BTU Calculation

Have to bypass a heat exchanger for a brine loop ( calcium chloride) for under floor heat for a curling rink. I have pump at 200 GPM and the required outlet temp required is 4.5C on the heat exchanger is there a calculation out there for this to figure out the BTUs required? No name plate on the existing heat exchanger was using waste heat off the discharge on a mycom compressor (ammonia) at around 95C. Looking at using a hot water on demand natural gas heater for the interim or something along those lines any suggestions?