09:52 Aug 5, 2009 

If it is about a transformer, the incoming line is called primary voltage, primary windings and the outgoing line is secondary. That's why Phong is confused with this.

10:39 Aug 4, 2009 

Without further context, I read this text as referring to a primary tank/reservoir on the right hand and a secondary tank/reservoir on the left hand - that is, there are 2 tanks/reservoirs.

01:42 Aug 3, 2009 

I see no reason to question the logic of the source text - and in particular, no reason to suggest that "the left tank should be the primary instead of the right tank".

Without more context we (I...) can only guess about the system configuration. But I suspect there might not just 2 but 4 containers: a LH and RH 'tank' and a LH and RH 'reservoir'.

In the absence of more context, I would assume the author knows what (s)he is writing about.