Control systems and methods for preventing evaporator coil freeze

Solution Overview

Problem

HVAC systems face challenges in preventing evaporator coil freeze, particularly in systems with multiple compressors, where variations in saturation suction temperature (SST) across the evaporator coil can lead to frost formation and damage, due to the inability to effectively modulate discharge air temperature (DAT) setpoints in dehumidification modes.

Innovation Solution

A controller in the HVAC system monitors the saturation suction temperature (SST) and algorithmically adjusts the discharge air temperature (DAT) setpoint to prevent evaporator coil freeze by increasing it when SST falls below a minimum threshold and decreasing it when SST returns above the threshold, thereby maintaining a safe operating range.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If the discharge air temperature (DAT) setpoint is reduced to increase latent capacity during dehumidification, then the latent cooling capacity is improved, but the evaporator coil temperature drops below the freezing point causing frost formation

Engineering Contradiction:

Improvelatent cooling capacityVSAvoidevaporator coil freeze

Core Design Contradiction:

ProductivityVSObject-affected harmful factors

Solution Approach 1:

The controller continuously monitors the saturated suction temperature (SST) from the evaporator coil and compares it against a minimum threshold. When the SST approaches the freezing point, the controller automatically increases the DAT setpoint to prevent frost formation. This closed-loop feedback mechanism allows the system to dynamically adjust the DAT setpoint based on real-time evaporator coil conditions, resolving the contradiction between maximizing latent capacity and preventing coil freeze.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent implements dynamic adjustment of the DAT setpoint based on real-time SST measurements. Instead of using a fixed DAT setpoint, the system continuously adapts the setpoint according to the evaporator coil's thermal conditions. This dynamic control enables the system to operate at lower DAT setpoints when the coil is warm (maximizing latent capacity) while automatically raising the setpoint when the coil approaches freezing temperatures, thus resolving the trade-off between productivity and safety.

Inventive Principle:

Principle #15Dynamics

2Device complexity

If a fixed DAT setpoint is used during dehumidification mode, then the control system is simple, but it cannot prevent evaporator coil freeze when SST drops below the minimum threshold

Engineering Contradiction:

Improvecontrol system complexityVSAvoidevaporator coil freeze prevention

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The controller continuously monitors the saturated suction temperature (SST) from the evaporator coil and compares it against a minimum threshold. When the SST approaches the freezing point, the controller automatically increases the DAT setpoint to prevent frost formation. This closed-loop feedback mechanism allows the system to dynamically adjust the DAT setpoint based on real-time evaporator coil conditions, resolving the contradiction between maximizing latent capacity and preventing coil freeze.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The system uses its own operational parameters (SST measurements from the evaporator coil) to automatically regulate itself. The controller monitors the coil temperature and self-adjusts the DAT setpoint without requiring external intervention or complex external control systems. This self-service approach maintains simplicity while improving reliability through automated protective action.

Inventive Principle:

Principle #25Self-service

3Reliability

If the DAT setpoint is increased to prevent evaporator coil freeze, then the reliability is improved, but the latent capacity of the HVAC system decreases

Engineering Contradiction:

Improveevaporator coil freeze preventionVSAvoidlatent cooling capacity

Core Design Contradiction:

ReliabilityVSProductivity

Solution Approach 1:

The patent implements dynamic adjustment of the DAT setpoint based on real-time SST measurements. Instead of using a fixed DAT setpoint, the system continuously adapts the setpoint according to the evaporator coil's thermal conditions. This dynamic control enables the system to operate at lower DAT setpoints when the coil is warm (maximizing latent capacity) while automatically raising the setpoint when the coil approaches freezing temperatures, thus resolving the trade-off between productivity and safety.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The system dynamically changes the DAT setpoint parameter based on the measured SST. When the evaporator coil temperature is safe, the system uses a lower DAT setpoint to maximize latent capacity. When the SST approaches the freezing threshold, the system increases the DAT setpoint to prevent freeze. This parameter change strategy allows the system to optimize productivity under safe conditions while ensuring reliability when temperatures become critical.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS11781793B2Control systems and methods for preventing evaporator coil freeze

Publication Date: 2023.10.10 LENNOX IND INC

AI summary

In an embodiment, a method of preventing evaporator coil freeze in a heating, ventilation and air conditioning (HVAC) system is performed by a controller in the HVAC system. The method includes determining a reference saturated suction temperature (SST) via a sensor disposed in relation to an evaporator coil in the HVAC system. The method also includes determining whether the reference SST is below a minimum SST threshold. The method also includes, responsive to a determination that the reference SST is below the minimum SST threshold, increasing a discharge air temperature (DAT) setpoint.