I need to understand whether the below sentence is parallel

Sentence:
There are fans but the hall has been quite airy.

I need to understand that are the two parts joined by 'but' completely parallel?

I assume it to be parallel but to my eyesight , they don't have the same structure though.

There are fans (There + helping verb + noun ) (Btw, what part of speech is there? A pronoun?)

The hall has been quite airy (Article +Noun+ has Been(present perfect tense) + adjective).

Please address this issue,
Also throw light on whether simple present/past tense could be parallel to the present/past perfect tense in sentences?

Any insights shall be deeply appreciated!!