In the author’s experimental configuration, a total of 10 cylinders of different volumes ranging from 1 922。66 mm3 to 206 088 mm3 were used。 For every volumetric of cylinder,  there  are  four  different  inner  diameters  of  the

discharge pipes, namely, 2 mm, 4 mm, 5 mm and 6 mm。 The length of discharge pipe is discrete and varies from 100 mm to 20 100 mm。 The lengths of every discharge piper are chosen according to condensation or no condensation occured in previous experiment。 Until the length of discharger pipe nears to the critical length, the condensation

occurs。  the  initial  status  parameters  of  compressed   air is that pressure is 0。5 MPa, temperature 15 ℃ , dew temperature of the air supply 2 ℃, atmosphere temperature 25  ℃, atmosphere humidity 35% – 40 %。

Fig。 1。    Experimental setup

3 Effect of Condensation for a Single Parameter

3。1    Internal condensations

As we known, internal condensation occurs in a pneumatic system when water droplets are produced as the temperature of compressed air drops to or sub-dew point temperature during expansion。 The effecting factors include atmospheric temperature and pressure, initial pressure, temperature and humidity of the compressed air and structural parameters of the system components such as inner volumes, effective sectional areas of the cylinder outlet, the reversing valve, the discharging pipe, and the length of discharging pipe。

In operating of pneumatic systems, atmospheric temperature and pressure, initial pressure and temperature of the compressed air are relatively stable。 Hence, in this paper, only the effects of the structural parameters on condensation are investigated。 The experiments[3–4] indicate that the effects of different structural parameters of the system components on internal condensation are different。 When the other structural parameters are fixed, for smaller cylinder volumes, longer lengths of gas pipe or bigger cross section of air pipes, internal condensation is more prone to occur。 Effective areas of cylinder and directional valve have no obvious effect on condensation。

Changing the volume of cylinder, effective area and length of discharging pipe, the distribution of experiment points at which the condensation or no condensation occurs are shown in Fig。 2。 Where, V is volume of cylinder, A is the effective cross section, and l is the length of discharging

pipe。

CHINESE JOURNAL OF MECHANICAL ENGINEERING ·643·

Similar to internal condensation, the effects of every structural parameter of system components on external condensation are also different。 Keeping the other structure parameters unchanged, the bigger volume of cylinder, the shorter length of discharging pipe, and the bigger cross section of discharging pipe, the external condensation are more prone to occur。

4 Condensation Analysis Based on Comprehensive Parameter

Fig。 2。 Effects of structure parameter of system on interior condensation

In order to investigate the effect of area at the outlet of cylinder and the effective area at outlet of valve on condensation, the area at the outlet of cylinder and the cross section of reversing valve were changed and the experiments were repeated as the procedure shown in Fig。 1。 One cylinder volume is 3 620 mm3, another is 5 640 mm3。 The regulator orifice inner diameter of each cylinder is 0。7 mm, 1。1 mm and 2。5 mm, and the effective area of directional valve are 2。7 mm2, 12 mm2 and 17。1 mm2, respectively。 The experimental results expressed that the effective area of cylinder outlet and directional valve have no effect on condensation。 Therefore, the volume of cylinder, effective cross section, and length of discharging pipe are focused on in this paper, and the outlet area of cylinder and effective area of regulator orifice are not be concerned。