Skin tank plumbing

[booke23]

I helped a guy on my moorings with changing the coolant on his boat (Beta 38 with a skin tank). We blead the skin tank and everything seemed ok. However after the engine had been running for a while and the thermostat opened, I was feeling the pipes going to the skin tank and realised the hot feed from the thermostat was plumbed into the bottom of the skin tank and the cool return was connected to the top of the tank.....ie the wrong way around!

 

Is there any possible reason for this configuration or has the boat builder made a mistake when plumbing it up? (the boat is only 4 years old).

[Tracy D'arth]

6 of one and half a dozen the other.

 

Plumbing hottest to the top gives some assistance to the pump by thermosyphon,  may be negligible. 

Plumbing hottest to bottom gives the highest heat transfer into the coldest water, again may be negligible.

 

It may however be easier to bleed air out the way it is, you can do it with the engine running.

[David Mack]

3 hours ago, Tracy D'arth said:

Plumbing hottest to bottom gives the highest heat transfer into the coldest water,

No it doesn't. The water temperature outside the skin tank will not vary very much from top to bottom when the boat is moving. But if you introduce the hot water at the bottom of the tank it will rise straight to the top, and some of it will enter the pipe back to the engine still warm. Introduce it at the top and it will tend to stay there, giving it more time to give up its heat to the canal. The water at the bottom of the tank will be cold by the time it enters the pipe back to the engine.

[Tracy D'arth]

Every skin tank I have seen has baffles to make the water follow a meandering route so I does not all rise to the top anyway.

If the tank is at the same temperature throughout its depth as you profess then the water going from the top to the engine will be the same temperature as the bottom.

You can't have it both ways! 

[David Mack]

I didn't say the tank was at the same temperature throughout its depth, I said the canal water outside will be at the same temperature over the depth if the tank.

 

Whether or not you have baffles, gravity will still take the hotter water in the tank to the top, its just that if you introduce hot water at the bottom, baffles will slow, but not prevent, its ascent.

[Tracy D'arth]

I will agree to disagree.

[booke23]

It does seem counter intuitive having the hot water enter via the bottom. I’ve had a look through @Tony Brooks website on his very informative section on cooling, but I couldn’t find any reference to which way round the tank should be plumbed. I’ve also tried searching for information on skin tank design and operation with no joy. I suppose it is quite a niche subject!

[Tony Brooks]

19 minutes ago, booke23 said:

It does seem counter intuitive having the hot water enter via the bottom. I’ve had a look through @Tony Brooks website on his very informative section on cooling, but I couldn’t find any reference to which way round the tank should be plumbed. I’ve also tried searching for information on skin tank design and operation with no joy. I suppose it is quite a niche subject!

 

FWIW I would put the hot in the top and take the cold out of the bottom so thermo-siphoning  would assist rather than inhibit circulation. I would use three of four horizontal baffles but some authorities show verticals. I can't get my head around that and fear it might prevent circulation if the top of the tank filled with air/gas.

[blackrose]

1 hour ago, Tracy D'arth said:

I will agree to disagree.

 

I agree with David. Hot water from the engine should be plumbed into the top of the skin tank for greater heat transfer. However the tank is baffled and all other things being equal, plumbing hot into the bottom will allow water to move to the top quicker and therefore result in reduced heat transfer compared to the other way around.

[Rincewind]

55 minutes ago, Tony Brooks said:

I would use three of four horizontal baffles but some authorities show verticals. I can't get my head around that and fear it might prevent circulation if the top of the tank filled with air/gas.

I came across this very scenario that Tony describes, vertical baffles in a skin tank that created an air lock at the top of the tank and led to overheating.  The only way to alleviate the problem was to drill & tap the tank above the air lock in order to vent it with small bore pipework to the header tank.

[booke23]

Thank you for all the comments. I spoke to him today and asked if he ever had any overheating issues and he has not (Although he has only been on canals). I have to say the point about a small airlock stopping circulation with the hot coming in the bottom is a very good one and after I mentioned this to him he is now considering swapping the pipes around.

[Tony Brooks]

11 hours ago, booke23 said:

Thank you for all the comments. I spoke to him today and asked if he ever had any overheating issues and he has not (Although he has only been on canals). I have to say the point about a small airlock stopping circulation with the hot coming in the bottom is a very good one and after I mentioned this to him he is now considering swapping the pipes around.

 

If that was me comment then it was referring to vertical baffles nut I can see taking the cold out of the top could well allow airlocks to form because the engine cooling inlet is probably lower then the skin tank top connection.

[ditchcrawler]

I have a vent in the top of my skin tank

[cuthound]

On 16/04/2022 at 13:29, ditchcrawler said:

I have a vent in the top of my skin tank

 

So do I. As that is the place where air is most likely to collect, I thought it would it would be standard practice.

[ditchcrawler]

3 hours ago, cuthound said:

 

  I thought it would it would be standard practice.

So would I