Valve Response

Thermostatic Expansion Valve Response

Cross-section of a Thermostatic Expansion Valve

Thermostatic expansion valves (TXVs) 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. SINDA/FLUINT 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 in SINDA/FLUINT allow a user 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 SINDA/FLUINT Sample Problem Appendix, Sample G: Vapor Compression, Part 2 (Included with SINDA/FLUINT installation).

FloCAD online training

Class times: May 2 & 4 from 10am to 2pm MT

Cost: $425

This online class will provide an introduction to fluid modeling components within FloCAD. The class will be held over a 2-day period, with daily sessions running approximately 4 hours each. The class uses a mixture of lecture, demonstrations, and self-paced tutorials to allow attendees to practice building fluid system models and interpreting results. The presentations will comprise 2 - 3 hours of each session, and the instructor will be available during the remainder of the time for questions during tutorials. Attendees must have basic working knowledge of SINDA and Thermal Desktop as these topics will not be covered but their usage is required for FloCAD.

Register here

Thermal Desktop, RadCAD, and TD Direct in-class training

Date: April 25-28, 2017, 8:00 a.m. to 5:00 p.m., daily
Location: Lakewood, CO

CRTech will be hosting introductory training for Thermal Desktop, RadCAD and TD Direct. Lecture and hands-on tutorials introduce attendees to basic Thermal Desktop and RadCAD usage and allow practice building models and interpreting results. The class will also introduce students to SpaceClaim direct modeling CAD interface and advanced meshing tools in TD Direct.

Daily Schedule

Day 1 and 2: Introduction to SINDA and Thermal Desktop
Day 3: Introduction to RadCAD
Day 4: Introduction to TD Direct

To learn more about this class and to register, visit our Training Page.

Anode and cathode of a flow battery

Using Sinaps? It is not too soon to get started with TD/FloCAD!

This webinar describes the process for converting from Sinaps to Thermal Desktop (TD) and FloCAD. This process includes using an exporter which works with Version 6.0 of the CRTech tool suite (expected to be released in May of 2017).

Come learn about the basics of TD/FloCAD, including many compelling features not available in Sinaps. The webinar will also cover how to manage the transition period, during which you may be using both programs simultaneously. This is also a chance to ask questions. 

If you missed this webinar, please contact us for the presentation material and recording.