# Need help with heat exchanger requirements [closed]

Disclaimer: Not an engineer

I need some help determining the required performance from a heat exchanger for a pseudo-instant hot water system on a boat. Using a heat exchanger, and a reserve of hot glycol, I want to heat domestic water. I have tried to find the answers and figure it out myself, but am not getting anywhere.

I have a diesel furnace that maintains a reserve of glycol at $$80^\circ\ C$$. I am happy to leave the furnace and hot glycol reserve out of the equation by stating that I simply have an unlimited supply of glycol at $$80^\circ\ C$$. I am also happy to state that the heat exchanger suffers no heat losses other than the desired heating of the water.

So here is the calculation I need help with:

If I want to raise the temperature of domestic water by $$20^\circ\ C$$ at a rate of $$8$$ litres per minute, how do I determine the required wattage of the heat exchanger in relation to the available flow of glycol?

$$Q=cm\Delta T$$
for heat energy $$Q$$, specific heat $$c$$, mass $$m$$, and temperature difference $$\Delta T$$. Differentiating with respect to time, we get the expression for power $$dQ/dt$$ as a function of mass flow rate $$dm/dt$$:
$$\frac{dQ}{dt}=c\frac{dm}{dt}\Delta T$$
From here, we need the density of water $$\rho$$. Substituting $$m=\rho V$$, we get the power as a function of the volume flow rate $$dV/dt$$:
$$\frac{dQ}{dt}=c\rho\frac{dV}{dt}\Delta T$$