## 1 Introduction  
Aggravated global warming has become a concern as the energy demands of emerging countries are increasing. Moreover, continuously increasing demand for air-conditioning and hot-water supply are to be expected, due to improved life standard. Therefore, enhanced energy saving of air-conditioning and hot-water supply systems is required. For these reasons, in the refrigeration and air-conditioning field, tubes with 5 mm outside diameter or smaller have been developed in Japan and elsewhere, and that has simultaneously attracted a lot of attention in environment-responsive technology, by reducing the net amount of the refrigerant through downsizing the heat exchanger. It is necessary to predict the condensation heat transfer and pressure drop characteristics in a small-diameter smooth tube for optimum thermal design of the heat exchangers.  
Many studies have been conducted about condensation characteristics using conventional-diameter smooth tubes with over 6.35 mm outer diameter [1; 2; 3; 4; 5; 6]. [PERSON] et al. [7] carried out experiments of horizontal small-diameter smooth tubes for condensation of R22, R134a, and R410A, and then a new condensation heat transfer correlation was proposed. [PERSON] et al. [8] conducted experimentson the condensation heat transfer in a horizontal single rectangular mini-channel with a hydraulic diameter of about 1 mm. [PERSON] et al. [9; 10] investigated the condensation heat transfer of R134a, R1234 yf, R1234 ze(E), R32, and R410A in a 4 mm outside diameter horizontal smooth tube.  
[PERSON] and [PERSON] [11] have proposed the general correlation of friction factor for single-phase flow, based on the experimental pressure drop of single-phase flow, using 4 mm outside diameter smooth and 10 kinds of microfin tubes. Furthermore, [PERSON] and [PERSON] [12] have proposed general correlations of friction factor for two-phase flow, while condensation heat transfer can be estimated using the friction factor for single-phase flow. However, these correlations of smooth and microfin tubes were based on experimental results of R410A and in a large mass velocity condition, from 320 to 1180 kg m\({}^{-2}\)s\({}^{-1}\). The low flow rate operation of the refrigerant, against the increase of the frictional pressure drop, is required by multi-pass flow channel. For a low mass velocity condition, [PERSON] et al. [13] have proposed general correlations of friction factor for two-phase flow, and condensation heat transfer in microfin tubes can be estimated using 3 kinds of refrigerants (R32, R410A, R152a), 4 mm outside diameter smooth and 10 kinds of microfin tubes. Therefore, there is a need to investigate heat transfer characteristics and accumulated experimental data in a low mass velocity condition.  
In this paper, the condensation heat transfer coefficient and pressure drop for R32, R410A, R152a, and R1234 ze(E) refrigerators, in a 4 mm outside diameter tube, at a low mass velocity condition from ca. 100 to 400 kg m\({}^{-2}\)s\({}^{-1}\), were determined. The general correlation for condensation heat transfer inside small-diameter smooth tubes was suggested, based on the obtained R32, R410A, and R152a values, and the same with the method of microfin tubes case [13], which were compared with the results for R1234 ze(E) and data for R404A and R290 from other researchers.