Advice needed on over heating engine

[Tacet]

3 minutes ago, Mike the Boilerman said:

 

 

From the depths of my memory I seem to recall a diesel is approx 50% efficient, i.e. of the energy being used in fuel, about half results in shaft output HP and the other hlfresults in heat and noise - mostly heat. So for any given engine, the shaft power output is roughly equal to the heat power output, and the heat power output needs to be dumped through the cooling system. This is why Tony is using the power output of the engine as a proxy for the heat transfer power to cool it, I suspect. 

Hmmm.  If the rads were capable of dissipating 5kw at 50/50 power/heat, you could cool a 13 hp engine.  Or make use of 13hp  more than if you were not using the rads, which is a significant contribution for a marginally cooled engine. I.e about the rule-of-thumb equivalent of 3 sq ft of skin tank

 

Tony is quite right when he doubts the rads will dissipate the nominal 5kw of the stove.  12000 BTU (the rating of the rads) is only about 3.5kw.  And that depends on a achieving a difference of 50 or 60 degrees C

[blackrose]

23 hours ago, alan_fincher said:

If you can provide a large enough external tank on the existing swim there is no downside to disabling or removing the existing one.  I'm with Tony that you are far less likely to suffer any air lock / bleeding problems if there is only one.  I fully accept your solution has worked fine for you, but I can imagine situations in a narrow boat where it might not.  "Keep it simple" is always best, to my mind.

 

Yes, I agree keeping it simple is best, and I think adding a second tank on the other swim is the simplest solution. There's nothing simple about cutting the existing tank out! It's unnecessary work.

12 hours ago, David Mack said:

 

No with 2 thicknesses of steel between the cooling water and that canal water you will get much less heat transfer - even if the thermal paste does its job. A new internal tank must have the existing shell plating as it's outside face - as in Blackrose's photo earlier in the thread.

I agree. Welding a complete 6-sided tank on is a bad idea. 

[blackrose]

12 hours ago, alan_fincher said:

I don't think it will work, and would strongly advise against it.  The likelihood that the new tank will exactly match the profile of the swim it is being added to seems slight to me, and I would expect this to be a most unpredictable arrangement.

 

I agree. On the other hand a four sided tank like the one I had welded in can still be pushed, prised and tacked into place if it doesn't align with the curve exactly.

9 hours ago, Robbo said:

 

I haven't kept up with the thread, but if you have central heating and radiators then just get a heat exchanger and pump.   Cheap, effective and useful (bar summer!).

I did try this on my boat before installing a skin tank. The cost was not insignificant and it didn't work. Not sure what I did wrong?

Edited August 13, 2018 by blackrose

[Tony Brooks]

The HP quoted on an engine is more or less (depending on the standards its tested to) the power available at the flywheel. AFTER all the  energy lost to the exhaust, cooling system, and friction losses are accounted for. The Beta figure for skin tank size is given in HP which again is the dynamometer figure which is the power available AFTER all the  energy lost to the exhaust, cooling system, and friction losses are accounted for. In other words it is a kind of proxy for the heat the cooling system has to dissipate because the heat to be dissipated and the engine HP are proportional.

 

As we don't have any thermal efficiency figures for the specific engine all we can use is the data we do have. If we knew the efficiency (not a range of possible efficiencies) then we could use the HP figure to calculate the amount of fuel used and from that guess the proportion lost t the cooling system but its not necessary in this case because the 1 sq ft per 4 HP already takes the efficiency etc. into account.  The only figure we have and can use is the rated HP.

 

Fitting a heat exchanger system to dump heat to the central heating will take heat out of the cooling system but will it be enough? It may or may not be but based on the only figures we have suggest to me that its a very good chance that it will not be. It seems to me to be an awful lot of work and expense for what looks like a marginal improvement in engine cooling. Its an excellent idea if you want to use waste engine heat to heat the boat but it does not look like a good idea for helping engine cooling in this case.

 

 

[Sea Dog]

10 hours ago, blackrose said:

I did try this on my boat before installing a skin tank. The cost was not insignificant and it didn't work. Not sure what I did wrong?

Such a tempting prospect from a boat heating perspective, but not really the ideal for engine cooling, eh.  I have fitted an additional small towel radiator in my bathroom paralleled off the return line from the calorifier. Three valves allow it to be isolated and throttled, but it works great with them all wide open.  Warmer bathroom in winter cruising, dry towels all year round, engine as happy as ever. Even this summer it was no issue having it in circuit with the bathroom window open. We digress...

 

[bizzard]

The jolly old fridge, or better still a freezer could be used for extra engine cooling by bashing inlet and outlet holes on either side of the cabinet and running the coolant pipe through and coiling and jumbling a few yards of piping inside it and then back to the engine. You would have some control of the amount of cooling too by the use of the fridges thermostat control.

[rusty69]

5 minutes ago, bizzard said:

The jolly old fridge, or better still a freezer could be used for extra engine cooling by bashing inlet and outlet holes on either side of the cabinet and running the coolant pipe through and coiling and jumbling a few yards of piping inside it and then back to the engine. You would have some control of the amount of cooling too by the use of the fridges thermostat control.

And could be attached to the hull with a fridge magnet!

[Tacet]

Looking at this another way at around 56 w/m K,  1 sq ft of 6mm steel will dissipate approximately 3.5kw (or 4.6hp) with a temperature differential of, say,  40 degrees (a guess at the average across the whole swim tank)  That is, assuming I have the U value and maths correct.

 

This is reasonably close to the rule of thumb of 1 sq ft for 4 hp.  But, of course, the hp from the engine is being consumed by propulsion and exhaust losses – not just in raising the temperature of the cooling water.  So the skin tanks are not as efficient (or is that inefficient?) as theory suggests.   No doubt blacking and rust comes into the equations.

 

So if the radiators are heated so are to lose their rated 1200 BTU (3.5kw), they could add the equivalent of 1 sq ft of skin tank to the existing 6 sq ft – which is not very much.  But allowing for the inefficiency (or is that now efficiency?) of skin tanks, it might be double that.   Or another way, it might allow the engine to develop and additional 4hp-8hp without overheating.

 

Assuming the plumbing was reasonably convenient, I would look to heat the radiators from the engine, as the principle of free cabin heating is satisfying and useful.  When the engine is in danger of overheating, bringing in the radiator circuit will give an initial cooling effect and a more modest continuing effect.   But it may well be insufficient in all circumstances.

[David Mack]

44 minutes ago, Tacet said:

Looking at this another way at around 56 w/m K,  1 sq ft of 6mm steel will dissipate approximately 3.5kw (or 4.6hp) with a temperature differential of, say,  40 degrees (a guess at the average across the whole swim tank)  That is, assuming I have the U value and maths correct.

You may well have the U value and maths of conduction through the steel correct, but there is another factor in play - the surface effect of getting heat from the coolant to the steel and from the steel to the canal water. The coolant immediately adjacent to the face of the steel will be cooler than the general body of coolant further away. The effect of this is to reduce the overall heat flow. Similarly on the canal water side. Turbulent flow will minimise this effect.

 

The 4hp per square foot rule is an umbrella allowance which takes into account all of these effects for a typical situation, including the relationship between engine output power (at the flywheel) and the amount of heat to be lost from the coolant.

[cuthound]

18 hours ago, Mike the Boilerman said:

 

 

From the depths of my memory I seem to recall a diesel is approx 50% efficient, i.e. of the energy being used in fuel, about half results in shaft output HP and the other hlfresults in heat and noise - mostly heat. So for any given engine, the shaft power output is roughly equal to the heat power output, and the heat power output needs to be dumped through the cooling system. This is why Tony is using the power output of the engine as a proxy for the heat transfer power to cool it, I suspect. 

 

The big turbocharged and intercooled diesels that I was familiar with in standby generators were about 45% efficient. I would expect a normally aspirated diesel engine, as used in as boat to be about 40% efficient.

 

Incidentally the current generation of petrol powered engines used in Formula One racing cars are reputedly 50% efficient, because of the turbocharger,  heat recovery from the turbo and regenerative braking. The latter two being converted to electrical energy to power a hybrid system.

Edited August 14, 2018 by cuthound
To remove a letter masquerading as a space.

[PeterF]

6 hours ago, Tacet said:

Looking at this another way at around 56 w/m K,  1 sq ft of 6mm steel will dissipate approximately 3.5kw (or 4.6hp) with a temperature differential of, say,  40 degrees (a guess at the average across the whole swim tank)  That is, assuming I have the U value and maths correct.

Not quite right, a 40degC temp difference through 6mm steel will transfer 35kW through 1 sq.ft., you have dropped a factor of 10 in your calc. In this type of heat transfer the conduction through the metal separator is usually insignificant to the convective heat transfer resistance between the fluid and the metal as has been pointed out elsewhere.

 

(56 x 40 / 0.006 / 3.281 / 3.281 = 34680 W)

[George and Dragon]

16 hours ago, PeterF said:

Not quite right, a 40degC temp difference through 6mm steel will transfer 35kW through 1 sq.ft., you have dropped a factor of 10 in your calc. In this type of heat transfer the conduction through the metal separator is usually insignificant to the convective heat transfer resistance between the fluid and the metal as has been pointed out elsewhere.

 

(56 x 40 / 0.006 / 3.281 / 3.281 = 34680 W)

Not that I remember anything about heat transfer but what is 3.281?

 

ETA  now I know: the reciprocal of 0.3048

Edited August 15, 2018 by George and Dragon
lightbulb moment

[PeterF]

3.281 ft per m

[George and Dragon]

You see, I would have converted the area to square metres

[Stilllearning]

On 13/08/2018 at 13:36, Michael Quelch said:

I already have a circulation pump for three radiators running the length of the boat to the back boiler on our squirrel stove. So could easily fit a heat exchanger into the circuit not to worried about the interior of the boat getting warm as that’s easily vented and would be free extra heating in the winter lol

 

That could be a potential option thank you. 

 

 

This is what I suggested in post #23

?

[Michael Quelch]

Hi Chaps, 

 

so so going off up the river is now off the cards for the time being, we are just going to pop out of the marina a very short distance about 30min chug to get out for a couple of days so the kids get too see a bit of the river. 

 

I have decided to go with a heat exchanger to the central heating system for a bit of extra cooling but also for the extra heating, I will be getting her out next year and have a larger skin tank fitted on the outside. 

 

As I fitting the engine hoses myself can someone please confirm if they are plumbed in correctly I.e in the right direction? Which way does the water flow around? 

 

The pipe ups from under the engine header tank is attached to the top of the skin tank and the bottom hose off the skin tank then goes to the gearbox cooler on the prm 150 

 

the first image is my engine and the second is a similar set up that I’ve found on the web showing the gear box cooler

 

 

[rogerc]

On 11/08/2018 at 15:59, Stilllearning said:

If there is an existing central heating system on the boat, you could connect it up to the engine cooling system. You will curse it on warm days, but it should do the job.

i added a heat exchanger on my boat its briliant in the winter and i have turned it on on a couple of occasions when the engine has got hot it gives a quick response the exchanger cost me £20 on ebay for a 20Kw. i also fitted a van heater from a scrap yard to create a drying cupboard. and i have 2 calorifiers so enough hot water for 8-10 showers on my 65 ft boat with a 42 HP engine.

[Chewbacka]

1 hour ago, rogerc said:

i added a heat exchanger on my boat its briliant in the winter and i have turned it on on a couple of occasions when the engine has got hot it gives a quick response the exchanger cost me £20 on ebay for a 20Kw. i also fitted a van heater from a scrap yard to create a drying cupboard. and i have 2 calorifiers so enough hot water for 8-10 showers on my 65 ft boat with a 42 HP engine.

Impressive, but how often do you have 9 showers in a day???

[MtB]

10 minutes ago, Chewbacka said:

Impressive, but how often do you have 9 showers in a day???

 

It might be two people having 4 1/2 showers each a day... 

 

 

[rogerc]

not often but it sleeps 8. we have had 11 including grandchildren. but plenty hot water is always useful. unfortunately it only stays hot for 1-2 days.

[ditchcrawler]

6 minutes ago, Mike the Boilerman said:

 

It might be two people having 4 1/2 showers each a day... 

 

 

O I'm into the half bit 

[system 4-50]

2 hours ago, Chewbacka said:

Impressive, but how often do you have 9 showers in a day???

But it might be 2 blokes and 2 - oh, never mind.