I have been given a question to do (given below.) I don't neccersarily want the question working out but a method (equations to apply and in what order) would be very much appreciated!A double pipe heat exchanger is made up from a length of 25mm i.d. steel pipe of 2.5mm thickness, and a length of 75mm i.d. pipe. The fluid in both pipes is water. Estimate the heat transfer coefficients at the outside and inside of the inner pipe for the following conditions.Outer pipe flow-rate 5kg/s @ 23? CInner pipe flow-rate 0.5kg/s @ 51?CUse the correlation for turbulent flowNu = 0.023 Re0.8 Pr0.33 (μ/μw)0.14The physical properties of water are: Cp of water @ 23?C = 4.187kJ/kgKCp of water @ 51?C = 4.184kJ/kgKk of water @ 23?C = 0.670W/mKk of water @ 51?C = 0.635W/mKI have also been given the density and viscosity of water at varying temperatures.
You'll have to reduce the coefficient of your design by increasing the heat transfer area.
Just okorder / and it'll do the computing for you in a more convenient way it includes solution, etc
However good your calculation, a simple thing like hard water can hit your coefficient for a six. In any case , you cannot increase the overall coeff by calculation and any number of iterations. If you ask me, i will suggest that you stick to the lower end of reported values, which will give some insurance. Organic vapours are tricky in that if any high boiling components are present they can form a fouling film quickly dropping the coefficient far below the designed values. You may mail me through my profile if you want a more detailed discussion.
Individual Heat Transfer Coefficient
You determine the transfer coefficient for each part of the boundry between the two fluids. That is made up of the film coefficient on the surface of the pipe, one for the inside and one for the outside. The type of fluid involved and the degree of turblence determines that. You may also have to add a fouling factor depending on the fluid involved. The other coeffient is for the conduction of the heat through the pipe material. This is the most straight forward portion of the calculation but you will find in the real world that it is almost nothing when compared with the film coefficients. See the reference below for more info.
