Calculating Fluid Flow through a Simple Heat Exchanger: Step-by-Step Guide

[lucy_james]

Hi guys this is my first ever homework post on here so be nice please :D.
1. I have attached an attempt on paint to give you a very rough idea of the very simple heat exchanger. The pipes are cooled by the air and it is a good few metres above the ground. For simplicities sakes let's say, for example, both vertical drops by the pipes are 5m in length and that it is 8m across at both sides of the heat exchanger. The heat exchanger in the middle is just a bunch of simple hairpins as the pipe travels downwards (there is no shell of any kind).


2. I've been given the flow rate and the diameter.

I've obtained area, velocity, Re number, density and viscosity quite simply.

I can choose any material for the pipe but I have to calculate the friction coefficient myself.

I've also got to perform calculations on pressure losses and temperature changes after the heat transfer.

Where the l on my crappy paint image is equal to the valve I've also got to calculate the flow conditions before the water enters the heat exchanger if the valves are both fully and half opened.

I know the pipes length and the reservoir diameters as well as how full up they are before the valve is opened. We're not assuming any losses due to pipe corners, just losses due to friction.

3. I really don't know where to go to continue. I'm sure there's something so obvious that I'm missing but I really don't know what. I feel like I haven't been given enough information.

I'm not looking for anyone to give me the answers, I just want somebody to give me an idea of the steps to take now to move towards the answers.

Can anybody help?

Please let me know if you don't understand my explanation. I'll be on here first thing tomorrow morning to clarify things further.

Thanks in advance for any help you can offer me.

Lucy

Homework Statement

Homework Equations

The Attempt at a Solution

Homework Statement

Homework Equations

The Attempt at a Solution

 

[lucy_james]

Forgot the attachment!

 

[Chestermiller]

You say the pipes are cooled by air. Do you have any information of the air flow velocity normal to the tube bank? Of course, you need to get the heat transfer coefficient on the air side. To do that, you need to use the correlation for flow over a single row of tubes. This should be in the Chemical Enginners Handbook, or in McAdams. The air side resistance is likely to dominate over the tube side resistance. Information on the shell side heat transfer in shell and tube heat exchanges should also give you some ideas.

 

[lucy_james]

Hi Chestermiller

Thanks for your response.

It's the friction coefficient and losses that I'm most in need of at the moment.

The air flow is at 20m/s. Do I need to know information on the heat before I can work out the friction coefficient and pressure losses? The latter two are the most important priority at this point.

Thanks

 

[Chestermiller]

I assume the air is cooling the liquid. If so, what happens to the viscosity of a liquid when it gets colder? How does this affect the pressure variations?

Do you know how to solve the fluid mechanics part of the problem if there were no heat transfer, so that the liquid were at a constant temperature? If not, see Chapter 7 of Transport Phenomena by Bird, Stewart, and Lightfoot. Why would you be interested in doing this? Because, if you can't solve that problem, you certainly won't be able to solve the problem with heat transfer. Also, solving the problem without heat transfer will give you a bound on the solution with heat transfer. Finally, it will give you some experience at handling a problem like this.

Transport Phenomena also gives correlations for heat transfer on the air side and on the fluid side. You need to improve your fundamentals background.