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

Advanced Pipes in FloCAD
Thursday November 14, 9-10am MT (8-9am PT, 11am-noon ET)
This webinar introduces advanced features for FloCAD pipes in addition to working with complex geometry. Complex geometry includes interior fins and surfaces for heat transfer, flow around enclosed objects, annular flow, concentric pipes, and more. FK Locators and TEEs as modeling objects will also be introduced.
Custom Heat Transfer and Pressure Drops
Tuesday November 19, 2-3pm MT (1-2pm PT, 4-5pm ET)
Do you know what the default assumptions are in FloCAD, and whether or not they apply in your situation? Do you know how far you can go past that starting point? The answer: pretty far. There are numerous mechanisms in FloCAD for adjusting factors, scaling uncertainties, and applying different or supplemental correlations. This webinar summarizes the options available to you to customize your flow models to make sure that they apply to each new situation you encounter.
Heat Exchangers: Detailed and System-level
Thursday November 21, 2-3pm MT (1-2pm PT, 4-5pm ET)
This is two webinars in one. The first explains the use and assumptions behind the FloCAD HX system-level modeling object. The second webinar describes detailed-level modeling of complex heat exchanger passages, including application of Compact Heat Exchanger (CHX) methods.
Starting in 2020, we will begin offering Introduction to Thermal Desktop and Introduction to RadCAD as either in-person training or online training, alternating between online and in-person every three months. The training uses lectures and demonstrations to introduce you to basic Thermal Desktop and RadCAD usage. Hands-on tutorials provide practice building models and interpreting results (tutorials are completed by students outside of the online class time).
 
The next training class will be an online format in January 2020:
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  • Introduction to RadCAD - January 23 from 9am to 12pm, Mountain time
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