Supply Fan Speed Control for Variable-Flow Constant-Volume HVAC
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Solution Overview
Problem
Constant-volume HVAC systems are inefficient and require costly, intrusive mechanical modifications to convert to variable flow operation, limiting energy efficiency and applicability.
Innovation Solution
A control system that reduces supply fan speed based on discharge air temperature sensor readings, using a controller and fan modulation device to adjust fan speed according to heating or cooling loads, without requiring mechanical components like air terminals or VAV diffusers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If constant-volume HVAC systems are used, then zone temperature control is maintained, but energy efficiency deteriorates
Solution Approach 1:
The patent applies dynamics by transitioning from constant-volume to variable-volume fan operation. The supply fan speed is dynamically adjusted based on real-time temperature differential measurements between supply air and zone air, allowing the system to adapt to varying heating and cooling loads while maintaining temperature control performance.
Solution Approach 2:
The patent changes the operating parameter of the supply fan from constant speed to variable speed. By modifying the fan speed parameter in response to temperature differential feedback, the system achieves improved energy efficiency while preserving the ability to maintain zone temperatures within acceptable ranges.
2Loss of energy
If VAV systems are installed, then energy efficiency is improved, but device complexity and cost increase
Solution Approach 1:
The patent extracts the essential variable-volume control function from complex VAV terminal units and concentrates it at the supply fan level. By implementing temperature differential-based fan speed control, the system achieves energy savings without requiring complex VAV boxes, control dampers, or terminal unit modifications in each zone.
Solution Approach 2:
The patent creates a universal control strategy that can be applied to both heating and cooling modes through a single temperature differential measurement approach. The same control algorithm and sensor configuration work for both heating (when supply air is cooler than zone air) and cooling (when supply air is warmer than zone air), eliminating the need for mode-specific control systems.
3Ease of operation
If mechanical modifications are made to convert constant-volume to VAV, then variable flow operation is achieved, but ease of installation deteriorates
Solution Approach 1:
The patent replaces complex mechanical VAV components (control dampers, VAV terminal units, mechanical flow control devices) with an electronic control system that adjusts fan speed. This substitution eliminates the need for mechanical modifications to ductwork and terminal units, requiring only the installation of temperature sensors and a fan speed control device.
4Loss of energy
If fan speed is reduced, then energy efficiency is improved, but airflow delivery may deteriorate
Solution Approach 1:
The patent implements feedback control by continuously monitoring the temperature differential between supply air and zone air and using this information to adjust fan speed. When the temperature differential indicates low heating or cooling loads, the fan speed is reduced to save energy. The system maintains adequate airflow delivery by ensuring the temperature differential remains within acceptable ranges that indicate proper thermal comfort.
Data Source
AI summary
A control strategy for supply fans in constant-volume heating, ventilating, and air-conditioning (HVAC) systems that reduces the speed of the fan at part-load conditions is provided. The invention consists of a constant-volume HVAC system, wireless discharge air temperature sensors, wireless hot source and cold source temperature sensors, and a wireless controller coupled to a fan modulation device. The controller includes a finite state machine that switches between a high-temperature control mode and a low-temperature control mode. The controller also includes a calculator that calculates a high temperature setpoint and a low temperature setpoint as a function of the hot source and cold source temperatures. In high temperature control mode, the controller compares the maximum discharge air temperature with the high temperature setpoint, and it commands the fan modulating device so that the maximum discharge air temperature remains close to the high temperature setpoint. In low temperature control mode, the controller compares the minimum discharge air temperature with the low temperature setpoint, and it commands the fan modulating device so that the minimum discharge air temperature remains close to the low temperature setpoint. Alternatively, the controller includes a calculator that computes a largest load as a function of previous fan command, discharge air temperatures, and readings from wireless zone temperature sensors. The controller increases the speed of the fan as the largest load increases, and reduces the speed of the fan as the largest load decreases.