Mixed Air Sensor Bypass Circuit for Economizer Damper Control
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Solution Overview
Problem
HVAC units face inefficiency when transitioning from free cooling to mechanical cooling due to the placement of mixed air temperature sensors after the evaporator coil, leading to incorrect temperature readings and unnecessary closure of outdoor air dampers, which reduces the effectiveness of free cooling.
Innovation Solution
A circuit with a relay and resistor is introduced to provide a false mixed air temperature signal to the economizer controller, allowing outdoor air to be maintained during mechanical cooling by mimicking a temperature at or above the set-point when mechanical cooling is initiated, thus preventing the closure of outdoor air dampers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If the mixed air temperature sensor is placed after the evaporator coil, then the sensor can accurately measure air temperature in space-constrained units, but the sensor incorrectly measures supply air temperature instead of mixed air temperature during mechanical cooling
Solution Approach 1:
A relay device is introduced as an intermediary between the temperature sensor and the economizer controller. The relay receives the actual temperature signal from the sensor and conditionally replaces it with a false temperature signal that mimics mixed air temperature at or above the set-point when mechanical cooling is detected, thereby resolving the measurement accuracy issue without changing sensor placement
Solution Approach 2:
The system changes the temperature parameter presented to the economizer controller based on the operational state. When mechanical cooling is detected via the Y1 signal, the relay transforms the temperature signal to reflect a different thermal condition (false mixed air temperature) that prevents erroneous damper closure, while maintaining the physical sensor position
2Ease of operation
If the economizer controller uses the temperature signal from the sensor located after the evaporator coil, then the controller can regulate mixed air temperature during free cooling, but the controller erroneously closes outdoor air dampers during mechanical cooling
Solution Approach 1:
The relay acts as a mediator that intercepts the temperature signal and selectively replaces it with a false signal when mechanical cooling is active. This prevents the controller from making erroneous decisions about outdoor air damper positioning during mechanical cooling while maintaining normal operation during free cooling
Solution Approach 2:
The system uses feedback from the Y1 mechanical cooling signal to control the relay's behavior. When Y1 indicates mechanical cooling is running, the relay provides feedback by sending a false temperature signal that prevents damper closure, creating a closed-loop control mechanism that adapts to operational conditions
3Power
If the unit transitions from free cooling to mechanical cooling, then the evaporator coil can meet higher cooling demands, but the false low temperature reading causes unnecessary closure of outdoor air dampers reducing free cooling effectiveness
Solution Approach 1:
The relay serves as an intermediary that prevents energy waste by blocking the erroneous temperature signal that would trigger unnecessary damper closure. This allows the system to maintain outdoor air intake during mechanical cooling, preserving free cooling effectiveness and reducing energy consumption
Solution Approach 2:
The relay applies preliminary anti-action by preemptively providing a false temperature signal that counteracts the erroneous low temperature reading. This prevents the controller from taking harmful action (closing outdoor air dampers) before it occurs, thereby maintaining energy efficiency during transitions between cooling modes
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enables the continued use of outdoor air during mechanical cooling when free cooling is available, optimizing energy usage by preventing unnecessary transitions to mechanical cooling and maintaining the benefits of free cooling.
Implementation Method 1
A relay and resistor are wired in series between the common terminal and the mixed air temperature set-point terminal
Data Source
AI summary
In an embodiment, a circuit for providing a mixed air temperature signal is provided. The circuit has a temperature input to an economizer controller, an air temperature sensor, a false air temperature device, and a switching device. The switching device has a switch and a switch actuating device. The switch connects the temperature input to the air temperature sensor when the switch is in a first state. The switch connects the temperature input to the false air temperature device when the switch is in a second state. The switch actuating device places the switch in the first state when a mechanical cooling signal is not sent. The switch actuating device places the switch in the second state when the mechanical cooling signal is sent.