I'm not an engineer and none of my education applies to thermodynamics, so if someone knows better than me, please correct me! That said:
I've been corresponding with this exact seller actually, and he's very helpful with advice regarding his heat exchangers.
I'm really interested if SMALLER plate chillers could be used. This seller's smallest plate chiller is only $44.00, but it's got a much smaller surface area than either the Shirron or the Therminator.
People seem happy with the Therminator and the Shirron, depending on who you ask, and depending on their tap water temperature.
I multiplied the length and width of the Therminator and the Shirron chiller by the number of plates and came up with the following estimates of their surface area:
Therminator: 675 square inches? (if it's 30 plates)
Shirron: @ 347 square inches? (if it's 10 plates)
The seller discussed above sells chillers that are 7.5 inches long, like the Therminator, but with fewer plates than the therminator and similar surface areas to the Shirron chiller.
For example, he sells a 7.5 inch chiller with 15 plates that has about 315 square inches of surface area, less than the Shirron but not by much, and the 20 plate version has 435 square inches; less than Therminator but more than Shirron. These are probably ok, but then you can go even cheaper with his products. He has a 7.5 inch chiller with only 10 plates and which costs only $44.00 plus shipping. This one has @ 217.5 square inches of surface area.
The fact that some people like the Shirron, and some like the Therminator without much complaint about the different capacities of both makes me think we probably haven't seen the smallest effective plate chiller for a batch of beer...
That said, I'm too chicken to dump $44 on a plate chiller that may be a waste of time, but if it did work it seems like it would be a great deal.
Is there anyone out there with some thermodynamics education who could help us out? Are we spending too much for excessively large heat exchangers?