Pressure Drop and Design of a Heat Exchanger

High flow (and hence pressure gradient) through a heat exchanger (either side) will promote turbulence and good thermal contact/transfer. In other words, it improves k-factor. However, greater velocity can cause erosion, as mentioned. Also it increases pumping costs. These factors must be weighed against each other to reach a good compromise.

In addition to the above, high flow can reduce fouling.

Pressure drop is always a factor in determining the compatibility of the exchanger with other characteristics of the attached pipe system.

Heat exchanger choice is a big subject. The high velocities and turbulence produce pressure drop. The pressure drop serves no useful purpose and is especially undesireable for gas heat transfer due to the pumping losses. You do not need high turbulence for heat transfer, microchannel heat exchangers are very compact and have laminar flow but if fouling is an issue then you would not use one.

It comes down to the fluids involved, temperatures, contaminants, serviceability, pressure, energy efficiency (read about factor 4).

Shell and tube heat exchangers are used a lot, but are pretty rudimentary. You can use them for gases but there are better solutions.

Hi Ask112233

One further point:- I would have thought that pressure drop in any heat exchanger would be defeating the purpose of the device (with a liquid such as hydraulic oil).

Pressure drop = work = heat!

If there was no cooling medium flowing to the cooler, the liquid to be cooled would INCREASE in temperature.

Obviously, this is not true for gases!

Pretty good, you just could not leave it can you, like you said

You have as you can see opened a can of worms

On the shell side you have baffles to ensure good cross current flow to improve the 'efficiency' of the heat exchange between shell nad tube side fluids.

I like to add that the baffles have a number of duty's. and example is when using steam, not only does the baffle ensure good cross section across the tubes. Which it also induces impingement to the steam, and what this does is when the steam strikes and object (impingement) it turns to condensate and releases it heat for transfer..... basically.

All this takes energy, and it has to come from some where, and where is the most efficient type of energy to transfer would that come from?.........pressure.

Hence pressure drop.

There is nothing bad about pressure drop, one just has to work with it, and not against it

You can not rewrite the laws of physics. Pressure drop is a product of this process.

High Pressure in the tubes - its cheaper to make small tubes that are sufficiently strong rather than a very large shell

other effects on smaller tubes..... better surface area ratios...trade off, more resistance ie higher pressure drop.

I also like to add, you do need turbulence for heat exchange, not enough, and you have a laminar flow across the surface, that insulates the core in the flow.

Turbulence flow is expressed by a dimensionless number called the Reynolds Number (Re). and the (Re) determinate value for turbulent flow can vary.

p911

Dear Mr.ask112233,

You have referred "And lastly, sometimes the hot fluid is in the tube side, and sometimes the hot fluid is in the shell side? What is the defining factor that decide if the hot fluid should be in the shell or tube side?"

If the gas or liquid contains more suspended particles/matter, for heat transfer purpose - whether it is heating medium or heated medium, it should be passed through inside the tube. If it is on shell side - flow area being larger than the tubular flow area, velocity will be very less and the suspended particles/matter will settle down and cause hindrance and cleaning will be a problem.

Higher Pressure drop will reflect on the Power Demand/Requirement. We have to strike a balance.

You have also referred ".......... shell and tube heat exchangers cannot be used for gas-gas service due to the tendency of high pressure drop across the shell and tube heat exchangers."

It is the designers choice based on the practical ease of cleaning etc., and there is no hard and fast rule on this. Ultimately the most economical eat transfer is the criterion.

A small correction. You have referred "why is it such an important perimeter in heat exchanger designs?

It should be referresd as PARAMETER and not PERIMETER - perimeter refers to the corcumference of a cylinder or a circle.

DHAYANANDHAN.S