Valve Response

Thermostatic Expansion Valve Response

Cross-section of a Thermostatic Expansion Valve

Thermostatic expansion valves (TXVs, also called TEVs) are often used in vapor compression based refrigeration and air conditioning systems. These valves adjust to allow more or less flow to achieve complete vaporization with adequate (but not excessive) superheat at the outlet of the evaporator.

TXVs “sense” the differential in temperature between the inlet and outlet of an evaporator. Unfortunately, there is a lag between the sensing of this temperature and its adjustment. Thermal Desktop and FloCAD can be used to analyze the dynamic stability of a TXV-controlled system: its ability to hold a set point after perturbations and to provide the necessary superheat.

For example, assume that there is currently too much superheat being produced, such that the TXV begins to open. In addition to lags and finite time constants in the sensing mechanism and valve pin motion, the newly released fluid must traverse the length of the evaporator, quenching heated sections as it does. By the time cooler vapor reaches the outlet, the system may overshoot and “hunt” for a stable set point. This difficulty in arriving at a stable set point is therefore termed evaporator or TXV “hunting.” Many time constants and lags are involved, making detailed modeling necessary. Hunting is undesirable not only from an efficiency viewpoint, but also because it leads to increased wear and tear of the valve and compressor.

Key to this analysis is the ability to calculate the forces on the TXV valve pin. These forces include not only the pressure difference across the diaphragm, but also the spring force and the frictional force. Inertia of the pin is also important. The ordinary differential equation (ODE) solvers allow you to define the equation of motion to be co-solved along with the thermohydraulic model to define the pin location. Once the pin position is known, the corresponding resistance of the TXV can be interpolated from the provided table of mass flow rate versus delta pressure.

The charts below show the resulting valve pin position and key temperature responses of such and analysis.

TXV transient response, pin position and temperature

Further details of the analysis can be found in the CRTech User Forum

dispersed vs. coalesced front

Tuesday, June 26, 2018, 1-2pm PT, 4-5pm ET

This webinar describes flat-front modeling, including where it is useful and how it works. A flat-front assumption is a specialized two-phase flow method that is particularly useful in the priming (filling or re-filling with liquid) of gas-filled or evacuated lines. It also finds use in simulating the gas purging of liquid-filled lines, and in modeling vertical large-diameter piping.

Prerequisites: It is helpful to have a background in two-phase flow, and to have some previous experience with FloCAD Pipes.

Register here for this webinar

FloCAD model of a loop heat pipe

Since a significant portion of LHPs consists of simple tubing, they are more flexible and easier to integrate into thermal structures than their traditional linear cousins: constant conductance and variable conductance heat pipes (CCHPs, VCHPs). LHPs are also less constrained by orientation and able to transport more power. LHPs have been used successfully in many applications, and have become a proven tool for spacecraft thermal control systems.

However, LHPs are not simple, neither in the details of their evaporator and compensation chamber (CC) structures nor in their surprising range of behaviors. Furthermore, there are uncertainties in their performance that must be treated with safety factors and bracketing methods for design verification.

Fortunately, some of the authors of CRTech fluid analysis tools also happened to have been involved in the early days of LHP technology development, so it is no accident that Thermal Desktop ("TD") and FloCAD have the unique capabilities necessary to model LHPs. Some features are useful at a system level analysis (including preliminary design), and others are necessary to achieve a detailed level of simulation (transients, off-design, condenser gradients).

CRTech is offering a four-part webinar series on LHPs and approaches to modeling them. Each webinar is designed to be attended in the order they were presented. While the first webinar presumes little knowledge of LHPs or their analysis, for the last three webinars you are presumed to have a basic knowledge TD/FloCAD two-phase modeling.

Part 1 provides an overview of LHP operation and unique characteristics
Part 2 introduces system-level modeling of LHPs using TD/FloCAD.
Part 3 covers an important aspect of getting the right answers: back-conduction and core state variability.
Part 4 covers detailed modeling of LHPs in TD/FloCAD such that transient operations such as start-up, gravity assist, and thermostatic control can be simulated.