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Mechanical water heater

Mechanical water heater

Mechanical water heater

Can I heat water mechanically using something like a torque converter?

I have a diesel motor powering a hydraulic pump for various work. I also want a supply of hot water. I will take what heat I can from the diesel motor's water jacket and exhaust, but if my water still isn't quite to desired temperature I'd like to provide the last energy maybe by turning a torque converter fixed at other end, then collecting heat from oil to water heat exchange.

Is there any merit in this idea?  

RE: Mechanical water heater

Certainly it CAN be done. Mechanical work in shows up as thermal energy out.

But one wouldn't think this is the easiest way to get this job done. If the engine has an alternator, an electric resistance heater will do the same thing but without adding as much complication to the system.

RE: Mechanical water heater

The water I'm heating is a constant flow to go through a diver's wetsuit. A situation where the water wouldn't be heated enough by the jacket and exhaust of the diesel engine would arise where engine wasn't working hard. I'm guessing I might want 10kW of heat going into a stream of 10 l/m.

I'd like to idle the engine when hydraulic demand is low and there is little work to do.

So a problem with using an alternator is when I have big demands is when the engine is revving slowest.

Back to taking heat from existing hydraulic power pack:
The torque converter might be totally unnecessary. Can I simply have a choke in my main hydraulic circuit with a heat exchange soon after it? A thermostat control of the choking device would govern my water temp.

Will the oil in the system suffer at this choke? Maybe need replacing much more often than normal because I'm generating 10kW by squeezing it?


RE: Mechanical water heater

Absolutely, a restrictor in the hydraulic system will turn mechanical work into heat.

RE: Mechanical water heater

Thanks Brian,
But I wondering why web searches of "mechanical water heater" and "hydraulic water heater" etc aren't showing me where others have done this.

Maybe they do it but don't bother publish! Or maybe people seldom have plenty of hydraulic power while little or no electric power (at same time as need for hot water.)

Or maybe I'm unaware of problems with creating heat in the hydraulic oil. Am wondering if the oil loses viscosity or something - will the system need (normally) excessive oil changes?

I've found a hydraulics forum... I'll see what they say.

RE: Mechanical water heater

You're not finding much in the way of searches because under normal circumstances, it's just not a good way to do this job. You are expending a whole lot of effort to make very useful mechanical power and then deliberately throwing it away!

If the waste heat from the engine is not sufficient for what you are doing, it's probably more efficient to get a dedicated gas-fired heater ...

RE: Mechanical water heater

Water brake engine dynos certainly do this, however it is a dissadvantage as the water must be cooled to keep constant temperature for consistent readings of power absorbed.

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RE: Mechanical water heater

I like the diesel fired water heater, but doubt I need it. I'm stingy on weight and capital and maintenance.

The thing is, I need a hydraulic powerpack regardless of water heating requirements. It will have a dedicated marine diesel of about 40kW. I'll get a good start on my heating requirements by using waste from the engine.

Then I'm after a neat solution, easily controlled by a thermostat, to "top up" the water heat requirement.

This might be as simple as having a coil of water pipe in the hydraulic oil tank that will extract heat from the oil, then deliberately forcing the heat of the oil to temperature required - about 43C I think.

I can't really see energy waste in this concept and I'm buying little equipment that wasn't needed anyway. But I don't know if the hydraulic system will appreciate the temperature and friction I would induce. Say I wanted 10kW of heating... it just sounds like a lot of energy shifting from mechanics to liquid (is that mechanical to latent??) in a small place... can I do that or will I wreck something?

Thanks for the tip on the water brake dyno.

I haven't heard anything from a hydraulics guru yet.

RE: Mechanical water heater

No matter how you generate your required 10kW the engine will not be at idle - it will be working fairly hard. Whether you use a direct water brake method, hydraulic choking method or generator method, to make the needed 10kW the engine will be working at about the same elevated rate. My preference would be the electric generator - the power output could be more easily controlled and you would have a handy standby generator of electrical power for other purposes. And the generator and controls would be off-the-shelf items - not experimental like the other methods.  

RE: Mechanical water heater

Those are fair comments thankyou yvesLLwelyn. The engine won't give me 10kW at idle. I was wrong to suggest alternator inappropriate because engine would be idling when it was used.

But then I must consider also, as soon as whatever "top up" heat generator kicks in because of lack of load on motor, then load increases and less "top up" is needed.

I don't think I can easily provide pto for a large alternator on this power pack. There are manufacturer's limits on pto from the front of the engine, and the bell housinging end is hogged by pumps. An alternator might also be an undesirably heavy option if I can instead use hydraulics I mostly already have.

So.... I'm keen (as you may have noticed winky smile) to get all heat requirements from engine's water jacket and exhaust, then from the oil of the hydraulic system. Seems no one has told me not to try this because it won't work, rather, consider all other methods first.

I calculated that I need about 22kW to  take 10 l/m of sea water from 10C to 43C. It would be interesting to know efficiency of engine at different revs and then my potential efficency at harnessing the inefficiency. That figure of 10 kW "top up" above was a guess.

Thanks for the input, people.

RE: Mechanical water heater

I would just run the engine at high idle to keep the coolant temp up.  The faster you run the engine the hotter the water coming out of the engine at no load.

RE: Mechanical water heater

I chartered a sailboat with twin 75 hp diesels.  Each one had a small hot water tank. The coolant from the engine passed through a coil in the water tank to heat it.

I didn't get a close enough look to notice whether sea water cooled the engine directly in a once through loop or if sea water cooled an engine coolant loop which then cooled the engine/heated the hot water tank.

Either way the water coming out of the tank was hot enough to scald.

I would think that the diesel on your boat generates enough waste heat that you most of the way to supply 10 l/m of hot water to your diver.  Not sure what the EGT on an idling diesel is but since you want to get creative, you could have a small tank, pass your coolant through a loop inside the tank, pass your exhaust gasses through a second loop, and provide a electric resistance heater as a backup in the event you can't get the desired heat input.

Depending on the length of the dives, you might be able to get away with a stored hot water system: The whole time you're steaming away to the dive site, the tank is being heated by waste heat so that when you get to the site you have 150 liters of 60C water.  As you begin to dispense the hot water, incoming 10C water is heated as close to 43C as thermodynamics allow and so it will take awhile for the bulk temperature in the water tank to fall below 43C.  Downside is added weight.

Found this sketch on Wikipedia

Might be right up your alley... good luck!

RE: Mechanical water heater

As for your question about using the hydraulics, technically, it shouldn't be a problem, but I think you will have a hard time implementing it.  The fluid is normally intended to carry heat away when it passes over restrictor plates or valves and such.  The heat increase would come from using a relief valve, but the problem you might have is the time it takes to overcome the thermal inertia of heating up all of your hydraulic fluid to the needed temperature.  Plus, you would probably want to bypass the hydraulic heat exchanger to let the fluid get warm enough for you to extract heat for the diver.  And that could be something which causes bigger problems than the original issue.  

Hydraulic systems try to keep fluid cool.  It seems that this might end up being a balancing act which keeps you dancing on the razor's edge between keeping it warm enough to heat the water, but also trying to keep the hydraulic fluid from overheating.  

Interesting problem, though.   

Engineering is not the science behind building.  It is the science behind not building.   

RE: Mechanical water heater

Thanks Etex, you've brought home an obvious point to me:

If I want water at 43C then we know the oil must be at least this temperature (actually it will be more - some thermal gradient is needed) when it leaves my heat exchange.

So I must consider how well my hydraulic circuit will behave at this sort of temperature. The resovoir, pump, ducting, etc will all need be kept insulated at a bit better than 43C lest I WASTE energy by letting it cool other than into my water.

When the 43C oil passes my point of restriction it will get hotter and should go next to the heat exchange.

Somebody mentioned water brakes above. And I started this thread thinking about a torque converter. These would be ways of generating heat after putting power through a hydraulic motor;  the larger hydraulic circuit of the poerpack would remain unheated.

But so much more gear! Just heating the main circuit could be so simple if the circuit didn't mind. I'm guessing it would need to operate at about 48C. I don't know enough about hydraulics yet but I'm going to drive a mechanic mad with this stuff real soon.

RE: Mechanical water heater


You may drive a mechanic mad very soon, but eventually, implementation of your planned arrangements will likely give you much more distress.  I wish you well, but I very much doubt the practicality of your ideas.

Valuable advice from a professor many years ago:  First, design for graceful failure.  Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs.  Only then can practicality and economics be properly considered.

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