How to implement PID control in simulink for when input and output have different units of measurement?

Hi,

I am trying to model a double pipe heat exchanger system where I would need to control the exiting hot fluid temperature using PID control where the input to the system is the flow rate of the hot fluid. The problem I am running into is connecting the exiting signal PID controller to the input of the plant, which in this case is a nonlinear s-function. Since the input to the plant is measured in units of m^3/s and the output is measured in units of degree C, how can I relate the exiting signal of the PID controller to the input of the plant so that the system response can be controlled?