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

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 will last 60 minutes and are designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts. 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.

May 31, 2018, 1-2pm (PT), 4-5pm (ET)

This webinar provides an overview of LHP design and operation, from a basic understand of components to a review of important performance considerations and limitations.

Many topics will be covered, from start-up issues to the purpose of the evaporator bayonet to capillary flow regulators to load balancing in parallel LHP units. However, we will cover these topics only in enough depth that you will be able to understand the reasons for various modeling approaches that will be covered in later webinars. In other words, this webinar will survey the various ways in which LHPs require a specialized approach to design analysis and simulation.

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Prerequisites: Basic understanding of two-phase thermodynamics and heat transfer.
Please register for Part 1 here

June 5, 2018, 8-9am (PT), 11am-noon (ET)

This webinar explains how the toolbox approach of Thermal Desktop and FloCAD can be used to design and simulate LHPs at a system level, where the focus is on predicting conductance of nominally operating LHP, including thermostatic control (variable conductance).

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Prerequisites: Basic understanding of Thermal Desktop and FloCAD operation as applied to two-phase systems. Basic familiarity with LHP components and operation (see Part 1).
Please register for Part 2 here

June 7, 2018, 8-9am (PT), 11am-noon (ET)

Modeling wick back-conduction in an LHP is critical to accurate prediction of the overall loop conductance and operating point. This prediction can't be separated from an understanding of what is happening in the wick core. This webinar presents time-honored methods of dealing with these complex topics in a relatively simple (if abstract) thermal/fluid network.Prerequisites:

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Basic understanding of Thermal Desktop and FloCAD operation as applied to LHP modeling (see Part 1 and Part 2).
Please register for Part 3 here

June 12, 2018, 1-2pm (PT), 4-5pm (ET)

This webinar explains how Thermal Desktop and FloCAD can be applied to simulate complex and transient phenomena in LHPs, including condenser design, start-up, thermostatic control, and gravity assist (evaporator below condenser). The design of an actual LHP will be used to demonstrate concepts; the implications of attaching large masses to the evaporators (cooled electronics and support structures) will become clear as a result.

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Prerequisites: Familiarity with LHP modeling approaches in TD/FloCAD (see Part 1Part 2 and Part 3).
Please register for Part 4 here