Mass flow characteristics and empirical modeling of R22 and R410A flowing through electronic expansion valvesCaractéristiques du débit massique et modélisation empirique de R22 et de R410A en écoulement dans les détendeurs électroniques

https://doi.org/10.1016/j.ijrefrig.2007.03.011Get rights and content

Abstract

The applications of electronic expansion valves (EEVs) into multi-type heat pumps and inverter heat pumps in building air-conditioning systems have increased for comfort environmental control and energy conservation. However, test data and mass flow models of EEVs are very limited in open literature. The objectives of this study are to investigate the mass flow characteristics of R22 and R410A through EEVs and to develop an empirical correlation for the prediction of mass flow rates of R22 and R410A through EEVs. Mass flow rates through six EEVs were measured by varying the EEV opening, inlet and outlet pressures, and the subcooling. Mass flow rates of R410A were compared with those of R22 at the same test conditions. Based on the experimental data, an empirical correlation for mass flow predictions in EEVs was developed by modifying the orifice equation. The predictions of the present correlation showed good agreement with the measured data with average and standard deviations of 0.76% and 5.9%, respectively. Approximately 92% of the measured data were within ±10% of the predictions.

Introduction

The expansion device controls the refrigerant mass flow and balances the system pressure in a refrigeration cycle. A capillary tube, a short tube orifice, and a thermostatic expansion valve (TXV) have been used as an expansion device in small refrigeration systems, air-conditioners, and chillers. Even though the capillary tubes and short tube orifices have advantages of simplicity, low cost and low starting torque of a compressor, they are not appropriate for a system that requires precise flow control for a wide range of flow rate. The TXV adopts a mechanical control method to obtain constant superheat at the evaporator outlet. Therefore, the response time of the TXV is relatively slow, and this slowness causes an unstable operating condition called superheat hunting [1]. Recently, multi-type heat pumps and inverter-type heat pumps have used electronic expansion valves (EEVs) instead of the conventional expansion devices [2], [3], [4]. The EEV affords a precise, fast, and stable flow control for a wide range of flow rate because it uses an active electronic control method based on an advanced control algorithm. The EEV into multi-type heat pumps allows more comfort control and energy conservation.

Most of the previous studies on expansion devices investigated constant-area expansion devices, such as capillary tubes and short tube orifices. Most capillary tube models modified the two-phase friction factor inside the tube based on the measured data [5], [6], [7]. Empirical correlations for the frictional pressure drop, mass flow rate, and delay of vaporization were also developed to predict the flow characteristics of refrigerants passing through capillary tubes [8], [9], [10], [11]. In addition, the existing empirical or semi-empirical models of flashing flow of refrigerants in short tube orifices were developed by modifying the orifice flow equation. Aaron and Domanski [12] developed an empirical correlation for R22 mass flow rate through short tube orifices at subcooled inlet conditions by modifying the single-phase orifice equation. Choi et al. [13] developed a generalized correlation for R12, R22, R134a, R407C, R410A, and R502 flowing through short tube orifices by performing dimensional analysis with the measured data.

Research on the mass flow characteristics of EEVs is very limited. Shanwei et al. [14] measured the mass flow characteristics through EEVs for different tapered needle valves and orifice inner diameters at the same inlet and outlet conditions. They concluded that there was no obvious relationship between mass flow rate and needle valve geometry (taper angle) at the same flow area. The mass flow rate remained constant with the same flow area even when the needle valve geometry was varied. However, they did not consider the effects of the orifice length inside the EEV. Choi and Kim [2] compared the performance of a heat pump having an EEV as an expansion device with that of a heat pump having a capillary tube for various refrigerant charge conditions. In general, for a wide range of operating conditions, the EEV system showed much higher performance than the capillary tube system.

Because the EEV has several advantages, it has been widely applied in an inverter-type heat pump system and a multi-type heat pump system. However, the experimental data and mass flow models for EEVs are very limited in open literature. Comprehensive studies on the flow characteristics of refrigerant flow through EEVs are needed to properly select and analyze the expansion device in multi-type heat pumps. The objectives of this study are to investigate the mass flow characteristics of R22 and R410A through EEVs and to develop an empirical correlation for the mass flow prediction through EEVs. The mass flow rates through EEVs were measured by varying the EEV opening, inlet and outlet pressures, and the subcooling. An empirical mass flow correlation was developed by incorporating a dimensionless correction coefficient in terms of EEV geometries and operating conditions into the orifice equation.

Section snippets

Experimental setup and test conditions

Fig. 1 shows the experimental setup used in the measurement of the performance of EEVs. The experimental setup included three loops: a refrigeration loop, a hot water loop, and a cold water/ethylene glycol loop. The refrigerant circulation loop consisted of a liquid pump, a mass flow meter, two plate type heat exchangers, and a test section. The inlet pressure of the test section was controlled by adjusting the liquid pump speed. The use of the liquid pump instead of a compressor allowed the

Experimental results and discussion

Fig. 2 shows the influence of subcooling on the mass flow rates of the two EEVs at the EEV opening of 31%. The EEV opening is defined as the ratio of the actual EEV opening to the full opening. Generally, the mass flow rate through the EEVs increased with the rise of subcooling at the EEV inlet. The refrigerant density at the EEV inlet increased with the increase of subcooling, resulting in higher mass flow rate at the same volumetric flow rate. In addition, the flashing point moved toward the

Development of an empirical correlation

In this study, an empirical correlation for the predictions of mass flow rate through EEVs was developed by modifying the single-phase orifice equation with consideration of EEV geometries and operating conditions. When liquid flows through an orifice without phase change, the mass flow rate can be determined by using the single-phase orifice equationm˙=C(πD24)2ρ(PinPd)1β4where C is the orifice coefficient and β is the ratio of the vena contracta diameter to the inlet diameter.

Aaron and

Conclusions

The performances of six EEVs for R22 and R410A were measured by varying the operating conditions and EEV geometries. The mass flow rates passing through EEVs increased with the increase of subcooling, inlet pressure, and EEV opening. Either, as the orifice length in the EEV increased at the same orifice diameter or as the orifice diameter increased at the same orifice length, the slope of the mass flow rate with respect to the subcooling, the inlet pressure, and the EEV opening increased. Based

Acknowledgements

This research was supported by a Grant (BA2-101-2-0-1) from the Carbon Dioxide Reduction & Sequestration Center, one of the 21st Century Frontier R&D Programs in the Ministry of Science and Technology of Korea.

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