Estimating passive ventilation performance

Passive ventilation performance can be estimated using simple calculations. The most commonly adopted approach is described and examples are given for open voids and air flow through a void former. 

The fresh air flow, Q, required to dilute the gas emissions, q, in the void to a defined level can be estimated using the following equation from CIRIA Report 149. Fresh air flow required (total under whole building), Q is given by  

Where q = surface emission rate of gas from the ground (total under whole building) (see Chapter 4) and = equilibrium gas concentration in the void (in this case expressed as the % value not the mathematical value i.e. for 1% use 1 in the equation rather than 0.01) (see Box 8.4). 

Required design concentration is 1% v/v of methane in the void and at outlets 

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Towards this objective, a theoretical model has been developed and based on this model, the response of the system has been estimated using the indigenously developed computer code CAPCIS (Code for Analysis of Passive Containment Isolation System). The numerical analysis performed takes into account the pressure transient because of the postulated initiating event (PIE) and the solution includes determination of levels in the two compartments of the isolation tank and the cumulative volumetric spill into the ventilation duct at any point of time during the transient. A case of a 200% break in inlet header is considered for the analysis and the pressure transient for the considered PIE is numerically simulated as shown in Fig. 10. Figure 11 shows the volumetric spillover during the entire transient. 

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