We have enclosed a revised quotation for your approval. (herewith sounds so British!!)
Briefly, in the scope of work in Phase I, shown in the revised quotation, four existing diameter 200mm fans will be replaced with new fans with a 600mm diameter. Technically, the improved works shall achieve 71 % of the actual total flow rate demand (600l/s) of the ABC room accordingly. (I don't understand this sentence. Do you mean to say the 600mm fans will acheive 71% of the total flow rate demanded by the ABC room?)
In order to prevent any interruption of normal services, the addition of the extra air conditioner in Phase II is highly recommended and should be implemented sooner, rather than later.