I don't no exactly what you do mean by dynamic condition. If you mean that the mass flow rate or temperatures are changing with time, then you need to make a transfer function for your system and find the time response, whatever it would be, say heat transfer rate, as a function of time. Steady state condition means that all variables are constants. This is in contrast with the dynamic mode.
I don't know exactly what you after mate, however, use the energy balance to find the amount of heat transfer from cold fluid to hot fluid by assuming that there is no heat losing, and the formula as below:
Cpcold * mass flowrate cold * temperature cold= Cphot * mass flowrate hot* temperature hot