Evaporator Superheat Research Articles

Model Free Control of the Refrigeration System Based on Minimum Stable Superheat

This paper applied model free control method to the refrigeration system evaporator minimum stable superheat control. Compared with conventional PID control, model free control method has fast convergence, anti-interference ability. Meanwhile, model free control can reduce the coupling effect between evaporator superheat and evaporating temperature and be well adapted to the variable load control requirements of the refrigeration system, which has good steady state performance and dynamic performance.

Gravity-Assisted Heat Pipe With Strong Marangoni Fluid for Waste Heat Management of Single and Dual-Junction Solar Cells

This study investigates the cooling of single and multijunction solar cells with an inclined, gravity-assisted heat pipe, containing a 0.05 M 2-propanol/water mixture that exhibits strong concentration Marangoni effects. Heat pipe solar collector system thermal behavior was investigated theoretically and semi-empirically through experimentation of varying input heat loads from attached strip-heaters to simulate waste heat generation of single-junction monocrystalline silicon (Si), and dual-junction GaInP/GaAs photovoltaic (PV) solar cells. Several liquid charge ratios were investigated to determine an optimal working fluid volume that reduces the evaporator superheat while enhancing the vaporization transport heat flux. Results showed that a 45% liquid charge, with a critical heat flux of 114.8 W/cm2, was capable of achieving the lowest superheat levels, with a system inclination of 37 deg. Solar cell semiconductor theory was used to evaluate the effects of increasing temperature and solar concentration on cell performance. Results showed that a combined PV/heat pipe system had a 1.7% higher electrical efficiency, with a concentration ratio 132 suns higher than the stand-alone system. The dual-junction system also exhibited enhanced performance at elevated system temperatures with a 2.1% greater electrical efficiency, at an operational concentration level 560 suns higher than a stand-alone system.

Dynamic modeling and self-optimizing operation of chilled water systems using extremum seeking control

Energy efficient operation of chilled-water systems is important for the energy saving of commercial buildings. This paper presents an extremum seeking control (ESC) scheme for energy efficient operation of chilled-water system, and presents a Modelica based dynamic simulation model for demonstrating the effectiveness of the proposed control strategy. The simulated plant consists of a water-cooled screw chiller and a mechanical-draft counter-flow wet cooling tower. The ESC scheme takes the total power consumption of chiller compressor and tower fan as feedback, and uses the fan speed setting as control input. The inner-loop controllers for the chiller operation include two proportional–integral (PI) control loops for regulating the evaporator superheat and the chilled water temperature. Simulation was conducted for three scenarios: fixed condition, change in evaporator inlet water temperature, and change in ambient air condition. The simulation results demonstrated the effectiveness of the proposed ESC strategy, and the potential for energy saving is also evaluated. Also investigated is the ESC windup issue caused by the limitation of the fan's maximum operation speed. A back-calculation anti-windup ESC scheme is applied and its effectiveness is validated by simulation results.

Optimal design of a micro-orifice for constant evaporator superheat in a small cooler

Abstract We present an optimal design for an orifice in a small cooler. The objective of the optimal design is to maintain constant superheat at the outlet of an evaporator while the flow rate and cooling load are changed. Four parameters are chosen for the optimal design: the diameter of the orifice, the aspect ratio between length and diameter, the entrance angle to the orifice, and the surface roughness. R-123 is used as the refrigerant. We perform a simulation to check the sensitivities of each parameter, and we determine the orifice diameter as the most sensitive design parameter among the four parameters to maintain the constant superheat. To find the optimal orifice diameter, experiments are performed on orifices of various diameters. To simulate the vapor-refrigeration cycle, the inlet condition of the orifice upstream flow is fixed at 3 bar and 60 °C. The superheat is measured at the outlet of the orifice while the cooling loads vary by 60, 80, and 100 W and the flow rate varies by 20–70 mL/min. An orifice diameter of 350 μm is selected as the optimal value to keep constant superheat at the evaporator outlet for various flow rates and cooling loads. The resulting optimal orifice design will be used in a small cooler.

Energy and irreversibility analysis of a cascade refrigeration system for various refrigerant couples

This study deals with energy and irreversibility analysis of a cascade refrigeration system employing various refrigerant couples, namely R152a–R23, R290–R23, R507–R23, R234a–R23, R717–R23 and R404a–R23, using a computer code developed for this aim. It is assumed that the refrigeration load is 1 kW, the refrigerated space temperature is −40 °C, and the environment temperature is 300 K, while the degrees of condenser subcooling and evaporator superheat are 5 and 7 °C, respectively, for all cases. Furthermore, the polytropic efficiencies of the compressors are assumed to be equal. It has been determined that the COP of the cascade refrigeration system increases and the irreversibility decreases with rising evaporator temperature and polytropic efficiency for all studied refrigerant couples. On the other hand, the COP of the cascade refrigeration system decreases and the irreversibility increases on increasing the condenser temperature and the difference between the saturation temperatures of the lower and higher temperature systems in the heat exchanger (Δ T). In all cases, the refrigerant couple R717–R23 has the highest COP and lowest irreversibility except for the limited ranges of polytropic efficiency (50–60%) and Δ T (13 K–16 K), while R507–R23 has the lowest COP and highest irreversibility. The refrigerant couple R152a–R23 has been found to be an alternative couple to R717–R23 for the above mentioned ranges of polytropic efficiency and Δ T. The refrigerant couples R134a–R23 and R290–R23 have placed in the middle range, and R404a–R23 can be considered as a replacement couple for R507–R23 in all cases.

On reducing evaporator superheat nonlinearity with control architecture

Evaporator superheat control is an important aspect of the operation of refrigeration and air conditioning systems; since the majority of cooling in these systems occurs through evaporation of two-phase refrigerant, the energy efficiency is dramatically improved by reducing the amount of superheat present. However, allowing refrigerant to leave the evaporator without completely vaporizing risks catastrophic damage to the compressor, so superior control is required at low superheat levels. One of the most significant challenges present in this control problem is the presence of significant nonlinearities in the response from the control input, e.g. expansion valve position, to evaporator superheat. This paper reveals how a particular control architecture inherently compensates for both the static and dynamic nonlinearities that dominate the valve-to superheat transient response. Furthermore, the control implementation only requires temperature measurements, which are frequently available in ordinary HVAC systems. Experimental results confirm the reduction of nonlinearities using the proposed approach, and the authors discuss the effect of actuator limitations on the nonlinearity compensation.

Superheat Control: A Hybrid Approach

This paper examines the perennial problem of evaporator superheat control. While standard mechanical devices can operate effectively under design conditions, many behave poorly as conditions vary or under transient operation, resulting in degraded system performance, such as thermostatic expansion valve (TEV) hunting. Technological advances enabling electronic control help alleviate these problems by allowing more sophisticated control approaches to regulating superheat—for example, electronic expansion valves (EEVs). However, poorly tuned EEVs can still exhibit undesirable behavior, and frequent valve adjustments raise concerns about device longevity. In this work, we propose a cascaded control approach, which regulates evaporator pressure and superheat and is achieved with a feedback control device that uses a hybrid of mechanical and electronic feedback. Analysis of the fundamental dynamic behavior of evaporator superheat motivates this approach, while experimental evaluation of two separate systems demonstrates the efficacy of the approach as compared to standard control devices, such as TEV and EEV.

Selecting PID Control Gains for Nonlinear HVAC&R Systems

While the dynamics of most HVAC&R processes are inherently nonlinear, a majority of HVAC&R systems utilize simple linear controllers, such as the proportional-integral-derivative (PID) controller. Although PID controllers are a time-tested and effective control strategy, most are improperly tuned or tuned for only one operating condition. As a result, these systems operate inefficiently and often exhibit poor performance. This study addresses the challenges of tuning PID controllers for HVAC&R systems. This paper presents recent advances in PID control design and extends and adapts these techniques for nonlinear HVAC&R systems. Two examples—namely temperature control of a variable air volume (VAV) unit, and evaporator superheat control using an electronic expansion valve—illustrate the efficacy of the techniques presented here.

Predictive functional control of an expansion valve for minimizing the superheat of an evaporator

In a previous paper, a Predictive Functional Control (PFC) method was proposed to control the evaporator superheat with an electronic expansion valve. It has been shown that superheat may be more accurately controlled by PFC than the conventional Proportional-Integral-Derivative (PID) control. In this paper, the proposed methodology is extended to regulate the condensing pressure. In order to study the influence of this control method on the Coefficient of Performance (COP), experiments are conducted on a refrigerating machine by changing the cooling capacity from 120 to 30 kW. As PFC improves disturbance rejection compared to a PID control, it is possible to reduce the superheat setting value and to prevent any unevaporated refrigerant liquid from reaching the compressor. As a consequence the use of PFC leads to an increase of COP which depends on operating conditions.

Control of air-conditioning systems in heating mode to enhance transient performance and steady-state efficiency

In this paper, a control framework for the air-conditioning system operated in heating mode is proposed. The control framework possesses a cascade structure for dealing with the fast and slow dynamics respectively contributed by the vapor compression cycle and the indoor dynamics. Under the control framework, the control performances associated with two strategies, the condenser superheat regulation (CSR) strategy and the evaporator superheat regulation (ESR) strategy are examined. It is shown experimentally that while ESR strategy demonstrates more superior transient response, CSR strategy is more advantageous in reducing the compressor power at steady state. In order to achieve both transient performance and steady-state power savings, the two strategies are combined in the way that ESR strategy is responsible for the control action during the startup/transient phase of operation, CSR strategy takes over the control when steady state is reached, and a bumpless compensator is used to smooth the control action during control switching. Experiments demonstrate that the combined strategy can simultaneously enhance transient performance and steady-state efficiency.

Study on predictive functional control of an expansion valve for controlling the evaporator superheat

A new method is proposed to control the evaporator superheat with an electronic expansion valve. The conventional proportional-integral-derivative (PID) control with invariable parameters cannot show good performance because of the variation of refrigeration unit parameters under disturbances. To solve this problem, this paper presents a method for the use of predictive functional controllers (PFCs) on superheat of an evaporator. This method is based on a physical model of the appliance studied to allow calculation of parameters needed for the use of PFCs. The control system created is incorporated into an industrial programmable logic controller and used for experiments on a refrigerating machine containing a shell and tubes evaporator with R410A as refrigerant fluid. The comparison between the two types of controller, i.e. PID and PFC, indicates that superheat may be more efficiently controlled by using the latter type of controller: the setting value is only slightly exceeded, there are only small oscillations of measured superheat, and the energy efficiency of the refrigeration unit may be improved.

Design of a steady-state detector for fault detection and diagnosis of a residential air conditioner

This paper presents a general methodology for developing a steady-state detector for a vapor compression system based on a moving window and using standard deviations of seven measurements selected as features. The feature thresholds and optimized moving window size were based upon steady-state no-fault tests and startup transient tests. The study showed that evaporator superheat and condenser subcooling were sufficient for determining the onset of steady-state during the startup transient. However, they misidentified steady-state during indoor temperature change tests where evaporator saturation temperature and air temperature change across the evaporator were needed for proper steady-state identification. Hence, the paper recommends including all fault detection and diagnosis (FDD) features in the steady-state detector to ensure the robustness of the detector because different features may play key roles with different transients.

97/03204 Parametric study of phase change thermal energy storage systems for space application

This study deals with energy and irreversibility analysis of a cascade refrigeration system employing various refrigerant couples, namely R152a–R23, R290–R23, R507–R23, R234a–R23, R717–R23 and R404a–R23, using a computer code developed for this aim. It is assumed that the refrigeration load is 1 kW, the refrigerated space temperature is −40 °C, and the environment temperature is 300 K, while the degrees of condenser subcooling and evaporator superheat are 5 and 7 °C, respectively, for all cases. Furthermore, the polytropic efficiencies of the compressors are assumed to be equal. It has been determined that the COP of the cascade refrigeration system increases and the irreversibility decreases with rising evaporator temperature and polytropic efficiency for all studied refrigerant couples. On the other hand, the COP of the cascade refrigeration system decreases and the irreversibility increases on increasing the condenser temperature and the difference between the saturation temperatures of the lower and higher temperature systems in the heat exchanger (ΔT). In all cases, the refrigerant couple R717–R23 has the highest COP and lowest irreversibility except for the limited ranges of polytropic efficiency (50–60%) and ΔT (13 K–16 K), while R507–R23 has the lowest COP and highest irreversibility. The refrigerant couple R152a–R23 has been found to be an alternative couple to R717–R23 for the above mentioned ranges of polytropic efficiency and ΔT. The refrigerant couples R134a–R23 and R290–R23 have placed in the middle range, and R404a–R23 can be considered as a replacement couple for R507–R23 in all cases.

Feed-forward control for a DX air cooling plant: Model simulation of a new system

A new feed-forward control concept for a Dx air cooling plant has been developed by mathematical modelling. This control system manipulates the compressor speed using the evaporator superheat signal and additionally adjusts the refrigeration duty by metering the required quantity of refrigerant into the evaporator. The simulation results are promising and indicate that further research work should be undertaken on this new control system.

Evaporator Superheat Control by An Electrically Driven Expansion Valve

Evaporator Superheat Control by An Electrically Driven Expansion Valve

System performance characteristics of an air conditioner over a range of charging conditions

A popular type of residential air conditioner in the USA is the split system. These systems have an outdoor unit which houses the condenser, condenser fan, compressor and controls. A separate indoor unit houses the evaporator, expansion device and evaporator blower. As the refrigerant charge of these systems is adjusted during field installation, the potential exists for not setting the charge exactly to the manufacturer's specifications. The objective of the work reported here was to quantify the influence of the refrigerant charge on the steady-state and cyclic operation of a residential split-system air conditioner. A 3 ton unit with capillary tube expansion was tested. The charge was systematically varied to determine its effect on system variables such as capacity, flow-rate, evaporator superheat, power consumption and seasonal energy efficiency ration (SEER). The results indicated a large degradation in capacity for undercharging than for overchar at a given temperature. However, the trends in capacity as a function of temperature for undercharging were different from those for overcharging. The difference in trends was found by close examination of the refrigerant flow control provided by the capillary tube. Other results indicated that small amounts (5%) of undercharging caused as much as a 6% drop in the SEER.

Evaporator critical heat flux in the double-wall artery heat pipe

The fact that heat is transferred into a heat pipe through the liquid-saturated evaporator wick gives rise to the so-called boiling limit on the heat pipe capacity. The composite nature of the double-wall artery heat pipe (DWAHP) wick structure makes the prediction of the evaporator superheat ( Δ T crit ) and the critical radial heat flux ( q r ) very difficult. The effective thermal conductivity of the wick, the effective radius of critical nucleation cavity, and the nucleation superheat, which are important parameters for double-wall wick evaporator heat transfer, have been evaluated based on the available theoretical models. Empirical correlations are used to corroborate the experimental results of the 2 m DWAHP. A heat choke mounted on the evaporator made it possible to measure the evaporator external temperatures, which were not measured in the previous tests. The high values of the measured evaporator wall temperatures are explainable with the assumption of a thin layer of vapor blanket at the inner heating surface. It has been observed that partial saturation of the wick (lean evaporator) causes the capillary limit to drop even though it may be good for efficient convective heat transfer through the wick. The 2 m long copper-water heat pipe had a peak performance of 1850 W at ∼23 W/cm 2 with a horizontal orientation.

Experimental investigation of the influence of lubricating oil on heat pump performance

Experimental results are presented which demonstrate the effect of lubricating oil on the COP and evaporator performance of refrigeration and heat pump systems. These are compared with the predictions of a simple theoretical analysis and it is shown that the agreement is satisfactory. It is demonstrated that evaporator heat extraction can be degraded by as much as 50 per cent and the COP by up to 30 per cent when compared to the performance indicated by the properties of pure refrigerants for systems containing up to 15 per cent oil. An interesting phenomenon is also demonstrated, wherein there is a clearly identifiable optimum setting for the evaporator superheat control, and the way in which this varies with oil concentration is indicated.