How Do I Calculate Internal Heat Transfer Rates in a Plate Chiller System?

[johnp909]

Hi There.

I'm currently working on a design project for school. Basically its a plate chiller which is lowered into a pot of hot liquid. The plates have channels in them thru which water will flow, the result being cooling of the hot liquid in the pot.

I've done a lot of research and reading in my fluid science book and I can't figure out the direction to go in my calcuations.

Here's what I know so far...

I have the specifics for the hot liquid. Thermal properties, mass volume, intended starting temperature and final temperature for the liquid etc. As well as a goal for the time that this process will take.

I know the total heat to be removed. Calculated that.

I know the heat transfer rate on the outside of the plates by convection. I calculated that using lumped system analysis and calculating coefficient of convection for a vertical plates. etc.

The thing that's got me crossed up is figuring out how to calculate the heat transfer rate inside the plate. I know the dimensions of the channels. At least as a starting point.

I understand that...

Heat Transfer Rate = mass flow rate x Cp x Difference in Temperature

and

Heat Transfer Rate = convection coefficient x cooling area x Difference in Temperature

The Thing that confuses me is that in this system the temperature of the hot liquid is changing over time as it's cooled this will slow the heat transfer rate. In addition the cooling fluid is raising temperature as it flows thru, therefore removing less heat at the end then at the begining.

It seems like LMTD only deals with constant inlet and outlet temperature situations. This seems different to me.

Thanks in advance of any advice or guidance. John.

 

[Chestermiller]

If the fluid residence time is short in the heat exchange is short, you can still assume quasi steady state and employ the LMTD.