Tension in this act, I think, depends mainly on the fact that the audience now know there is more to come out, and the speed at which Priestley allows it to actually come out - the audience is now waiting for the inevitable revelations.
The Inspector has told us that all of them might be collectively responsible for Eva Smith's death. We're waiting to find out how.
There is another tension. On one side there is Sheila ("he knows... HE KNOWS" she tells us) who knows that the Inspector is about to bring the whole thing crashing down on everyone, and is genuinely scared and worried (just like the audience are). On the other are Mr and Mrs Birling, both of whom try to intimidate the Inspector and control the situation, terrifying Sheila.
You see this most clearly when Mrs Birling's snooty remarks are quickly twisted into her interrogation, and the eventual revelation that she had something to do with the girl. Yet even this doesn't knock her confidence, and she keeps up her arrogance. We know she's going to bring on something worse - but she doesn't. And, when her remarks eventually point the suspicion of the play - not at her, but at her son - you feel the release of the dramatic tension as the trap of the play snaps shut on him.