I've learnt to model plant loop in energyplus by using PlantApplicationGuide documentation but I wasn't able to find any guide to model airloop...do you have any suggestion?

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Though not as thoroughly explained as the PlantApplicationGuide you can try to look at example idf files located in the ExampleFiles folder of your EnergyPlus installation.

If you look there you can find two html files.

ExampleFiles.html which lists what is inside each idf file.

ExampleFiles-ObjectsLink.html which indexed by E+ objects.

It is also a good practice to search first for your question since it might been asked before.