Frankly speaking, I am not an engineer. But I know little bit physics.

To lift 250 watts of heat from my system, I want to use a water-air heat exchanger;
i.e. water will take out the heat from metal block (which is hot) and will give out that heat
to ambient (through fan). In this process, water will become hot and then cold. So there will
be one inlet & one outlet in the heat exchanger. Water will thus be circulated in closed loop.
Metal block can be inside a vacuum chamber.

I want to calculate water flow rate for this.
I am using following equation :

q = D*F*c*(T_out - T_in),
where q = heat to be removed
D = density of water
F = water flow rate (liters/minute)
c = specific heat of water
T_out = temp. of water coming out of heat exchanger
T_in = temp. of water going into the heat exchanger

I want to keep the metal block at 30 deg C. Also ambient temperature is 22 deg C.

In above equation, I do not know what value I should put for T_in & T_out.

Also, this equation does not contain surface area of contact between metal block & water.

Does this surface area matter in calculation of flow rate ?

Any help will be highly appreciated.