This article provide the information about the fan trays are they hot-swappable.

The fan trays are hot-swappable field-replaceable units (FRUs). The fans are not field-replaceable.

The statement means that the fan tray (the entire module or assembly that holds the fans) is a field-replaceable unit (FRU), meaning it can be replaced easily in the field. However, the individual fans within the fan tray are not field-replaceable, so if one of the fans in the tray fails, you would need to replace the entire fan tray, not just the fan itself.

Refer this document for more details.SRX4200 Cooling System