# Double Pipe Heat Exchanger Flow Hot Cold

Based on sources I referred, I got to know that a double pipe heat exchanger operates in a way where hot water flows in a pipe and comes out as cold water and cold water flows into another pipe and comes out as hot water. I dont understand the logic of flowing hot water to remove heat. for example if have a hot metal plate and I would like to take the heat away. why do I have to flow hot water in one of the pipes? Because in most examples, that is how a double pipe heat exchanger is shown to be:

One pipe: hot water in, cold water out Another pipe: Cold water in, hot water out

How will flowing hot water in one of the pipes help?

I think you're getting confused about what a heat exchanger does.

A heat exchanger isn't (normally) a cooling device like a radiator. Instead it's a way of transferring heat from one fluid to another. There is a good description of how a heat exchanger works on Wikipedia.

For example, in a desalination plant you heat seawater to boiling and condense the vapour to produce hot fresh water. The hot fresh water is passed through a heat exchange to heat the incoming sea water, which reduces the amount of (expensive) energy needed to heat the sea water.

Heat exchangers obviously have a cooling effect, because they pump heat around so they inevitably cool the object the heat is coming from. However their main aim is to transfer heat from one fluid to another. That's why a heat exchanger has two fluid circuits.

• Thank you very much for explaining. So you mean that a heat exchanger is practically used only to transfer heat among fluids where we dont want them to have contact with each other? In my application, will a single pipe be enough then?
– Vin
Apr 28, 2015 at 9:01
• @Vin: yes, if you're just trying to cool a metal plate then use a single pipe. Well, you might have the pipe branch into lots of smaller pipes to cover the surface of the plate. But it's still just a single water flow system. Apr 28, 2015 at 9:04
• Ok will do. However, do you know any references online on how I could design this pipe, for example how big the diameter and length should be. Because if it were a double pipe heat exchanger, I would be able to use the log mean temperature difference formula n so on for the determination of diameter and length. But I am not really sure how the design for a single pipe should be.
– Vin
Apr 28, 2015 at 9:17
• one more question. Wouldn't using a double pipe heat exchanger be more effective since heat will be transferred from metal plate to the outer pipe and then the inner pipe. so the surface area to transfer the heat out is higher compared to using one pipe?
– Vin
Apr 28, 2015 at 13:03

The goal isn't to cool the heat exchanger itself. If you wanted to do that, you'd obviously just want to flow cold water into it (and you could do that through a single pipe circuit, no need for a second circuit).

The purpose of the heat exchanger is to allow heat exchange without cross contamination of the carrier fluids. Usually this is for safety purposes and/or to achieve modularity (e.g., if different types of fluids are necessary on each side of the heat exchanger). For example, if you are heating a swimming pool, you don't want the water humans are swimming in to pass through the heater directly. If the heater is electric, there is the possibility of the water in the swimming pool becoming live if electrical insulation in the heater fails. Also, the heater might not be capable of handling the chlorine content of the water.

More obviously, heat exchangers that prevent cross contamination are useful in nuclear power plants. The water that picks up heat from the fuel rods is carefully kept separate from the water that runs through the steam turbines. A heat exchanger is used to allow heat transfer through these separate circuits of water.

• So you mean that a heat exchanger is practically used only to transfer heat among fluids where we dont want them to have contact with each other? In my application, will a single pipe be enough then?
– Vin
Apr 28, 2015 at 8:58
• If you can't see any disadvantages to running the same fluid through the both the heat source and the heat sink in your application, then obviously the heat exchanger can be eliminated, and a single circuit used instead, to save costs. Apr 28, 2015 at 9:04
• @I might be using only cold water as cooling medium, so a single circuit should be enough then. However, do you know any references online on how I could design this pipe, for example how big the diameter and length should be. Because if it were a double pipe heat exchanger, I would be able to use the log mean temperature difference formula n so on for the determination of diameter and length.
– Vin
Apr 28, 2015 at 9:15
• @Vin If the heat sink is the limiting part of your process, and you aren't making your own radiator, you should consult the data sheet for the radiator. It should allow you to figure out the flow rate you need for a given hot site temp, ambient temp and wattage needing to be dissipated. Then it's down to selecting an appropriate pump, taking to account pressure drops from friction through the piping system. E.g., pipeflowcalculations.com/pressuredrop In practice, I'd just oversize the pump 2x rather than do the calculations. Apr 28, 2015 at 9:39
• besides calculators, are there any particular list of formulas which I can use to construct my pipe with the calculated dimensions? oversize the pump 2x means 2x bigger compared to the ..?
– Vin
Apr 28, 2015 at 12:24