Thermostatic Expansion Valve Research Articles

The novel use of phase change materials in refrigeration plant. Part 2: Dynamic simulation model for the combined system

A dynamic mathematical model for coupling the refrigeration system and PCMs has been developed in this paper. Overall the model consists of the following basic components: a compressor, a condenser, an expansion valve, an evaporator cooler and a PCM heat exchanger. The model developed here, is based on a lumped-parameter method. The condenser and evaporator were treated as storage tanks at different states, which have a superheat region, a two-phase region and a sub-cooled region. In the single-phase region the parameters are considered homogeneous whereas in the two-phase region, the intensive properties are considered as in thermal equilibrium. The compressor model is considered as an adiabatic process; an isentropic efficiency is employed in this process. The expansion process in the thermostatic expansion valve is considered as an isenthalpic process. The PCM is treated as a one-dimensional heat transfer model. The mathematical simulation in this study predicts the refrigerant states and dynamic coefficient of performance in the system with respect to time. The dynamic validation shows good agreement with the test result.

Application of refrigeration system in electronics cooling

An experimental investigation of the transient response of the vapor compression refrigeration (VCR) system to rapid change in evaporator (simulated electronics) heat load is presented. In this study, the VCR system is designed and constructed specifically for applications to cool high heat flux electronics and high-end computers. Temperature and pressure data were measured at pre-selected locations to study the behavior of the refrigeration system to alterations in evaporator heat load. Results show that the VCR system maintains the junction temperature of the simulated electronics at a much lower temperature compared to conventional air-cooling systems. The maximum temperature was registered near the exit of the evaporator cold plate. Experimental evidence shows an oscillation in temperature with time at the evaporator cold plate and the thermostatic expansion valve prior to attaining steady-state condition, and that the thermostatic expansion valve and the evaporator time constants are equal; and have a value of 70 s. Also, for analytical and numerical models of heat transfer in evaporator cold plate, results shows that the assumption of one-dimensional temperature distribution is unrealistic.

Dynamic simulation and analysis of a water chiller refrigeration system

In this paper, a lumped-parameter dynamic model of a water chiller refrigeration system based on mass and energy balance principles is developed. First the component models for an evaporator, compressor, condenser and a thermostatic expansion valve (TEV) were derived. These models were integrated to develop an overall model of a water chiller system. A control-oriented approach to model development was taken and the effect of control inputs such as compressor operational frequency and TEV opening fraction on the output performance of the system was investigated. The transient response characteristics show that the thermal system responses are much slower than pressure and mass flow rate responses revealing a two time-scale property of the system. Steady state performance of the system is also analyzed and graphical relationships between refrigerant mass flow rate, suction vapor superheat temperature, operation frequency and thermostatic expansion valve opening fraction are presented.

Impact of Area Contact Between Sensor Bulb and Evaporator Return Line on Modular Refrigeration Unit: Computational and Experimental

In the past, virtually all commercial computers were designed to operate at temperatures above the ambient and were primarily air-cooled. However, researchers have always known the advantages of operating electronics at low temperatures. This facilitates faster switching time of semiconductor devices, increased circuit speeds due to lower electrical resistance of interconnecting materials, and reduction in thermally induced failures of devices and components. Depending on the doping characteristics of the chip, performance improvement ranges from 1% to 3% for every 10°C lower transistor temperature can be realized. The IBM S/390 high-end server system is the first IBM design which uses a conventional refrigeration system to maintain the chip temperatures below that of comparable air-cooled systems, but well above cryogenic temperature. In previous work, the focus was to study the effect of variation of evaporator outlet superheat on the flow through thermostatic expansion valve at varying evaporator temperature. The effect of change in bulb location and effect of bulb time constant on the hunting at the evaporator has been reported. The effect of area contact on the stability of the system is been predicted theoretically. Mechanical analysis is performed in order to check the stresses induced. The evaporator return line and the sensor bulb are simply attached. The effect of area contact is further studied experimentally on an experimental bench.

Effect of the Location and the Properties of Thermostatic Expansion Valve Sensor Bulb on the Stability of a Refrigeration System

The combination of increased power dissipation and increased packaging density has led to substantial increases in chip and module heat flux in high-end computers. The challenge has been to limit the rise in chip temperature. In the past, virtually all commercial computers were designed to operate at temperatures above the ambient. However, researchers have identified the advantages of operating electronics at low temperatures. The primary purpose of low-temperature cooling using a vapor compression system are faster switching times of semiconductor devices, increased circuit speed due to lower electrical resistance of interconnecting materials, and a reduction in thermally induced failures of devices and components. Achievable performance improvements range from 1% to 3% for every 10°C lower transistor temperature, depending on the doping characteristics of the chip. The current research focuses on IBM’s mainframe, which uses a conventional refrigeration system to maintain chip temperatures below that of comparable air-cooled systems, but well above cryogenic temperatures. Although performance has been the key driver in the use of this technology, the second major reason for designing a system with low-temperature cooling is the improvement achieved in reliability to counteract detrimental effects, which rise as technology is pushed to the extremes. A mathematical model is developed to determine the time constant for an expansion valve sensor bulb. This time constant varies with variation in the thermophysical properties of the sensor element; that is, bulb size and bulb liquid. An experimental bench is built to study the effect of variation of evaporator outlet superheat on system performance. The heat load is varied from no load to full load (1 KW) to find out the system response at various loads. Experimental investigation is also done to see how the changes in thermophysical properties of the liquid in the sensor bulb of the expansion valve affect the overall system performance. Different types of thermostatic expansion valves are tested to investigate that bulb size, bulb constant, and bulb location have significant effects on the behavior of the system. Thermal resistance between the bulb and evaporator return line can considerably affect the system stability, and by increasing the thermal resistance, the stability can be further increased.

Impact of TXV and Compressor in the Stability of a High-End Computer Refrigeration System

The combination of increased power dissipation and increased packaging density has led to substantial increases in chip and module heat flux in high-end computers. The challenge is to limit the rise in chip temperature above the ambient. In the past, virtually all commercial computers were designed to operate at temperatures above the ambient. However, researchers have identified the advantages of operating electronics at low temperatures. Until recently, large-scale scientific computers used direct immersion cooling of single-chip modules. The current research focuses on mainframes (computer system), which uses a conventional refrigeration system to maintain chip temperatures below that of comparable air-cooled systems, but well above cryogenic temperatures. Multivariable control of compressor speed along with thermostatic expansion valve (TXV) opening can give better stability and performance. TXV is a mechanical controlling device used in the refrigeration system. The compressor is the only mechanical-working component in the refrigeration cycle that circulates refrigerant through the system continuously. Hence, controlling the compressor is an important aspect. The control objective is defined as improving the transient behavior of the vapor compression cycle for the refrigeration system operating around an evaporator set-point temperature. The system behavior is studied in two cases, TXV being the only control element in the first case, while TXV and a compressor both act as control elements in the other case.

Performance analysis and improvement of air-to-water chiller for application in wide ambient temperature range

Some air-to-water chillers can work well in summer but meet problems in low temperature ambience. When the ambient temperature decreases into a certain level, the cooling capacity of the chiller drops rapidly or in severe situation the compressor stops running. In order to expand the applicable condition, performance variation of an air-to-water chiller in a wide range of ambient temperature is analyzed based on simulation, and a new type air-to-water chiller is presented, in which an auxiliary capillary tube is installed parallel to the thermostatic expansion valve (TEV) as well as variable number condenser fans are used. A suitable control strategy of the chiller is proposed, and the performance of the chiller across an assorted range of ambient temperature range is also predicted. The operation of the capillary tube and the cooling fans are found to be a function of the coolant temperatures and the unique chiller operation has been tested for a period of more than a year.

Pressure drop and flashing mechanisms in refrigerant expansion devices

This paper examines a novel pressure drop mechanism as well as flow choking conditions that determine mass flow rate in refrigerant expansion devices. For this study, an ideal situation is considered where an expansion device such as a short tube orifice or a thermostatic expansion valve is modeled as an ideal isentropic nozzle. In addition, a liquid with a certain initial degree of superheat is first expanded in the converging nozzle down to the exit section without any phase transition. At the exit section where the metastable liquid jet flashes to produce a complex axisymmetric two-phase flow, a shock wave may terminate the overall expansion process. The model presented here is based on experimental observations in short nozzles, where the metastable liquid in the central core undergoes a sudden phase transition in the interfacial region, giving rise to a high-speed two-phase flow. A simple 1-D analysis of the radial evaporation wave based on the theory of discontinuities from gas dynamics leads to the Chapman–Jouguet (C-J) solution. Flow choking issues are examined and numerical examples are presented for three common refrigerants: R134a, R-22, and R-600a. Results suggest that the evaporation wave may be the flow controlling mechanism in these devices.

Dynamic simulation of air-to-water dual-mode heat pump with screw compressor

A dynamic model of air-to-water dual-mode heat pump with screw compressor is presented here. The high-pressure and low-pressure segments are divided into three control volumes, including the refrigerant inside tube, the tube wall and the fluid outside tube that is water or air. Time dependent ordinary differential equations are obtained from the mass and energy balances for each control volume. As the compressor, thermostatic expansion valve (TEV) body, and reversing valve have very small thermal inertias, steady-state models are applied for the compression, throttling, and leakage processes. The relationship between the temperature of the saturated liquid–vapor mixture in TEV’s bulb and the temperature of the refrigerant vapor at the evaporator exit is described with a time dependent ordinary differential equation. System simulation is finally carried out with ‘predictor–corrector’ and ‘adaptive integration step’ methods. Simulated results are in good agreement with the measured data, which lead to conclusion that the model can be used as a tool for the product development.

An experimental and numerical study of hunting in thermostatic-expansion-valve-controlled evaporators

The dynamic behaviour of a thermostatic-expansion–valve (TEV)-controlled dry-evaporator is studied experimentally and numerically. Although the linear model of the TEV together with the distributed model of the evaporator is able to predict the stable dynamic response of the system adequately, it fails to reproduce the hunting behaviour that is observed under certain operating conditions. A scrutiny of the experimental data reveals the possible existence of hysteresis in the system. The distributed model including the experimentally determined input-output characteristics of the TEV is able to reproduce the main features of the hunting oscillations well. The amplitude and frequency of these oscillations depend on the static superheat setting, the heat load of the evaporator and the time constant of the TEV bulb.

Modeling and experimental evaluation of an automotive air conditioning system with a variable capacity compressor

A steady state computer simulation model has been developed for refrigeration circuits of automobile air conditioning systems. The simulation model includes a variable capacity compressor and a thermostatic expansion valve in addition to the evaporator and micro channel parallel flow condenser. An experimental bench made up of original components from the air conditioning system of a compact passenger vehicle has been developed in order to check results from the model. The refrigeration circuit was equipped with a variable capacity compressor run by an electric motor controlled by a frequency converter. Effects on system performance of such operational parameters as compressor speed, return air in the evaporator and condensing air temperatures have been experimentally evaluated and simulated by means of developed model. Model results deviate from the experimentally obtained within a 20% range though most of them are within a 10% range. Effects of the refrigerant inventory have also been experimentally evaluated with results showing no effects on system performance over a wide range of refrigerant charges.

The effects of improper refrigerant charge on the performance of a heat pump with an electronic expansion valve and capillary tube

For inverter heat pumps and multi-type heat pumps, conventional expansion devices such as capillary tubes, short tube orifices, and thermostatic expansion valves (TXVs) are being gradually replaced with electronic expansion valves (EEVs) because of the increasing focus on comfort and energy conservation. In this study, the effects of off-design refrigerant charge on the performance of a water-to-water heat pump are investigated by varying refrigerant charge amount from −20% to +20% of full charge in a steady state, cooling mode operation with expansion devices of capillary tube and EEV. The characteristics of the heat pump with an EEV are compared with those with a capillary tube. The capillary tube system is more sensitive to off-design charge as compared with the EEV system. Cooling capacity and COP of the EEV system show little dependence on refrigerant charge, while those are strongly dependent on outdoor conditions. In general, for a wide range of operating conditions the EEV system shows much higher performance as compared with the capillary tube system. The performance of the EEV system can be optimized by adjusting the EEV opening to maintain a constant superheat at all test conditions.

Experimental evaluation of electronic and thermostatic expansion valves performances using R22 and R407C

An experimental study to evaluate the energetic performances in steady-state and in transient operating modes of an electronic and thermostatic expansion valve is presented. Both valves have been assembled to feed an air cooled evaporator connected to an experimental vapour compression plant with a water cooled condenser operating with a semihermetic compressor. The performances of the valves have been examined at different conditions when the experimental plant works with R22 and with a substitute as the non-azeotropic blend R407C that is chlorine free. Indeed the HCFC designated as R22 contains chlorine that is harmful for the ozone layer and must be replaced in the future. The final results of this study show an overall better performance of the electronic expansion valve compared with the thermostatic expansion valve under transient conditions while in steady-state conditions both the valves are equal in performance. These results apply to both R22 and R407C.

An experimental evaluation of the vapour compression plant performances in presence of R407C leaks using an electronic expansion valve

Chlorofluorocarbons and hydrofluorocarbons (HFCs) used as working fluids in the vapour compression plants, have to be replaced by new substances because of their ozone depletion potential. Zeotropic mixture of HFCs refrigerants that are environment-friendly substances are often employed. The zeotropic mixtures with a large glide temperature could cause problems in the refrigeration control system when a leak occurs because their composition modifies. This paper presents a comparison of the energetic performances, in presence of leaks, when a thermostatic valve and an electronic expansion valve are used in a refrigeration plant, working with the zeotropic mixture designated as R407C (R32/R125/R134a 23/25/52% in mass)—this is the most suitable substitute of the HCFC22. The vapour leaks are simulated at the inlet of the evaporator and at the liquid receiver. Experimental results show that a good adaptability to mixture leaks is related to the electronic expansion valve, while better energetic performances are obtained using the thermostatic expansion valve as long as it is usable.

Performance instability of a refrigerator with its evaporator controlled by a thermostatic expansion-valve

An evaporator's time-constant ( t c) equation has been developed as part of a mathematical model describing the time-dependent behaviour of a refrigeration system. The magnitudes of the evaporator's t c were approximately 2 and 5 s, respectively for 100 and 50% loads imposed on the refrigerator. As a result of a step change in the inlet's water-temperature, the magnitudes of both t c and the two-phase heat-transfer coefficient ( f tp) for the refrigerant system were altered. Although a step decrease in the inlet-water's temperature led to large increase in t c, the system's behaviour stabilized quickly because f tp had also increased significantly. The predictions of this investigation showed that such an analysis is highly sensitive to the accuracy of the equations used to calculate both t c and f tp and this makes the experimental verification of this model desirable.

Detection of liquid mass fraction at the evaporator exit of refrigeration systems

The article presents four methods of detecting droplets in the stream of superheated vapor at the evaporator exit of refrigeration systems: (a) an energy balance, (b) a thin-film resistance (MEMS) sensor developed for this project, (c) a laser and photodiodes for measuring light scattered by entrained droplets, and (d) an exposed beaded thermocouple. Three configurations of the plate evaporator are examined, with refrigerant flow controlled by: (1) a thermostatic expansion valve, (2) a manual valve, and (3) a separate liquid injector used to examine the detection of controlled flow of droplets. The signals were recorded at both slow (0.5 Hz) and fast (40 Hz) rates in order to characterize the unsteady flow exiting the evaporator. The presence of liquid in superheated vapor at the evaporator exit indicates nonequilibrium conditions and maldistribution of two-phase refrigerant within the evaporator. The performances of the MEMS sensor and the beaded thermocouple when exposed to small liquid mass fractions in a superheated vapor stream are examined to assess the feasibility of using each instrument to detect and control liquid mass fraction instead of superheat at evaporator exit and improve distribution among evaporator plates. Results show that the MEMS sensor is more sensitive to liquid mass fraction (LMF) in superheated vapor than the thermocouple at lower values. Both instruments in current versions exhibit a saturation point beyond which they can no longer detect increases in LMF.

Effect of sudden changes in evaporator external parameters on a refrigeration system with an evaporator controlled by a thermostatic expansion valve

A theoretical model is used to investigate the effect of sudden changes in the evaporator external parameters on the behaviour of a refrigeration system with a dry-expansion evaporator controlled by a thermostatic expansion valve. The analysis showed that even under stable operation, sudden changes in chilled water inlet conditions may result in unstable system for a certain period of time. Results were obtained for the 50 and 25% partial loads. The inlet chilled water temperature and mass flow rate were suddenly increased or decreased with factors F t and F m respectively. Instability of the system parameters such as evaporator and superheat temperatures, and TEV-outlet mass flow rate followed the sudden changes. The bulb temperature remained stable. Since the evaporator and the bulb temperatures are control parameters of the TEV-outlet mass flow rate, then the instability of the latter must be mainly due to the evaporator temperature instability. The unstable region vanished completely for the 50% partial loads, and continued for the 25% partial loads. The evaporator may get flooded depending on the factors applied. For example, with the 50% partial load, the evaporator has more potential to flood when sudden decrease is applied. The new stable values showed to be different than the initial ones due to the new balanced system operating parameters.

Simulation of the performance of alternative refrigerants in liquid chillers

The impact of refrigeration systems on the environment can be reduced by (a) the use of alternative refrigerants which are less harmful to the environment and (b) the optimization of systems and control strategies to deliver increased levels of energy efficiency. Mathematical modelling offers the opportunity to test the performance of systems under different operating conditions and with alternative refrigerants. Dynamic models allow comparison of both transient and steady state behaviour and this is of particular importance for liquid chillers since these systems can operate under transient conditions for long periods. This paper covers the development of a general dynamic model for the simulation of liquid chillers. Brief descriptions of the system component models are given, including a semihermetic reciprocating compressor and thermostatic expansion valve as well as a shell-and-tube evaporator and condenser. The paper demonstrates the application of the model to simulate the performance of a liquid chiller retrofitted with a range of alternative refrigerants. The performance of the system is determined in terms of cooling capacity, power consumption and coefficient of performance for a range of different operating conditions. The relative performance of each refrigerant is discussed and the preferred alternative identified for typical applications.

Curvature of the vapour pressure curve

This paper brings out the existence of the maximum in the curvature of the vapour pressure curve. It occurs in the reduced temperature range of 0.6–0.7 for all liquids and has a value of 3.8–4.8. A set of 17 working fluids consisting of several refrigerants, carbon dioxide, cryogenic liquids and water are taken as test fluids. There exists also a minimum close to the critical point which can be observed only when a thermodynamically consistent functional form of the vapour pressure equation is chosen. This feature, in addition to throwing some light on the behaviour of the vapour pressure curve, could provide some useful inputs to the choice of working fluids for vapour pressure thermometers and thermostatic expansion valves.

Experimental analysis of a transfer function for an air cooled evaporator

A transfer function to model a direct expansion air cooled evaporator, inserted in a vapor compression refrigeration plant, is deduced by means of experimental analysis. For inlet air temperatures onto the evaporator and refrigerant mass flow rate variable in appropriate ranges, the evaporator dynamic behavior is simulated by a linear model with delay. The results of transfer function are compared with experimental data, obtained by applying both step inputs and periodic changes to the refrigerant mass flow rate. The influence of the hunting, typical of a thermostatic expansion valve, is also estimated experimentally and then validated by the transfer function, obtaining a good agreement. These results could be applied to obtain a control algorithm for the refrigerant mass flow rate feeding the evaporator, by varying the speed of the compressor motor.