Economizer Target Shift for Combined Free and Mechanical Cooling
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Solution Overview
Problem
HVAC units face challenges in simultaneously utilizing free cooling and mechanical cooling without producing undesirably cold discharge air, which can lead to evaporator coil freezing and discomfort, as existing controllers either stop using outdoor air or produce excessively cold air when combining both cooling methods.
Innovation Solution
An HVAC unit controller adjusts the economizer to maintain a reduced free cooling target temperature and signals for mechanical cooling when additional cooling demand is met, ensuring the discharge air temperature remains above freezing while continuing to use outdoor air, thereby preventing cold discharge air and coil freezing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the economizing function continues to maintain the initial free cooling target temperature when combining free cooling and mechanical cooling, then the discharge air temperature drops below the target temperature, but the economizer stops utilizing outdoor air and the system effectively uses only mechanical cooling
Solution Approach 1:
The system dynamically adjusts the free cooling target temperature based on operating conditions. When mechanical cooling is activated, the controller automatically reduces the target temperature from the initial free cooling setpoint to a lower value that accounts for the additional cooling capacity provided by the evaporator coil, allowing both systems to work together effectively
Solution Approach 2:
The controller continuously monitors the discharge air temperature and cooling demand, using this feedback to determine when to activate mechanical cooling and how much to reduce the free cooling target temperature, ensuring optimal coordination between the two cooling modes
2Temperature
If the system utilizes only outdoor air as supply air when combining free cooling and mechanical cooling, then the discharge air temperature becomes undesirably cold below freezing point, but this creates conditions where the evaporator coil freezes
Solution Approach 1:
The system changes the target temperature parameter dynamically based on the cooling mode. When mechanical cooling is combined with free cooling, the controller reduces the free cooling target temperature to a level that prevents the discharge air from becoming too cold, thereby preventing evaporator coil freezing while still utilizing outdoor air
Solution Approach 2:
The controller proactively reduces the free cooling target temperature before the discharge air temperature can drop to dangerous levels. This preliminary adjustment prevents the evaporator coil surface temperature from reaching the freezing point of moisture in the air, avoiding ice formation before it occurs
3Temperature
If the system combines free cooling and mechanical cooling with reduced free cooling target temperature, then the discharge air temperature remains comfortable and above freezing, but requires precise control coordination between economizer and mechanical cooling
Solution Approach 1:
The controller performs multiple functions: it manages the economizer damper position, sets the free cooling target temperature, monitors discharge air temperature, and controls mechanical cooling activation. By integrating these functions into a single control logic, the system achieves coordinated operation without requiring separate complex control systems for each function
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 approach allows for effective simultaneous use of free and mechanical cooling, maintaining comfortable discharge air temperatures and preventing evaporator coil freezing, while conserving energy by optimizing the use of outdoor air.
Implementation Method 1
A blower moves air past the evaporator coil, transferring heat from the air to the evaporator coil
Implementation Method 2
The evaporator coil contains a refrigerant. A blower moves air past the evaporator coil, transferring heat from the air to the evaporator coil
Implementation Method 3
Free cooling utilizes the cooler outdoor air as some or all of the air volume
Data Source
AI summary
A method and apparatus for cooling a supply of air is provided. A unit controller receives a cooling demand signal. In response to the cooling demand signal, the unit controller adjusts an economizer according to an economizing function. The economizing function is configured to achieve an initial free cooling target temperature as a discharge air temperature. The unit controller receives an additional cooling demand signal. In response to the additional cooling demand signal, the unit controller reduces the free cooling target temperature. The unit controller adjusts the economizer according to the economizing function, with the economizing function configured to achieve the reduced free cooling target temperature as the discharge air temperature. Additionally in response to the additional cooling demand signal, the unit controller signals a unit to perform mechanical cooling on a supply of air received from the economizer.