Camping shower heat exchanger 1

[GrahamB]

A question from a very rusty physics student... pls forgive my lack of knowledge.

I'm trying to make a simple heat exchanger to heat water for a camping shower. The heat source will be the car's cooling system. My basic design concept is a coil of copper tube enclosed in a vessel. The vessel will have water circulated from the car heater circuit, inlet temperature approx 100oC. The copper tube will be 7mm OD and 5 mm ID and the flow rate of the water to be heated will be 4 - 5 litres per minute. I don't know the flow rate of the water in the car heater circuit but I do know it is "enough" because commercial versions of my heat exchanger can be purchased.

My question: what length of copper tube will I need to achieve a temperature rise of say 40oC (from 10oC heated to 50oC)

Any assistance would be appreciated... precision is not vital, I just don't want to build it with say 3 metres of tube and then find I really need 5 metres!

Thanks in advance,
GrahamB

 

[Goorah]

This is a relatively simple heat tranfer problem; however, being "rusty" myself in heat exchanger design, I could not answer your question specifically without opening up my "dusty" heat exchanger design books.

There is however another way to go about this since you are in the "copycat" mode: as long as you say commercial versions are available, do a "competitive review" and either go to the mfct's website if they have one and so some research; or go to a dealer and look at one. How big is the HX? Only so much tubing of a given diameter could fit into a given size shell. What size is the shower circulating pump? This will give you an idea of the P-drop, What is the apparent flow between the shower water and the engine cooling water: parallell, counter, X-flow? Much can be learned by just looking at it.

It seems very fuel inefficient to me to run the engine just to take a shower!! ... unless of course the camper is under way on the road .... but then one might be bumping down the road and might slip on the soap!!

I have a camper which has a 4 gallon propane-fire water heater on board which works quite well for showers and dishwashing. It is made by the company "Dometic".

 

[prex]

I think you should review your requirements.
Supposing you want to use a single tubing, your requirement on mass flow corresponds to a quite high speed (more than 4 m/s) and the heat exchange requirement is of the order of 14 kW. I wonder wether car's motor can supply this, at least when running idle.
I estimate at least 12 meter length for your tube, so you see my point (however the length will greatly depend on the flow arrangement of car cooling water in your vessel and could be much more than that).
I agree with Goorah: try to refine your approach by inspecting an existing similar equipment.
prex
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