperature control.
2. Description of Prior Art
Modern buildings typically use heating, ventilating, and air-conditioning (HVAC) systems to control indoor temperature, pressure, ventilation rate and other variables. Prior to the oil embargo of the 1970s it was common to design HVAC systems with constant-speed fans and with temperature controls that would re-heat cooled air or mix heated air with cooled air to maintain space temperature. HVAC systems with constant speed fans are called constant-volume systems.
There are three common types of constant-volume HVAC systems that serve multiple zones. One of these three is the single-duct re-heat system. These systems have a single supply duct that delivers cooled air to each zone re-heat coil. Re-heat coils add heat to the cooled air to keep the zone space temperature close to a setpoint. In rare cases the supply duct delivers hot air that is re-cooled by zone re-cooling coils.
A second type of constant-volume system is the dual-duct constant-volume system. Dual-duct systems deliver heated air and cooled air all the way to each zone terminal unit with separate hot air ducts and cold air ducts. The hot air duct has a heating coil and heating valve that are used to keep the hot air duct temperature close to a setpoint. The cold air duct has a cooling coil and a cooling valve that are used to keep the cold air duct temperature close to a setpoint. Zone terminal units mix the heated air with the cooled air to keep the zone space temperatures close to a setpoint.
The third common kind of constant-volume system is the multi-zone system. A multi-zone system is a special kind of dual-duct system where the hot air duct and the cold air duct are short, and are referred to as the hot deck and the cold deck, respectively. The mixing dampers are close to the fan, and are integrated with the hot deck and the cold deck. The mixing dampers for each zone mix heated air from the hot deck with cooled air from the cold deck to keep the zone temperature close to a setpoint. The hot deck and cold deck are packaged with the fan and other components of the system.
Constant-volume HVAC systems are inefficient. In states with strict energy codes, such as California, they are effectively prohibited in new construction. For HVAC systems that serve multiple zones, it is now common to use variable-air-volume (VAV) systems.
VAV systems have variable-speed fans that are controlled so that the amount of simultaneous heating and cooling or re-heating is significantly reduced. There are two common kinds of VAV systems: single-duct and dual-duct. Single-duct VAV systems supply cooled air to each zone terminal unit, where it is metered with a control damper when cooling is required or re-heated when heating is required. When heating, the amount of cooled air is reduced to a low level by the terminal controls. Dual-duct systems deliver heated air and cooled air all the way to each zone terminal unit with separate air ducts. Dual-duct VAV terminal units have independent dampers that modulate hot airflow rate to heat and modulate cold airflow rate to cool. Unlike the dual-duct constant-volume system, the dual-duct VAV system does very little mixing. Most of the time it supplies a variable amount of hot air when heating and a variable amount of cooled air when cooling. It only mixes air when the amount of heating or cooling is close to zero so that adequate ventilation air is provided.
Although constant-volume systems are less common in new construction, there is still a large installed base that serves billions of square feet of commercial buildings. Since they are inefficient, many retrofit strategies have been developed to modify their design and operation in order to make them more efficient.
One approach is to convert constant-volume systems to VAV systems. Typical VAV conversions for single-duct or dual-duct constant-volume systems involve replacing the constant-volume terminal units with VAV terminal units, adding terminal controls, adding a supply duct static pressure sensor, adding a variable frequency drive (VFD) to the fan, and adding a controller to regulate the supply duct pressure by modulating the fan speed with the VFD. These conversions are very expensive and intrusive because of the mechanical modifications. The spaces served by the system may have to be evacuated while the construction work is performed.
A typical VAV conversion for a multi-zone system involves disabling the hot deck, adding terminal units with control dampers and reheat coils to each zone supply duct, adding terminal controls, adding a supply duct static pressure sensor, adding a variable frequency drive (VFD) to the fan, and adding a controller to regulate the supply duct pressure by modulating the fan speed with the VFD. This approach is very expensive and very intrusive because it requires significant mechanical modifications.
Another way to convert constant-volume systems to VAV is to use VAV diffusers. U.S. Pat. No. 6,736,326 to Hunka, U.S. Pat. No. 6,176,777 to Smith et al., and U.S. Pat. No. 5,556,335 to Holyoake all describe VAV diffusers that can be used to convert constant-volume systems to VAV operation. This approach still requires mechanical modifications. Furthermore, all of the diffusers must be replaced for this method to work properly, and most zones have more than one diffuser.
For single-duct constant-volume systems, Liu, Claridge, and Turner (Continuous Commissioning Guidebook for Federal Energy Managers, DOE, 2002) add a VFD to reduce the fan speed during after-hours operation. During occupied hours the fan is operated at full speed. This strategy does not save energy for systems that are shut off after hours. Even when there is after-hours operation, this method is not cost effective unless the system is large because the energy savings are limited.
For dual-duct constant-volume systems, Liu and Claridge (Converting Dual-Duct Constant-volume Systems to Variable Volume Systems Without Retrofitting the Terminals, ASHRAE Transactions, Vol. 101, Part 1, 1999, pp. 66-70) describe a means for improving energy performance without retrofitting terminal units. They add a damper to the hot duct and use it to control the pressure in the hot duct. This strategy still requires a mechanical modification, which is intrusive and requires that the system be shut down. It also requires the installation of pressure sensors in the hot air duct and cold air duct.
For multi-zone constant-volume systems, Liu, Claridge, and Turner (Continuous Commissioning Guidebook for Federal Energy Managers, DOE, 2002) describe a means for improving the energy performance by adding a VFD to the supply fan and controlling the supply fan speed so that the most-open mixing damper is 95% open to the hot deck in the heating season. In the cooling season their strategy controls the fan speed so that the most-open mixing damper is 95% open to the cold deck. The command to the VFD comes from a Proportional-Integral-Derivative (PID) controller that takes the most-open damper position as input. This strategy requires that position sensors be added to the mixing dampers. Position sensors are expensive and difficult to install. Resistive position sensors are prone to vibration-induced premature failure. This strategy cannot be applied to single-duct constant-volume systems because they do not have mixing dampers.
Johnson (Johnson, G. A., 1984, “Retrofit of a Constant Volume Air System for Variable Speed Fan Control,” ASHRAE Transactions, 90(2B), 201-212.) describes a system for retrofitting single-zone constant volume air-handling units to VAV operation. Johnson's system does not apply to air-handling units that serve multiple zones. It also does not apply to single-zone systems that heat and cool. The units on which it was demonstrated were cooling-only units. Johnson's approach is to control the supply air temperature to a fixed setpoint with the cooling valve or outdoor air dampers, and then adjust the supply fan speed based on the average zone temperature so that the average zone temperature is maintained at a setpoint.