Sir, As per the Go p 62/2017 dtd 30. 06. 2017 the fin plate heat exchanger use in the manufacture of ship or vessels by the yard the same would be eligible to purchase @ 5%
As per your query hsn code 84195020, the gst rate is 18%.
Solar water heater and system 5% Gst.
Machinery, plant or laboratory equipment, whether or not electrically heated (excluding furnaces, ovens and other equipment of heading 8514), for the treatment of materials by a process involving a change of temperature such as heating, cooking, roasting- 18% Gst.
Anyway you can confirm the gst rate by checking your purchase bill with the above hsn code.