3。2 External condensations

During cooling of compressed air due to expanding, heat transfer takes place between the air and surface of components, as well as the atmosphere near the outer surface of components, which causes the temperature decrease dramatically。 When the temperature drops below the dew point, the water molecules separate from atmosphere and form to water droplet, condensation creates。 If the temperature of the atmosphere is enough lower, the water droplet will change to ice[2–3]。

The factors effecting on the external condensation include temperature, pressure and humidity of atmosphere, initial pressure, temperature and humidity of compressed air, structure parameters of system component such as inner volume of cylinder, effective cross section area at outlet of cylinder, effective cross section area of reversing valve, effective cross section area and length of discharging pipe。 In this paper, only the effects of structure parameters of system component were studied。

The condensation in a pneumatic system is a quite complicated phenomenon involving many factors。 The analysis on effects degree of every factor relates to hydrodynamics, heat transfer theory and phase transformation, the calculation are troublesome and the prediction measures of condensation are difficult to find out, that is why the study on condensation in pneumatic system strongly depends upon empirical and semi-empirical analysis method based on a large number of experimental data。

In all of the factors, the effects of the structural parameters of system components are very important。 Each of structural parameter effects the condensation in different trend and degree, which brings a lot of difficulties to analyze data when the condensation in pneumatic system was studied by experiment method。

It is particularly important to adopt the method of theoretical analysis or dimensional analysis and choice the comprehensive parameter with certain physical interpret to describe the effect of every structural parameters of component on condensation phenomenon in pneumatic system when we studied the condensation in pneumatic system。

4。1 Bringing up of comprehensive parameters based relative humidity of compressed air

The different single effect factor on condensation is reflected by the effect on status parameters and movement parameters of compressed air。 The charging-discharging air system is shown in Fig。 3。

Fig。 3。    Filling and exhausting processes

The following assumptions were made: (1) The solid boundary of system is set as controller; (2) The boundary of the system is superimposed with that of controller at the beginning/initial moment。

From   Clapeyron’s   equations,   Dalton’s   law,   and the

JIN Yingzi, et al: Comprehensive Parameter for Analyzing Condensation in Pneumatic System

definition formula of humidity, the humidity of compressed air  in  any  position  of   system   can   be   derived  from Eq。 (1)[10]:

Namely, at the end moment of cylinder discharging, the humidity of compressed air reaches to a maximum。 The condensation  in  pneumatic   system  is  caused  by    water

droplet produced when the humidity of compressed air    in

system reaches to hundred percent during the operation process of cylinder。 In the following study, the maximum humidity at the inlet of container is analyzed。 Set x = l,

where (x, t ) is the relative humidity, x0  is the initial absolute

humidity of compressed air, p is the pressure, p∞(x, t) is the saturated  vapor  pressure  in  plane  liquid  surface  under a