Evaporator Capacity Estimation for Variable Water Flow Control
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Solution Overview
Problem
HVAC chillers face challenges in controlling the evaporator's cooling capacity due to unknown water flow rates, leading to overcorrection at low rates and sluggish responses at higher rates, and existing methods like flow meters or pressure sensors are either expensive or inaccurate.
Innovation Solution
Estimating the water flow rate through the evaporator based on the degree of opening of the expansion valve, pressure differential across the valve, and change in enthalpy per unit mass of refrigerant, allowing for the calculation of cooling capacity without directly measuring water flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a flow meter is added to the water circuit to measure flow rate, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent uses refrigerant as an intermediary substance to indirectly measure water flow rate. By measuring refrigerant mass flow rate through the expansion device and using the heat exchange relationship between refrigerant and water, the system derives water flow rate without direct water measurement, thus avoiding complex water circuit instrumentation
Solution Approach 2:
The patent replaces mechanical flow measurement systems (flow meters, pressure sensors in water circuit) with a thermodynamic calculation approach. By substituting mechanical measurement with thermodynamic relationships and refrigerant-based measurement, the system achieves accurate water flow rate determination with simpler equipment
2Device complexity
If water pressure sensors are used to determine approximate flow rate, then device complexity is reduced, but measurement precision deteriorates
Solution Approach 1:
The patent introduces refrigerant mass flow rate as an intermediary measurement that provides more accurate information about water flow rate than pressure sensors. The refrigerant flow measurement through the expansion device, combined with thermodynamic calculations, yields superior precision while keeping device complexity low
Solution Approach 2:
The patent changes the measurement parameter from water pressure (which gives only approximate flow rate) to refrigerant mass flow rate (which provides precise water flow rate information through heat exchange relationships). This parameter transformation enables accurate measurement without complex water circuit instrumentation
3Device complexity
If the chiller controller does not know the water flow rate, then device complexity is reduced, but control precision deteriorates
Solution Approach 1:
The patent implements a feedback mechanism where the controller continuously calculates water flow rate based on refrigerant mass flow rate and heat exchange relationships. This calculated feedback enables precise outlet temperature control by allowing the controller to adapt to actual water flow conditions without adding complex sensing equipment
Solution Approach 2:
The patent replaces mechanical feedback systems (flow meters, pressure sensors) with a thermodynamic calculation-based feedback approach. By substituting physical measurement feedback with calculated feedback from refrigerant parameters, the system achieves precise temperature control while maintaining simple device architecture
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 method effectively maintains the desired outlet temperature of chilled water by accurately estimating the evaporator's cooling capacity, ensuring consistent performance across varying water flow rates without the need for expensive flow measurement equipment.
Implementation Method 1
a pressure differential across the expansion valve
Implementation Method 2
Within the evaporator, vaporizing refrigerant cools a supply of water
Implementation Method 3
vaporizing refrigerant cools a supply of water
Implementation Method 4
vaporizing refrigerant cools a supply of water
Data Source
AI summary
A method of controlling a refrigerant chiller system is particularly suited for chillers where the water being chilled (or some other liquid) flows through the chiller's evaporator at a flow rate that is variable and is not directly known. To effectively control the chiller and maintain the temperature of the water leaving the evaporator at a desired target temperature, the cooling capacity of the chiller's evaporator is estimated based the degree of valve opening of an expansion valve, a pressure differential across the expansion valve, and a change in enthalpy per unit mass of the refrigerant flowing through the evaporator. In some embodiments, the chiller system includes multiple refrigerant circuits that are hermetically isolated from each other.