The fluid flowing through the tubes will be decided  majorly on ease of operation , frictional effects and purpose whether it is  to be cooled or heated up.For example in Gas +liquid heat ex changer ( steam condenser) it is difficult to push steam through the tubes and hence liquid is made as tube side fluid.In liquid +Liquid type the one which is having low viscosity is generally be the tube fluid.And finally hot fluid is chosen as shell side fluid for effective heat transfer and of course it vary from one application to other.

It is relevant and depends on multiple factors.

If one of the liquid is corrosive it will depend on what type of steel/material is used for tubes or shell and their relative cost. Generally on shell side you have low pressure drop. So depending on the loop with low pressure drop you can select the fluid. High pressure liquids are generally kept on the tube side as tubes have high pressure rating compared to same thickness of shell.

From only heat transfer point of view you should look at which fluid has lower deltaT or specifically Cp_min*deltaT with the ambient temperature. The fluid with low value should be on shell side/outer tube side.

It is also important to have the hot fluid on the tube side in order to minimize the heat losses to the environment, which are of course dependent on the temperature gradient. Therefore, the lower the temperature close to the heat exchanger enclosure, the lower the losses (or the lower the required thickness of the insulating material).

The fluid flowing through the tubes will be decided  majorly on ease of operation , frictional effects and purpose whether it is  to be cooled or heated up.For example in Gas +liquid heat ex changer ( steam condenser) it is difficult to push steam through the tubes and hence liquid is made as tube side fluid.In liquid +Liquid type the one which is having low viscosity is generally be the tube fluid.And finally hot fluid is chosen as shell side fluid for effective heat transfer and of course it vary from one application to other.

 I agree too with Konijeti Ramakrishna.

You need to know the kind of process you will do on the heat exchanger. It´s Different a double pipe heat exchanger for a Tube and Shell, for a radiant heat exchanger. You need to see where is the direction of the heat flux on the different case. 

best regard.

it's convenient to flow the fluid with higher pressure through the tubes for safety/mechanical reasons. The selection of which fluid is going to flow through the shell/tubes should be take into account mechanical factors, not only thermal factors.

it is also very important the viscosity (the pressure drop through the heat exchanger is always higher in the tubes than in the shell). That's why in oil coolers, for example, the oil flows usually through the shell. But, again, the pressure of the oil should be lower than the cooling fluid.

Temperature of the fluid isn't usually taken into account: in condensers (power plants, refrigeration units,...), the hot fluid (steam, refrigerant,...) flows through the shell; in evaporators, in chiller units, the hot fluid (the water) flows through the shell, too.

I recommend you the design codes published by TEMA or ASME.

T.Steven (2008) found that there is a report of optimum pressure drops: (∆P)_c/(∆P)_h ⋍1.73 to maximize the performance of the heat exchanger by making its design, such as pressure drop cold side are approximately 1.7 times larger than the pressure drops of the hot side. In absolute terms, it is possible to lose more cold side pressure, therefore, choose to pass the cold flow in the tubes.

Tine Stevens. (2008). Design Methodologies for Efficient Heat Transfer in Micro Systems.

A corrosive fluid is better to flow in the inner tube. Also a high pressure fluid should be on the inner tube side.

Yes I agree with Taherian that the corrosive fluid should flow in the inner tube just like what we have in HVAC water cooled chillers where the water flows in the inner tubes. This done for easy maintenance because after a while some scaling may occur in the inside surface of these tubes which needs scrubbing and cleaning to remove these deposits.

Thank you everyone for your response. So, the decision about which fluid should flow through inner or outer tubes is mainly on the basis of Thermal and mechanical design.

Thank you Prof. Ramakrishna.Your reply was the most useful one.

Thank you Omar for recommending a book!!

Another reason: fluid having high heat transfer coefficient  is normally flow through inner tube and fluid (normally gas) having low heat transfer coefficient flows through shell, because less area available in inner side compared to outer and hence heat transfer can be balance by higher heat transfer area at outer side, and therefore fins are provided on gas side (outer side). 

Hello Jayanti

I am sorry ;i dont agree with your explanation when you say that "fluid having high "heat transfer coefficient". The heat transfer coefficient is not a property of the fluid.It depends on flow parameters and other thermophysical properties. Therefore,we dont know the value of heat transfer coefficient in advance.

Or may be you mean that we should "predict" the heat transfer coefficients of the two flow scenarios by using empirical correlations and then decide on the basis of heat transfer coefficient values. If yes, then your approach is correct.

Thank you.

You are correct Vipin in your first answer as you mentioned that there is no fluid having high heat transfer coefficient  because for fluids the heat transfer coefficient depends on flow properties like velocity,pressure,temperature....etc and on fluid properties like viscosity,density, volatility.....etc and so this will involve a complex approach in the  decision making.

Thank you Prof. Kamal

Hi, it is a typical optimization case. Designer has to considers properties of fluids and their application. e.g. Corrosion, viscosity, phase change, heat loss, fouling, pressure and temp., mechanical construction, ease of maintenance,  capital and running cost, Probability of mixing, leak and hazards. 

All these shall govern fluid allocation of sides and thermal & mechanical design.

Sometimes project specific requirements also needs to be considered. 

I agree with answer given by Amey Majgaonkar.

I agree with the answer of Prof. Konijeti Ramakrishna 

Obviously, this is an optimization case and must be considered for every heat exchanger based on it's conditions.

But, there is some general rules to select the fluid which must be flow in the tubeside. For example, in Shell and Tube heat exchangers, it is better to choose whichever fluid appears higher in the following list to pass inside tubes:

1- Cooling water,

2- Corrosive fluid,

3- A fluid which is fouling,

4- The less viscous of two fluids,

5- The fluid under higher pressure,

6- The hotter fluid,

7- The fluid of less volume.

And, as I know there is no difference in the case of a double tube heat changer, someone can follow above rule. If the shell-side heat transfer coefficient is small compared to the tube side, finned tubes should be considered.