Prediction of Condensation
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5 Prediction of Condensation

The possibility of having condensation within a composite wall is predicted by plotting the variation of temperatures and vapour pressures across the wall, as shown in Figure 2. Figure 2 shows three curves:

(a) Curve for structural temperature (¾ ¾ )

The curve shows the variation of dry bulb temperature throughout the structure.

(b) Curve for saturation vapour pressures (------)

The curve indicates the saturated vapour pressure corresponding to the structural temperatures at different locations.

(c) Curve for diffused vapour pressures (--.--.--)

The curve shows the variation of diffused vapour pressure throughout the structure.

When the curve for saturation vapour pressures falls below the curve for diffused vapour pressures at section X-X (Figure 2) where the structural temperature is below the dewpoint, condensation occurs at this section X-X.

The three curves in Figure 8 are useful in:

(a) finding the relative humidity values across a composite wall, and

(b) examining the suitability of various wall elements from condensation point of view.

 

5.1 Procedures to Predict Condensation

The procedures to plot the vapour pressure and temperature gradients are as follows:

(a) Calculate both the thermal and vapour resistances of the individual layers.

(b) Find D T and D Pw across each layer from the individual resistance using D T/Rt and D Pw/Rvt.

(c) Establish T and Pw at each layer boundary.

(d) Calculate the humidity ratio from Pw and Pat.

(e) Read corresponding dew-point temperature from psychrometric chart.

(f) Plot the dewpoint and structural temperature curves as shown in Figure 9.

(g) Check whether any condensation occurs or the dewpoint curve is kept above the dry bulb temperature curve.