HVAC control system with adaptive flow limit heat exchanger control

1. An adaptive flow limit controller for controlling a flow rate of a fluid through a heat exchanger, the controller comprising a processing circuit configured to:

use an adaptive model to determine a threshold flow rate of the fluid through the heat exchanger at which a gradient of a temperature difference of the fluid across the heat exchanger with respect to the flow rate of the fluid through the heat exchanger has a threshold gradient value; and

operate at least one of a valve, damper, fan, pump, or actuator to maintain the flow rate of the fluid of through the heat exchanger at less than or equal to the threshold flow rate.

2. The adaptive flow limit controller of claim 1 , wherein the adaptive model defines a relationship between the temperature difference of the fluid across the heat exchanger and the flow rate of the fluid through the heat exchanger as a function of an adaptive model parameter.

3. The adaptive flow limit controller of claim 1 , wherein the processing circuit is configured to update an adaptive model parameter of the adaptive model using one or more measurements of the temperature difference of the fluid across the heat exchanger and the flow rate of the fluid through the heat exchanger.

4. The adaptive flow limit controller of claim 1 , wherein the gradient of the temperature difference of the fluid across the heat exchanger with respect to the flow rate of the fluid through the heat exchanger is a function of both the flow rate of the fluid through the heat exchanger and an adaptive model parameter of the adaptive model such that:

the threshold gradient value corresponds to a first threshold flow rate of the fluid through the heat exchanger when the adaptive model parameter has a first parameter value; and

the threshold gradient value corresponds to a second threshold flow rate of the fluid through the heat exchanger, different from the first threshold flow rate, when the adaptive model parameter has a second parameter value different from the first parameter value.

5. The adaptive flow limit controller of claim 1 , wherein the threshold gradient value is a predetermined value stored in memory of the adaptive flow limit controller.

6. The adaptive flow limit controller of claim 1 , wherein the processing circuit is configured to generate the threshold gradient value as a function of:

a first gradient value that prioritizes energy consumption and corresponds to a first temperature difference of the fluid across the heat exchanger; and

a second gradient value that prioritizes occupant comfort and corresponds to a second temperature difference of the fluid across the heat exchanger, the second temperature difference being less than the first temperature difference.

7. The adaptive flow limit controller of claim 1 , wherein the processing circuit is configured to generate the threshold gradient value as a function of a user-adjustable weight that indicates a priority of energy consumption relative to occupant comfort.

8. A method for controlling a flow rate of a fluid through a heat exchanger, the method comprising:

using an adaptive model to determine a threshold flow rate of the fluid through the heat exchanger at which a gradient of a temperature difference of the fluid across the heat exchanger with respect to the flow rate of the fluid through the heat exchanger has a threshold gradient value; and

operating at least one of a valve, damper, fan, pump, or actuator to maintain the flow rate of the fluid of through the heat exchanger at less than or equal to the threshold flow rate.

9. The method of claim 8 , wherein the adaptive model defines a relationship between the temperature difference of the fluid across the heat exchanger and the flow rate of the fluid through the heat exchanger as a function of an adaptive model parameter.

10. The method of claim 8 , further comprising updating an adaptive model parameter of the adaptive model using one or more measurements of the temperature difference of the fluid across the heat exchanger and the flow rate of the fluid through the heat exchanger.

11. The method of claim 8 , wherein the gradient of the temperature difference of the fluid across the heat exchanger with respect to the flow rate of the fluid through the heat exchanger is a function of both the flow rate of the fluid through the heat exchanger and an adaptive model parameter of the adaptive model such that:

the threshold gradient value corresponds to a first threshold flow rate of the fluid through the heat exchanger when the adaptive model parameter has a first parameter value; and

the threshold gradient value corresponds to a second threshold flow rate of the fluid through the heat exchanger, different from the first threshold flow rate, when the adaptive model parameter has a second parameter value different from the first parameter value.

12. The method of claim 8 , wherein the threshold gradient value is a predetermined value stored in memory of the adaptive flow limit controller.

13. The method of claim 8 , further comprising generating the threshold gradient value as a function of:

a first gradient value that prioritizes energy consumption and corresponds to a first temperature difference of the fluid across the heat exchanger; and

a second gradient value that prioritizes occupant comfort and corresponds to a second temperature difference of the fluid across the heat exchanger, the second temperature difference being less than the first temperature difference.

14. The method of claim 8 , further comprising generating the threshold gradient value as a function of a user-adjustable weight that indicates a priority of energy consumption relative to occupant comfort.