the afffect of the diffrent flow rate in heat exchanger?
In general with higher flow rates the overall heat transfer coefficient will increase a bit and with lower flow rates it will decrease. This is because the change in velocity will affect the film coefficient portion of the overall heat transfer coefficient. So with a higher flow rate you will get slightly better heat transfer but you will also have a greater mass flowing through the exchanger. The net result will be the transfer of more heat but the actual temperature of the fluid exiting the exchanger will likely be lower because of the extra mass involved. Other things that are affected by changes in flow rate are the pressure through the exchanger (Higher flow rates leads to higher pressure drops. Lower flow rates lead to lower pressure drops.) and in some cases possible fouling of the exchanger. This is specially true in case were there are solids in the liquid.
the higher the mass flow rate through HE lower would be the rate of heat exchange as the working fluids come in contact(direct or indirect as the case may be) for a shorter period of time and thus the change in temperature of the fluids would be little.
