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Mike Adams

dual coil calorifier/engine heating

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I shall be fitting some radiators (6 small ones) in the boat and thinking of changing to a dual heating coil calorifier to get hot water from the webasto. One heating from the engine, one from the webasto. Standard stuff using a webasto diesel heater with its own self contained circuit. It would be nice to get some free heat from the engine while moving. Has anyone extracted heat from the calorifer via the calorifer coil to feed the rads while the engine is heating it via the other coil? Otherwise I am thinking of puting another heat exchanger in the water outlet from the engine before it goes to the skin tank. Also what size header tank would be needed for the webasto?

Thanks

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My boat has a twin coil calorifier, and is fitted with an additional pump in the Webasto circuit. This pump tranfers heat from the hot water (heated by the engine when cruising) to the radiators. 

 

However when using this pump the radistors only take the chill off, rather than heat the boat, as they do when the Webasto is operating.

 

A plate heat exchanger between thecengibe circuit and heating circuit would work better in my opinion, and is on my list of things to do.

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Its very difficult to extract useful heat from the engine and can be damaging.

My boat had such a system when we purchased it and I have removed it.

Modern diesels, especially DI's, are very efficient and unless done carefully you will just prevent the engine from reaching its correct temperature. On the (few) days when you are thrashing down the Thames (In Summer?) there will be heat available, but for battery charging or gentle cruising in winter (when you need heat) the engine likely needs all the heat it can generate.

 

Taking heat direct from the engine is hard enough, taking it via the calorifier and two loops is a non starter.

 

................Dave

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Does anyone have any stats about how much power a say 30hp diesel needs to dissipate via its cooling system when thrashing up The Thames at full chat? And in comparison, how much it needs to dissipate when pottering along at 2mph in still water?

 

The (small) surface area required on the skin tank seems to illustrate the figures are smaller than a boat's-worth of radiators would radiate.

 

 

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3 minutes ago, Mike the Boilerman said:

 

Does anyone have any stats about how much power a say 30hp diesel needs to dissipate via its cooling system when thrashing up The Thames at full chat? And in comparison, how much it needs to dissipate when pottering along at 2mph in still water?

 

The (small) surface area required on the skin tank seems to illustrate the figures are smaller than a boat's-worth of radiators would radiate.

 

 

 

 

30 HP = 30 x 746 watts = 22380 watts but that is usable power. A diesel may be around 35% efficient so that 22380 watts is just 35% of the energy in the fuel. Some of the 65% is lost in friction etc and the majority dumped as waste heat so lets say (blind guess) 50% of the total energy is dumped as heat.

 

Total energy = 22380/35 x 100 = 63943 Watts  so  the waste heat is 63943 /2 = 331791 watts or about 3 kW.

 

However remember a 30hp engine will not be developing 30 BHP  when connected to a prop on a displacement boat, it will be whatever HP is  developed at maximum torque assuming the prop is properly specified. At canal speed the engine will be developing less than 10hp, I have seen figures quoted as low as 2 to 3 HP on a 60ft boat.

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I assume diesels are about 50% efficient so an engine producing 10Hp will also give out 10Hp or about 7Kw of heat energy. The thermostat will keep a modern engine at the correct temperature unless the flow though the calorifier coil is excessive and that can be controlled by an orrifice/valve. But as been said above it is the temperature difference that is important and I don't think having a two stage process ie into the calorifier and back out through the 2nd coil would give enough temperature difference to get the rads hot.

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2 minutes ago, Tony Brooks said:

However remember a 30hp engine will not be developing 30 BHP  when connected to a prop on a displacement boat, it will be whatever HP is  developed at maximum torque assuming the prop is properly specified. At canal speed the engine will be developing less than 10hp, I have seen figures quoted as low as 2 to 3 HP on a 60ft boat.

I suppose if you know how much fuel you are using and its calorific value you can work it out from that. I think 5Hp would be the minumum because you have friction losses, alternator, water pump etc and the engine is not running at its most efficient.

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11 minutes ago, Tony Brooks said:

50% of the total energy is dumped as heat.

 

 

I'd imagine of that 50% dumped as heat, perhaps half goes out the exhaust as hot products of combustion, and half is dissipated by the water cooling system. Frictional losses get translated into heat I'd say, but are small. 

 

So when pootling at 3hp, there are further 3hp being dissipated in heat from exhaust and cooling, perhaps 1.5hp into the coolant alone. A shade over 1kW. Enough to heat one double radiator perhaps. 

 

 

 

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24 minutes ago, Tony Brooks said:

I have seen figures quoted as low as 2 to 3 HP on a 60ft boat.

This seems to be backed up by @peterboat's figures from his electric wide beam. 2 to 3kW, or 3 to 4 HP to push it along in deep water at 3mph. Electric motors being much closer to fully efficient than a diesel will ever be and able to provide a direct measure of the power used. It still needs a cooling fan though, so some of that power is ending up as heat, so maybe 2 to 3 HP to turn the prop. Very little of my Beta 43's supposedly 43HP output from the crank shaft ever gets used to push the boat. The 43HP is for maximum rev's, which it has never reached. Most of the time pootling along at 1500rpm or so. @Mike the Boilerman's figures seem reasonable for a typical narrowboat in terms of power available from the cooling system. Engines don't like being overcooled. People taking out the thermostat to "cure" an overheating problem is a bad idea. The cauliflower cooling circuit is often independent of the engine thermostat, and only the relative sizes of the hoses to restrict the amount of coolant that goes to the calorifier, compared to the skin tank.

 

Jen

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31 minutes ago, Tony Brooks said:

 

 

30 HP = 30 x 746 watts = 22380 watts but that is usable power. A diesel may be around 35% efficient so that 22380 watts is just 35% of the energy in the fuel. Some of the 65% is lost in friction etc and the majority dumped as waste heat so lets say (blind guess) 50% of the total energy is dumped as heat.

 

Total energy = 22380/35 x 100 = 63943 Watts  so  the waste heat is 63943 /2 = 331791 watts or about 3 kW.

 

However remember a 30hp engine will not be developing 30 BHP  when connected to a prop on a displacement boat, it will be whatever HP is  developed at maximum torque assuming the prop is properly specified. At canal speed the engine will be developing less than 10hp, I have seen figures quoted as low as 2 to 3 HP on a 60ft boat.

If we assume 50% is usable power and 50% waste heat, and 30hp is 22kw, then 22kw is usable power and 22kw is waste heat.

 

looking again at your calculation you went a bit wrong.

total energy you calculated as 64kW (63943W) so half would be 32kW.  Hence big skin tanks on rivers.

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I’d say taking heat from the calorifier is bad for two reasons, one being that you won’t get more than a kw or two out of it (because it’s not a particularly large or efficient heat exchanger). And secondly because the calorifier is connected directly to the engine and so there is a risk of over-cooling at low power / idling etc. The calorifier as intended will heat up after 30 mins / an hour or so and once the water is hot, will no longer take any heat from the engine thus there is no risk of prolonged over cooling.

 

So I fitted a plate heat exchanger on the skin tank circuit, ie after the thermostat and thus no risk of overcooling. The rads don’t get as hot as they do with the Mikuni but I think I undersized the heat exchanger. It was rated at 12kw but of course you have to understand what that means - it refers to the heat transferred across a large temperature gradient. In reality you are starting with water not that hot (Beta43 standard thermostat is 72C) and the heat flowing across the exchanger is proportional to the temperature difference so you are never going to get the rads up to 72C. But you will get closer to 72C with a bigger exchanger.

 

As to the detail of the plumbing I used a small pump from Solarproject which so far has been faultless, along with 2 of their low operating pressure one way valves. These mean that when I run the Mikuni it doesn’t back flow through the heat exchanger, and when I run the exchanger pump it doesn’t back flow through the Mikuni.

 

In practice when cruising at narrow canal speeds the rads get reasonably hot and certainly make a big difference to cabin temperature. When traversing a flight of locks (engine mostly at idle) they only get warm. Which just goes to show the risk of overcooling if you took the heat off before the thermostat.

 

I also changed the thermostat for an 80C one which has helped a bit.

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6 minutes ago, Chewbacka said:

If we assume 50% is usable power and 50% waste heat, and 30hp is 22kw, then 22kw is usable power and 22kw is waste heat.

 

looking again at your calculation you went a bit wrong.

total energy you calculated as 64kW (63943W) so half would be 32kW.  Hence big skin tanks on rivers.

0.5 litres per hour into the Mikuni gives its 4kw rated output, not 100% efficient so let’s say 5kw energy input. Around 1.5 litres/hr is typical Beta 43 consumption at canal speeds, so 15kw. Let’s say a diesel is 33% efficient so that leaves 10kw as waste heat. Some to exhaust, most to coolant (exhaust is cooled by transferring heat to coolant in a Beta engine). So enough to run some rads. The problem is that heat only flows “down hill” so if the coolant is 40C and the canal is 5C, plenty of heat is dumped to canal via skin tank, but coolant at 40C is not much use for heating rads. So it is not just heat but also temperature that has to be considered.

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I've used the calorifier as a heat exchanger to warm up the radiators.  I have a temperature sensor on the calorifier so I can see if it drops the temperature.  I've fitted a switch to turn on the boiler pump without affecting the boiler.

On this boat the radiators will get quite warm, but not really hot. A similar arrangement on my last boat made the rads much hotter.  The temperature on the calorifier doesn't drop, so the heat going in from the engine is the same or more than is being taken out.  The engine feed to the calorifier is at full temperature, i.e. thermostat fully open,  even when just normally cruising, so it isn't overcooling the engine.

It does help to take the chill off the boat and dry the towels, but doesn't get as hot as with the boiler.  It needs careful balancing between the rads and calorifier (I've got 22mm full flow valves on each circuit) to get the best heat.

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28 minutes ago, Jen-in-Wellies said:

This seems to be backed up by @peterboat's figures from his electric wide beam. 2 to 3kW, or 3 to 4 HP to push it along in deep water at 3mph. Electric motors being much closer to fully efficient than a diesel will ever be and able to provide a direct measure of the power used. It still needs a cooling fan though, so some of that power is ending up as heat, so maybe 2 to 3 HP to turn the prop. Very little of my Beta 43's supposedly 43HP output from the crank shaft ever gets used to push the boat. The 43HP is for maximum rev's, which it has never reached. Most of the time pootling along at 1500rpm or so. @Mike the Boilerman's figures seem reasonable for a typical narrowboat in terms of power available from the cooling system. Engines don't like being overcooled. People taking out the thermostat to "cure" an overheating problem is a bad idea. The cauliflower cooling circuit is often independent of the engine thermostat, and only the relative sizes of the hoses to restrict the amount of coolant that goes to the calorifier, compared to the skin tank.

 

Jen

 

20 minutes ago, nicknorman said:

I’d say taking heat from the calorifier is bad for two reasons, one being that you won’t get more than a kw or two out of it (because it’s not a particularly large or efficient heat exchanger). And secondly because the calorifier is connected directly to the engine and so there is a risk of over-cooling at low power / idling etc. The calorifier as intended will heat up after 30 mins / an hour or so and once the water is hot, will no longer take any heat from the engine thus there is no risk of prolonged over cooling.

 

So I fitted a plate heat exchanger on the skin tank circuit, ie after the thermostat and thus no risk of overcooling. The rads don’t get as hot as they do with the Mikuni but I think I undersized the heat exchanger. It was rated at 12kw but of course you have to understand what that means - it refers to the heat transferred across a large temperature gradient. In reality you are starting with water not that hot (Beta43 standard thermostat is 72C) and the heat flowing across the exchanger is proportional to the temperature difference so you are never going to get the rads up to 72C. But you will get closer to 72C with a bigger exchanger.

 

As to the detail of the plumbing I used a small pump from Solarproject which so far has been faultless, along with 2 of their low operating pressure one way valves. These mean that when I run the Mikuni it doesn’t back flow through the heat exchanger, and when I run the exchanger pump it doesn’t back flow through the Mikuni.

 

In practice when cruising at narrow canal speeds the rads get reasonably hot and certainly make a big difference to cabin temperature. When traversing a flight of locks (engine mostly at idle) they only get warm. Which just goes to show the risk of overcooling if you took the heat off before the thermostat.

 

I also changed the thermostat for an 80C one which has helped a bit.

When I had the diesel engine [twin thermostat model] I used to have a 140 litre heat store which the engine, whispergen, rayburn and bubblestove all used to heat. The heatstore used to heat the cauliflower by gravity and the central heating by pump, it was effective to the point that the skin tank very rarely got warm never mind hot! The engine did run at 88 degrees according to the gauge.

Jen is right I use just over 3 KW to do 3MPH in deep water and a an electric blower is needed to keep the electric motor cool the original fan wasnt up to the job as the motor doesnt spin fast enough at cruising speed

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I extracted engine heat via a £30 ebay plate exchanger in  the cooling circuit - and it worked well.  In colder weather, the boat is nicely warmed whilst cruising and as you tie-up - so you have plenty of time to stir the solid fuel fire into action.   I also put a towel radiation in the engine/calorifier circuit which was nice too - both summer and winter.

 

It was a twin thermostat engine, so there was no chance of over-cooling the engine to a significant degree.  As Nick Norman says, you could put it after the thermostat in a single thermostat engine or, if the plumbing runs made it easier, some sort of pipestat controlling the circulation pump.

 

 

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I have done it and it does work for me... I hate the idea of the heat going to waste on a cruise.

 

I just have to remember to turn off the pump when i stop or my lovely hot water is no longer hot...

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3 hours ago, Tacet said:

extracted engine heat via a £30 ebay plate exchanger in  the cooling circuit - and it worked well.  In colder weather, the boat is nicely warmed whilst cruising and as you tie-up - so you have plenty of time to stir the solid fuel fire into action.

Me too, except our heat exchanger is out of an old combi gas boiler, probably rated at about 25 KW. The engine (25 hp LW ) heats the 5 rads nicely when cruising, but not as quickly as the 5kw webasto water heater.

 

The heat exchanger is circulated by a 12v pump which doesn't cut in until the engine thermostat starts passing water at more than 45 deg measured on the outside of a copper pipe.

 

I'm absolutely delighted with the resulting comfort level and the engine never gets over cooled.

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