Evaporator Design

Hi,

As part of a university project I am replacing the evaporator in a system that I removed from a fridge, however I am unsure of the parameters that I need to ensure that the new evaporator works with the best efficiency. I was hoping that someone could point me in the right direction with what to start looking at as to what is important, how can I calculate pressure in the current evap to ensure that I get the correct length in my new evap? I know that there is currently a 70g charge of R404 in the system as we have already changed the compressor..

Thanks in advance

Tom

First define efficiency in a way that you understand.

If you define it as the energy absorbed by the evaporator over the energy you use and your evaporator is a static one, re-think your definition.

If it occurred to you to use one with forced circulation change career quickly!
Once you have a definition you will probably know what to do.

If you CAN measure the pressure drop of the actual evaporator, designing a new one with less pressure drop is in the right track but it will be very hard to design having to fit the same space. If you do, please keep around because we’ll need you in the future!

Hi Tom
You can measure the pressure of the current evap inlet and outlet with a sensor or gauge for pressure drop, measure temps too. Also measure air on/off coil or space temps. Could you twll us what type of heatexchanger it is/was.
Improvements would be using thinner wall for heat transfer, increase surface area, increase flow of refrigerant and/or secondary refrigerant (air) or improving the geometry of the heat exchanger possibly using the maxima minima equation in a fancy way.
Calcualte the mass flow rate of refrigerant - this is from the fixed speed compressor displacement less losses & system pressure drops. Are you going to change the metering device? if so there could be gains here.
Use an enthalpy chart & psychometric chat to help with calcs. By creating a higher split between evap and space temp heat transfer is faster but it will remove more moisture.
You could add a suction/liquid heat exchanger which allows for a smaller coil size for the same kWs.